Species range expansion constrains the ecological niches of resident butterflies.
AimChanges in community composition resulting from environmental changes modify biotic interactions and affect the distribution and density of local populations. Such changes are currently occurring in nettle-feeding butterflies in Sweden where Araschnia levana has recently expanded its range northward and is now likely to interact with the resident species (Aglais urticae and Aglais io). Butterfly occurrence data collected over years and across regions enabled us to investigate how a recent range expansion of A. levana may have affected the environmental niche of resident species.\n\nLocationWe focused on two regions of Sweden (Sk[a]ne and Norrstrom) where A. levana has and has not established, and two time-periods (2001-2006, 2009-2012) during its establishment in Sk[a]ne.\n\nMethodsWe performed two distinct analyses in each region using the PCA-env and the framework described in Broennimann et al. (2012). First, we described the main sources of variation in the environment. Second, in each time-period and region, we characterized the realized niches of our focal species across topographic and land use gradients. Third, we quantified overlaps and differences in realized niches between and within species over time.\n\nResultsIn Sk[a]ne, A. levana has stabilized its distribution over time while the distribution of the native species has shifted. These shifts depicted a consistent pattern of avoiding overlap between the native species and the environmental space occupied by A. levana, and it was stronger for A. urticae than for A. io. In both regions, we also found evidence of niche partitioning between native species.\n\nMain conclusionsInterspecific interactions are likely to affect local species distributions. It appears that the ongoing establishment of A. levana has modified local biotic interactions, and induced shifts in resident species distributions. Among the mechanisms that can explain such patterns of niche partitioning, parasitoid-driven apparent competition may play an important role in this community.