Patterns of species richness and introduced ... - Wiley Online Library

REGULATED RIVERS: RESEARCH & MANAGEMENT

Regul. Ri6ers: Res. Mgmt. 17: 699 – 707 (2001) DOI: 10.1002/rrr.631

PATTERNS OF SPECIES RICHNESS AND INTRODUCED SPECIES IN NATIVE FRESHWATER FISH FAUNAS OF A MEDITERRANEAN-TYPE BASIN: THE GUADIANA RIVER (SOUTHWEST IBERIAN PENINSULA) ´ NCHEZb CASIMIRO CORBACHOa,* AND JUAN MANUEL SA ´ rea de Biologıá Animal, Uni6ersidad de Extremadura, A6da. de El6as s/n, Grupo de In6estigacioń en Conser6acioń, A 06071 Badajoz, Spain b ´ rea de Biologıá Animal, Uni6ersidad de Extremadura, A6da. de El6as s/n, Grupo de In6estigacioń en Conser6acioń, A 06071 Badajoz, Spain a

ABSTRACT In this study, we analysed the factors affecting species richness and introduced species component patterns in native fish faunas of 30 streams of the Middle Basin of the Guadiana River. From a principal component analysis and a stepwise multiple regression analysis performed on a data matrix composed of ten hydrological and biotic variables, we showed that: (1) fish species richness increased with stream length and watershed area, (2) the number of native species in a stream declined as channelizations and river regulation (constructions of dams) are higher, whereas introduced species increased in the same way, (3) the two main negative factors affecting native ichthyofaunas affected dissimilar ecological areas: channelizations, which depend on land-use intensity of floodplain, mainly occurred in lower reaches of streams, but construction of dams mainly took place in upper sections of rivers, (4) the length of the remaining well-preserved reaches in a stream appeared to be the only factor accurately predicting native fish species richness, and (5) native fish faunas of small isolated streams are more vulnerable to habitat alteration than those of large streams. Both isolation and fragmentation of populations were recorded, so the conservation status of native and highly endemic fish fauna of the study area is extreme. Protection of the few still extant, well-preserved small streams and upper reaches, habitat restoration of channeled areas, and inclusion of the need for native fish fauna conservation in long-term public planning of water use become a priority. Fish communities appear to be a sensitive indicator of biological monitoring to assess environmental degradation. Copyright © 2001 John Wiley & Sons, Ltd. KEY WORDS:

channelization; conservation; endemic ichthyofauna; exotic species introduction; fragmentation; isolation; river regulation; stream order

INTRODUCTION Freshwater fish fauna of the Iberian Peninsula shows a distinctive position within European ichthyofauna. Over 80% of native species belonging to the Cyprinidae, Cobitidae and Cyprinodontidae families are endemic to the area (Almaça, 1995; Elvira, 1995). The isolation from other European catchments, together with the watershed discreteness and climatic factors, appears to be the main causes leading to the differentiation of many independent and isolated fish populations (Doadrio et al., 1991). In this context, the Guadiana River (southwestern Spain) native ichthyofauna shows special features with regard to other Iberian basins (Doadrio et al., 1991; Almaça, 1995). It has local endemic species (Anaecypris hispanica and Barbus microcephalus), and both threatened (Blennius flu6iatilis and Acipenser sturio) and migratory species (Petromyzon marinus, Anguilla anguilla and Alosa alosa) are still commonly recorded. Iberian– Northern African affinities for endemisms (Barbus and Anaecypris) are also cited, while native Salmonidae species are absent owing to the lack of mountain streams. These important criteria are often overlooked as conservation priorities. ´ rea de Biologıá Animal, Universidad de Extremadura, Avda. de * Correspondence to: Grupo de Investigacioń en Conservacioń, A Elvas s/n, 06071 Badajoz, Spain. E-mail: [email protected]

Copyright © 2001 John Wiley & Sons, Ltd.

