Funk & Dunlap 1999

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1759 Colonization of high-elevation lakes by long-toed salamanders (Ambystoma macrodactylum) after the extinction of introduced trout populations W. Chris Funk and William W. Dunlap Abstract: We surveyed high-elevation lakes for long-toed salamander (Ambystoma macrodactylum) larvae and trout in the northern Bitterroot Mountains of Montana, U.S.A., in 1978, 1997, and 1998. Our objectives were to (i) test whether trout exclude salamander populations; (ii) determine whether lakes in which trout h
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  Colonization of high-elevation lakes by long-toedsalamanders (   Ambystoma macrodactylum  ) after the extinction of introduced trout populations  W. Chris Funk and William W. Dunlap Abstract : We surveyed high-elevation lakes for long-toed salamander (  Ambystoma macrodactylum ) larvae and trout inthe northern Bitterroot Mountains of Montana, U.S.A., in 1978, 1997, and 1998. Our objectives were to ( i ) testwhether trout exclude salamander populations; ( ii ) determine whether lakes in which trout have gone extinct have sincebeen colonized by salamanders; and ( iii ) estimate the rates of population extinction and colonization in lakes neverstocked with trout. In agreement with previous work on the interactions between trout and long-toed salamanders, trouteffectively excluded salamander populations from lakes. Somewhat surprisingly, however, salamanders managed tocolonize lakes after the extinction of trout populations despite evidence of low levels of interpopulation dispersal inthese salamander populations. In lakes never stocked with trout there was no evidence of a decline in salamanderpopulations; 2 of these lakes were colonized and no populations went extinct. Résumé  : Nous avons inventorié des lacs de haute altitude afin d’évaluer l’abondance des larves de la Salamandre àlongs doigts (  Ambystoma macrodactylum ) et des truites dans la chaîne nordique Bitterroot Mountains du Montana,É.-U., en 1978, 1997 et 1998. Nous avions pour objectif ( i ) d’établir si la présence de truites exclut les populations desalamandres, ( ii ) de déterminer si les lacs d’où les truites sont disparues ont été recolonisés par les salamandres, et( iii ) d’estimer les taux d’extinction et de colonisation des populations dans les lacs qui n’ont jamais contenu de truites.Tel que prévu à la lumière des travaux antérieurs sur les interactions truites-salamandres, les truites ont de fait éliminéles populations de salamandres des lacs. Étonnamment, cependant, les salamandres ont réussi à coloniser les lacs aprèsl’extinction des populations de truites en dépit du faible taux apparent de dispersion inter-populations chez cespopulations de salamandres. Dans les lacs qui n’ont jamais contenu de truites, les populations de salamandres sonrestées stables, 2 des lacs ont été colonisés et aucune population n’est disparue.[Traduit par la Rédaction]  1767Funk and Dunlap Introduction In the last decade, evidence has accumulated suggesting aworldwide decline in amphibian species above and beyonddeclines being experienced by all taxa (Blaustein and Wake1990; Wyman 1990; Wake 1991; Wake and Morowitz 1991;Blaustein 1994; Phillips 1994; Sarkar 1996). Some specieshave completely vanished, many show signs of reduced pop-ulation sizes or overall numbers, while others appear stable(Pounds 1991; Crump et al. 1992; Pounds and Crump 1994;Weller and Green 1997; Meyer et al. 1998; Williams andHero 1998). The decline in amphibian populations is of par-ticular concern in the American and Canadian West, wherethe population sizes and ranges of a relatively high propor-tion of species have decreased in the latter half of this cen-tury (Blaustein and Wake 1990; Weller and Green 1997).A number of factors have been explored as potentialcauses of amphibian population declines. Among the mostobvious, and likely most important, causes are site-specificfactors such as habitat destruction, habitat fragmentation,and the introduction of predator and competitor species(Wyman 1990; Green 1997; Kiesecker and Blaustein 1998;Tyler et al. 1998 b ). Global factors may also be involved. Forexample, increased UV-B radiation has been shown to de-crease