DEVELOPMENT OF EXPECTATIONS OF LARVAL AMPHIBIAN ASSEMBLAGE STRUCTURE IN SOUTHEASTERN DEPRESSION WETLANDS

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  • 1219

    Ecological Applications, 10(4), 2000, pp. 12191229q 2000 by the Ecological Society of America

    DEVELOPMENT OF EXPECTATIONS OF LARVAL AMPHIBIANASSEMBLAGE STRUCTURE IN SOUTHEASTERN DEPRESSION

    WETLANDS

    JOEL W. SNODGRASS,1,2,3 A. LAWRENCE BRYAN, JR.,2 AND JOANNA BURGER1

    1Division of Life Sciences and Consortium for Risk Evaluation with Stakeholder Participation (CRESP),Environmental and Occupational Health Sciences Institute, Rutgers University, Piscataway, New Jersey 08855 USA

    2Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, South Carolina 29802 USA

    Abstract. We surveyed larval amphibians and fish in 25 relatively pristine depressionwetlands on the upper Atlantic coastal plain of South Carolina to examine relationshipsamong hydroperiod length, fish presence/absence and larval amphibian assemblage struc-ture. Our goals were to test the application of general models of lentic community structureto Southeastern depression wetlands and to develop expectations of larval amphibian as-semblage structure at reference sites. Amphibian species richness showed a unimodal patternalong a hydroperiod gradient, with wetlands that contained water for 810 mo/yr havingthe highest species richness. Wetlands that contained water for longer periods (i.e., driedonly during severe drought) often contained fish and had relatively low amphibian speciesrichness. Most species occurred along a restricted portion of the hydroperiod gradient, andsome species were found almost exclusively in wetlands with fish. Associations among theoccurrence of species led to relatively discrete breaks in assemblage structure along thehydroperiod gradient. Canonical correspondence analysis of catch-per-unit-effort data iden-tified four groups of wetlands with similar assemblage structure: (1) short (drying in spring),(2) medium (drying in summer), and (3) long (drying in fall or semi-annually) hydroperiodwetlands without fish; and (4) long hydroperiod wetlands with fish. Our results suggestthat general models of community structure in lentic systems are applicable to southeasternisolated wetlands and should form the basis for developing expectations of larval amphibianassemblage structure in these systems.

    Key words: amphibians; biological monitoring; Carolina bays; community structure; fish; hy-droperiod; reference conditions; Savannah River Site, South Carolina; species richness; wetlands.

    INTRODUCTION

    Development of biological assessment and monitor-ing programs to evaluate aquatic resources has con-centrated on what has become known as the referencecondition approach (Reynoldson et al. 1997). The ref-erence condition approach involves comparing condi-tions at a site of interest (to be assessed or monitored,or both) to conditions that are representative of agroup of minimally disturbed sites organized by se-lected physical, chemical, and biological characteris-tics. The reference condition approach has been suc-cessfully applied in aquatic systems of Canada (Rey-noldson et al. 1995), Australia (Parsons and Norris1996, Marchant et al. 1997), the U. K. (Wright 1995)and the U.S. (Karr and Chin 1997) using macroinver-tebrates and fish.

    The application of the reference condition approachrequires a basic understanding of community structurein the aquatic system under study so that expectationsunder minimally disturbed conditions can be devel-oped. Thus, one of the challenges of defining reference

    3 Present address: Department of Biology, Towson Uni-versity, Towson, Maryland 21252 USA.

    Manuscript received 28 August 1998; revised 5 July 1999;accepted 28 July 1999; final version received 19 August 1999.

    conditions is incorporating variation associated withnatural environmental gradients that influence thegroup of organisms being used in biological monitoringprograms. The ability of biological monitoring pro-grams to provide early and accurate detection of humanimpacts to aquatic systems depends on variation amongreference sites (Reynoldson et al. 1997). If variationdue to natural environmental gradients among refer-ence sites is not accounted for, impacts that cause ashift that is similar to movement along the gradientwill not be detected. For example, if species richnessvaries along an environmental gradient, then an impactthat reduces species richness at a site on the high spe-cies richness end of the gradient will not be detecteduntil the impact become severe. Inclusion of ecologicalprinciples and conceptual models of community struc-ture in the development of biological monitoring pro-grams can increase the efficiency and accuracy of theseprograms (Hart 1994). Specifically, environmental gra-dients identified as strong structuring forces in con-ceptual models of community structure in aquatic sys-tems should be included in sampling plans for definingreference conditions.

