Biotic and Abiotic Factors Influencing the Distribution of theHuachuca Springsnail (Pyrgulopsis thompsom)

Yi-jiun Jean Tsai and Kelsey MaloneyConservation Biology Internship Program

andA. Elizabeth Arnalda

Division of Plant Pathology and MicrobiologyDepartment of Plant Sciences

University of ArizonaTucson,Arizona 85721 USA

ABSTRACTThe Huachuca springsnail (Pyrgu/opsis thompsoni) is a species of concern

inhabiting springs of the HuachucaMountains in southeasternArizona, USA. Wedocumented springsnail distribution and examined the major abiotic and biotic factorsinfluencing springsnail abundance at eight spring channels before and after the onset ofseasonal summer rains in 2003. Of the abiotic factors examined (total dissolved solids, pH,distance trom spring source, spring channel, water temperature, and dissolved oxygen),only spring channel and total dissolved solids were strongly associated with springsnailabundance. However, correlation of total dissolved solids with pH, temperature, dissolvedO2, and distance downstream necessitated further exploration of these explanatoryvariables. We found that sampling locations with P. thompsoni were characterized bycooler, more oxygenated, and less-occluded water conditions relative to sampling locationswithout springsnails. Springsnail abundance was positively associated with abundance ofcaddis fly larvae (He/icopsyche sp.) but not significantly associated with abundance of aco-occurring snail (Physa sp.). The most important outcomes of this study were thedocumentation of major factors associated with springsnail abundance, and recovery ofspringsnails at greater distances downstream than previously documented.

INTRODUCTIONThe Huachuca springs nail (Pyrgulopsis thompsoni, Hydrobiidae), a candidate for

federal listing (United States Department ofInterior: Fish and Wildlife Service 2005),occurs in springs of the Huachuca Mountains and the San Rafael Valley in southeasternArizona, USA (Hurt 2004). In 1992, J. Landye surveyed sixteen springs in the northernHuachuca Mountains, finding P. thompsoni in nine of these areas (Landye b,unpublisheddata). Although finding these nine populations was the primary factor in the US Fish andWildlife Service's decision not to list P. thompsoni at the time, neither the ecologicalfactors influencing springsnail occurrence nor springsnail population sizes were examined.No data regarding P. thompsoni have been published in over ten years, during whichsoutheastern Arizona has experienced a significant drought. Average precipitation at thebase of the Huachuca Mountains has been substantially below normal in the last decade,raising concerns about springsnail persistence in previously surveyed springs.

The goals of our study were to (1) census focal springs in the Huachuca Mountainsfor P. thompsoni and (2) examine abiotic and biotic factors associated with springsnailpresence and abundance. While many factors are likely to affect the distribution of theHuachuca springsnail, we specifically explored the relationships between springs nailabundance and the abundance of co-occurring snails (Physa sp.) and caddisfly larvae(Helicopsyche sp., Trichoptera), as well as a suite of abiotic factors (total dissolved solids,pH, distance trom spring source, spring channel, water temperature, and dissolvedoxygen).

aCorresponding author; E-mail: Amold@ag.a.Iizona.edubpresent affiliation: US Fish and Wildlife Service.

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Joumal of Freshwater Ecology, Volume 22, Number 2. June 2007

MATERIALS AND METHODSMuch of the Huachuca Mountains in southeastern Arizona lies within the Fort

Huachuca military base (310 35' N, 1100 20' W). Local water sources range from extensivestream networks to small wallows and isolated seeps. Most are subject to human impactdue to roads, hiking trails, and military or recreational activities.

During the summer of2003, we surveyed eight spring-flow channels in theHuachuca Mountains for P. thompsoni (Table I). We conducted our surveys before theonset of summer rains (June 2003) and following the start of rainy season (late July 2003).Each survey lasted approximately two weeks.

We established data collection points at each spring source and at 5 m intervalsdownstream. With the exception of 15 m of Sawmill Spring and 5 m of Cave Spring 1where pipes blocked access, McClure, Cave Springs 1 and 2, Sawmill Spring, andHuachuca Springs I, 2, and 3 were sampled for P. thompsoni over their entire lengths.Because of its length (> 1,000 m), we sampled only the first 500 m of Garden CanyonSpring.