survivorship during the early stages of developmentin some amphibian species (Blaustein et al. 1994; Blausteinet al. 1997; Anzalone et al. 1998; Lizana and Pedraza 1998).This could result in decreased population growth rates andan increased probability of population extinction.However, the degree to which extinctions of individualamphibian populations will result in the overall decline of species depends on a critical parameter: the rate of colo-nization of empty habitat patches. Metapopulation theorypredicts that if population extinctions are balanced by colo-nizations, then the set of populations (i.e., the meta-population) in question will persist (Levins 1970; Hanskiand Gilpin 1991). However, if the rate of population extinctionexceeds the rate of colonization, the metapopulation is pre-dicted to decline.Although previous authors have discussed the importanceof considering dispersal and colonization for understandingthe species-wide affects of amphibian population extinctions Can. J. Zool.  77 : 1759–1767 (1999) © 1999 NRC Canada 1759 Received November 30, 1998. Accepted July 20, 1999. W.C. Funk. 1 Division of Biological Sciences, University of Montana, Missoula, MT 59812, U.S.A. W.W. Dunlap.  MathSoft, Inc., Mount Vernon, WA 98274,U.S.A. 1 Author to whom all correspondence should be addressed(e-mail: wcfunk@selway.umt.edu).  (Gill 1978; Sjögren 1991; Sjögren Gulve 1994; Hecnar andM’Closkey 1996; Driscoll 1997; Vos and Chardon 1998),there is little empirical evidence concerning the capacity of amphibians to colonize empty habitat patches (SjögrenGulve 1994; Hecnar and M’Closkey 1996). Genetic differ-entiation among populations tends to be high in amphibiansrelative to other taxa, suggesting that gene flow, and hencedispersal, is limited in amphibians (Larson et al. 1984;Slatkin 1985; Gascon et al. 1996; Gascon et al. 1998). Theresults of studies using direct methods to measure amphibiandispersal generally support the same conclusion: amphibiansare highly philopatric (Daugherty and Sheldon 1982; Crump1986; Driscoll 1997). These results suggest that amphibiancolonization rates may be low as well.In view of these low dispersal rates, the critical question,therefore, is whether dispersal is great enough to allow thecolonization of empty habitat patches at a rate equal to orgreater than the rate of population extinction. We addressthis question for a set of long-toed salamander (  Ambystomamacrodactylum ) populations in high-elevation lakes in thenorthern Bitterroot Mountains of Montana, U.S.A.Many of these salamander populations were extirpated bythe introduction of trout to these historically fishless lakesearlier this century. However, with the cessation of troutstocking in 1984, some of these trout populations have goneextinct, allowing for possible colonization by salamanders.We surveyed these lakes in 1978, 1997, and 1998 to ( i ) testwhether trout exclude salamanders; ( ii ) determine whetherlakes in which trout have gone extinct have since been colo-nized by salamanders; and ( iii ) estimate the rates of coloni-zation and population extinction in lakes never stocked withtrout. Materials and methods Study area All lakes surveyed are located in the northern Bitterroot Moun-tains (Fig. 1). Surveyed lakes range in size from approximately0.0020 to 1.0000 km 2 and in elevation from approximately 1790 to2470 m (estimated from 1 : 24 000 scale U.S. Geological Surveymaps) (Table 1). The township–range format for locations is asused in all U.S. geological survey and U.S. Forest Service maps.Because of impassable waterfalls, these lakes were historically fish-less but served as habitat for breeding populations of long-toed sal-amanders and spotted frogs (  Rana luteiventris ). However, many of the lakes were stocked with various species of trout ( Oncorhynchusclarki ,  Oncorhynchus mykiss ,  Salvelinus fontinalis ) earlier this cen-tury. Stocking of these lakes by the Montana Department of Fish,Wildlife, and Parks continued until 1984 (C. Clancy, personal com-munication). Surveys We surveyed 42 lakes in 1978, 1997, and 1998 for long-toed sal-amander larvae and trout. Survey procedures differed between theearlier surveys (1978) and later surveys (1997 and 1998). In 1978,surveys involved carefully