Recei6ed 7 August 2000 Re6ised 29 December 2000 Accepted 8 January 2001

´ NCHEZ C. CORBACHO AND J.M. SA

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Endemic fish faunas of small and isolated aquatic systems are highly vulnerable to habitat alteration (Sheldon, 1988; Moyle and Williams, 1990; Maitland, 1995). Iberian and, particularly, the Guadiana Basin’s native ichthyofauna present a serious conservation problem, with most species being actually threatened, owing mainly to poor land-uses, water pollution, habitat loss, river regulation, abusive water extraction to benefit agriculture, etc. (Almaç a, 1995; Elvira, 1995, 1996). Introduced species become an additional negative factor (e.g. Moyle and Williams, 1990; Reinthal and Stiassny, 1991; Bianco, 1995; Crivelli, 1995), and the major agent contributing to the loss of aquatic biodiversity (Moyle and Leidy, 1992). The present situation of Iberian fish fauna is similar to that of other Mediterranean-type areas, the basic reason for this being the intense competition between humans and fishes for limited supplies of fresh water (Moyle and Williams, 1990; Moyle, 1995). Iberian and Mediterranean ichthyofaunas have received little attention, this even though an urgent need for conservation assessment and plans of action are required (Moyle and Williams, 1990; Crivelli and Maitland, 1995; Moyle, 1995). Species conservation status has just been reviewed (e.g. Almaç a, 1995; Bianco, 1995; Elvira, 1995), and the factors contributing to the decline of native fish faunas have also been listed (Maitland, 1995; Elvira, 1996), but only from a qualitative approach. There are, however, few analytical studies concerning the multiple factors involved in the development of a comprehensive framework for native freshwater fish faunas conservation (e.g. Moyle and Williams, 1990). In this paper, we present an integrated landscape-level approach at an entire basin scale (1) to identify the factors regulating both native and introduced species richness patterns, (2) to assess the relative influence of these factors, and (3) to contrast the predictions made about fish community structure and the relative success of fish invasions into native faunas, and their implications for environmental management and fish conservation assessment procedures. For these purposes, we selected a Mediterranean area of the southwestern Iberian Peninsula, the Middle Basin of the Guadiana River (MBGR), which is considered to be of great interest regarding freshwater fish fauna (Doadrio et al., 1991).

STUDY AREA AND DATA ANALYSIS The study area extended to the MBGR (25000 km2; Figure 1). We surveyed 30 first-, second- or third-order tributaries comprising over 1700 km of stream length, selected according to Horton’s method (Horton, 1932) over a 1:200000 scale map. Mediterranean climate of MBGR, with average annual precipitation, ranges from 450 to 1000 mm (about 90% from November to April), produces small streams that have strong seasonal patterns of flow, with low-flows and extended droughts in the rainless summer, and high-flows in winter and spring. Land-use is based mainly on a dry agriculture scheme (cereal crops, sunflower, olives and vines predominate), together with holm-oak ‘dehesas’, a semi-natural open forested habitat typical of the southwest Iberian Peninsula. However, in recent decades over 200000 ha have been converted into irrigated lands. A total of 27 dams have been constructed to benefit agriculture (over 250 km, 13% of the total streams length being dammed); in the same way, stream channels flowing through irrigated lands have often been greatly modified to accommodate them to intensive agricultural use, mainly by channeling river courses (210 km, 10%). As a result, a variable human disturbance, from highly disturbed to relatively undisturbed watersheds, can be documented, providing an excellent area in which to examine these effects on fish faunas. From each stream selected, we measured the following hydrological ‘abiotic’ variables: (1) stream tributary order (hereafter, ORD, in the tables) as the category of stream with respect to the main waterway; i.e. first-, second- or third-order tributaries (Figure 1); (2) length (km) of the river (LEN) from the source to the mouth; (3) watershed area (ARE) as surface (km2) of drainage basin of each river; and (4) mean land-use intensity (USE) from upstream to downstream; land-use schemes immediately bordering the stream or riparian channel were considered only (we coded and rated them according to an increasing intensity; i.e. from 1 to 10 for undisturbed forests, shrublands, ‘dehesas’, afforestations, grasslands, extensive cultivations, intensive cultivations, irrigated lands, urbanizations and urban lands respectively); (5) number of dams (DAM) within a stream; (6) ratio of stream length dammed (in %; Copyright © 2001 John Wiley & Sons, Ltd.

