Species range expansion constrains the ecological niches of resident butterflies
ABSTRACTAimChanges 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 whereAraschnia levanahas recently expanded its range northward and is now likely to interact with the resident species (Aglais urticaeandAglais io). Butterfly occurrence data collected over years and across regions enabled us to investigate how a recent range expansion ofA. levanamay have affected the environmental niche of resident species.LocationWe focused on two regions of Sweden (Skåne and Norrström) whereA. levanahas and has not established, and two time-periods (2001-2006, 2009-2012) during its establishment in Skåne.MethodsWe performed two distinct analyses in each region using the PCA-env and the framework described in Broennimannet 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.ResultsIn Skåne,A. levanahas 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 byA. levana, and it was stronger forA. urticaethan forA. io. In both regions, we also found evidence of niche partitioning between native species.Main conclusionsInterspecific interactions are likely to affect local species distributions. It appears that the ongoing establishment ofA. levanahas 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.