    The syntheses of general models of community struc-ture in lentic systems (Wiggins et al. 1980, Schneider

  • 1220 JOEL W. SNODGRASS ET AL. Ecological ApplicationsVol. 10, No. 4

    and Frost 1996, Wellborn et al. 1996) suggest that ex-pectations of amphibian community structure at lenticreference sites can be developed. Models of communitystructure in lentic systems suggest that interactionsamong the abiotic constraints of hydroperiod length,predation, and life history characteristics of individualspecies will produce predictable patterns of communitystructure along a hydroperiod gradient from temporaryponds to permanent lakes (Wiggins et al. 1980, Schnei-der and Frost 1996, Wellborn et al. 1996). Species thatpersist in temporary ponds (that dry annually) musthave the ability to survive dry periods (e.g., terrestrialadult forms, encysted eggs, or ability to move to otheraquatic habitats) and complete the aquatic phase oftheir life cycle before ponds dry. In more permanentponds and lakes (that never dry or dry only duringdroughts), larger predators can complete their life cycleand are expected to eliminate smaller species typicalof temporary ponds. Because traits that increase fitnessat one point on the hydroperiod gradient can decreasefitness on other portions of the gradient, distinct com-munities will develop along the gradient (Wellborn etal. 1996). This lentic community model is particularlyapplicable to larval amphibian assemblages (Heyer etal. 1975, Wilbur 1984).

    In this paper we use existing conceptual models oflentic community structure to design and carry out asampling plan for defining reference conditions for lar-val amphibian assemblages in depression wetlands ofthe Savannah River Site on the upper Atlantic coastalplain of southeastern North America. We sampled lar-val amphibians in 22 wetlands spanning the range ofannual hydroperiod length (from permanent wetlandsto wetlands that hold water for two to three months ayear), and with and without fish populations. We se-lected hydroperiod length and fish presence/absencebecause general models of community structure in len-tic systems suggest these will be strong structuring fac-tors. Because we selected wetlands based on hydro-period length and fish presence/absence, with a prioriexpectations of patterns, our analyses represent a testof the ability of models of community structure in lenticsystems to predict patterns in southeastern depressionwetlands.

    METHODS

    Study site

    We sampled amphibians on the 780 km2 SavannahRiver Site (SRS), located on the northern shore of theSavannah River on the upper coastal plain of SouthCarolina. The SRS is a U.S. Department of Energynuclear production site that was established in the early1950s. For security reasons, access to a large bufferarea around SRS has been controlled since 1951. Nat-ural habitats within this buffer zone have been pro-tected from human disturbances and represent some ofthe most pristine habitats in the region. There are .300

    depression wetlands located within the SRS boundary(Kirkman et al. 1996), most of which have not beenimpacted by site operations. For these reasons, the SRSrepresents a unique opportunity for development of ex-pectations of amphibian assemblage structure in un-disturbed depression wetlands of the region.

    Depression wetlands (some of which also are re-ferred to as temporary ponds, Carolina bays or poco-sins) are common features of the Atlantic coastal plain.Fish populations naturally occur in 1020% of the de-pression wetlands on the upper Atlantic Coastal Plainof South Carolina (Snodgrass et al. 1996) and depres-sion wetlands are important breeding sites for manyamphibians (Gibbons and Semlitsch 1991). Waterchemistry of depression wetlands at the SRS is typicalof softwater, acidic systems (Newman and Schalles1990). Dissolved oxygen levels normally range be-tween 0% and 50% saturation, varying both temporallyand spatially (Schalles and Shure 1989); and pH rangesbetween 4.9 and 6.1 (Schalles et al. 1989). Annualhydroperiods follow a typical pattern of winter re-charge and spring to late summer drawdown or com-plete drying (Pechmann et al. 1989, Lide et al. 1995,Semlitsch et al. 1996).

    To determine larval amphibian assemblage structurewe haphazardly selected a subset of 25 wetlands froma larger group of 99 wetlands for which we had a prioriknowledge of wetland hydroperiod and fish presence/absence (Snodgrass et al. 1996). The 25 wetlands wereselected to include the range of annual hydroperiodlength (from 3 to 12 months) and included six wetlandswith fish populations. Wetlands were selected such thatthey were distributed across the landscape of the SRS.The numbers used to refer to wetlands are those as-signed by Schalles et al. (1989) who also provides in-formation on wetland locations within the SRS.

