Biodiversity and Conservation 11: 137–147, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

Concordance of species richness patterns amongmultiple freshwater taxa: a regional perspective

JANI HEINODepartment of Biology, University of Oulu, POB 3000, 90401 Oulu, Finland; Present address:Department of Biological and Environmental Sciences, University of Jyväskylä, POB 35,40351 Jyväskylä, Finland (e-mail: jani.heino@oulu.fi)

Received 10 October 2000; accepted in revised form 26 February 2001

Abstract. Geographical gradients in species richness and the degree to which different taxa show congru-ent patterns remain unknown for many taxonomic groups. Here, I examined broad-scale species richnesspatterns in five groups of freshwater organisms; macrophytes, dragonflies, stoneflies, aquatic beetles andfishes. The analyses were based on provincial distribution records in Denmark, Norway, Sweden and Fin-land. In general, variation in species richness across provinces was concordant among the groups, butstoneflies showed weaker negative relationships with the other taxonomic groups. Species richness in mostgroups decreased with increasing latitude and altitude, and a considerable part of the variation was ex-plained by mean July temperature. However, stoneflies showed a reversed pattern, with species richnesscorrelating positively, albeit more weakly, with mean provincial altitude. Nevertheless, combined speciesrichness of all five taxa showed a strong relationship with mean July temperature, accounting for 74% ofvariation in provincial species richness alone. Such temperature-controlled patterns suggest that regionalfreshwater biodiversity will strongly respond to climate change, with repercussions for local communityorganization in freshwater ecosystems in Fennoscandia.

Key words: beetles, congruence, dragonflies, freshwater biota, freshwater fishes, macrophytes, speciesrichness, stoneflies

Introduction

Considerable research effort has recently been devoted to describing spatial patternsof biodiversity, yet our knowledge of the distribution of diversity at broad spatialscales remains poor (Gaston et al. 1995; Gaston and Williams 1996). This is a majorconcern, since biodiversity is diminishing at an extremely rapid rate (Ehrlich andWilson 1991; Soule 1991; Sala et al. 2000). The lack of knowledge on species rich-ness patterns at different spatial scales hinders us from effectively predicting andconserving biodiversity at different levels of organization. For example, geographicalpatterns in species richness are unsatisfactorily known for several taxonomic groups,as is also the degree to which different taxa show concordant patterns (Prendegastet al. 1993; Gaston et al. 1995; Gaston 1996; Prendegast and Eversham 1997).

Compared with many terrestrial groups, patterns and determinants of fresh-water biodiversity are poorly known (Allan and Flecker 1993; Crow 1993; Vinson and

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Hawkins 1998; Rundle et al. 2000). At broad spatial scales, in particular, quantitativeinformation on species richness patterns are largely lacking for many freshwater taxa,fish and amphibians being notable exceptions (e.g. McAllister et al. 1986; Abell et al.2000; but see France 1992; Prendegast et al. 1993 for some other taxa). This is largelydue to the fact that most freshwater research has traditionally focused on describingphenomena within or among local systems (e.g. Minshall 1988). However, given therecent emphasis on the effects of regional processes on local communities (Cornelland Lawton 1992; Ricklefs and Schluter 1993; Huston 1999; Maurer 1999), fresh-water research would also benefit from increased attention to broad-scale patterns inspecies richness.

Few freshwater studies have addressed the concordance of community structureand species richness among different taxa, and most of these have compared indi-vidual streams or lakes within regions (Jackson and Harvey 1993; Allen et al. 1999a;Kilgour and Barton 1999; Paszkowski and Tonn 2000). In general, different taxa seemto show broadly concordant community patterns among local systems (Jackson andHarvey 1993; Paszkowski and Tonn 2000), although the environmental determinantsof these patterns may vary among taxa (Jackson and Harvey 1993; Allen et al. 1999b).However, the degree of concordance in species richness may be low among sometaxonomic groups (see Allen et al. 1999a), and almost certainly varies with scale(Gaston and David 1994; Reid 1998).

Many freshwater taxa show broadly similar geographical distribution patterns,corresponding to climatic variation along latitudinal and altitudinal gradients (e.g.McAllister et al. 1986; Rorslett 1991), although there are also few propable excep-tions to these general patterns (see e.g. Crow 1993). Here, I examined geographicalpatterns in species richness of freshwater macrophytes, dragonflies, stoneflies, bee-tles and fishes in Fennoscandia. My objectives were to examine (i) the broad-scaleenvironmental determinants of species richness, and (ii) the degree of concordance inspecies richness patterns among different taxonomic groups.