scanning the water column, water sur-face, and bottom of each lake from shore for salamander larvae andtrout as described by Thoms et al. (1997). At each lake, surveyscontinued until either the entire perimeter of the lake had beensearched or a trout had been seen. Therefore, in 1978 all lakeswere thoroughly surveyed for trout, but only those lakes withouttrout were thoroughly surveyed for salamander larvae. No salaman-der larvae were ever seen in lakes containing trout, but because theentire perimeters of these lakes were not searched, it is not knownwhether or not salamander larvae were actually absent from all of these lakes.In contrast, in 1997 and 1998 we surveyed the entire perimetersof all but the 2 largest (Bass Lake and Big Creek Lakes) of the 42lakes from shore for both salamander larvae and trout as describedabove. Approximately one-quarter of the perimeters of Bass Lakeand Big Creek Lakes were surveyed. In addition, if no salamanderlarvae and (or) trout were detected in a given lake by searchingfrom shore, snorkel surveys were employed to confirm their pur-ported absence. Snorkel surveys for salamanders consisted of slowly swimming three different 25-m transects running parallel toand approximately 2 m from the shoreline. In these surveys, boththe water column and substrate within approximately 2 m eitherside of the transect line were carefully checked for salamander lar-vae (Tyler et al. 1998 a ). Snorkel surveys for fish consisted of slowly swimming three different randomly chosen 50-m transectsrunning parallel to and approximately 3 m from the shoreline. Inthese surveys, the water column within approximately 3 m eitherside of the transect line was carefully searched for trout. In no casedid snorkel surveys give a different result from surveys conductedfrom shore, suggesting that shore surveys were sufficient for deter-mining the presence or absence of salamanders and trout. Data analysis Whether or not trout exclude salamander larvae from lakes wastested by comparing the proportion of lakes without trout that cur-rently contain salamander larvae with the proportion of lakes withtrout that currently contain salamander larvae, using Fisher’s exacttest. This test generates an exact probability that two variables arestatistically independent. All Fisher’s exact tests were performedusing Zaykin and Pudovkin’s (1993) program CHIRXC.Since the 18 lakes containing trout in 1978 (Table 1) were notthoroughly searched for salamander larvae during the 1978 sur-veys, salamander larvae found in these lakes during the 1997 and1998 surveys cannot unequivocally be said to be the descendants of colonists. They could also be the descendants of individuals thatexisted sympatrically with the introduced trout populations. Thus,to test whether or not lakes in which trout populations have goneextinct have been colonized by salamanders, we used a statisticalapproach. We first assumed that the proportion of the 12 lakes withtrout in the 1997 and 1998 surveys (Table 1) that also contain sala-mander larvae is representative of the proportion of the 18 lakeswith trout in the 1978 surveys that also contained salamander lar-vae. We then tested whether the proportion of the 6 lakes in whichtrout went extinct (Table 1) that currently contain salamander lar-vae is greater than that which we would expect if the salamanderlarvae found in these lakes were simply the descendants of individ-uals that had managed to coexist with trout. This was done usingFisher’s exact test to test whether the proportion of the 6 lakes inwhich trout have gone extinct that currently contain salamanders issignificantly greater than the proportion of the 12 lakes in whichtrout have persisted that contain salamanders.A second assumption of this approach is that the lakes in whichtrout went extinct are a random subsample of the lakes that con-tained trout in 1978 with respect to the capacity of each lake tosupport the coexistence of salamander larvae and trout. Two factorsthat affect the suitability of high-elevation lakes for both salaman-der larvae and trout, and, therefore, which likely affect a givenlake’s capacity to support the coexistence of larvae and trout, arewater depth and elevation (Bahls 1992; Tyler et al. 1998 a ; Vos