Aim Biotic interactions are an important factor structuring ecological communities but data scarcity limits our understanding of the impact of their response to climate and land use changes on communities. We studied the impact of a change in species assemblage on biotic interactions in a community of closely-related butterflies. Specifically, we examined the impact of the recent range expansion of Araschnia levana on the resident species, with a particular focus on natural enemies, parasitoids, shared with other butterfly species in the assemblage. Location Sweden. Time period Two years (2017-2018). Major taxa studied Nettle-feeding butterflies (Aglais urticae, Aglais io, Araschnia levana, and Vanessa atalanta) and their parasitoids. Methods We assessed parasitism in 6777 butterfly larvae sampled in the field from 19 sites distributed along a 500 km latitudinal gradient, and every two weeks throughout species' reproductive seasons. We identified the parasitoid complex of each butterfly species and their overlap, and analysed how parasitism rates were affected by species assemblage, variations in abundance, time, and the arrival of A. levana. Results Parasitoids caused high mortality, with substantial overlap across the four host species. The composition of the host community influenced parasitism rates and this effect was specific to each species. In particular, the rate of parasitism in resident species was comparatively higher at sites where A. levana has been established for longer. Main conclusions Parasitoid pressure is a significant source of mortality in the nettle-feeding butterfly community studied. Variations in butterfly species assemblages are associated with substantial variations in rates of parasitism. This is likely to affect the population dynamics of their butterfly host species, and, potentially, the larger number of species with which they interact.

Altered parasitism of a butterfly assemblage associated with a range-expanding species