    Amphibian sampling

    We sampled larval amphibians during three discreteperiods (winter, 18 February28 March; spring, 21April19 May; and summer, 23 June11 July; all 1997)using hoop traps, minnow traps (metal), and dipnets.These sampling periods encompassed the larval periodsof all amphibians known to utilize isolated wetlandsfor breeding on the SRS (Gibbons and Semlitsch 1991).During each sampling period, we deployed 13 hooptraps and 310 minnow traps in each wetland. Thenumber of traps used in each wetland was dependenton wetland size and the number of mesohabitats (e.g.,floating vegetation, open water) present. Traps weredeployed so that all mesohabitats were sampled. Trapswere checked every 24 hr for one 72-hr period duringeach discrete sampling period.

    During each sampling period, we also conducted ac-tive sampling using dipnets. Two individuals searchedall mesohabitats and dipnetted all amphibians sighted.Blind sweeps with dipnets also were made in all me-sohabitats. Duration of active sampling varied from 20

  • August 2000 1221AMPHIBIANS AND BIOLOGICAL MONITORING

    to 90 minutes depending on wetland size at the timeof sampling and the number of mesohabitats present.

    Most salamanders and adult amphibians, and all fish,were identified in the field and returned to the wetland.All tadpoles were preserved in 10% formalin and re-turned to the laboratory for identification. Tadpoleswere identified according to Altig (1970) and Travis(1981). We collected voucher specimens of all am-phibian species encountered.

    Analyses

    For all analyses we used only larval forms with thefollowing exceptions. Three totally aquatic salaman-ders (Amphiuma means, Siren lacertina, and S. inter-media) were encountered in our study area. When theseaquatic salamanders were present we captured them atlow frequencies. We included both adult and larvalaquatic salamanders in analyses and assumed the pres-ence of either indicated successful reproduction. Am-bystoma talpoideum has both terrestrial and paedo-morph adult forms (Semlitsch 1985). We included pae-domorphs and larval A. talpoideum in our analyses.Finally, Notophthalmus viridescens has aquatic larvaland adult forms that are sometimes separated by animmature terrestrial eft (Healy 1974). We included bothaquatic forms of N. viridescens in our analyses andassumed the presence of either indicated successful re-production.

    As a relative measure of wetland hydroperiod, wecalculated a wetland drying score as the number oftimes a wetland was visited and it held water (Snod-grass et al. 1996). Wetlands were visited five times,approximately evenly distributed, during 1990 andagain five times between September 1996 and August1997 (the annual hydrodrological cycle encompassingour sampling period). Our drying scores ranged from1 (i.e., water present 10% of the time) for wetlandswith relatively short hydroperiods to 10 for wetlandswith long hydroperiods.

    We tested three predictions of current lentic com-munity models: (1) species richness will exhibit a un-imodal pattern along a gradient of increasing hydro-period length; (2) decreased species richness in longerhydroperiod wetlands is correlated with the presenceof large predators (i.e., fish); and (3) distinct breaks inassemblage structure will occur along the hydroperiodgradient.

    We tested hypothesis (1) using a quadratic equationof the form

    2y 5 a 1 b x 2 b x1 2

    where y is species richness and x is drying score. Sig-nificance of both b1 and b2 indicates a significant un-imodal pattern of species richness along the hydroper-iod gradient. We also tested the null hypothesis thatthe quadratic model did not provide a better fit than asimple linear model using an F test (Zar 1984).

    We tested hypothesis (2) using a linear regressionapproach to ANCOVA. The model used was

    y 5 a 1 b x 1 b x 1 b x x1 1 2 2 3 1 2

    where y is species richness, x1 is drying score, and x2is presence of fish coded as a dummy variable. In thismodel, a and b1 are the intercept and slope, respec-tively, of the line relating species richness to dryingscores among wetlands without fish, and b2 and b3 arethe intercept and slope, respectively, of the line relatingspecies richness to drying scores among wetlands withfish. For both the quadratic and the ANCOVA models,dependent variables were log transformed to moreclosely approximate the assumptions of the models.

    To test hypothesis (3) we used canonical correspon-dence analysis (CCA; ter Braak 1988) and a test forclustered distribution of wetland larval amphibian as-semblages in ordination space (Matthews 1998). CCAis a direct gradient analysis ordination technique thatincorporates a linear regression step, relating assem-blage structure directly to environmental variables, inthe reciprocal averaging algorithm (ter Braak 1986).Inclusion of the linear regression steps constrains theordination axis so as to summarize variation related toenvironmental variables. We included drying scoresand fish presence/absence (coded as a nominal variable)to investigate the distribution of amphibian larvaealong the hydroperiod and fish presence/absence gra-dients. We used catch-per-unit-effort (number of am-phibianstrap2124 hr trap period21 1 number of am-phibians/hr of active sampling) as dependent variablesin the CCA. To test for significant variation in assem-blage structure along environment...

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