Materials and methods

Data sets

Biogeographical provinces in Fennoscandia (Denmark, Sweden, Norway and Fin-land; 54–71◦ N and 5–32◦ E) were used as sampling units, as data for invertebrategroups was available only as province records. For provincial delineations and dis-cussion on the applicability of province records in biogeography, see Lillehammer(1988) and Väisänen et al. (1992), respectively. Species richness data from 78 prov-inces (Figure 1) for five freshwater taxonomic groups were analysed: macrophytes,dragonflies, stoneflies, aquatic beetles and teleost fish. Since the provinces were ofunequal size, I corrected the number of species in each province using the formula:

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Scor = Sorig/Az

where Scor is the corrected number of species, Sorig the original number of species ina province, A is the area of a province, and z is a constant in a typical species–arearelationship. Since the value of z has been found to vary between 0.12 and 0.17 incontinental environments (Pianka 1988), I chose an intermediate value of 0.15 forthis study (see Lahti et al. 1988).

Data for macrophytes were obtained from the distribution maps of Hulten (1971).The original, relatively accurate distributional data were transformed to correspond tothe provincial delineations. Data for 76 species of freshwater macrophytes occuringpredominantly in lentic or lotic environments (i.e. submerged, floating-leaved andemergent species) were included. Emergent macrophytes incorporated only speciesshowing clear affinity to freshwater habitats, and which do not occur widely in othermoist habitats. Neither brackish water nor introduced species were included.

Data for the distribution of dragonflies (Odonata) were derived from Askew (1988)and references therein. Original distribution data were transformed to provincialrecords. Altogether 56 species of dragonflies were regarded as having breedingpopulations in the study area.

Data for stoneflies (Plecoptera) provided by Lillehammer (1988) represents a typ-ical entomological approach to tabulate species’ distributions within the biogeograph-ical provinces of Fennoscandia. Furthermore, records presented in Kuusela (1996) forFinland were also included. In total, 42 species of stoneflies occur in the study area.

Data in Holmen (1987) and, Nilsson and Holmen (1995) for aquatic beetles existin a similar format as that for stoneflies. In total, 13 species of Gyrinidae, 21 speciesof Haliplidae, two species of Noteridae, one species of Hygrobidae and 155 speciesof Dytiscidae occur in the study area.

Data for fish were taken from Curry-Lindahl (1985) for Scandinavia and Koli(1990) for Finland. Because the species’ distributions were presented in maps, thedata were tranformed to provincial distribution tables. Only freshwater teleost fishwith breeding populations in the study area were included. Extinct species, intro-duced species and those not breeding regularly in Fennoscandian freshwaters wereomitted from the analyses. Due to incomplete taxonomical and distributional data,whitefish (Coregonus spp.) were combined into a single collective species. With theserestrictions, 35 species of freshwater teleost fish occur in the area.

Six large-scale environmental variables were included. Mean latitude, longitudeand altitude of each province were taken from maps. Altitude was scaled accordingto the maps of Hulten (1971) as follows: 1 = 0–100 m a.s.l., 2 = 100–200 m a.s.l.,3 = 200–500 m a.s.l., 4 = 500–1000 m a.s.l., 5 = more than 1000 m a.s.l. If thevalues overlapped within a province, 0.5 were added to the lower value. Further-more, three variables describing climate conditions were derived from the maps ofHulten (1971): mean July temperature, the duration of growing season (days in ayear with mean temperature above +4 ◦C), and the number of hours of sunshine in a

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year. The climatic variables were approximated to represent mean conditions, yet thesmaller-scale variation within a province may be considerable.

Statistical methods

Stepwise multiple regressions with forward selection were used to relate provincialspecies richness records for each taxa to environmental variables. This type of regres-sion analysis selects independent variables in the model, in order, according to theamount of variation they explain in the dependent variable. The analysis is completedonce no additional variable explains a significant amount of the remaining variation.I used a Bonferroni correction for multiple tests, setting entry and removal criteria at aP = 0.008 (0.05/6), because there were six independent variables (see Sokal and Rolf1995). This was done to prevent the random inclusion of independent variables in themodel. Furthermore, Spearman rank correlation coefficients were calculated amongeach taxa to reveal whether species richness showed concordant patterns across prov-inces among taxonomic groups. Corrected provincial counts of species number wereused in all statistical analyses.

