Butterfly abundances but not patch occupancy buffered by vegetation heterogeneity during climatic extremes

As extreme climatic events are becoming more frequent and intense, we need to understand what mechanisms could buffer populations from climate change impacts to ensure their persistence. Fragmented populations may be especially vulnerable to climate change, as exemplified by species with metapopulation dynamics, whose persistence is determined by a precarious balance between frequent local extinctions and recolonizations of interconnected habitat patches. We investigated whether variation in vegetation productivity between and within habitat patches can buffer metapopulation dynamics in years of drought. Using 24 years of long-term monitoring data from the Glanville fritillary butterfly (Melitaea cinxia) for over 4000 habitat patches, we modelled temporal trends in butterfly habitat patch occupancy and abundance as a function of vegetation productivity and its heterogeneity during the growing season, using yearly Normalized Difference Vegetation Indices (NDVI) estimated from satellite data. We then tested whether these vegetation heterogeneity effects differed between years depending on the magnitude of drought, measured as the cumulative climatic water balance during the growing season. Across years, butterfly occupancy was consistently highest in patches with low to intermediate vegetation productivity. However, once patches were occupied, the effects of vegetation heterogeneity on butterfly abundances differed between years, showing opposite trends in dry vs wet years. In drier years, butterflies tended to be more abundant in patches of higher mean productivity, while in wet years, abundances tended to increase with higher levels of within-patch vegetation heterogeneity. These results indicate that there can be a mismatch between patch occupancy and abundance dynamics, with patch (re)colonization success not responding to variation in the conditions that promote population growth during more extreme years. Nevertheless, our results suggest that differences in vegetation productivity within and between habitat patches can create enough variation in local population dynamics to support the metapopulation during extreme years. The type of vegetation heterogeneity needed for this support, however, varies with the type of climatic perturbation.