At each data collection point, we counted individuals of P. thompsoni, Physa sp.,and Helicopsyche sp. on up to fifteen haphazardly chosen rocks, leaves, and twigs acrossthe width of the spring. Then, we collected and sifted three substrate samples from thespring bottom using a cylindrical scoop (5.06cm3) and a nylon sieve (2mm mesh) andquantified the abundance of these organisms. Two substrate samples were collected fromareas closest to each bank, and one was taken from the center of the spring channel. Datafrom these sampling methods were combined for analysis.

We examined water chemistry at randomly selected data collection points alongeach spring channel. Dissolved oxygen, pH, conductivity, temperature, and total dissolvedsolids were measured with a Hach Sension 156 multiparameter meter (Hach; Ames, Iowa,USA).

We conducted statistical analyses using JMP (Statistical Discovery Software; Cary,North Carolina, USA). Examination of the explanatory variables indicated that pH,temperature, dissolved oxygen, and total dissolved solids were significantly correlated withdistance downstream (Table 2). Moreover, pH was significantly correlated with dissolvedoxygen and total dissolved solids, and dissolved oxygen was significantly correlated withtotal dissolved solids (Table 2). Therefore, by including only spring channel, watertemperature, and total dissolved solids as explanatory variables, we developed a multipleregression model that avoided multicollinearity (Table 3). Extra-sum-of-squares F-testsshowed that this model did not differ significantly in quality from a richer modelcontaining those explanatory variables and the abundance of Physa sp., Helicopsyche sp.,or both. Because analyses of data collected in June and July 2003 yielded similar results,we combined data from the two survey periods.

Table 1. Spring channels surveyed for Pyrgulopsis thompsolli in the Huachuca Mountains,Arizona,USA,in Juneand July 2003. GPScoordinatesindicateeachspringsource.

UTM UTM Elevation Total # data % pointsSpring channel Easting Northing (m) length collection with

(m) points P. thompsolli

Garden Canyon Spring 560252 3480611 1,615 >1,000 108 83.3McClure Spring 559302 3482377 1,844 25 6 66.6Cave Spring 1 560187 3483409 1,848 45 8 62.5Cave Spring 2 45 10 60.0Sawmill Spring 561311 3478439 2,180 65 10 60.0Huachuca Spring 1 558068 3487099 1,717 155 32 3.1Huachuca Spring 2 40 9 ILlHuachuca Spring 3 5 2 0.0

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Table 2. Correlation coefficients and significance values for variables examined insprings of the Huachuca Mountains. Coefficients represent Pearson productmoment correlations using all available values (N = 81 comparisons per pair ofvariables). TDS =total dissolved solids. Asterisks indicate significantcorrelations (alpha =0.05); t indicatesmarginallysignificantassociation.

Table3. Finalmultipleregressionmodelexaminingeffectsof springchannel,temperature,and total dissolvedsolids (IDS) on log-transformedabundanceofP. thompsoni. R2=0.56. For the whole model, F7,J7=3.07, P = 0.0277. Asteriskindicates statistical significance with alpha = 0.05; t indicates a strongrelationship that is marginally significant.

Table 4. Means and standard deviations for abiotic variables measured at spring channels inthe Huachuca Mountains with and without P. thompsoni. Asterisks indicatesignificant differences based on non-parametric Wilcoxon sign-rank tests (alpha =0.05); t indicates marginally significant difference further supported by multipleregression analysis.