Results

Determinants of species richness

The amount of variation in species richness explained by environmental variablesranged from 27% for stoneflies to 84% for dragonflies (Table 1). For macrophytes,dragonflies, beetles and fish, mean July temperature was entered first in the regres-sion models, and accounted for most of the variation in species richness. Formacrophytes, additional variation was explained by longitude and altitude, both ofwhich were negatively related to species number. Variation in the species number ofdragonflies was further explained by latitude, longitude and the length of the growingseason, and that of beetles by hours of sunshine in a year. In contrast to the othergroups, variation in the number of stonefly species was positively related to altitude,although the regression model explained less than third of the variation. Neverthe-less, a regression model for the combined species richness of all five taxa accountedfor 86% of the variation. This model incorporated mean July temperature, latitude,longitude and hours of sunshine as the most important variables explaining variationin species richness (Table 1).

Concordance of species richness patterns

As indicated by correlation coefficients, variation in species richness was relativelysimilar among most taxonomic groups (Table 2). Among macrophytes, dragonflies,

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Table 1. Regression models for the relationships between species richnessand environmental variables. Forward selection procedure was used, and theentry and removal criteria were set at P < 0.05/6. Cumulative R2 incorpo-rates the variable at each step and those preceding it. Corrected provincialcounts of species number in each taxa were used in all analyses. All modelswere significant at P < 0.001.

Model Variable Coefficient R2

Macrophytes Constant 8.193Mean July temperature 2.742 0.716Longitude −0.466 0.807Altitude −2.593 0.840

Dragonflies Constant 49.825Mean July temperature 0.789 0.691Latitude −1.258 0.738Longitude 0.862 0.803Growing season 0.127 0.841

Stonefiles Constant 6.392Altitude 2.264 0.274

Beetles Constant −82.199Mean July temperature 6.642 0.580Hours of sunshine 0.003 0.679

Fishes Constant −21.056Mean July temperature 2.445 0.722

All taxa Constant 346.210Mean July temperature 5.401 0.738Latitude −6.376 0.794Longitude 2.502 0.821Hours of sunshine 0.004 0.856

Table 2. Spearman rank correlation coefficients for the concordance of spe-cies richness patterns among different taxa. Combined species richness did notinclude the taxa with which the correlation coefficient was calculated. All cor-relations were significant (P < 0.001).

Taxa Dragonflies Stoneflies Beetles Fishes Combined

Macrophytes 0.839 −0.431 0.771 0.835 0.832Dragonflies −0.398 0.776 0.809 0.833Stoneflies −0.460 −0.439 −0.524Beetles 0.792 0.780Fishes 0.848

beetles and fishes, the coefficients were positive and varied from 0.771 to 0.839. Mac-rophytes and dragonflies showed the strongest degrees of concordance in the variationof species number. However, correlation coefficients among species richness of stone-flies and of the other groups were always negative and generally rather low, varyingfrom −0.398 to −0.460 (Table 2). Nevertheless, these relationships were statisticallysignificant (P < 0.001). Concordance among combined species richness and species

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richness in each taxon was also relatively high. Fish, dragonflies and macrophytesalone predicted relatively well variation in the combined species richness of the othergroups, coefficients being 0.848, 0.833 and 0.832, respectively (P < 0.001). Again,stoneflies showed a weaker negative relationship to variation in the combined speciesrichness of the other taxa (Table 2).

Geographically, species number in most taxonomic groups attained highest valuesin the provinces located along a belt from Denmark through southern Sweden tosouthern Finland (Figure 1). For macrophytes and beetles, highest number of spe-cies occurred in parts of Denmark and southernmost Sweden, while dragonflies andfish attained highest diversity in southern Finland and southern Sweden. By contrast,stoneflies showed a reversed pattern with most species rich provinces being locatedin the northernmost parts of the study area, and secondarily along the Scandinavianmountain range.

Discussion

Species richness in most higher taxonomic groups generally decreases with increas-ing latitude and altitude, and several hypotheses have been proposed to explain thesepatterns (Stevens 1989; Huston 1994; Rosenzweig 1995; Brown and Lomolino 1998).For instance, variation in temperature and energy have been regarded as major cor-relates of species richness at broad spatial scales (Currie 1991; Wright et al. 1993).Similarly, in this study, a major proportion of the variation in species richness ofmacrophytes, dragonflies, beetles and fish was accounted for by variables related totemperature, suggesting a strong climatic control on freshwater biodiversity in Fenno-scandia. For fish, such climatically controlled diversity patterns have been extensivelyreported elsewhere (e.g. McAllister et al. 1986; Tonn 1990), and species’ distributionsin the other freshwater groups have also been found to show a close correspondenceto broad-scale climatic factors (e.g. Abell et al. 2000).