Regul. Ri6ers: Res. Mgmt. 17: 699 – 707 (2001)

SPECIES RICHNESS AND INTRODUCED SPECIES IN NATIVE FRESHWATER FISH FAUNAS

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Figure 1. Map of MBGR, showing the selected streams for the present study. Stream tributary order (in brackets), dams (reservoir in black) and stretches channelized (dotted line) are also indicated

%DAM), i.e. length occupied by reservoirs with respect to total length of stream; (7) ratio of stream length channelized (%CHAN); and (8) length (in km; WPRE) of the stream remaining well-preserved, taking into account both stream channel and riparian corridor. Data on fish distribution per stream were derived from Doadrio (1986) and Doadrio et al. (1991), and mainly from our own and most recent unpublished studies (Corbacho, 1997). Doadrio’s samples were collected using electrofishing, performed on one to three 50 m long stretches in each stream, depending on their length. We sampled fishes by a combination of fishing techniques, with a similar sampling effort applied in all the streams; a total of 128 sampling stations, each located approximately 15 km apart along the streams were surveyed in the study area. The primary methods were a large electrofishing apparatus (220–250 V, 50 Hz, 2500 W, 10 A), completed with the use of a 20 m long seines in more than 2 m deep, and interviews with fishermen. Species abundance in each sampling area was rated as ( + ) presence, 2 –20 individuals as rare, 20 – 50 common, 50 – 100 abundant and + 100 very abundant, but these categories were not considered in the analysis. All the samples were collected from August to October during low-flow periods, when the streams were reduced to a series of small isolated water mass, thus facilitating the fishes capture because of availability of preferred habitats being specially limited (Angermeier and Schlosser, 1989). We adopted taxonomic, biogeographic and conservation status of species according to Elvira (1995) (Table I). From the complete species list of each stream, we quantified the following ‘biotic’ variables: (9) total species richness (SPE), including all, even very rare, species captured; (10) number of native species Copyright © 2001 John Wiley & Sons, Ltd.



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Table I. List of all species cited for the selected streams in the study area Native species (V) (V) (V) ++ (V) + (V) ++ (R) + (NT) + (NT) + (NT) + (R) (NT) + (NT) + (V) (V) (E)

Petromyzon marinus Linnaeus, 1758 Anguilla anguilla Linnaeus, 1758 Alosa alosa Linnaeus, 1758 Anaecypris hispanica Steindachner, 1866 Barbus comiza Steindachner, 1865 Barbus microcephalus Almaç a, 1967 Barbus sclateri Gunther, 1868 Chondrostoma polylepis Steindachner, 1865 Leuciscus pyrenaicus Gu¨ nther, 1868 Rutilus lemmingii Steindachner, 1866 Tinca tinca Linnaeus, 1758 Tropidophoxinellus alburnoides Steindachner, 1865 Cobitis paludica de Buen, 1930 Gasterosteus aculeatus Linnaeus, 1758 Blennius flu6iatilis Asso, 1801

Introduced species Oncorhynchus mykiss Walbaum, 1792 Esox lucius Linnaeus, 1758 Carassius auratus Linnaeus, 1758 Cyprinus carpio Linnaeus, 1758 Gobio gobio Linnaeus, 1758 Lepomis gibbossus Linnaeus, 1758 Micropterus salmoides Lace´ pe`de, 1802 Gambusia holbrooki Girard, 1859 The conservation status in the Iberian Penıńsula (E, endangered; V, vulnerable; R, rare; NT, not threatened) and the endemic species (+, Iberian endemism; ++, Guadiana Basin endemism) are indicated.

