Extreme weather affects colonization-extinction dynamics and the persistence of a threatened butterfly.
Abstract
Extreme weather events can be expected to increase in frequency in the future. Our knowledge on how this may affect species persistence is, however, very limited. For reliable projections of future persistence we need to understand how extreme weather affects species' population dynamics. We analysed the effect of extreme droughts on the host plant Succisa pratensis, colonization-extinction dynamics, and future persistence of the threatened marsh fritillary Euphydryas aurinia. Specifically, we studied a metapopulation inhabiting a network of 256 patches on Gotland (Sweden), where the summer of 2018 was the driest ever recorded. We analysed how the frequency and leaf size of host plants changed between 2017 and 2019, based on 6,833 records in 0.5-m2 sample plots. Using turnover data on the butterfly from 2018 to 2019 we modelled local extinction and colonization probabilities. Moreover, we projected future population dynamics with an increasing frequency of extreme years under three different management strategies that regulate the grazing regime. Our results show a substantial decrease in both frequency (46%) and size (20%) of host plants due to the drought, which taken together may constitute a 57% loss of food resources. The butterfly occupancy decreased by over 30% between 2018 and 2019 (from 0.36 to 0.27). The extinction probability increased with increasing 'effective area' of the patch (taking quality reduction due to grazing into account), and the colonization probability increased with increasing connectivity and ground moisture. Projections of future dynamics showed an increasing risk of metapopulation extinction with increasing frequency of years with extreme droughts. The risk, however, clearly differed between management strategies. Less grazing in years with droughts decreased the extinction risk considerably. Synthesis and applications. Extreme weather events can have profound negative impacts on butterflies and their host plants. For the marsh fritillary, an increased frequency of extreme droughts can lead to extinction of the entire metapopulation, even in a large and seemingly viable metapopulation. Increased grazing, due to fodder deficiency in dry years, may lead to cascading negative effects, while active management that reduce grazing in years with droughts can almost completely mitigate these effects.