In contrast to the other taxa, only a minor proportion of the variation in stoneflyspecies richness was explained by the large-scale geographic and climatic variables.Furthermore, stoneflies showed a positive relationship to province altitude and lati-tude. As a group, stoneflies are largely restricted to stream environments, attaininghighest diversities in mountainous regions (Wiggins and Mackay 1978; Lillehammer1985, 1988; Ward 1992). Thus, the habitat preferences of stoneflies probably haveconsiderable effects on their broad-scale distribution patterns, whereas the broad-scale climatic variables per se are probably not adequate predictors of stonefly speciesrichness. Instead, altitudinal variation within a region, which generally correspondsto regional stream riffle area, might explain a considerable part of the variation inregional stonefly diversity. However, more explicit GIS approaches will be used toevaluate the influence of regional riffle area on stoneflies in a detailed future study.

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In general, concordant spatial patterns in species richness among different taxamay result from: (i) random mechanisms; (ii) biotic interactions among different taxa;(iii) common environmental determinants; or (iv) spatial covariance in different envi-ronmental factors that independently account for diversity variation in different taxa(Gaston 1996; Gaston and Williams 1996). Here, with the exception of stoneflies, therelatively high degree of concordance among taxa is almost certainly due to similarresponses by different taxa to climatic conditions rather than to biotic interactions.Nevertheless, if local systems were compared, relatively high degrees of concordancecould also be generated through biotic factors (see Jackson and Harvey 1993, Pasz-kowski and Tonn 2000); such patterns might be predicted for macrophytes, aquaticinsects and fish. For instance, macrophytes create structural complexity, thus provid-ing refugia for aquatic insects from fish predation (e.g. Crowder and Cooper 1982;Gilinsky 1984). Similarly, fish may seek insect prey or refugia in macrophyte beds(e.g. Tonn and Magnuson 1982; McEadie and Keast 1984), and, therefore, a closemechanistic link should exist between these groups at a local scale. Alternatively, thehigh degrees of concordance among macrophytes, dragonflies and beetles could alsoresult from common preferences for and adaptations to lowland warm-water habitats,i.e. similar responses to habitat templets (e.g. Southwood 1977, 1988). However, thedegree to which such local interactions or responses to local habitats might affectorganisms’ broad-scale distribution patterns is likely to be minor.

Also historical factors may contribute to the present-day distributional patternsof freshwater biota (see Koli 1990; Tonn et al. 1990). Most of Fennoscandia wascovered by ice until 10 000 years ago, and thereafter large areas of land remainedsubmerged under postglacial waters. Strong latitudinal patterns in species richnessmay mirror the influence of such historical factors, and that some species have notbeen able to disperse to the northernmost regions after the latest glacial period (Pielou1991; Oswood et al. 2000). However, evidence for rapid colonization of streams byinvertebrates and fish after the retreat of glaciers has been obtained in North America(e.g. Milner 1994). Therefore, most freshwater taxa have likely attained their lati-tudinal limits to correspond to the large-scale climatic and vegetational conditions,suggesting that historical factors alone cannot account for concordance in speciesrichness among different freshwater taxa in Fennoscandia.

Concordance in the species richness of macrophytes, dragonflies, beetles, fish, andcombined species richness across provinces was generally high. As already indicated,this would suggest that similar mechanisms are affecting species richness, and thatthese groups could be used as indicators of biodiversity at a provincial scale in Fenno-scandia. However, the degree of congruence in species richness patterns varies withscale (Gaston and David 1994; Prendegast and Eversham 1997), and thus should beapplied with caution (see Prendegast et al. 1993; Gaston and Williams 1996). Never-theless, at broad spatial scales, even such crude congruence patterns could be used fordetermining high diversity areas (Reid 1998), within which local freshwater systemsshould be given additional attention and conservation priority (see Abell et al. 2000).

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However, local systems harbouring rare community types or single species with highconservation value (see Haila and Margules 1996; Angermeier and Winston 1997)may also exist in low diversity regions, and should also be considered in biodiversityassessments.

To conclude, provincial species richness in most of the considered taxonomicgroups seemed to be under strong climatic control along latitudinal and altitudinalgradients. Therefore, if climate will change, considerable changes in the distributionpatterns of freshwater biota and provincial biodiversity are to be expected (see Wardand Stanford 1982; Tonn 1990; Poff 1997; Sala et al. 2000). Nevertheless, all groupsof freshwater organisms, e.g. stoneflies, are unlikely to show congruent distributionalshifts with the other taxa, at least if their present-day species richness patterns areconsidered. Undoubtedly, such distributional shifs will strongly affect regional spe-cies pools and, consequently, the organization of local freshwater communities innorthern Europe.

Acknowledgements

I thank Timo Muotka and two anonymous referees for commenting on earlier versionsof the manuscript.

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