(NAT); and, (11) number of introduced species (INT). We also calculated (12) the relative importance of introduced species in fish fauna (%INT) as the introduced/total fish species ratio, an index of the degree to which fish faunas have been contaminated by exotic species (‘zoogeographic integrity coefficient’, Bianco, 1990). Native ichthyofauna of MBGR include 15 species, with nine taxa (60%) being endemic to Iberian basins; three migratory species are also recorded (Table I). In addition, eight non-native species now inhabit the study area, representing about 35% of the entire species richness. The introduction of exotic species by humans in the Iberian Peninsula has a long history, dating back, at least, to the 17th century (Elvira, 1995). The introductions have continued ever since, mainly in the present century, and the recent spread of alien species has been recorded in the last few decades; only four out the 31 watershed surveyed are actually free from exotic species (Corbacho, 1997). Data analysis included a combination of statistical techniques to assess the relative effects of abiotic factors on fish assemblages. Prior the analysis we (x+ 1)1/2 transformed the original variables to assure normality (Zar, 1996). On the transformed data matrix, we first performed a principal components analysis (PCA) to obtain the few explainable components (PC) accounting for the maximum variance (Tabachnick and Fidell, 1989). From the general patterns we obtained by PCA, we rated the relative importance of factors on fish fauna by performing a stepwise multiple regression analysis (Tabachnick and Fidell, 1989). Variables in the models were selected according to a significant (pB 0.05) proportion of residual variation explained after all other variables were included. Non-parametric Mann– Whitney U-test and Spearman rank correlation tests were also performed (Zar, 1996). All the analyses were run on the Statistica 5.1 program (Stat Soft Inc., 1997). Copyright © 2001 John Wiley & Sons, Ltd.



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RESULTS The PCA produced three independent components that explained over 77% of total variance. PC1 accounted for 32% of variance, and loaded heavily mainly for abiotic variables, such as stream length, watershed area and length of well-preserved reaches of stream left; in the same way, it loaded for biotic ones related to fish species richness (Table II). This axis was interpreted as a gradient of species-area. Hence, the greater the stream length, watershed area and length of well-preserved reaches, the higher the fish species richness, this general pattern involving both native (Figure 2) and introduced species. To a lesser extent, construction of dams appeared to be associated with large streams. PC2 (28% of variance) related fish fauna conservation status to the extent that a stream remained unaltered. Thus, the greater the well-preserved length of stream, the higher both total and native species richness, which in turn involved a lesser relative importance of introduced species. In contrast to this, channelization of streams, but also river regulation by dams appeared to have a mainly negative and similar effect (Table II). PC3 (18% of variance) was totally related to stream habitat alteration, this being mediated by mean land-use intensity Table II. Loadings on the three first components and cumulative variance in the PCA of the combined data set for biotic and abiotic variables of streams selected in the study area PC Variable

1

2

3

Stream length Watershed area Mean land-use intensity % stream length dammed % stream length channelized Well-preserved length of stream Total fish species richness Native species richness Number of introduced species Ratio of introduced species

0.914 0.895 0.130 0.410 0.026 0.623 0.571 0.458 0.654 0.035

0.211 0.165 −0.088 −0.382 −0.501 0.546 0.733 0.819 0.009 −0.900

0.072 0.097 −0.833 0.684 −0.695 0.257 −0.047 0.034 −0.177 −0.100

Variance explained Cumulative variance (%)

3.176 0.318

2.792 0.279

1.769 0.177

Data were (x+1)1/2 transformed to assure normality.

Figure 2. Plot of native fish species richness on length of remaining well-preserved reaches in streams of the study area Copyright © 2001 John Wiley & Sons, Ltd.



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Figure 3. Plots of proportion (%) of total stream length channelized (open squares; bold line) and dammed (black squares; dotted line) in relation to mean land-use intensity bordering the stream corridor

