effects of pond area, habitat heterogeneity and isolation. Christer Br6nmark. Department of Animal Ecology, University of Lund, S-223 62 Lund, Sweden.
Oecologia 9 Springer-Verlag1985
Oecologia (Berlin) (1985) 67:127-131
Freshwater snail diversity: effects of pond area, habitat heterogeneity and isolation Christer Br6nmark Department of Animal Ecology, University of Lund, S-22362 Lund, Sweden
Summary. A large number of eutrophic ponds were surveyed for the presence of freshwater gastropods. Factors thought to influence the distribution of the snails were evaluated. As the investigated area has a homogeneous geological background physicochemical factors probably have a low effect on the local distribution of snails. There was a significant, positive regression between pond area and the number of gastropod species in the pond, but the regression only explained a minor part of the variation in species numbers. Multiple regression of an extended number of variables, associated with habitat complexity and dispersal, indicated that, in addition to area, macrophyte diversity and the mean number of gastropods in the 5 closest ponds (~qs) were important in explaining the distribution of gastropods. An increase in the number of macrophyte species increases the number of available microhabitats and refuges from predators. $5 probably influences the dispersal rates between ponds. The gastropods in this area are thought to have additional dispersal modes, besides aerial dispersal with birds, and this probably increases the immigration rates and/or decreases the extinction rate.
The distribution of species in insular habitats has been a major topic in modern ecology. A number of hypothesis have been presented to explain the relationship between the number of species present on an island and the area of the island (see e.g. Simberloff 1974; Connor and McCoy 1979; Abbott 1980; Mcguiness 1984). The first hypothesis simply explains the increase in species numbers with area as a passive sampling process - larger islands are hit by more propagules and this results in higher immigration rates. This hypothesis does not invoke any biological processes and it has been suggested that it should be considered as a null hypothesis in island biogeographical studies (Connor and McCoy 1979). The "habitat diversity hypothesis", in contrast, emphasises the importance of habitat heterogeneity. The number of habitats, each with a specific set of associated species, usually increases with island area and in such cases the number of species increases with island area. According to the equilibrium theory of island biogeography, as developed by MacArthur and Wilson (1967), the number of species on an island is a function of immigration and extinction rates. Immigration is a negative function of distance from the source pool, while extinction is inversely related to island area. The theory predicts the number
of species (S) at equilibrium to be a positive function of area (A) as S = CW. A z value lower than the predicted 0.262 (Preston 1962) has been proposed for islands with high immigration rates (MacArthur and Wilson 1967). However, low z values could also be a result of low extinction rates or a combination of both (Johnson and Simberloff 1974). High z values (z = 0.44) were obtained from sets of islands with a small area as compared to the territory size of the bird species investigated (Rusterholz and Howe 1979). Several authors have pointed out that a z value of 0.25 does not necessarily have any biological meaning, but rather is expected for mathematical reasons (Connor and McCoy 1979; Abbott 1983; Connor et al. 1983; but see also Sugihara 1981; Martin 1981). Recently, a fourth hypothesis has been presented (Sousa 1976; McGuiness 1984). The "disturbance hypothesis" deals with the effects of catastrophic events on island communities. The intensity and frequency of disturbances are supposed to decrease with increasing area and thus the effect on species diversity is larger the smaller the island. The majority of island biogeography studies have focused on terrestrial organisms, especially landbirds on oceanic islands (see reviews by Simberloff 1974; Abbott 1980; Gilbert 1980). Freshwater habitats can be visualized as islands of water surrounded by land, and several studies have applied the equilibrium theory to freshwater organisms, such as plants (Keddy 1976), plankton (Hubbard 1973), molluscs (Sepkoski and Rex 1974; Lassen 1975; Aho 1978a, b; Browne 1981; Dillon and Benfield 1982), waterbeetles (Nilsson 1984), fish (Barbour and Brown 1974) and birds (Nilsson and Nilsson 1978). Studies of freshwater gastropods have shown that the equilibrium theory has low predictive power in oligotrophic waters (Aho 1978a, b). However, as the water gets more eutrophic, hydrochemical factors no longer restrict the species distribution and the importance of the equilibrium model increases (Lassen 1975; Aho 1978a, b; Dillon and Benfield 1982). The present study investigates the distribution of freshwater gastropods in a large number of eutrophic ponds. The importance of pond area, habitat diversity and dispersal between ponds is considered. Material and methods
The study area is situated 40 km south of Lund, southern Sweden (Fig. 1) and is rich in eutrophic ponds. Since the
128 Table
Lymnaea stagnalis L. peregra L. auricularia L. palustris L. glutinosa Physa fontinalis Planorbarius corneus Planorbis planorbis Anisus vortex
o 0 Q~
~
o
oooo~ 1 7 6 o ~0 o
0 o
o
1. List of gastropod species found in the study area
o
~5
Bathyomphalus contortus Gyraulus albus G. crista Hippeutis eomplanatus Aeroloxus lacustris Viviparus fascia tus Bithynia tentaculata B. leaehi Valvata eristata
o o
T a b l e 2. Mean, range and standard deviation of the variables used in the analyses. Distances are expressed in meters and area in hectares. Sg= number of gastropod species, S,, = number of macrophyte species, logA=log area, Dist=distance to closest pond, Dis%=mean distance to the five closest ponds, Ns0=number of ponds in a 50 ha square, S~ear= number of gastropod species in the closest pond, g5 = mean number of gastropod species in the five closest ponds
~ x/ J
o O
(]
o
~ o o
o
s
(3%%
~ o
0
Variable
d
o
Sg Sm logA Dist Dist s Nso S. . . . $5
o
~9
*. c::7
* b 0
0
~ '%0 ~
O
~
& ....
