Long-Term Changes in Four Populations of the Spiny Toad, Bufo spinosus, in Western France; Data from Road Mortalities

Abstract

Habitat fragmentation is widely recognized as a contributor to the decline of biodiversity, with amphibians one of the key groups impacted. To understand the effects of habitat fragmentation on amphibian populations requires long-term data sets showing population trends. In this paper, road mortalities were employed as proxies to describe long-term numbers of four populations of the spiny toad Bufo spinosus in western France during a 17-year period. Road mortalities were found during all months in all populations but were most frequent during October, November and December, the main migratory period. Large females were found significantly more frequently during these migration months, forming 45% of the total sample, compared with their presence from January to September (34.4%). The long-term trends were evaluated using regression analysis of the logarithmic (loge) transforms of annual counts as dependent variables against year as the independent variables. All coefficients showed no significant departure from the 0 hypothetical coefficients, indicative of population stability. This was supported by jackknife analysis, which showed good agreement of the pseudo-regression coefficients with the true equations. Stepwise regression of potential climate impacts on toad numbers suggested rainfall levels in October adjusted to 2- and 3-year lags were involved in driving population change. Road mortality counts were also made during 2020 and 2021 when human movement restrictions were in place due to the COVID-19 pandemic. To estimate the potential impact on this disturbance in the methodology, the Poisson distribution was used to estimate potential differences between what would have been expected counts and the observed counts. The results indicate that the observed mortalities were significantly lower than expected in all four populations.