Biodiversity of aquatic insects

Article

Biodiversity of aquatic insects: spatial gradients and environmental correlates of assemblage-level measures at large scales Jani Heino1,2 Finnish Environment Institute, Research Programme for Biodiversity, P.O. Box 413, FI-90014, University of Oulu, Finland

1

(correspondence). E-mail: [email protected] Department of Biology, University of Oulu, P.O. Box 3000, FI-90014 University of Oulu, Finland.

2

Received 15 July 2008; accepted 29 October 2008; published 12 March 2009

Abstract Biodiversity embraces multiple facets of the variability of nature, although most research has dealt separately with population-, species- and assemblage-level measures. This review concentrates on aquatic insect biodiversity and the assemblage-level measures, such as species richness, assemblage compositional variation, taxonomic distinctness and functional diversity. Most studies on aquatic insects have considered biodiversity patterns based on surveys of local assemblages along geographical and environmental gradients, while there is a virtual lack of studies that have considered regional grain sizes (i.e. the size of the observational unit). Latitudinal gradients at both regional and local grain are ambiguous in aquatic insects, as different studies have found either higher or lower local diversity in the tropics than in the temperate zone. Other geographical patterns in aquatic insect diversity may also be relatively weak, as suggested by subtle among-ecoregion differences in both local species richness and assemblage composition. An ecological explanation for the absence of strong geographical gradients is that local environmental features may not necessarily vary with geography, and these factors may override the influences of historical and climatic influences on local diversity.

Evidence from

large-scale studies suggests, however, that not only various local habitat and ecosystem variables, but also those measured at the watershed, regional and geographical scales are needed to account for variation in the species richness and assemblage composition of aquatic insects. Within regional species pools, species richness and assemblage composition in both lotic and lentic ecosystems vary most strongly along gradients in habitat size and acidity.

Knowing how α-, β- and γ-

diversity of aquatic insects vary along geographical and environmental gradients has important implications for the conservation of biodiversity in freshwater ecosystems.

Such knowledge is

not yet well-developed, and two aspects should be considered in future research. First, further survey research on lotic and lentic ecosystems is necessary for improving our understanding of general biodiversity patterns.

DOI: 10.1608/FRJ-2.1.1

Second, given that freshwater ecosystems are facing a severe

Freshwater Reviews (2009) 2, pp. 1-29 © Freshwater Biological Association 2009

Heino, J.

biodiversity crisis, the implementation of representative networks of freshwater protected areas would certainly benefit from increased understanding of patterns in aquatic insect biodiversity. Keywords: Assemblage composition; congruence; environmental gradients; functional biodiversity; higher taxon surrogates; lakes; latitudinal gradient; ponds; rivers; species richness; streams.

Introduction

based on surveys of local ecosystems within small regions (Vinson & Hawkins, 1998; Jeppesen et al., 2000).

Biodiversity embraces multiple facets of the variability

A decade ago, Vinson & Hawkins (1998) reviewed

of nature, ranging from genes to species, and from

factors affecting stream insect biodiversity at local,

ecosystems to biomes (Gaston & Spicer, 1998).

Most

catchment and regional scales. While their review focused

research has traditionally been based on populations,

on patterns at different scales, especially those at the

species and assemblages (Angermeier & Schlosser, 1995).

within-stream scales, large scales received less attention.

This review will concentrate on assemblage-level measures

However, a large number of large-scale studies spanning

of biodiversity using aquatic insects as a model group.

catchments, ecoregions and countries have emerged since

Assemblage-level measures in this context include species

the publication of that paper. One aim of the present

richness, assemblage composition, taxonomic variability

paper is thus to update and complement the information

and functional diversity. The approach in this review is

provided by the previous reviews on stream insect

general, however, and most consideration will be grounded

biodiversity (Vinson & Hawkins, 1998; Meyer et al., 2007;

on general ecological knowledge.

Clarke et al., 2008). The present review concentrates on

General patterns of biodiversity include diversity–

large-scale patterns across natural environmental gradients,

latitude (e.g. Willig et al., 2003), diversity–productivity (e.g.

and the reader should consult previous reviews for further

Waide et al., 1999), diversity–disturbance (e.g. Petraitis et

insights into small-scale aspects of aquatic insect ecology

al., 1989) and diversity–heterogeneity relationships (e.g.

(Ward, 1992;Allan & Castillo, 2007; Vinson & Hawkins, 1998)

Vinson & Hawkins, 1998). These relationships have been

and anthropogenic influences on freshwater ecosystems

studied across a wide variety of spatial scales. Most of the

and biodiversity (Malmqvist & Rundle, 2002; Allan, 2004;

research on latitudinal gradients has dealt with macro-

Dudgeon et al., 2006). The emphasis in this review will

scales (i.e. wide spatial extents and large-sized grids), and

be on aquatic insects, because they typically account for

these studies have largely concentrated on terrestrial and

more than 80 % of taxa in freshwater macroinvertebrate

marine taxa. Many macro-scale studies on terrestrial and

assemblages (e.g. Sandin, 2003; Briers & Biggs, 2005).

marine taxa show strong latitudinal gradients in species richness (Rosenzweig, 1995; Gaston & Blackburn, 2000;

Importance of spatial scale

Willig et al., 2003; Hillebrand, 2004). By contrast, few studies have examined freshwater taxa in this respect,

Spatial scale embraces two components (Wiens, 1989).

and the studies that exist have considered latitudinal

First, grain refers to the size of the observational unit. In

variation in local richness (Vinson & Hawkins, 2003;

aquatic studies, grain may be related to units sampled by a

McCreadie et al., 2005). Similarly, although numerous

small sampling device (e.g. Surber sampler), parts of whole

studies in the terrestrial realm have concentrated on

ecosystems (e.g. a stream riffle) or whole ecosystems

macro-scales when examining diversity–productivity and

(e.g. a pond). In macroecology, grain sizes are typically

diversity–heterogeneity relationships (Rosenzweig, 1995),

much larger than these, and macroecological studies

freshwater studies have considered these relationships

are often based on large-scale grids, regions or countries

© Freshwater Biological Association 2009

DOI: 10.1608/FRJ-2.1.1

Aquatic insect biodiversity

(Gaston & Blackburn, 2000). Second, extent refers to the area

as α-diversity, and between-habitat diversity as β-

over which the sampling units are distributed. In aquatic

diversity. In this review, the first one refers to the number

studies, extent may be a drainage basin, an ecoregion or

of species in a local ecosystem, while the latter refers

even a continent. In the present review, emphasis will be

to variation in assemblage composition between local

given to grain sizes that comprise parts of ecosystems and

ecosystems. While α-diversity thus reflects the importance

whole ecosystems and will be directed at extents spanning

of local abiotic and biotic factors, β-diversity is related

from single drainage basins and ecoregions to countries

to the way in which species respond to environmental

and continents.

heterogeneity along ecological gradients.

Finally, γ-

The importance of considering grain and extent is

diversity refers to regional species richness, and is the

closely related to the fact that patterns and associated

sum of differences among habitats in species composition.

processes typically change with regard to the spatial

It is thought to be determined primarily by large-scale

scales considered (Levin, 1992; Whittaker et al., 2001).

evolutionary, historical and climatic factors. In this review,

For example, the determinants of species richness may

γ-diversity refers to the number of species in separate

differ considerably when streams versus large-scale grids

geographical regions, large-scale grids or drainage basins.

(e.g. 100 km × 100 km) are considered across the same

Another approach to understanding the effects of

spatial extent (Ricklefs, 1987). In the former case, not only

spatial scale relates to the importance of environmental

historical and climatic processes determine species richness,

variables measured at different scales. This approach is

but also local-scale factors, such as disturbance, stress and

closely associated with the idea of environmental filters

heterogeneity. By contrast, when large-scale grids are

(Fig. 1) (Tonn, 1990; Poff, 1997) and has been followed in

considered across the same spatial extent, it is unlikely that

a number of studies on freshwater macroinvertebrate

‘mean’ local habitat conditions override the influences of

assemblages (Johnson & Goedkoop, 2002; Sandin, 2003;

historical and climatic factors on species richness. These

Townsend et al., 2004; Heino et al., 2007a). These studies

scale-dependent differences mainly stem from the fact

have generally considered multiple nested scales, with the

that local-scale environmental factors are homogenised

assumption that factors at different scales are hierarchically

across large-scale grids.

Another reason is that local

structured, and that factors at larger scales determine, at

species richness is pooled over all local assemblages when

least in part, those at smaller scales (Frissell et al., 1986;

large-scale grids are considered, yielding a measure of

Hildrew & Giller, 1994). In this review, both of these

regional species richness. In this vein, it is also important

approaches will be considered, yet a clear distinction will

to note that latitudinal gradients are typically stronger

be made between (i) spatial scale in terms of grain and

when large-scale grids are examined, whereas weaker

extent and (ii) variables measured at different spatial scales.

species richness–latitude relationships may emerge when local ecosystems are considered (Hillebrand, 2004). This pattern may result from different species and species

Large-scale patterns in species diversity

groups responding differently to local-scale environmental factors that do not necessarily co-vary with latitude.

Geographical patterns in regional diversity

More generally, these notions suggest that variation in biodiversity may be best explained by different factors at

Few studies have considered latitudinal gradients in

different grain sizes and geographical extents (Rosenzweig,

aquatic insect diversity. Most of the existing studies have

1995; Whittaker et al., 2001; Blackburn & Gaston, 2002).

been based on surveys of local ecosystems, but a couple

Diversity measured at different scales has typically

of studies have examined patterns across large grain size

been expressed as α-, β- and γ-diversity (Whittaker, 1975).

and considered regional species richness. For example,

Within-habitat diversity has typically been considered

Boyero (2002) examined the regional species richness of

DOI: 10.1608/FRJ-2.1.1

Freshwater Reviews (2009) 2, pp. 1-29

Heino, J.

mayflies and dragonflies in the New World. A general finding was that of the four large regions compared, after region area was accounted for, the regional species richness

Continental species pool

of mayflies was lowest in tropical South America and highest in Central America. A corresponding analysis of dragonflies showed that Central America was again the most diverse region, whereas North America was home for History and Climate

the lowest number of species. When the area of each region was not taken into account a different pattern emerged.

Watershed

Ecosystem

Macrohabitat

Decreasing spatial scale of environmental filters

Regional species pool

Mayfly diversity was clearly highest in North America and lowest in Central America, whereas dragonfly diversity was by far highest in tropical South America and lowest in temperate South America. The degree to which the four regions comprised too large a grain size for a meaningful analysis is unknown, and more detailed analyses based on smaller grain size would allow more useful insights into the latitudinal patterns of aquatic insect diversity. A comparison of major aquatic insect groups across Europe shows that there are significantly negative species richness–latitude relationships in mayflies, dragonflies and stoneflies, while other groups do not exhibit significant latitudinal patterns (Table 1). However, most aquatic insect orders attain their highest levels of diversity in France, as opposed to the more southerly countries of Portugal and Greece. Lowest levels of diversity occur in Latvia,

Microhabitat

the Netherlands, Denmark, Norway and Finland. These among-country differences reflect two potential suites of factors behind geographical gradients in diversity. First,

ASSEMBLAGE

Fig. 1. A schematic model of environmental filters affecting local assemblages. The continental species pool is determined by speciation and extinction processes at very large spatial and temporal scales. Filters at the largest scale in this scheme are history (e.g. speciation, extinction, dispersal) and climate (e.g. temperature, precipitation, energy) which determine the structure of regional species pools. Within the limits of regional species pools, there are filters at four levels that eventually determine local assemblages. These are watershed (e.g. vegetation, hydrological regimes), ecosystem (e.g. temperature, water chemistry), macrohabitat (e.g. depositional vs. erosional habitats, macrophyte cover) and microhabitat (e.g. macrophyte structural complexity, substratum particle size) filters. These filters determine diversity, composition, as well as functional and taxonomic variability of assemblages through species traits. Only species with suitable traits are able to overcome the challenges presented by the filters at each scale. For original ideas, see Tonn (1990) and Poff (1997). © Freshwater Biological Association 2009

southern and central European countries with major mountain ranges, such as France, Germany, Spain, Italy and Austria, support high diversity, whereas lowland countries, such as the Netherlands, Latvia and Denmark, support low diversity in most aquatic insect groups. This suggests that large altitudinal variation increases the heterogeneity of freshwater habitats and variability of thermal conditions, which has positive effects on diversity.

Also, if geographically separated mountain

ranges are present, each mountain range may support unique species, contributing to high diversity per country. Second, diversity decreases to the north of Europe, with diversity being clearly higher in the southerly countries, such as Spain and Italy, than in the northerly countries, DOI: 10.1608/FRJ-2.1.1

DOI: 10.1608/FRJ-2.1.1

158

-

148

-

114

58

67

142

358

France

Bulgaria

Italy

Greece

Portugal

Spain

63

84

87

-

97

-

75

558

138

57

91

146

82

147

70

-

110

125

101

126

101

117

50

33

12

25

37

10

35

36

-0.692**

Stoneflies

Plecoptera

124

-

54

60

89

-

79

-

-

67

61

37

73

62

66

-

56

65

60

64

23

52

53

-0.437

Bugs

Heteroptera

1113

-

231

269

342

-

579

-

-

-

358

349

480

287

419

-

410

201

294

354

202

269

-

-0.193

Beetles

Coleoptera

1173

355

216

150

382

-

465

-

-

305

307

176

322

251

266

-

197

188

166

219

188

190

214

-0.448

Caddisflies

Trichoptera

1309

-

-

-

-

-

635

-

-

342

556

327

703

239

411

-

520

197

528

506

-

503

539

-0.127

Midges

Chironomidae

Note: The number of Odonata species in Finland is actually 54. For comparability purposes, the figure given is that from http://www.freshwaterecology.info.

* P < 0.05; ** P < 0.01

Europe

-

-

Romania

77

77

71

84

69

73

Ukraine

114

Poland

-

52

85

-

Belgium

Switzerland

51

UK

53

116

36

Netherlands

52

59

122

43

Denmark

Austria

60

Sweden

54

Slovakia

49

Latvia

45

46

97

46

Norway

143

53

Finland

-0.805*

Dragonflies

Germany

-0.629**

Correlation

Odonata

Czech

Mayflies

Country

Ephemeroptera

Table 1. Numbers of species in major aquatic insect orders in European countries. Data were compiled from http://www.freshwaterecology.info (web site accessed on December 28, 2007); note that data for some countries and orders may have been updated on the website since. Information for many orders was not available for a number of countries (-). Note that information for the family of non-biting midges (Chironomidae: Diptera) may still be inadequate even in countries for which the numbers of species are given. Spearman rank correlations between species richness and latitude (south latitude of each country) are also shown. Countries are arranged on an approximate north–south gradient. Aquatic insect biodiversity


Heino, J.

such as Finland and Norway. This suggests that large-

preferences, with mid-latitude regions possibly providing

scale historical and climatic factors have some effects on

a wide range of habitats suitable to various kinds of taxa.

aquatic insect diversity along long geographical gradients.

However, the fact that other studies have found either

For example, Pleistocene glaciations may have had some

higher (Stout & Vandermeer, 1975; Lake et al., 1994;

effects on the present-day patterns of diversity, with some

Jacobsen et al., 1997) or lower (Stanford & Ward, 1983;

species having not yet reached all suitable areas. One caveat

Arthington, 1990; McCreadie et al., 2005) taxa richness

should be considered in this comparison. Taxonomic

in tropical than in temperate streams suggests that more

and faunistic traditions in each country may affect the

comprehensive studies, especially in tropical streams and

degree to which observed diversity reflects clear patterns.

spanning several tens of degrees of latitude, are needed

For some insect groups, such as non-biting midges,

to resolve whether stream insects do or do not show clear

historical faunistic work may differ considerably among

latitudinal gradients.

countries. This is critical for comprehensive comparisons

At a scale smaller than continents, some studies have

of aquatic insect diversity between countries and regions.

examined latitudinal patterns in the local species richness

Latitudinal species richness patterns may vary with

and assemblage composition of freshwater macroinverte-

the habitat types of the considered taxa. Ribera et al. (2003)

brates. In North Europe, where ecoregional delineations

found that the diversity of lotic beetles was negatively

largely follow latitude, there are differences between

related to latitude across countries in Europe, whereas

ecoregions in both species richness (Sandin & Johnson,

lentic beetles did not show a significant latitudinal pattern.

2000; Heino et al., 2002) and assemblage composition

The authors interpreted this finding as an indication

(Sandin & Johnson, 2000; Heino et al., 2007a). However,

that the higher historical persistence of lotic than lentic

these patterns are typically rather weak, and species

habitats contributed differently to diversity patterns

richness does not show a linear response to latitude

in the two groups of species. Proving the generality

from southern to northern ecoregions. Studies that have

of this hypothesis clearly awaits further studies on

directly examined species richness–latitude relationships

other aquatic insect groups and geographical regions.

in stream macroinvertebrates in Sweden and Finland have corroborated these findings (Sandin, 2003; Heino,

Geographical patterns in local diversity

unpublished manuscript). By contrast, it has been found that the species richness of stream macroinvertebrates

Patterns detected at regional scales may change when local

declines with latitude across Alaska (Wrona et al., 2006).

ecosystems are considered. Based on global literature on

This finding is in accordance with a previous study

local genus richness, Vinson & Hawkins (2003) found no

that found that stream insect genus richness declines

clear linear latitudinal richness trends for stream mayflies,

linearly from 40°N to 70°N in North America (Vinson &

stoneflies and caddisflies. By contrast, the highest levels

Hawkins, 2003). Similarly to lotic macroinvertebrates,

of genus richness were generally attained at mid-latitudes

studies of lentic macroinvertebrates have generally found

in North America and South America, although there was

no linear declines in species richness with latitude and

much scatter around the relationships. Secondary peaks

ecoregional delineations (Johnson, 2000; Vamosi et al.,

occurred in the tropics. While these patterns may result

2007). By contrast, part of the variability in the assemblage

partly from inadequate data and taxonomic knowledge in

composition of lake macroinvertebrates is related to

tropical regions, they also suggest that stream insects may

latitude (Johnson & Goedkoop, 2002; Johnson et al., 2004).

deviate from patterns detected for many major terrestrial

The two most probable reasons for the weak

taxa (Willig et al., 2003). A reason for this discrepancy may

relationships between latitude and species richness or

be that mayflies, stoneflies and caddisflies contain genera

assemblage composition are that (i) the local environmental

and species showing highly differential temperature

features of freshwater ecosystems do not necessarily


DOI: 10.1608/FRJ-2.1.1


show clear latitudinal variation (e.g. substratum particle

been directed to the relationship between local species

size, moss cover, current velocity, gradient), and (ii) local

richness (LSR) and regional species richness (RSR). Two

environmental factors override the influences of large-

general patterns have been suggested for LSR–RSR plots

scale historical and climatic factors on local assemblages.

(Cornell & Lawton, 1992). First, LSR may increase linearly

This reasoning is undeveloped, however, because single

with RSR, suggesting that local factors do not limit LSR,

surveys of local assemblages do not typically span large

assemblages being mainly under regional control. Second,

latitudinal gradients.

Thus, new large-scale datasets

if LSR attains an asymptote with increasing RSR, then it

may change our views about the latitudinal diversity

is suggested that local assemblages are saturated, being

patterns shown by freshwater macroinvertebrates.

mainly under local control. Although the approach in

In addition to patterns along latitudinal gradients, other

general has been criticised, particularly because highly

geographical patterns are also of interest. For example, east-

interactive assemblages may show linear LSR–RSR

to-west gradients in species richness have been found for

relationships (Hillebrand, 2005) and because such plots

stream insects across North Africa (Beauchard et al., 2003).

cannot be considered as evidence of competition (He et al.,

This pattern was explained by a combination of climatic,

2005), the examination of LSR–RSR relationships may at least

hydrological and biogeographic factors, with the latter

provide hints as to whether regional factors are important at

relating to historical dispersal processes. Similar historical

all (Witman et al., 2004; Harrison & Cornell, 2008).

influences of dispersal after the last glacial period may

LSR–RSR relationships for freshwater macroinverte-

also be related to east–west and south–north gradients of

brates have typically been derived using a riffle site as the

stream macroinvertebrate species richness in other regions,

local arena and a drainage basin as the region. Research

although the combination of geographical gradients with

on Finnish streams revealed that mean LSR was linearly

local environmental features may be more influential.

related to RSR, suggesting that regional species pools

More complex geographical gradients in species richness

limited LSR at least to some degree (Heino et al., 2003a).

and assemblage composition have also been suggested

A similar linear relationship was found for southeastern

by findings from surveys of streams across ecoregions

Australian streams (Marchant et al., 2006). The Australian

in New Zealand (Harding et al., 1997), North America

dataset contained a much larger number of species than the

(Hawkins & Vinson, 2000) and North Europe (Heino

Finnish dataset, suggesting that even at very high levels of

et al., 2002). While stream and lake macroinvertebrate

LSR and RSR, a linear relationship may prevail. In contrast,

assemblages show some differences between ecoregions,

based on a much more limited number of species, a strongly

differences are typically rather weak (Hardin et al.,

asymptotic relationship between LSR and RSR was found

1997; Hawkins et al., 2000; Johnson, 2000; Hawkins

for stream blackflies in the New World (McCreadie et al.,

& Vinson, 2000).

However, even weak relationships

2005). A study on Swedish streams and lakes yielded a

suggest that local assemblages are, at least in part,

weakly asymptotic mean LSR to RSR relationship in both

constrained by large-scale climatic and historical factors.

habitat types (Stendera & Johnson, 2005). The patterns detected in this study were probably confounded by the

Local–regional species richness

fact that RSR estimates for each ecoregion were based on

relationships

different numbers of streams and lakes, and a correction for the number of sampled sites might have been necessary

Theoretical considerations and empirical evidence suggest

for valid comparisons of LSR–RSR relationships (Marchant

that species richness in local assemblages is constrained

et al., 2006). Finally, a study on Palaearctic predaceous

not only by local factors, but also by regional species pools

diving beetles showed that LSR was positively related to

(Ricklefs, 1987; Cornell & Lawton, 1992; Griffiths, 1997;

RSR, even when compensations were made for differences

Angermeier & Winston, 1998). Much recent interest has

in local pond characteristics (Kholin & Nilsson, 1998). This

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Heino, J.

finding suggested a regional enrichment of LSR, and that

in local diversity. However, proving the predominance

a suite of historical and environmental explanations might

of weak latitudinal diversity patterns would necessitate

be needed to account for variation in the species richness of

comprehensive surveys of tropical freshwater ecosystems

predaceous water beetles in ponds. The findings from the

and those that span several tens of degrees of latitude

above studies clearly indicate that there is no single type

across major biomes. A fruitful approach in this context

of LSR–RSR relationship in freshwater macroinvertebrates.

would be to determine the relative influences of latitude, and of regional, watershed, ecosystem and habitat factors

Relative influences of variables at different scales

on local aquatic insect diversity, as has been done in studies conducted within smaller geographical extents.

Such

studies should not only be directed towards lotic systems, Conceptual models suggest that local assemblages are

for which some data already exist, but also towards lentic

structured by environmental factors at multiple scales

systems, for which our knowledge of geographical diversity

(Fig. 1) (Tonn, 1990; Poff, 1997). These suggestions have

patterns is more limited. Understanding geographical

been corroborated by empirical evidence. For example, the

diversity patterns would be a valuable asset in determining

assemblage composition of both stream and lake macroin-

and implementing geographically representative networks

vertebrates has been found to be determined by multiple

of protected areas for the conservation of freshwater

variables, ranging from local habitat and ecosystem features

biodiversity.

through watershed characteristics to regional climate and geographical factors (Townsend et al., 2003; Johnson et al., 2004; Sandin & Johnson, 2004; Heino et al., 2007a).

Species diversity–environment relationships within regions

In general, variables at small scales have been suggested to be the most important, both in terms of explained

The drainage basin is a natural regional unit in lotic ecology

variability in assemblage composition and in the ecological

(e.g. Frissell et al., 1986), and lake districts have similarly

sense, as small-scale variables are directly associated with

been given considerable recent attention in lentic ecology

the organisms. However, in a study covering a large

(e.g. Kratz & Frost, 2000). It is not surprising, therefore, that

geographical extent in Europe, geographical position

there have been many within-region surveys of lotic and

and ecoregion overrode the influences of local factors on

lentic assemblages. In the following, I will review findings

stream assemblages (Johnson et al., 2007). Also, species

from surveys of aquatic insect assemblages at geographical

richness in streams has been found to be best explained by

extents smaller (i.e. single ecoregions, drainage basins and

models including variables acting at different spatial scales

lake districts) than those considered above (i.e. several

(Sandin, 2003).

ecoregions and countries). I will first review research on (i) diversity–disturbance, (ii) diversity–productivity,

Synthesis and implications

(ii) diversity–acidity, (iv) diversity–heterogeneity and (v) diversity–habitat size relationships separately in both

Aquatic insect biodiversity varies geographically, although

lotic and lentic ecosystems. I will also consider similarity

present evidence suggests that these patterns are complex.

in the environmental relationships of species richness

Both species richness and assemblage composition

and assemblage composition. I will end this section by

typically show some variability among ecoregions and

addressing variation in species richness and assemblage

drainage basins, yet among-site heterogeneity within

composition with regard to the nested subset pattern.

regions largely masks the influences of climate and history

Biotic interactions, including predation and competition,

on local assemblages. Such a weak influence of large-scale

will be considered below only if there are shifts in species

factors is likely to contribute to weak latitudinal gradients

interactions associated with these major environmental


DOI: 10.1608/FRJ-2.1.1


gradients. Readers should consult previous reviews for

than the latter type of streams (Lake et al., 1986; Williams,

the importance of species interactions at small within-

1996). This is not surprising, because drying out of streams

ecosystem scales (Giller & Malmqvist, 1998; Allan &

presents severe challenges to the life history adaptations

Castillo, 2007).

and physiological performance of aquatic insects.

