Guild diversity impacts demographic outcomes of novel species interactions following range shifts

Novel competitive interactions between native and range shifting species can precipitate local extinction of native species. However, increased biological complexity within recipient communities may prevent native species loss by decreasing the strength of novel competition experienced by any one species. This phenomenon, termed 'biotic resistance', is commonly applied in invasion ecology, but has received little attention in the context of climate induced range shifts. Here we investigate the effects of biotic resistance in competition between resident native and range-shifting damselflies in a region of Scotland newly colonised by the range-shifter, using competitive mesocosm treatments across multiple life stages and experimental temperatures. