Increasingly uncommon common toads: multidecadal, ongoing abundance decline of a widespread amphibian despite volunteer conservation action

Abstract Wide ranging and abundant species, often termed common species, play critically important roles in ecosystem functioning. However, conservation practice usually focuses on rare and declining species, including monitoring their state, while population trends of common and widespread species are often logistically difficult to evaluate compared to rare habitat specialists. This is particularly the case for small nocturnal vertebrates such as common amphibians, where abundance data are typically absent across most of their range. Building on previous work, we used national scale volunteer-collected data across Switzerland and Britain to investigate population trends over four decades for Europe’s two most common amphibian species, common toads ( Bufo bufo ) and common frogs ( Rana temporaria ). We included an average of 86 populations per year with an average count of 87,900 toads and 74,000 frogs in Switzerland and 80 populations per year with an average count of 75,800 toads in Britain, resulting in a total count of nearly 5.5 million toads across the entire study period. Overall, toad abundance declined in all decades since 1973 in Switzerland (33% abundance reduction) and since 1985 in Great Britain (41% reduction), but there were significant regional recoveries in Britain since 2013. Common frog abundance trends in Switzerland varied and remained positive overall but population growth rate declined after the year 2000. The impacts of such large-scale and ongoing abundance loss of common species may affect the ecosystem services provided by these species and require effective conservation action that expands beyond protected areas.