A

comprehensive recent review of the effects of drought Species diversity and major environmental

on stream biota can be found elsewhere (Lake, 2003).

gradients in lotic systems Diversity–productivity relationships Diversity–disturbance relationships

In addition to diversity–disturbance relationships, the

Floods are major natural causes of disturbance in running

effect of productivity on diversity has been studied

waters (Power et al., 1988; Resh et al., 1988; Poff, 1992)

extensively.

and constitute important determinants of diversity (Resh

diversity–productivity relationship takes many forms,

et al., 1988; Lake, 2000). Diversity is typically negatively

ranging from positive, unimodal and negative to non-

related to flow disturbance intensity and frequency

significant (Waide et al., 1999; Mittelbach et al., 2001). It is

(Robinson & Minshall, 1986; Death, 2002) and positively

possible that productivity in terms of stream water nutrient

related to benthic habitat stability (Death & Winterbourn,

concentration or periphyton biomass may also show

1995; Death, 2002).

Recent reviews have suggested that the

By contrast, none of the studies

positive, unimodal or negative relationships to the species

reviewed by Vinson & Hawkins (1998) suggested that the

richness of stream macroinvertebrates. When productivity

intermediate disturbance hypothesis would fit the species

ranges from very low to moderate, a positive relationship

diversity patterns of stream macroinvertebrates, although

may occur (Perry & Sheldon, 1986). Such a relationship

intermediate levels of disturbance may be associated with

may also be common in arctic and subarctic streams, which

higher diversity in other systems (Connell, 1978; Petraitis

have generally very low to moderately low nutrient levels.

et al., 1989). However, more recent studies have found

When nutrient levels range from very low to very high, a

highest diversity at sites perturbed by intermediate levels of

unimodal relationship may be found. Such a relationship

disturbances, leading to a unimodal diversity–disturbance

may be common in temperate regions with a wide range in

relationship (Townsend et al., 1997). This is due to the fact

stream water nutrient levels. Finally, when nutrients range

that high-level disturbances prevent abundant species from

from moderate to very high levels, a negative relationship

attaining a dominant position and competitive advantage

may be found. Such a relationship may also be common

(Huston, 1994), yet disturbances of intermediate level do

in temperate regions, where anthropogenic effects have

not lead to the exclusion of most disturbance-sensitive

increased nutrient inputs to streams. However, a negative

species. However, very few studies on lotic macroinver-

relationship may also occur at relatively low levels of

tebrates have examined these relationships across multiple

productivity (Malmqvist & Eriksson, 1995). Malmqvist

streams (Townsend et al., 1997; Death, 2002), whereas

& Eriksson (1995) suggested that the negative relationship

a number of within-stream studies have also examined

resulted, at least in part, from productive lake outlets being

the effects of disturbance on macroinvertebrate diversity

dominated by few abundant and competitively dominant

(Matthaei et al. 1999, 2000; McCabe & Gotelli, 2000). These

species, such as highly aggressive net-spinning caddis

small-scale studies, however, are out of the scope of this

larvae (see also Giller & Malmqvist, 1998). It should be

review.

emphasised, however, that nutrient levels may not be

Another form of disturbance affecting stream macroin-

the sole source of productivity affecting stream insects,

vertebrates is drought (Lake, 2003). In this regard, some

although nutrients are most closely associated with

studies have compared intermittent and permanent

autochthonous production in these systems. It should also

streams, and found that the former support fewer species

be noted that the above predictions are largely theoretical

DOI: 10.1608/FRJ-2.1.1


10

Heino, J.

rather than proven correct by empirical evidence.

Diversity–acidity relationships

Empirical studies are thus needed to ascertain if the form of

Among the most important stress factors in stream

the diversity–productivity relationship varies with regard

ecosystems is acidity. Species richness–acidity relationships

to regional range in potential productivity. These studies

have been studied in regional studies in widely-separated

should not only consider in-stream productivity, but also

geographical areas, ranging from Sweden (Malmqvist &

the productivity of the riparian zone.

Hoffsten, 2000) and Britain (Townsend et al., 1983) to the USA (Rosemond et al., 1992) and New Zealand (Collier et

Interactive effect of disturbance and productivity

al., 1990). Acidity typically leads to a low diversity of stream

on diversity

macroinvertebrates and such patterns have been observed

Disturbance and productivity may act in concert in affecting

in a number of studies spanning gradients from pH values

diversity, and this relationship is typically expressed as

4 to 7. In general, considerable reduction in the occurrence

the dynamic equilibrium model (Huston, 1994).

This

and richness of macroinvertebrates is typically seen at

model predicts that the unimodal relationship between

pH values below 5.5 (Otto & Svensson, 1983; Townsend

disturbance and diversity in the intermediate disturbance

et al., 1983; Hämäläinen & Huttunen, 1990; Larsen et al.,

hypothesis moves in relation to the productivity of a

1996). However, the findings of general reductions of

system, producing a contour of decreasing diversity with

species richness with acidity have also been challenged

high disturbance and high productivity. Such patterns

(Winterbourn & Collier, 1987; Collier et al., 1990; Dangles

of diversity result from two opposing forces: population

et al., 2004; Petrin et al., 2007). These discrepancies may be

growth rate and disturbance. First, local processes, such

related to the fact that acidity has both direct and indirect

as competition, dominate and reduce diversity under high

effects through food availability on species richness (Otto

population growth rates and low disturbances. Second,

& Svensson, 1983), or that species from different regions

other local processes, such as extinction, are important

may possess differing sensitivity to acid stress (Petrin et

at very low levels of growth rate and high disturbance

al., 2007). Furthermore, Petrin et al. (2007) suggested that

frequencies that thus reduce diversity. Very few studies

macroinvertebrate species richness decreases with acidity

have tested this model, because it necessitates surveys at

if it is of anthropogenic origin, while species richness may

large among-stream scales, making experiments unfeasible.

remain virtually unaltered if acidity is of natural origin.

However, Death & Zimmerman (2005) tried to overcome

Based on surveys in southern (acidity of anthropogenic

this problem, and examined simultaneously the effects of

origin) and northern Sweden (acidity of natural origin),

both disturbance and productivity on macroinvertebrate

they showed that the species richness of mayflies decreased

species richness in a set of streams. In general they found

with acidity in both regions, whereas the species richness

weak support for the two above-mentioned diversity

of caddisflies showed a negative response to acidity in

models and suggested instead that species richness was

the former region. The species richness of stoneflies did

a function of time since the last disturbance, this being

not vary along acidity gradients. Thus, the overall species

associated with the recovery of the autotrophic production

richness–acidity relationships for lotic macroinvertebrates

base for macroinvertebrates (Death & Zimmerman, 2005;

as a whole may be driven by a few taxonomic groups, while

see also Death & Winterbourn, 1995; Bond & Downes 2000;

others are more or less indifferent with regard to acidity.

Death, 2002).

For example, stoneflies in general may be rather indifferent with regard to acidity, and their species richness typically shows no clear response to acidity (Malmqvist & Hoffsten, 2000). However, despite no effects on species richness, the species composition of stonefly assemblages may, as


DOI: 10.1608/FRJ-2.1.1


11

in many other insect groups, change along gradients of

(Minshall et al., 1985a; Hildrew & Townsend, 1987; Allan

acidity (Petrin et al., 2007).

& Castillo, 2007; Cummins, 1996), and suggest that species richness increases from the headwaters (orders 1 to 3) to

Diversity–heterogeneity relationships

mid-order rivers (orders 4 to 6) and then declines with

Habitat heterogeneity has a positive effect on species

increasing river size (orders 7 to 12). These mid-order peaks

richness in general (Huston, 1994), and the same is true for

in species richness have been explained by higher levels

stream macroinvertebrates at various spatial scales (Vinson

of habitat heterogeneity and higher numbers of thermal

& Hawkins, 1998). Studies conducted at small scales, for

niches in mid-sized rivers than in smaller or larger rivers

example, within streams based on samples of stones, have

(Minshall et al., 1985b). In general, empirical evidence

associated variation in species richness to various measures

corroborates the prediction of the RCC that the species

of habitat heterogeneity, diversity and complexity. For

richness of stream macroinvertebrates increases with

example, species richness is typically positively related to

stream size up to mid-order rivers. Such patterns have been

habitat complexity in terms of stone surface features and

found in widely-separated geographical regions, including

plant cover at small scales (Hart, 1978; Hart & Horwitz,

Europe (Malmqvist & Hoffsten, 2000; Céreghino et al., 2003;

