Recent, substantial, and unexplained declines of the common toad Bufo bufo in lowland England

Abstract

A decline in numbers of a large common toad (Bufo bufo) population in south-east England during the 1990s, together with anecdotal reports of similar trends in other toad populations, prompted a nation-wide survey of this species. The survey also included the common frog (Rana temporaria) as a control for which there was no comparable evidence of recent decline. A questionnaire requesting information on the fate of toad and frog populations in the last 15 years of the 20th century was distributed to professional and amateur herpetologists during autumn 2001. Ninety-five respondents provided data on a total of 277 sites, including 232 frog and 202 toad populations in England, Scotland and Wales. More than 80% of the reported sites were rural for both species. Rural frog populations were generally doing well in most parts of Britain, with almost as many increasing as decreasing populations and a high proportion of stable populations. Rural toads also showed no overall trend across the country as a whole. However, regional analysis indicated that although toads were faring at least as well as frogs in the north and west of Britain, they were declining seriously in central and eastern/south-eastern areas. In these regions 50% or more of toad populations have experienced recent declines, whereas frogs have fared as well as they have elsewhere. Toads breeding alone have fared significantly worse than toads breeding at sites where frogs were also present. Reasons for apparently toad-specific declines in lowland England remain unknown.

Introduction

Amphibian declines at many disparate sites across the world have become a cause of increasing international concern over the past decade or so (e.g. Blaustein and Wake, 1990, Berger et al., 1998, Lips, 1999). The most dramatic events have occurred in apparently pristine habitats, mostly at high altitudes and often in the tropics. Climate change and disease, perhaps linked in some cases, are widely attributed as causative agents and the extinction of the golden toad (Bufo periglenes) in Costa Rica may be the first documented casualty of global warming (Pounds et al., 1999). However, severe amphibian declines in Europe pre-dated those that have received more recent attention elsewhere in the world (Hoolahan et al., 2000). In Britain, at least four species (Rana temporaria, Bufo bufo, B. calamita and Triturus cristatus) experienced severe decreases around the middle of the 20th century (Cooke, 1972, Beebee, 1975, Beebee, 1976). These declines were primarily attributable to land use changes, especially agricultural intensification.

By contrast, the last decades of the 20th century were times of more mixed fortunes for British wildlife. There were causes for optimism with the recovery of some mammals (such as otters Lutra lutra) and birds (such as red kites Milvus milvus) but also continuing concerns. Many plants and songbirds characteristic of open farmland declined as fast or faster than in previous decades during the 1980s and 1990s (Sotherton, 1998, Donald et al., 2001, Vickery et al., 2001). Surveys of widespread amphibians in the 1970s indicated that although declines were still occurring, they were less severe than before (Cooke and Scorgie, 1983). However, by the 1980s there was preliminary evidence that the common toad (Bufo bufo) was experiencing a new round of declines in parts of lowland England (Hilton-Brown and Oldham, 1991). It was the substantial decline during the 1990s, for no obvious reason, of a previously large common toad population near Lewes in East Sussex that prompted this new examination of two widespread amphibians. The Lewes locality was scheduled by the national statutory conservation agency as a Site of Special Scientific Interest in the late 1980s specifically on account of its amphibian assemblage, and particularly because of its large population (several thousands) of common toads. By the late 1990s this population had decreased to low hundreds of animals, though populations of other species of amphibians at the site remained unchanged. Similar recent decreases of B. bufo had been noticed at several other sites by local naturalist groups (Foster, 1996). In most cases there were no obvious reasons for the declines, and anecdotal evidence also suggested that they might be toad-specific. As at Lewes, common frogs were often faring well in the same area that common toads were decreasing. We therefore designed a questionnaire to seek information from volunteer and professional herpetologists about the recent fates of frog and toad populations across Britain, and the results of this investigation are reported below.