Andrew Blaustein and Deanna Olson (of the U.S. Forest Service) conducting their yearly census of toads in the central Cascade Range of Oregon. Declining Amphibian Populations in Perspe_ctive .
· ecies have become extremely rare and these repons. Below, I briefly su~manze By Andrew R. Blaustem ~thers recently may have gone extinct. For some.o~ the m_ajor issues concemmg the The National Science Board (1989) re- example, the golden toad (Bufo periglenes) amphtbtan dechne problem. . . cently concluded that an ongoing and un- from Costa Rica was abundant as recently as Ecologicallmportance of Amphibians precedented loss of the variety ~d numbers 1987 and has become extremely rare since Amphibians are integral components of of species around the world eXJsts. Indeed, then (Crump etal. 1992). The gastric brood- manyecosystemsandtheymayconstitute ~e members of all taxa are affected. As part of ing frog (Rheobatrachus silus) from Austra- highest fraction of vertebrate biom~s m this overall "biodiversity crisis" there have liahasnotbeenseensince 1979(fyler 1991_). certain ecosystems (e.g. Burton and L~ens been numerous recent repons suggesting However, notalJ amphibian species are m 1975). Because of the important contrtbu that the populations of amphibian species in decline and there are species whose popula- tion of amphibians to trophic dynamics in _a a wide array of geographic regions and hab- lions seem to be persistent in the same areas variety of communities, a decline in therr itats apparently have declined or h~ve expe- where populations are declining. Moreover, numbers could have important impacts on rienced range reductions (e.g. reviewed by most of the repons on declining amphibian other organisms. Adult amphibians play Barinaga 1990; Blaustein and Wake 1990; populationsarebasedprimarilyonanecdotal major carnivore roles in many systems and Phillips 1990). information. The Jack of long-term data serveaspreyinothers(DuellmanandTrueb Disappearance of frogs, toads, and regarding amphibian populations makes it 1986); larval amphibians can be important salamanders have been reported on all con- difficult to assess the overall significance of tinents where amphibians are found. Some
8 Park Science predators and herbivores as well as prey Pechmann et a!. (1991) monitored the Besides oven habitat destruction and nat (Duellman and Trueb 1986) in aquatic hab breeding population sizes of four amphibian ural population fluctuations, other proposed itats. species at one site in South Carolina for 12 causes include introduction of exotic species Moreover, a.rnphibians may serve as good years. They showed that the populations of such as fish and bullfrogs, disease, ultraviolet bioindicators ofenvironme ntal stress (1) due three species fluctuated and one species in radiation, and acidification. Most of these to their permeable skin that readily absorbs creased over that time span. hypothetical explanations have not been ex toxic substances, (2) because they are not In addition to the subjects of the the long amined experimentally. protected by hair orfeathe rs, (3) because their term studies cited above, populations ofother In the Pacific Northwest, where there have eggs are not encased in hard shells, and (4) amphibian species have disappeared from beenrangereductionsinanumberofspecies, because many species may come in contact portions of their historical ranges (without unusual egg mortality, and disappearances with both terrestrial and aquatic stresses due concomitant shifts in their ranges) and have oflarge numbers oftadpoles, weare continu to their complex life cycles. failed to reestablish at such sites for periods ing to accumulate long-term population data Long-term Studies longer than their estimated maximum life at a variety of sites for a number of species. There have been few studies where am span. For example, this has occurred in We also are investigating the role of patho phibian populations were sampled at least several species in western North America, gens and ultravio1etradiation in the declines once per year on a long-term basis. Of those including the Cascades (Rana cascadae), of several species of amphibians from Ore long-term studies that have been conducted, Red-legged (R. aurora),and western-spotted gon. (R . pretiosa) frogs and the western toad some show populations that are in decline, Blaustein is a professor in the Department of whizle others do not. Two long -term studies (Bufo boreas). In addition, the eggs of Zoology, Oregon State University, Corvallis. from Europe have documented declines. several of these species have experienced and co-chair ofthe Pacific Northwest section of Semb-Johansson 's (1989) study of common unusually high mortality in recent years the /UCN's Declining Amphibian Populations toads (B ufo bufo) on islands off the Norwe (Blausteinetal. 1993). Inthesameregionas Task Force. gian coast is one of the best examples of a these species, populations of the Pacific treefrog (Hyla reg ilia) seem to be persistent. Literature Cited long-term population study ofan amphibian. Barinaga, M. 1990. Volhere have all the froggies gone? Toads were monitored for 24 years (1966 to Causes Science 247:1033·1034. 1989). Theirnumbers declined dramatically Habitat destruction and habitat a! teration Beebee, T.J.C., R.J. Flower, A. C. Stevenson, S.T. Patrick, from 1966-1975 and have remained low. probably are the major causes for decline in P.G. Appleby, C.Retcher, C. Marsh, J. Natkansi, B. amphibian populations. However, due to Rippey, and R.W. Battatbee. 1990. Decline of the Beebce eta!. (1990) have documented the Natterjack toad Bufo calamita in Britail: Paleoecological, decline of the natlerjack toad (B ufo calimata) lack of long-term data it is difficult to distin documentary and experimental evidence for breedng site in Great Britain over a 20 year period. guish between human-induced causes and acidifiCation. Biological Conservation 53: 1·20. Studies in the U.S. show different popula natural population fluctuations. For exam Blaustein, A.R., D.G. Hokit, R.K O'Hara, and R.A. Holt. tion dynamics in different regions. Com and ple, very little is known about the population 1993. Pathogeric fungus contributes lo amphibian losses in the Pacifte Nor1hwest. Biological Conservation. In Press. Fogleman's (1984) study documented the dynamics of golden toads and gastric brood Blaustein, A.R and D. B. Wake. 1990. Declining amphibian local extinction of leopard frogs (Rana ing frogs, so it is possible they may be in poptiations: A global phenomenon? Trends in Ecology pipiens) in Colorado across a number ofs ites. dormancy and may again be common when and E\Qiution 5:203·204. In this study, six populations of R. pipiens conditions are better. Buton, T.M. and G. E. Likens. 1975. Salamanderpopliations were examined for 10 years (1973-1982). and biomass in the Hubbard Brook experimental forest, New Hampshire. Copeia 1975:541-546. Reproductive failure was seen in 1973 at one The author (Andrew Blaustein) holding a Com, P.S. andJ.C. Fogelman. 1984. Extinction of montane site and by 1981 noR. pipiens were seen at deviee that filters out ultraviolet light. The filters are placed over developing eggs in popliations of the rorthem leopard frog ( R111a pipiens) in Colorroo. ..boola~ of Herpetology 18:147·152. any site. Rana pipiens was absent from the experiments investigating the role of Cn.mp, M.l., F.R. Hensley, and K.L Clark. 1992. Apparent area at the end of the study. ultraviolet radiation on am phiban declines. Kagarise Sherman and Morton (1993) dedineofthegoldentoad: Undergroundorextinct?Copeia 1992:413-420. documented the population changes over 20 Duellm<11, W.E. and l. Trueb. 1986. Biology of Amphibi<11s. years in B ufo canorus at Tioga Pass, Califor McGraw-Hill, New YOlk. nia. Comprehensive surveys of breeding Jaeger, R.G. 1980. Density-dependent
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