The cause of global amphibian declines: a developmental endocrinologist's perspective

Abstract

Greater than 70% of the world's amphibian species are in decline. We propose that there is probably not a single cause for global amphibian declines and present a three-tiered hierarchical approach that addresses interactions among and between ultimate and proximate factors that contribute to amphibian declines. There are two immediate (proximate) causes of amphibian declines: death and decreased recruitment (reproductive failure). Although much attention has focused on death, few studies have addressed factors that contribute to declines as a result of failed recruitment. Further, a great deal of attention has focused on the role of pathogens in inducing diseases that cause death, but we suggest that pathogen success is profoundly affected by four other ultimate factors: atmospheric change, environmental pollutants, habitat modification and invasive species. Environmental pollutants arise as likely important factors in amphibian declines because they have realized potential to affect recruitment. Further, many studies have documented immunosuppressive effects of pesticides, suggesting a role for environmental contaminants in increased pathogen virulence and disease rates. Increased attention to recruitment and ultimate factors that interact with pathogens is important in addressing this global crisis.

Fig. 1.

Tiered approach showing interactions of the many factors that contribute to amphibian declines. Effects at any given level have no impact on higher levels, but interact with other factors on the same level and impact factors at lower levels. The five ultimate factors at Level 3 affect multiple factors at Level 2, which in turn interact with each other and ultimately contribute to amphibian declines due to death and decreased recruitment (Level 1). Line weights at Level 3 reflect rankings as described in the text (‘Horizontal interactions at Level 3’).

Fig. 2.

A communal wood frog (Rana sylvatica) deposition site in Concord (Middlesex County) Massachusetts from 1988 (A). Well over 100 egg masses are shown just on the surface, indicating a local population of over 200 breeding adults. Similar numbers bred at this site in the previous 2 years, but in this same time frame only two adults were ever seen outside of the breeding season. (B—F) Breeding sites for boreal toads (Bufo boreas) along Del Peurto Creek in Stanislaus County, California. B shows a pair of copulating toads with the strings of eggs wrapped around the pair. C and D show views of one ephemeral pool where a single clutch of eggs (which can be up to 10,000 individuals) has hatched in 1996. E and F show changes in a second nearby deposition site from 1995 (E) to 1999 (F). The breeding site is on the left in both panels with Del Peurto Creek on the right. Arrow in E shows the location of the small breeding pool shown in panels C and D. Because of anthropogenic activity, water flow has changed at the site, these ephemeral breeding pools have disappeared, and breeding has not occurred here for at least 4 years.