1991; Douglas & Lake, 1994; Dudley et al., 1986; Downes

Arscott et al., 2005), North America (Minshall et al., 1985b;

et al., 2000; Heino & Korsu, 2008). Positive relationships

Grubaugh et al., 1996) and South America (Tomanova et al.,

between stream substratum heterogeneity and species

2007). However, in studies where only small to mid-sized

richness have also been found among sites within streams

streams are sampled, species richness may be positively

(Cowie, 1985). By contrast, although habitat heterogeneity

related to stream size (Brönmark et al., 1984; Ward, 1986;

could be envisaged to account for among-stream variation

Malmqvist & Mäki, 1994; Malmqvist & Eriksson, 1995;

in species richness, evidence for such relationships is scant

Wiberg-Larsen et al., 2000; Paller et al., 2006), although

(Vinson & Hawkins, 1998). Reasons for the virtual absence

exceptions to these patterns also exist (Winterbourn et al.,

of a species richness–habitat heterogeneity relationship at

1981; Malmqvist & Hoffsten, 2000; Voelz & McArthur,

the among-stream scale are unknown, but possibly result

2000; Heino et al., 2008). When species richness attains

from stronger impacts of other physical and chemical

an asymptote or decreases in mid-sized rivers, a possible

factors that override the influences of habitat heterogeneity

reason for such patterns is that sampling effort has not

on species richness. Alternatively, stream ecologists may

been sufficient in mid-sized reaches, as opposed to more

not have measured heterogeneity at the among-stream

adequate effort in the headwater reaches (Malmqvist

scale as perceived by, or is important for, aquatic insects.

& Hoffsten, 2000; Heino et al., 2008). An alternative

It is also unknown if habitat heterogeneity is positively

explanation is that localised changes in environmental

related to stream size. It is possible, however, that habitat

conditions (e.g. substratum particle size, macrophyte

heterogeneity increases with stream size in small to mid-

cover) disrupt expected species richness–habitat size

sized rivers, but larger rivers do not necessarily possess

relationships (Wright & Li, 2002; Heino & Paasivirta, 2008).

more heterogeneous habitats (see below).

It should also be emphasised that data from the short and steep catchments in New Zealand and Australian drainage

Diversity–habitat size relationships

basins do not support the RCC predictions (Winterbourn

The river continuum concept (RCC) provides a theoretical

et al., 1981). Note that habitat size for lotic ecosystems in

basis to lotic ecology, for example, by relating river size

the above-mentioned studies has been based on a number

and longitudinal position to variation in functional feeding

of measures, including stream order, catchment area,

group composition and macroinvertebrate species richness

distance to source and stream width. Another important

(Vannote et al., 1980). The various predictions of the RCC

aspect to note is that not all studies have examined a

have been summarised several times since its formulation

single river continuum, but have been based on surveys of

DOI: 10.1608/FRJ-2.1.1


12

Heino, J.

different-sized streams scattered across a drainage basin

Diversity–productivity relationships

or an ecoregion. This may increase the importance of

Productivity in lentic ecosystems has typically been

localised environmental influences, causing patterns to

described by proxy variables, such as nutrient levels and

differ from the predictions of the RCC.

chlorophyll-a concentration (Dodson et al., 2000; Jeppesen et al., 2000; Declerk et al., 2005; Barnett & Beisner, 2007).

Species diversity and major environmental

However, very few studies have directly tested diversity–

gradients in lentic systems

productivity relationships in lentic macroinvertebrates. Brodersen et al. (1998) found that the species richness and

Diversity–disturbance relationships

Shannon’s diversity index of littoral macroinvertebrates

Disturbance in terms of hydroperiod has both direct and

were negatively related to these measures of productivity.

indirect influences on diversity in lentic littoral and pond

Given that the lakes they studied ranged from mesotrophic

ecosystems. Permanent ponds typically support higher

to highly eutrophic, negative relationships can be easily

numbers of species than temporary ponds, and diversity

envisaged. Chase & Leibold (2002) observed two different

increases with increasing degree of permanency (Spencer

types of species richness–productivity relationships in

et al., 1999; Urban, 2004; but see Larson, 1985). This is

pond macroinvertebrates with varying spatial scale. When

likely to be due to physical limitation on the life history

analysed with ponds as replicates, a unimodal relationship

characteristics of various taxa (Schneider & Frost, 1996;

was found in α-diversity. By contrast, when analysed over

Spencer et al., 1999), although the determination of species

ponds in a watershed, a positive relationship emerged for

richness and assemblage composition along the lentic

γ-diversity. They explained these differing relationships as

permanency gradient may be more complex (Wellborn

an indication that, in watersheds with highly productive

et al., 1996). Wellborn et al. (1996) reviewed literature on

ponds, β-diversity was higher due to multiple stable

invertebrate assemblages along this gradient and found that

states, while β-diversity was lower in low-productivity

various species occurred in either only temporary habitats,

watersheds.

only permanent but fishless habitats, or only permanent

contributed to differences in γ-diversity. Despite these

fish containing habitats. These patterns were suggested

rigorous studies, more studies in lentic ecosystems, with

to be driven by the relative roles of: (i) drying that selects

macroinvertebrates as the model group, are needed before

for traits allowing rapid development and physiological

any general conclusions about the diversity–productivity

capacity to cope with disturbance; (ii) winter oxygen stress

relationship can be made.

These differences in species turnover

that eliminates fish; and (iii) types of top predators that vary in different types of lakes, being dragonfly larvae in

Diversity–acidity relationships

permanent fishless lakes and fish in least disturbed lakes.

Diversity–acidity relationships have been relatively

The interaction between disturbance and predation should

well studied in lentic ecosystems. A typical finding has

also be manifest in overall assemblage composition and

been that the overall species richness of lentic macroin-

species richness, and different groups of aquatic insects

vertebrates decreases strongly with natural (Bradford

are likely to show differing diversity patterns along the

et al., 1998) and anthropogenic (Schindler et al., 1989)

permanency–predation gradient. This freshwater habitat

acidity. However, to my knowledge, no study has tried to

gradient offers interesting possibilities for further studies

determine the actual roles of these two sources of acidity

trying to tease out the effects of disturbance and predation

in affecting lentic macroinvertebrates. Decreasing trends

on the species richness and assemblage composition of

between species richness and acidity have been found for

aquatic insects.

both ponds (Friday, 1987) and lakes (McNicol et al., 1995; Hynynen & Meriläinen, 2005; Arnott et al., 2006). These trends are typically driven by the negative responses of


DOI: 10.1608/FRJ-2.1.1

13


species in a few taxonomic groups, with mayflies among

only organic bottoms and sparse cover of macrophytes at

aquatic insect taxa being generally highly sensitive to

best supported much lower species richness than forest

acidity in lakes (Meriläinen & Hynynen, 1990; Carbone et

lakes with a heterogeneous bottom of both organic and

al., 1998). However, some taxa may show no significant

inorganic material, as well as more developed macrophyte

relationships to acidity (Pollard & Berrill, 1992), and

beds.

diversity patterns opposite to those of whole assemblages

relationship between the species richness of diving beetles

may even be detected due to species benefiting indirectly

and macrophyte cover and complexity in lakes (Nilsson et

from acidic conditions (Bendell & McNicol, 1987). These

al., 1994; Nilsson & Söderberg, 1996).

This finding is in agreement with a positive

species may not necessarily prefer acidic conditions, but are excluded from neutral lakes due to biotic interactions.

Diversity–habitat size relationships

For example, fish are often excluded from acidic lakes,

Species–area relationships have been studied extensively

this providing an opportunity for macroinvertebrates

in various systems, and positive relationships have

sensitive to fish predators to thrive in acidic lakes (Eriksson

been found in a large number of studies (Rosenzweig,

et al., 1980; Bendell & McNicol, 1987; McNicol et al., 1995).

1995; Gaston & Blackburn, 2000). Habitat size has also

Among the most well-known examples in this regard are

been found to be important for the species richness and

larval dragonflies, waterstriders and waterboatmen, some

assemblage composition of aquatic insects in temporary

species of which may be more frequent and abundant in

ponds (Nilsson & Svensson, 1995; Kholin & Nilsson, 1998),

fishless habitats (Macan, 1965; Bendell & McNicol, 1995;

permanent ponds (Oertli et al., 2002) and lakes (Allen et

McNicol et al., 1995). Thus, similarly to disturbance, acidity

al., 1999; Heino, 2000). However, non-significant species

has both direct and indirect effects on species richness and

richness–habitat size relationships have also been found

assemblage composition in lentic ecosystems.

for some groups of aquatic insects in ponds (Larson, 1985; Oertli et al., 2002). For lakes, habitat size and habitat

Diversity–heterogeneity relationships

heterogeneity are typically correlated, and a positive

Habitat heterogeneity, diversity and complexity are

relationship between species richness and lake size may

potentially important determinants of diversity in lentic

thus result from heterogeneity effects. Given that habitat

littoral and pond ecosystems (Eadie & Keast, 1984; Rahel,

heterogeneity may account for more variation in species

1984). Similarly to lotic ecosystems, a number of studies

richness than lake size, heterogeneity is likely to be behind

have examined diversity–heterogeneity relationships

strong species–area relationships in surveys of lakes.

at small within-ecosystem scales.

