Scaling distributional patterns of butterflies across multiple scales: Impact of range history and habitat type

AimAssessing the threat status of declining but yet widespread species poses a challenge to applied ecologists. Previous studies using a common metric to describe the spatial aggregation of occurrences across multiple scales, the fractal dimension D-ij, have suggested that species' distributional trends may be deduced from readily understandable spatial patterns: Expanding species are expected to show more aggregated spatial distributions (higher value of D-ij) than declining species (lower value of D-ij). Here, we revisited these predictions using a large-scale empirical dataset on Finnish butterflies. LocationFinland. MethodsFor each butterfly species (n=97) and across three spatial scales (grid squares of 10km, 50km and 100km), we calculated the area of occupancy (AOO(i)) as the sum of occupied grid squares. We employed values of AOO(i) to derive the D-ij for each butterfly species. We then used these metrics to compare the changes in spatial patterns of distribution (AOO(i) and D-ij) between two time periods, 2000-2002 and 2009-2011. ResultsMajority of the studied butterfly species showed declining areas of occupancy (at the scale of 10km, AOO(10)) and fractal dimensions (across the scales from 10km to 100km, D10-100) between the two study periods. In contrast to predictions, AOO(10) and D10-100 showed negative impacts on the AOO(10), an observation that may be explained by the high proportion of declining species in our data. Butterfly species with the greatest fractal dimensions at regional scales (D10-100) in the years 2000-2002 showed both positive long-term distributional trends and most notable northern recent range limit shifts. Main conclusionsOur results were in most cases congruent with the prediction of higher fractal dimension values in expanding compared to declining species. As a novel observation, many butterflies expanded northwards in spite of their occurrences getting simultaneously more scattered, particularly in southern Finland.