Extreme weather affects colonization-extinction dynamics and the persistence of a threatened butterfly

1. 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. 2. 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 6833 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. 3. 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. 4. 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. 5. 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.