Not surprisingly,

Similarly to species richness, the assemblage composition

these studies have typically found positive relationships

of lentic macroinvertebrates may vary strongly with lake

between macroinvertebrate diversity, abundance and

size and habitat structure (Heino, 2000).

habitat complexity, including that of macrophyte beds (Carpenter & Lodge, 1986; Brown et al., 1988), although

Multiple environmental variables determine

exceptions also exist (Cyr & Downing, 1988; McAbendroth

diversity

et al., 2005a).

Evidence about diversity–heterogeneity

relationships based on among-ecosystem surveys is more

Vinson & Hawkins (1998) called for multi-factor analyses in

limited. However, Heino (2000) found that the species

studies on aquatic insect biodiversity. Since the publication

richness of littoral macroinvertebrates was strongly related

of their review, a number of studies have examined the

to a composite habitat heterogeneity variable, comprising

relative importance of multiple environmental variables

information about substratum diversity, heterogeneity

on aquatic insect biodiversity. Such studies have been

of organic material distribution, and heterogeneity and

conducted in both lotic (Brosse et al., 2003; Sandin,

diversity of macrophytes.

2003) and lentic ecosystems (Nilsson & Svensson, 1995;

DOI: 10.1608/FRJ-2.1.1

In general, bog lakes with


14

Heino, J.

Heino, 2000). In general, most studies have noted that a

pH and water colour were the most important variables

comprehensive understanding of variation in the diversity

determining variability in both species richness and

of aquatic insects requires the measurement of multiple

assemblage composition of streams in northern Sweden.

environmental variables, including ecosystem-scale (e.g.

Wiberg-Larsen et al. (2000) also found that both the species

pH, nutrients, temperature) and habitat-scale variables (e.g.

richness and assemblage composition of caddisflies were

substratum characteristics, current velocity, macrophytes).

strongly related to stream size in Denmark. In all of these

This reasoning may be true in most cases, but exceptions

cases, the environmental gradients were apparently strong

may occur when one dominant environmental gradient

enough to modify diversity, and variability in species

has been surveyed and others are not influential. In such

richness and assemblage composition followed the same

cases, variation in diversity may be explained adequately

gradients. By contrast, in situations where there are several

by a single environmental gradient (Townsend et al.,

equally strong and uncorrelated environmental gradients,

1983; Wiberg-Larsen et al., 2000). Studies that have not

species richness and assemblage composition may vary

found significant relationships between species richness

differently along environmental gradients. This is true in

and environmental features may be complicated by a

situations where α- and β-diversity vary independently, i.e.

simultaneous survey of multiple ecosystem types (e.g.

showing an anti-nested pattern (see below).

springbrooks, headwater streams, large rivers, lake outlets; Malmqvist et al., 1999).

Surveys of different

Nestedness

ecosystem types, with each harbouring a certain suite of characteristic species, may thus lead to non-significant

Ecological assemblages within the limits of a regional

diversity–environment relationships.

Alternatively, if

species pool show a number of generalised patterns (Leibold

significant diversity–environment relationships have not

& Mikkelson, 2002; Heino, 2005a). One such pattern is

been detected, it is possible that influential environmental

nestedness. A perfectly nested pattern occurs if species in

variables were not measured at all. Biodiversity surveys

depauperate assemblages are proper subsets of those in

should therefore include a large number of explanatory

progressively more diverse assemblages, with common

variables and use multivariate methods to help to identify

species occurring in all assemblages and rare species

various factors of potential importance.

tending to occur only in diverse assemblages (Patterson & Atmar, 1986). Thus, if a set of assemblages is perfectly

Do species richness and assemblage composition vary along the same gradients?

nested, the most species-rich site includes all species detected at lower-richness sites.

Several studies have

found a significant nested subset pattern in various taxa This question is of relevance to conservation, as not only

and environments, implying that nestedness is a common

species richness (α-diversity), but also spatial changes

pattern in nature (D.H. Wright et al., 1998). However,

in assemblage composition (β-diversity) should be

the virtual ubiquity of nestedness has recently been

considered in conservation planning.

Reports of the

challenged, and only 10 % to 40 % of the matrices studied

environmental relationships of both species richness and

by Ulrich & Gotelli (2007) were actually significantly

assemblage composition have been rather unambiguous.

nested.

Not surprisingly, both these components of diversity

analyses stem from the fact that, in the former study, null

appear to respond similarly to environmental gradients.

models for significance testing were liberal (D.H. Wright et

For example, Townsend et al. (1983) found that pH was

al., 1998), whereas the latter study comprised multiple and

the most important variable for both species richness and

typically more conservative null models (Ulrich & Gotelli,

assemblage composition in southern English streams.

2007). Earlier studies on the nestedness of aquatic insects

Malmqvist & Hoffsten (2000) found that stream size,

have typically been based on the liberal null models, such


These discrepancies between the two meta-

DOI: 10.1608/FRJ-2.1.1

15


as that of the nestedness calculator (Atmar & Patterson,

these environmental and organismal relationships of

1993), yet the results have been ambiguous. The published

nestedness could be easily envisaged based on information

studies on aquatic insects have found either that these

from terrestrial systems (D.H. Wright et al., 1998) and other

assemblages are significantly nested (Nilsson & Svensson,

aquatic taxa (Soininen, 2008), further studies are required

1995; Malmqvist 1999; Malmqvist & Hoffsten, 2000; Baber

to prove the generality of these findings in aquatic insects.

et al., 2004; Heino, 2005a; McAbendtroth et al., 2005b; Monaghan et al., 2005; Heino et al., 2009) or non-nested

Synthesis and implications

(Malmqvist & Eriksson, 1995; Nilsson & Svensson, 1995; Malmqvist et al., 1999; Urban, 2004). However, even when

Aquatic insect assemblages within the limits of a

significant nestedness has been found in studies of aquatic

regional species pool show many interesting patterns.

insects, it has often been rather weak. Boecklen (1997)

The relationships of species richness and assemblage

noted earlier the same pattern for aquatic invertebrates in

composition to major environmental gradients may

general, and suggested that, because these assemblages

sometimes be rather predictable, although the explanatory

contain various kinds of organisms many of which possess

power in statistical models is typically rather low. In

high colonisation rates, there is only weak nestedness at

general, habitat size and acid stress appear to structure

best. Weak nestedness means that there are somewhat

aquatic insect assemblages most strongly in both lotic

anti-nested patterns, which might mean checkerboard

and lentic ecosystems, although there are some notable

distributions of species pairs and high β-diversity (Gotelli &

differences between streams, rivers, ponds and lakes.

McCabe, 2002; Heino et al., 2009).

Understanding the species richness–environment and

Although a considerable number of studies have

assemblage

composition–environment

relationships

assessed the degree of nestedness, very few studies have

has a number of implications for the conservation and

considered the determinants of this pattern in aquatic

management of α-, β-, and γ-diversity. First, conservation

insects. McAbendroth et al. (2005b) found that nestedness

of α-diversity benefits from the understanding of species

in pond macroinvertebrates was primarily driven by

richness–environment relationships, although the degree

habitat size, and secondarily by habitat features and

of nestedness should also be considered (Boecklen, 1997).

isolation.

Interestingly, species contributing to perfect

If assemblages are perfectly nested, then the conservation

nestedness (i.e. nested species) varied in a number of

of the most species rich site protects all species. By contrast,

organismal characteristics from species not contributing

if nestedness is weak, as has been found in many studies

to nestedness (i.e. idiosyncratic species). Nested species

on aquatic insects, then rare species are not restricted

showed stronger spatial structure, weaker dispersal ability

to species rich sites only. In such cases, an alternative

and narrower ecological tolerances than idiosyncratic

conservation strategy should be followed, with interest in

species. These suggestions corroborate well the findings

both idiosyncratic species and species-rich sites. Second,

from boreal streams (Heino et al., 2009). In this study, the

given the rather predictable assemblage composition–

most important correlate of nestedness in stream insect

habitat size relationships, different-sized streams and lakes

assemblages was stream size. Furthermore, idiosyncratic

are needed to conserve β-diversity. Thirdly, the protection

species occurred, on average, at more sites than nested

of sites supporting characteristic assemblage types and

species, mirroring the restricted distributions of several

sites at different parts of environmental continuums is also

nested species that were inclined towards species-rich

important for the conservation of γ-diversity.

sites. Idiosyncratic and nested species also differed in niche position and niche breadth, with idiosyncratic species having less marginal niche positions and wider niches than nested species (Heino et al., 2009). Although DOI: 10.1608/FRJ-2.1.1


16

Heino, J.