Variable

Temperature (0C)Dissolved O2(mgIL)IDS (mgIL)pHDistance downstream (m)

P. thompsoni present18.4:1:2.085.44:1:0.86

261.68 :I: 42.407.88:1:0.31

178.93:1: 145.13

P. thompsoni absent

21.19:1: 2.435.03:1: 1.45

273.93:1: 46.527.91:1: 0.38

161.85:1: 169.73

P

0.0001*0.0026*0.0550t0.84430.1656

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Variable 1 Variable 2 Correlation coefficient P

Distance pH 0.4916 0.0001*Distance Temperature -0.3189 0.0037*Distance TDS -0.3076 0.0052*Distance Dissolved O2 0.3129 0.0045*Dissolved O2 pH 0.7619 0.0001*Dissolved O2 Temperature 0.0767 0.4963TDS pH -0.3227 0.0033*TDS Temperature 0.1296 0.2487IDS Dissolved O2 -0.3929 0.0003*Tem£erature____ pH 0.2165 0.0522t

Variable Parameters DF SS F P

Spring channel 5 5 30.76 3.40 0.0261*Water temperature I I 2.67 1.47 0.2415IDS I I 6.45 3.56 0.0764t

RESULTS AND DISCUSSIONWe recorded 7,276 springs nails among seven of the eight focal spring channels in

the Huachuca Mountains during June and July 2003. We found significantly more snails inJuly (4176 snails), following the onset of summer rains, than in June (3100 snails; t =2.3051, P = 0.015).

In contrast to congeners whose distributions are confined to areas immediatelysurrounding spring sources (Hurt 2004), we found P. thompsoni farther downstream thananticipated. Spring channels (excluding Garden Canyon) showed an increase of springs nailabundance to a local maximum 5-15 m trom the source before decreasing downstream(Fig. I). Within the first 500 m, Garden Canyon Spring showed a similar pattern:springsnail abundance peaked relatively near the spring source and then decreaseddownstream (Fig. I). However, living springsnails were found in Garden Canyon Spring upto 510 m downstream Irom the source, and it is likely that P. thompsoni persists atdistances beyond the scope of our study. This finding is of particular importance, as itsuggests that (I) P. thompsoni has a greater tolerance of environmental variability thandocumented before, and/or (2) P. thompsoni has better dispersal capabilities thanpreviously believed.

Our final model showed that the abundance of P. thompsoni was influencedsignificantly by spring channel and marginally by total dissolved solids (Table 3). Thesignificant relationship among total dissolved solids, pH, dissolved oxygen, and distancedownstream (Table 2) suggests that any of these four factors could underlie the observedresults. Therefore, we examined the abiotic characteristics of data collection points withand without P. thompsoni. Conditions at points with springsnails differed significantly intemperature, dissolved O2, and total dissolved solids trom locations without springsnails(Table 4). Across all sampling points. springsnails were present in sites characterized bycooler, more oxygenated, and less-occluded water conditions (Table 4).

oi~~~~~~immc1.~~~~_W*_B~~~~amR__B~D~_BBW*8

Distance from spring source (m)

Figure I. Number of Pyrgulopsis thompsoni observed at 5m intervals along focal springswith> 15 snails (Huachuca Spring I, 2, and 3 are not shown). The first 500m ofGarden Canyon spring were sampled. A total of26 springsnails (June) and 43springsnails (43) were found over the last 110 m (data are not shown).

216

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These conditions were also generally favored by Physa and Helicopsyche, whichwere three times (Physa) to thirty times (Helicopsyche) more abundant in samplinglocalities containing P. thompsoni (Table 5). Over the entire dataset, the abundance ofHelicopsyche was positively correlated with that of P. thompsoni. However, Physa and P.thompsoni abundances were not significantly associated (Table 5), suggesting that thesetwo snails may compete or differentially partition habitat or food resources in sharedsprings. Sampling points with Physa were characterized by somewhat warmer and morevariable water temperatures (mean = 20.5°C :f:2.39; 95% confidence interval =19-22°C)and slightly higher pH values (mean = 8.06:f: 0.18, 95% confidence interval =7.9-8.2)than points with P. thompsoni. In contrast, sites containing Helicopsyche sp. were similarin terms of pH, temperature, and other characteristics to those containing P. thompsoni.The overall correlation between Helicopsyche and P. thompsoni abundance suggests thatthe presence of this caddis fly species may be a useful indicator of appropriate habitat forHuachuca springsnails.