bordering the riparian corridor. Thus, the higher the mean land-use intensity, the higher the percentage of stream length channelized, but the lower the ratio of damming (Figure 3). The stepwise multiple regression analysis showed that the only factor significantly affecting both total and native species richness was the extent to which a stream remain well-preserved (R 2 = 0.39 and R 2 =0.47, pB0.001 in both cases), measured in term of stream corridor length in a good conservation status (Table III). The number of introduced species and its relative importance in native fish faunas appeared to be complex variables, so only a little variance was accounted for in the predictor variables shown. Introduced species richness was significantly predicted, exclusively by watershed area (R 2 = 0.20, pB 0.05), whereas the relative importance of exotic species into native ichthyofauna was explained by the length of well-preserved reaches left in a negative way and the ratio of stream length channelized in a positive one (R 2 = 0.34, p B0.01), respectively (Table III). Stream tributary order appeared to be also an important factor affecting total, native and introduced species richness (Table IV). Thus, total fish species (Mann –Whitney U-test, U= 29, p B 0.001), native species (U= 37.5, p B 0.01), and introduced species richness (U= 37.0, p B 0.01; n1 = 18 and n2 = 12, in all cases) were higher in first order tributaries than in second –third ones, even though no significant differences were found between the two groups for any of the significant variables that emerged formerly (Table IV). A significant negative effect of stream channelization was, however, recorded in second and third order tributaries on both number of native species (Spearman correlation test, r= −0.62, pB 0.05) Table III. Summary of stepwise regression analysis of each of the biotic variables of fish faunas versus abiotic ones in streams of study area Dependent model NAT= 0.325+0.686 WPRE WPRE (0.47) INT =1.614+0.452 ARE ARE (0.20) %INT = 5.693−0.464 WPRE+0.584 %CHA WPRE (0.22) %CHAN (0.12)

R2

p

0.47

***

0.20

*

0.34

**

Tabled entries include equations for the models (with intercepts and slopes for each of the variables selected), proportion of total variance accounted for the variables (in brackets, ranked in order of importance), proportion of variance accounted for by the models (R 2) and probability (p). * pB0.05, ** pB0.01, *** pB0.001. Data were (x+1)1/2 transformed. Copyright © 2001 John Wiley & Sons, Ltd.



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Table IV. Summary statistics and Mann–Whitney U-test results for abiotic and biotic variables in relation to stream tributary order Stream tributary order Variable Stream length (km) Watershed area (km2) Mean land-use intensity Number of dams % stream length dammed %stream length channelized Well-preserved length (km) Total fish species richness Number of native species Number of introduced species Ratio of introduced species

1st 76.7942.0 (18) 13269 1947.3 (18) 4.991.3 (18) 1.190.9 (18) 8.2910.3 (18) 10.7916.2 (18) 48.9923.9 (18) 10.194.1 (18) 6.793.7 (18) 2.691.0 (18) 28.2916.2 (18)

2nd–3rd

p

50.29 15.0 (12) 499.69 304.1 (12) 5.09 1.0 (12) 1.19 0.8 (12) 14.49 12.6 (12) 12.8919.7 (12) 34.4913.4 (12) 4.392.5 (12) 2.692.5 (12) 1.191.1 (12) 39.8941.2 (12)

* NS NS NS NS NS NS *** ** ** NS

NS: p\ 0.05, * pB0.05, ** pB0.01, *** pB0.001. Sample sizes are given in brackets.

and relative importance of introduced species (r= 0.73, pB 0.01; n = 12 in both cases; Figure 4), but not in large tributaries (r = −0.34 and r = 0.28, respectively, p \ 0.05, n= 18).

DISCUSSION With an entire basin approach, fish species richness of streams in MBGR was correlated to stream length and watershed area (see also e.g. Sheldon 1988), this being in accordance with the ubiquitous species – area relationships (McArthur and Wilson, 1967). The increasing habitat area and habitat diversity which occurs in large rivers appear to be the main responsible factors, although in streams subject to strong seasonal environmental variations, habitat features are mediocre predictors (e.g. Gorman and Karr, 1978; Angermeier and Schlosser, 1989). In the study area, however, poor and intensive land-use systems of stream corridors greatly affected species richness pattern of native fish fauna. In streams highly disturbed by channelization, an increase of

Figure 4. Effect of channelization on relative component of exotic species into native fish faunas in first (black squares, bold line) and second –third (open squares, dotted line) order tributaries Copyright © 2001 John Wiley & Sons, Ltd.