andChardon 1998). To check this assumption, we looked for system-atic differences in surface area (as a surrogate for water depth) andelevation between lakes in which trout went extinct and lakes inwhich they persisted, by plotting log (surface area) and elevationfor each lake (Fig. 5). We then used Fisher’s exact test to testwhether the proportion of lakes in which trout went extinct differedbetween the 9 lakes with the smallest log (surface area) and the 9 © 1999 NRC Canada 1760 Can. J. Zool. Vol. 77, 1999  lakes with the largest log (surface area) or between the 9 lakes atthe lowest elevation and the 9 lakes at the highest elevation.Change in the proportion of lakes that have never been stockedwith trout that contained salamander larvae between the 1978surveys and the 1997 and 1998 surveys was tested using Fisher’sexact test. Results Of the 42 lakes surveyed in 1997 and 1998, 12 (28.6%)contained trout and 29 (69.0%) contained salamander larvae(Table 1, Figs. 1 and 2). Only 2 (16.7%) of the 12 lakes withtrout also contained larvae, whereas 27 (90.0%) of the 30lakes without trout contained larvae (Table 1, Figs. 1 and 2).The proportion of lakes with trout that also contained larvaeis significantly different from the proportion of lakes with-out trout that contained larvae (  p  < 0.0001; Fig. 2). This sug-gests that trout excluded larvae from lakes.By 1998, trout populations had gone extinct in 6 (33.3%)of the 18 lakes that contained trout in 1978 (Table 1, Figs. 1and 3). Salamander larvae were present in 5 (83.3%) of the 6lakes in which trout went extinct, whereas only 2 (16.7%) of the 12 lakes in which trout persisted contained larvae (Ta- © 1999 NRC Canada Funk and Dunlap 1761 Fig. 1.  Map of the 42 lakes surveyed for long-toed salamander (  Ambystoma macrodactylum ) larvae and trout in the northern BitterrootMountains of Montana, U.S.A., in 1978, 1997, and 1998. Symbols indicate whether or not lakes were stocked and the presence orabsence of salamander larvae and trout in the 1978 and 1997–1998 survey sessions as specified.  ble 1, Figs. 1 and 3). The proportion of lakes containing lar-vae in which trout went extinct was significantly differentfrom the proportion of lakes containing larvae in which troutpersisted (  p  = 0.0128; Fig. 3). This suggests at least some of the lakes in which trout went extinct were colonized by sala-manders. © 1999 NRC Canada 1762 Can. J. Zool. Vol. 77, 1999 Drainage Lake designation and name LocationMill Creek WOO T11N R21W S22 SW1/4Carlton Creek CAR, Carlton L. T11N R21W S27 NE1/4LCA, Little Carlton L. T11N R21W S27 SE1/4One Horse Creek MOH, Reed L. T11N R21W S33 NE1/4SON, South One Horse L. T11N R21W S33 SE1/4Sweeney Creek MIL, Mills L. T10N R21W S8 SE1/4HOL, Holloway L. T10N R21W S8 SE1/4DLK, Duffy L. T10N R21W S8/9DUF T10N R21W S9 NW1/4NDF T10N R21W S4 SW1/4PET, Peterson L. T10N R21W S10 SW1/4SWE, Sweeney L. T10N R21W S20 NW1/4ESW T10N R21W S20 NW1/4South Fork, Lolo Creek ASC T10N R21W S30 NW1/4Bass Creek BAS, Bass L. T10N R21W S30 SW1/4Kootenai Creek WKO, West Middle Fork L. T9N R22W S11 SE1/4EKO, East Middle Fork L. T9N R22W S11 SE1/4NKO, North Kootenai L. T9N R22W S11 NE1/4SKO, South Kootenai L. T9N R22W S13/14Sharrott Creek UDS T9N R21W S27 NW1/4LDS T9N R21W S27 NW1/4SH2 T9N R21W S22 SE1/4SH3 T9N R21W S27 NE1/4BOB T9N R21W S22 SE1/4RUT T9N R21W S27 NW1/4CLA T9N R21W S27 SW1/4McCalla Creek SMC T9N R21W S28 SE1/4DMC T9N R21W S33 NW1/4Big Creek BCL, Big Creek Lakes T9N R22W S33/34/ T8N R22W S5SFL, South Fork L. T8N R22W S17 SE1/4WSF T8N R22W S16 SW1/4ASF T8N R22W S16 SW1/4PRL, Pearl L. T8N R22W S8/17UPR T8N R22W S17 NW1/4STG T8N R22W S16 NW1/4HID, Hidden L. T8N R22W S11 NW1/4UHD T8N R22W S11 SW1/4BHD T8N R22W S11 NE1/4TCR T8N R22W S13 NW1/4Sweathouse Creek GLE, Glen L. T8N R22W S13 NW1/4Bear Creek BEA, Bear L. T8N R22W S21 SE1/4BRY, Bryan L. T8N R22W S29 SW1/4 Note:  A question mark indicates a lake and year in which the incidence of   A. macrodactylum  was notdetermined with confidence (see Materials and methods). Table 1.  Incidence of long-toed salamander (  Ambystoma macrodactylum ) larvae and troutpopulations in lakes in the northern Bitterroot Mountains of Montana, U.S.A., in 1978 and 1997–1998.
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