Malcom et al. (2005) found that the closely related species Pyrgulopsis bernardinaoccurred at a wider range of temperatures (l4-22°C) than was observed for P. thompsoniin our study (95% confidence interval: 17.5-19.3°C). Malcom et al. (2005) also indicatedthat P. bernadina occurs at a slightly wider range of pH values (7.6-8.0) than we found forP. thompsoni (95% confidence interval: 7.8-8.0). These data suggest that relatively narrowabiotic conditions dictate the occurrence of P. thompsoni in springs of the HuachucaMountains.

The importance of individual spring channels (Table 3) suggests that intrinsicfactors, such as probability of drying during prolonged drought and local structuralcharacteristics, may play an unexplored but important role in concert with the abioticfactors studied here (Harman 1972, Brown 1991, Mladenka and Minshall 200 I, Hurt 2004,Malcom et al. 2005, Gerald and Spezzano 2005). Together, these findings indicate theimportance of protecting the seven distinct spring channels in which P. thompsoni wasfound.

Table 5. Means and standard deviations for abundance of Physa sp. and Helicopsyche sp.at spring channels in the Huachuca Mountains with and without P. thompsoni. P-values are based on non-parametric Wilcoxon sign-rank tests for log-transformed abundance data. The asterisk indicates a significant difference(alpha =0.05); t indicatesa marginallysignificantdifference.Superscriptaindicates the significant correlation over the entire dataset for the abundance ofHelicopsyche sp. and P. thompsoni (correlation coefficient = 0.3979,P<O.OOOI,based on log-transformed data).

Organism

Physa sp.Helicoscyphe sp."

P. thompsoni present

1.83 :f: 7.49.92 :f: 20.6

P. thompsoni absent0.66 :f: 4.620.34 :f:1.l2

P

0.0801t0.0001*

ACKNOWLEDGMENTSFunding from the National Science Foundation for the Conservation Biology

Internship Program at the University of Arizona (NSF DEB-0080078) is gratefullyacknowledged. We thank Sheridan Stone at Fort Huachuca for helping us design and carryout this study, and Cheryl Craddock, Rob Robichaux, Bob Steidl, Guy McPherson, KarlFlessa, David Maddison, and Alex Wilson of the University of Arizona, Jacob Malcom atthe San Bernardino National Wildlife Refuge, Kristin Stanford of Northern IllinoisUniversity/Franz Theodore Stone Laboratory, and Julie Ray of Old Dominion Universityfor mentoring and research advice.

217

LITERATURECITEDBrown, K. M. 1991. Mollusca: Gastropoda.Pages 285-314 in Thorp, 1.H. and

A. P. Covich (Editors). Ecology andclassificationof North American fteshwaterinvertebrates. Academic Press Inc., San Diego.

Gerald, G. W. and L. C. Spezzano Jr. 2005. The influenceof chemical cues andconspecitic density on the temperatureselectionof a fteshwater snail (Melanoidestuberculata). Journal of ThermalBiology 30:237-245.

Harman, W. N. 1972. Benthic substrates:Their effect on fresh-waterMollusca. Ecology53:271-277.

Hurt, C. R. 2004. Genetic divergence, populationstructure and historical demographyofrare springsnails (Pyrgulopsis) in the lower ColoradoRiver basin. MolecularEcology 13:1173-1187.

JMP. The StatisticalDiscovery Software. Cary,North Carolina.Malcom, J., W. R. Radke, and B. K. Lang. 2005. Habitat associationsofthe San

Bernardino Springsnail,Pyrgulopsis bernardina(Hydrobiidae). Journal ofFreshwater Ecology 20:71-77.

Mladenka. G. C. and G. W.Minshall. 2001. Variation in the life history and abundanceofthree populations of Bruneau hot springsnails(Pyrgulopsis bruneauensis). WesternNorth American Naturalist 61:204-212.

United StatesDepartment ofInterior, Fish and WildlifeService. May 11,2005.Endangered and threatenedwildlife and plants; review of native species that arecandidates or proposed for listing as endangeredor threatened; annual notice offindingson resubmitted petitions; annualdescription of progress on listing actions;proposed rule. Federal Register 70:24903, 24930.

218Received: 21 April 2006 Accepted: 6 August 2006