Quantifying the long-term decline of the West European hedgehog in England by subsampling citizen-science datasets

Abstract

It is increasingly important to be aware of trends in species abundances in order to be able to act ahead of possible irrecoverable declines and extinctions. Long-term monitoring of species is generally used to determine how a species is faring, which is essential knowledge for conservation planning and design. However, monitoring programmes that encompass large areas and long timespans are rare or non-existent for the vast majority of species. Citizen-science-based datasets provide a wealth of data on past and present species’ occurrences but are often biased to a large extent. We evaluate the potential use of such datasets by subsampling datasets collected over different time periods to detect trends in the long-term temporal abundance of West European hedgehogs (Erinaceus europaeus), a species which is thought to be in decline in parts of its geographic range. We used subsampling as a means to account for quantitative differences between two non-systematic datasets of West European hedgehog occurrences in England; one dataset was collected by the public between 1960 and 1975 and one was collected between 2000 and 2015. Here, we confirm and quantify previous anecdotal evidence of a long-term decline of the species throughout England. We find that although the West European hedgehog is still widespread in England, a 5.0 to 7.4 % decline in occupied grid cells was observed when comparing the 2000–2015 dataset to the previous survey in 1960–1975 after adjusting for differences in effort. This suggests that the decline of the relative abundance of West European hedgehogs is moderate in England, 25 % being an amber alert for birds of conservation concern in the UK. Importantly, we show that subsampling disparate citizen-science datasets is a useful tool for monitoring species population trends.