,
O
2 - 14 4 - 30 0.003- 2.33 10 -230 48 -414 1 - 14 2 - 13 4.6 - 11.8
~?-+S.D. 7.95_+ 7.83 13.73+ 5.79 0.38_+ 0.52 86.67+__99.72 161.99_+80.78 7.92__ 3.25 7.26+_ 2.66 7.88_+ 1.66
n 115 45 115 114 104 113 99 44
o
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; ....
r
Range
O
%
km
Fig. 1. Map of the investigation area area has a homogeneous geological background (glaciofluvial moraine deposits), the chemical composition of the water should not influence the local distribution of the freshwater gastropods. Polluted or otherwise disturbed ponds were not included in the analysis. The youngest ponds are around 100 years old, but considering the high immigration rate of freshwater gastropods (Lassen 1975; Barnes 1983) this timespan should be long enough to allow colonization of all potential species. The surface-area of the ponds were obtained from an 1:10,000 map with computer digitizer technique. The size of the smallest ponds was estimated in the field. The bottom substrate is very similar in all ponds surveyed. It consists of mud with a high organic content and occasional stones and pieces of wood. The shores are lined with emergent vegetation. The sampling was performed in September-October 1981 and 1982. The vegetation zones were sampled with a pond net and, in addition, vegetation, stones and woody debris were inspected visually for the presence of gastropods. A total of 18 species of gastropods were found in the 115 ponds sampled (Table 1). The number of macrophyte species in 45 of the sampled ponds were obtained from Jens~n (1971). Only species associated with water were included. A certain turnover of macrophyte species probably have occurred in the 10 years between the surveys, but since the ponds are expected to be in equilibrium regarding the number of macrophyte species, the species number in an individual pond is expected to
be close to constant (Jens6n pers. comm.). The number of macrophyte species (S,,) in the ponds were considered as a measure of habitat diversity and entered as an independent varaible in the multiple regression analysis. The other independent variables were: pond area, distance to the closest pond (Dist), mean distance to the five closest ponds (Dis%), number of ponds in a 50 ha square with the pond under consideration as centre (Nso), number of gastropod species in the closest pond (Sn~_,~) and mean number of species in the five closts ponds ($5). The five last variables were thought to influence the immigration rate of the freshwater snails. The number of gastropod species per pond (Sp was entered as the dependent variable. The SPSS statistical package was used for the regression analysis. In order to trace possible effects of the number of macrophyte species on the number of gastropod species, the species-area residuals were calculated. The species-area residual (S) is the portion of the species number in a given pond that remains unexplained by the species-area regression (i.e. S = S - 0 . 4 5 A ~
(Lomolino 1982).
The occurrance of breeding birds in the ponds was obtained from Karlsson (pers. comm.). Only bird species associated with water, and thus liable to act as snail vectors, were considered. Results
The range, mean and standard deviation of the variables used in the analyses are shown in Table 2. Four models of regression between the number of gastropod species (Sp and area (A) were calculated. The expo-
129 Table 3. Result of the multiple regression with the number of gastropod species (Sg) as the dependent variable 14-
12:
99
~ ;
Independent variables
a_ 1 0 {o
o~64-
~
~
~
~
9
log A log o~5 log S,,
9176 9
m 2-
==
o.o'ol
o3~
o',~
Regression coefficients
;
1;
Standardized regr, coeffic.
0.085 0.849 0.355
0.392 0.410 0.364
Constant
- 0.591
Multiple R=0.902
R 2=0.823
A R E A (ha)
Fig. 2. Species-area relationship for freshwater gastropods. log Sg=0A14 log A +0.454, R 2 =0.162, P < 0.001, n= It5
65" 4a
3' 2
40-
35
1.
304 25-_-2o-~
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5
~149
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1;
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25
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NUMBER OF M A C R O P H Y T E SPECIES
Fig. 4, The relationship of species-area residuals (~) with the number of aquatic macrophyte species. }=0.130S,,- 1.337, R2 =0.152, P
