Biological Conservation 67 (1994) 251-254 PATHOGENIC FUNGUS CONTRIBUTES TO AMPHIBIAN LOSSES IN THE PACIFIC NORTHWEST Andrew R. Blaustein, D. Grant Hokit, Richard K. O'Hara Department of Zoology, 3029 Cordley Hall, Oregon State University, Corvallis, Oregon 97331-2914, USA & Richard A. Holt Department of Microbiology, Nash Hall 220, Oregon State University, Corvallis, Oregon 97331-3804, USA (Received 2 August 1992; revised version received 7 April 1993; accepted 23 April 1993) Abstract Worldwide declines in amphibian populations have been the subject of numerous recent reports, and numerous hypotheses have been constructed to address the causes. There is no evidence for a single cause for the declines. We identify and describe the spread of a pathogenic jungus that appears to be largely responsible for egg mortality in one population of western toad Bufo boreas. This is the first study documenting contemporary mortality in an amphibian population with identification of the attributed pathogenic species. The fungus we identify is circumglobally distributed and we suggest that this fungal infection could also be a major contributor to other amphibian populations declines. Keywords: amphibian declines, pathogens, fungus, Bufo, Saprolegnia. INTRODUCTION Worldwide declines in amphibian populations have been the subject of numerous recent reports (e.g. Honegger, 1978; Semb-Johansson, 1989; Barinaga, 1990; Beebee et al., 1990; Blaustein & Wake, 1990; Phillips, 1990; Wyman, 1990; Wake, 1991; Crumpet al., 1992). Range reductions, decreases in populations and egg mortality have been described. Hypothesized causes tbr the declines include habitat destruction, chemical pollution, acid precipitation, increased ultraviolet radiation, introduction of exotic species, harvesting by humans and natural population fluctuations (Blaustein & Wake, 1990; Phillips, 1990; Wyman, 1990; Pechmann et al., 1991). There is no evidence for a single cause for the declines (Blaustein & Wake, 1990). The causes for the apparent declines have been espe- Biological Conservation 0006-3207/94/$07.00 © 1994 Elsevier. Science Limited, England. Printed in Great Britain 251 dally perplexing in relatively undisturbed areas of western North America (Roberts, 1986; Blaustein & Wake, 1990; McAllister & Leonard, 1990, 1991; Fellers & Drost, 1993). However, in this paper, we identify and describe the spread of a pathogenic fungus that appears to be largely responsible for egg mortality in one population of western toad Bufo boreas. Unusual egg mortality has occurred in this population every year since 1989 (Blaustein & Olson, 1991). A previously ubiquitous species in western North America (Stebbins, 1985), the western toad has undergone drastic popula- tion declines and is considered threatened in much of its range (Federal Register, 1991). This is the first study documenting a major mortality event in amphibians as it occurred, in which the cause for the mortality was attributed largely to a pathogen, with subsequent identification of the pathogenic species. Moreover, the fungus we identify is worldwide in distri- bution and we suggest that fungal infection could be a major contributor to declines in other amphibian popu- lations. Although there is great potential for pathogens causing amphibian population reductions (Smith et al., 1986; Hunter et al., 1989; Bradford, 1991; Aho, 1990) they have been largely overlooked and not carefully documented in this regard. MATERIALS AND METHODS Since 1980 we have been studying the western toad in Oregon at several locations (Olson et al., 1986; Olson, 1988; Blaustein & Olson, 1991). In 1992, we observed B. boreas breeding activity at Lost Lake (Linn County, Oregon, USA, 97 km east of Albany on US Highway 20; elevation 1220 m) from 26 to 29 April. We marked (by toe clipping) all individuals within the breeding population and estimated the total number of eggs laid. We monitored the development of eggs where they were laid, macroscopically and with a hand lens. We