Among-taxon congruence in diversity

invoked

similar

environmental

responses

as

the

mechanism contributing to co-varying patterns of assemblage composition in pond insects. By contrast,

Understanding the degree to which different taxonomic

Heino et al. (2003b) stated that, due to differing responses

groups show congruent biodiversity patterns is an

to geographical location and major environmental

important field of study in conservation biology (Gaston,

gradients, stream insect taxa showed weak congruence in

1996b) and environmental assessment (McGeogh, 1998).

biodiversity. Thus, the responses of different taxonomic

However, despite its importance, relatively few studies

groups to the same environmental gradients are most likely

have examined congruence in the diversity patterns of

to be responsible for the degree of congruence. Responses

aquatic organisms in general (Allen et al., 1999; Paavola et

to correlated environmental factors may also contribute to

al., 2003; Heino et al., 2005) and aquatic insects in particular

congruence in some situations, whereas biotic interactions

(Briers & Biggs, 2003; Heino et al., 2003b; Bilton et al.,

and random draw are less likely explanations at the grain

2006). The studies that have appeared thus far have led to

of local ecosystems and the extent of single drainage basins.

differing conclusions. Heino et al. (2003b) found that the

However, random draw from the regional species pool

species richness and assemblage composition patterns of

may contribute to strong congruence if local ecosystems are

stream mayflies, stoneflies, caddisflies and midges were

surveyed across long geographical gradients (Heino et al.,

weakly, though sometimes significantly, correlated at the

2003b). These suggestions are speculative, and given that

across-ecoregions scale in Finland. Within ecoregions, a

very few studies on aquatic insects have considered among-

few cross-taxon correlations were slightly higher, but even

taxon congruence, further studies in various regional and

these correlations lacked the predictive power required

environmental settings are needed to resolve this question.

for them to be useful in practice. These findings generally

Knowing the degree to which different taxonomic

comply with those for the same four insect groups in

groups show congruent diversity patterns is potentially

a single boreal drainage basin (Heino & Mykrä, 2009).

valuable in the conservation context.

Bilton et al. (2006) also studied among-taxon congruence

showing strongly congruent patterns with other taxonomic

in species richness and assemblage similarity in several

groups, a good biodiversity indicator taxon should possess

pond macroinvertebrate taxa. They found that, although

a number of additional qualities. These include occurrence

species richness was weakly correlated among taxonomic

in various environmental conditions, cost-efficiency,

groups, assemblage composition patterns showed stronger

applicability at different scales, and it should be easily

among-group correlations. Sánchez-Fernández et al. (2006)

sampled, easily identified and ecologically well-known

found that aquatic beetles were relatively good indicators

(Noss, 1990; McGeogh, 1998). The concept of biodiversity

of species richness in various stream macroinvertebrate

indicator taxa has only rarely been considered in aquatic

taxa, with correlations ranging from non-significant for

insect studies. Briers & Biggs (2003) found that coenagriid

molluscs and mayflies to significant and rather strong for

damselflies and limnephilid caddisflies were the most

aquatic bugs, stoneflies, caddisflies and total richness of the

promising groups as indicators of pond biodiversity.

studied taxonomic groups.

Bilton et al. (2006) considered aquatic beetles as the most

Why should some taxonomic groups show strong correlations in biodiversity?

In addition to

useful indicator group for pond biodiversity assessment,

There are four main

because they conform best of the studied taxa to the

mechanisms behind strong among-taxon congruence:

qualities listed above. Similarly, Sánchez-Fernández et al.

(i) similar environmental responses; (ii) responses to

(2006) also regarded aquatic beetles as a good indicator

different, but correlated environmental features; (iii) biotic

taxon. However, the degree to which aquatic beetles

interactions; and (iv) random draw from the regional

perform as surrogates in the biodiversity assessment of

species pool (Gaston & Williams, 1996). Bilton et al. (2006)

streams and lakes is uncertain, and recent research on


DOI: 10.1608/FRJ-2.1.1

17


boreal bog and forest lakes suggests that aquatic beetles

A few studies have examined functional biodiversity

are so sporadically distributed that they cannot be used

along

multiple

non-anthropogenic

environmental

effectively as an indicator taxon (Heino et al., unpublished).

gradients.

It is thus likely that there is no single, universal indicator

describing variation in the functional structure of stream

taxon among aquatic insect taxa, and that indicator

insect assemblages of mountain streams. They found that

groups need to be adjusted according to regional and

functional structure varied primarily with periphyton

environmental settings. This may also mean that indicator

biomass and particle size, although the longitudinal

groups are unlikely to be useful in most situations.

position was also important. Heino (2005b) concentrated

Finn & Poff (2005) used nine traits for

on the macroinvertebrates of boreal lowland streams,

Patterns of functional diversity

and used a combination of functional feeding groups (Cummins, 1973) and habit trait groups (Merritt &

Although the functional approach has a long history in

Cummins, 1996). Functional feeding groups refer to the

ecology (Statzner et al., 2001a), there has been increasing

feeding modes and approximate food type of macroinver-

interest in the topic recently due to greater recognition of

tebrates, whereas information on where food is obtained

the importance of functional biodiversity to ecosystem

(e.g. on stones versus within sediments) is used to derive

functioning (Kinzig et al., 2002). Early examinations of

habit trait groups.

macroinvertebrate functional feeding groups (Cummins,

spatially-structured and environmentally-related variation,

1973; Cummins & Klug, 1979) in the context of the river

with pH, stream size and moss cover being among the

continuum concept provided first glimpses of the utility

most important variables. Three measures of functional

of the functional approach when studying the longitudinal

diversity (functional richness, diversity and evenness)

gradient in lotic ecosystems (Vannote et al., 1980; Minshall

decreased with water acidity, canopy cover and water

et al., 1985a; Minshall, 1988). By contrast, few studies have

colour, and increased with moss cover. In a similar study

examined patterns in the functional structure and diversity

on the functional biodiversity of lake littoral macroinverte-

of freshwater macroinvertebrate assemblages along

brates, a general finding was that functional structure and

multiple non-anthropogenic environmental gradients,

diversity varied strongly along gradients in macrophyte

although the need for such studies has been clearly

cover and lake area (Heino, 2008a). The above findings

identified (Poff, 1997; Usseglio-Polatera et al., 2000). Using

were somewhat surprising, because some authors have

functional structure, or more generally species traits, as

suggested that the trait structure of assemblages varies only

response variables instead of taxonomic composition may

little along natural environmental gradients, and virtually

increase our understanding of the relationship between

no variation is related to geographical location (Charvet et

freshwater biodiversity and ecosystem functioning.

al., 2000; Statzner et al., 2001b). These discrepancies between

However, although most recent studies have certainly

studies are apparently due to the types of functional traits

addressed multiple important traits (Gayraud et al.,

used and analytical methods. However, despite these

2003; Lamouroux et al., 2004; Poff et al., 2006), there has

methodological differences, theory suggests that functional

typically been rather little division between the traits that

traits should be strongly related to environmental gradients,

are truly functional in terms of ecosystem functioning

be they of natural or anthropogenic origin (Poff, 1997).

Functional structure showed both

and those that portray various life history characteristics

A question pertaining to the utility of functional

of stream macroinvertebrates. Yet, directly emphasising

traits in the description of freshwater macroinvertebrate

the relationships between particular traits and ecosystem

biodiversity has been tackled recently: do functional traits

functioning might further increase our understanding of

provide more and complementary information to that

freshwater ecosystems.

provided by species? This question is important, because if there are no clear differences in assemblage patterns

DOI: 10.1608/FRJ-2.1.1


18

Heino, J.

between the functional trait and taxonomic approaches,

found that the taxonomic distinctness of aquatic beetle

then it may not be necessary to allocate taxa to various

assemblages responded more weakly than species richness

functional categories.

Recent studies have suggested

to anthropogenic alteration of inland waters in Spain. This

that both functional groups and taxa show relatively

finding is, in part, in accordance with those from Finland,

similar assemblage patterns along major environmental

where neither taxonomic distinctness nor species diversity

gradients (Finn & Poff, 2005; Heino et al., 2007a).

indices varied between near-pristine and anthropogenically

Furthermore, diversity measures based on functional

altered streams (Heino et al., 2007b). However, because

groups (e.g. number of functional groups) have been

taxonomic distinctness and species diversity indices were

found to be strongly related to similarly derived taxonomic

at best weakly correlated, both groups of indices should

measures (e.g. number of species) in both lentic and lotic

be used in biodiversity assessment. By contrast, Marchant

macroinvertebrates (Heino, 2008a; Heino et al., 2008).

