A documented amphibian decline over 40 years: Possible causes and implications for species recovery

Amphibians are declining worldwide, but lack of long-term regional data makes identifying possible causes difficult, hindering conservation efforts. We evaluated whether habitat destruction, terrestrial habitat adjacent to ponds and the physico-chemical characteristics of ponds could explain the regional and local decline of the spadefoot toad (Pelobates fuscus) in Sweden.Analyses of aerial photos and field observations revealed that out of all of the known calling sites of the species since 1959, 26% did not exist in 2000. The road traffic intensity adjacent to existing ponds indicated that in 1997–2003 it was higher near ponds where calling males had disappeared (N = 240) compared to sites where calling males were present (N = 84). The soil-type adjacent to ponds with calling males was more sandy than at ponds where calling males had disappeared (instead dominated by till). By including road traffic intensity and proportion of sandy soils adjacent to ponds, a logistic regression model correctly classified 82% of the ponds into their correct category.Of 36 ponds investigated in 2004, we found evidence of successful reproduction (tadpoles) in 53%. Unsuccessful reproduction seemed to be associated with eutrophication and low coverage of submerged macrophytes. In an area with low road traffic intensity and sandy soils, restoration of ponds started in 1996, and the number of calling males increased from a maximum of 77 in 1993–1996 to 146 in 2006. Our results indicate that habitat destruction has likely contributed to the regional decline of P. fuscus, but also that local factors such as soil type, traffic intensity and reproductive failure may also help explain the decline of the species.