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the relative importance of introduced species was recorded, which led to low native fish species richness assemblages (cf. Schlosser, 1991). Significant effects were, however, not related to damming, though its negative influence (see Moyle and Leidy, 1992) could be concealed in the study area by the strong relationship that exists between river regulation (number of dams) and watershed area (r= 0.49, pB 0.05, n=30) or stream length (r = 0.71, p B0.001, n = 30), so both native and introduced species richness were also implicated. As a conclusive result, in MBGR, the length of remaining well-preserved reaches in a stream became the only factor that determined native fish species richness, which in turn affected relative importance of alien species into fish fauna (Figure 2). In MBGR, therefore, habitat alteration, mainly by channelization, but also river regulation through use of dams, act in concert to determine that: (1) the modified streams support native fish communities of lower species diversity than the natural ones; and that (2) the streams that present high levels of human disturbance appear to be particularly vulnerable to exotic species invasions. Mediterranean-type native ichthyofauna is adapted to an irregularly fluctuating environment. Introduced species cannot adapt to the highly seasonal flow regimes of undisturbed or unregulated streams (‘environmental resistance hypothesis’, Moyle, 1986), but the alteration of rivers improves conditions for them (e.g. Reinthal and Stiassny, 1991; Ross, 1991; Moyle and Light, 1996). In this sense, a recent and rapid dispersal of alien species has been described in MBGR following stream alteration (Corbacho, 1997). It is uncertain, however, whether the associated decline of native species is caused by the habitat alteration, the introductions of species, or both working together. Native species show a high resistance capability in the face of alteration processes, and so this rarely affect species richness (Schlosser, 1990). In MBGR, however, our results suggest that the introduction of alien species associated to habitat alteration took place (Schlosser, 1990; Reinthal and Stiassny, 1991). Disastrous effects of the large river regulation (i.e. channelization and damming) upon the species richness have been recorded (Welcomme, 1985). In the study area, however, the autochthonous fish fauna of small streams appears to be more vulnerable to habitat alteration than the large river ones. The great habitat availability and habitat diversity in large rivers, which can give refuge to native species in the face of high alteration levels, appears to be the factor affecting this. In small streams, the effect of alteration processes spread to entire basin, and local extinctions of native species were recorded. The extreme situation of the native and highly endemic fish fauna of MBGR become aggravated by the disaggregated spatial distribution that exhibit channelizations and dam locations. The channelizations mainly affect the lower reaches of streams located in flat landscapes where intensive land-use predominates (Spearman rank correlation land-use intensity versus percentage ratio of channelization (r= 0.45, p B0.01, n=30). The construction of dams, however, takes place mainly in relatively well-conserved wild rough landscapes of upper sections of rivers (r = −0.40, pB 0.05, n= 30; Figure 3). Two aspects become highly disturbing (see also Moyle and Williams, 1990; Moyle and Leidy, 1992): (1) lower sections of rivers support the much higher fish species richness of streams (Gorman and Karr, 1978), and (2) the well-conserved both upper reaches and isolated small streams are the only actual refuges to threatened and small Iberian endemisms (Rutilus, Leuciscus, Tropidophoxinellus, Anaecypris, Cobitis) (Doadrio et al., 1991). The negative effects of both channelizations and dams upon native fish faunas are, therefore, cumulative, with both isolation and fragmentation of the populations being recorded (Sheldon, 1988). On this basis, the protection of the still extant few well-preserved both entire streams and upper reaches becomes a priority (Sheldon, 1988). Preventing exotic fish species invasions should also be a major consideration in order to protect native fish assemblages. However, the need for restoration of stretches affected by channelization must be emphasized, because this appears to be the principal long-term means by which successful native fish conservation will be achieved (Maitland, 1995). Native fish fauna conservation requirements must be taken into account in long-term public planning of water use (Maitland, 1995). The entire catchment of a stream must be considered (Moyle and Leidy, 1992), as the fish communities reflect watershed conditions, and the basis for using biological monitoring of fish to assess environmental degradation (Fausch et al., 1990; Schlosser, 1991). Copyright © 2001 John Wiley & Sons, Ltd.



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ACKNOWLEDGEMENTS

A. Lo´ pez, E. Costillo and C. Fuentes assisted in field studies. The authors thank C. Almaç a, R. Mora´ n and two anonymous referees for useful comments on an earlier version of this manuscript. Thanks also to J. McCue and D. Parejo for English translations. This research was supported, in part, by a grant provided by Confederacio´ n Hidrogra´ fica del Guadiana to C. Corbacho.

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Copyright © 2001 John Wiley & Sons, Ltd.