(2007) found that taxonomic distinctness values of stream insect assemblages from anthropogenically altered sites

Patterns in the taxonomic variability of assemblages

were often below expectations of random draws of species from the species pools (see also Clarke & Warwick, 1998), and taxonomic distinctness was also negatively correlated

Species richness and diversity indices have traditionally

with a water quality gradient that described variation in

been used to assess variability in biodiversity across

turbidity and total phosphorus. Thus, it was suggested that,

near-pristine and anthropogenically altered ecosystems

due to a high number of species and taxonomic variability

(Magurran, 2004). However, it has been suggested that

in the dataset, a taxonomic distinctness index was able to

biodiversity measures should not only portray variability in

separate near-pristine and anthropogenically altered sites

species richness and relative abundances, but also take into

(Marchant, 2007). Accordingly, taxonomic distinctness

account the evolutionary relatedness of species (Harper &

indices in the other two studies on aquatic insect assemblages

Hawksworth, 1994). For this purpose, Clarke & Warwick

may not have performed well due to a more limited

(1998) devised a set of taxonomic distinctness indices that

number of species or higher taxa. Given that measures

take into account the taxonomic relatedness of species.

of taxonomic variability of assemblages are important

Thus, an assemblage that harbours distantly-related

in biodiversity assessment, not only environmental

species from, for example, different phyla is more diverse

researchers but also biodiversity researchers should

than an assemblage with the same number of species from

include taxonomic distinctness indices in their toolbox.

just one family. Taxonomic distinctness indices have been widely used in marine studies (Warwick & Clarke, 1998; Rogers et al., 1999; Ellingsen et al., 2005), but research on

Higher taxon surrogates for species-level patterns

the taxonomic variability of assemblages has just begun with aquatic insects.

The higher taxon approach in biodiversity research has

Three recent studies on aquatic insects have addressed

usually been directed at examining genus and family

taxonomic variability based on the taxonomic distinctness

richness as surrogates for species richness. These studies

indices. These studies have not only considered variability

have generally concluded that the numbers of higher taxa

in near-pristine ecosystems, but have also examined

are typically strongly correlated with species richness

biodiversity along anthropogenic gradients. Campbell &

(Williams & Gaston, 1994; Balmford et al., 2000; Gaston,

Novelo-Gutierrez (2007) studied the response of taxonomic

2000). By contrast, very few studies have directly examined

distinctness of odonate assemblages to dam impoundment

cross-taxonomic level patterns in assemblage composition,

and detected reduced values after the construction

so it is uncertain if patterns in assemblage composition are

of hydroelectric impoundment.

similarly strongly congruent between taxonomic levels as

Abellán et al. (2006)


DOI: 10.1608/FRJ-2.1.1

19


with taxon richness patterns. It has been suggested that

Conclusions

assemblage patterns between different taxonomic levels are strongly congruent in regions where there are low

Biodiversity

species-to-genus and species-to-family ratios, whereas

assemblages are complex. This complexity is seen in

patterns

exhibited

by

aquatic

insect

congruence diminishes in regions with high species

the environmental relationships of both species richness

diversity, resulting from high degrees of adaptive radiation

and assemblage composition.

(Hawkins & Norris, 2000). These suggestions remain to be

environmental variables are needed to understand

studied rigorously with aquatic insects.

variation in the diversity of aquatic insects. Although

In general, multiple

Freshwater research has traditionally concentrated

a few major environmental gradients, such as habitat

on examining the importance of taxonomic resolution

size and acidity, typically account for a modest amount

in bioassessment, whereas few biodiversity studies

of variability in aquatic insect assemblages, other minor

have addressed higher taxa as surrogates of species-

factors are also often required to understand variation in

level diversity. A number of lessons can be drawn from

diversity. This is especially true in cases where there is no

bioassessment studies. First, species richness has often

single dominant environmental gradient which has an

been found to be correlated with genus and family richness

overriding effect on biodiversity. Knowing assemblage–

(Guerold, 2000; Hill et al., 2001; Marshall et al., 2006). Also,

environment relationships is valuable for conservation

patterns of assemblage composition of different taxonomic

planning, and generally sites along different parts of major

levels are typically strongly correlated (Bowman &

environmental gradients should be protected to conserve

Bailey, 1997; Marshall et al., 2006; Heino, 2008b). Second,

α-, β- and γ-diversity of aquatic insect assemblages (Clarke

higher taxa often respond similarly to species to major

et al., 2008).

environmental gradients (Furse et al., 1984; Marchant

However, our knowledge of various aspects of aquatic

et al., 1995; Feio et al., 2006). Third, although genus- and

insect biodiversity is woefully incomplete. For example,

family-level identifications of stream macroinvertebrates

understanding one of the oldest patterns in ecology, the

are less expensive and time-consuming than species-

latitudinal diversity gradient (Hawkins, 2001), is severely

level identifications (Marshall et al., 2006), the latter are

hindered by the lack of both distributional data at broad

necessary when the interest is in community ecology and

scales and comprehensive surveys of aquatic insect

species distribution for conservation purposes (Furse et

assemblages across long south–north gradients. Achallenge

al., 1984; Bailey et al., 2001; Lenat & Resh, 2001). Studies

to future studies is thus to survey local assemblages along

addressing the higher taxon approach for biodiversity

latitudinal gradients. More than that, researchers should

and conservation purposes have generally corroborated

aim at comparing the relative importance of geographical

the findings from bioassessment research, with patterns

position, regional factors, watershed characteristics, and

in taxonomic richness and assemblage composition being

ecosystem and habitat variables for aquatic insect diversity

highly similar at species, genus and family levels (J.F. Wright

across large geographical extents. There are emerging

et al., 1998; Heino & Soininen, 2007). Thus, because higher

possibilities for such studies in Europe, and recent research

taxa show patterns similar to those of species, they can be

on streams has already touched this topic (Johnson et al.,

used in rapid biodiversity assessments, the goal of which

2007). However, there are severe problems in studies of

is to assess and rank a large number of sites according to

local ecosystems across large geographical extents. One

their relative biodiversity value. Such approaches may

such problem relates to the species-level identification of

be the only option for studies in tropical regions lacking

the aquatic larval stages of insects. Given that taxonomic

taxonomic information at the species level, and higher taxon

knowledge is unevenly distributed among countries,

surrogates have thus been used as estimates of species-level

large-scale surveys in poorly known areas could utilise

biodiversity patterns (Jacobsen et al., 1997; Jacobsen, 2004).

higher taxa as surrogates for local species-level diversity

DOI: 10.1608/FRJ-2.1.1


20

Heino, J.

patterns at least in the early phases of inquiry into large-

headwater lakes. Streams and ponds have been used as

scale patterns (Jacobsen, 2004).

Such surrogates have

arenas for testing ecological theory for a long time, and

proved promising in previous environmental assessment

given the easier sampling of small than large freshwater

and biodiversity studies (J.F. Wright et al., 1998).

ecosystems, this emphasis on streams and ponds is likely

In addition to species richness and assemblage

to continue. However, because freshwater biodiversity

composition, other measures of biodiversity should

is in severe crisis (Dudgeon et al., 2006), we should

be given more attention in research on aquatic insect

have no excuse for neglecting surveys of large rivers

assemblages. For example, it is still a largely unsettled

and headwater lakes across extensive regions.

question as to whether measures of the taxonomic variability

by knowing biodiversity patterns in various types of

of assemblages are useful in assessing environmental

freshwater ecosystems, will we be able to predict, conserve

degradation of biodiversity in freshwater ecosystems.

and manage aquatic insect biodiversity effectively enough.

Only

Both negative and positive results have emerged from studies that have used this approach (Heino et al., 2007b;

Acknowledgements

Marchant, 2007). However, even though measures of taxonomic variability of assemblages would not perform

The author wishes to thank Kai Korsu, Timo Muotka,

well in determining anthropogenic degradation of

Janne Soininen, Colin S. Reynolds, and two anonymous

freshwater ecosystems, the fact that they are at best weakly

reviewers for constructive comments on earlier versions of

correlated with various other measures of biodiversity

this paper. The preparation of this paper was supported by

suggests that they should be utilised more widely in

a grant from Maj and Tor Nessling Foundation.

studies on aquatic insect assemblages (Heino et al., 2008). Functional biodiversity should also be given more

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DOI: 10.1608/FRJ-2.1.1

Author Profile Jani Heino is a researcher working at the Finnish Environment Institute, Research Programme for Biodiversity. He received his PhD from the University of Jyväskylä in 2002. His current research lies at the nexus of large-scale ecology, community ecology, landscape ecology and conservation biology, with the main aim to understand spatial and temporal variation of biodiversity across and within aquatic ecosystems. He also uses the information on ecological patterns in guiding the conservation and management of aquatic ecosystems. His study subjects comprise a wide range of aquatic ecosystems (e.g. streams, rivers, littorals) and groups of organisms (e.g. invertebrates, bryophytes, algae). Freshwater Reviews (2009) 2, pp. 1-29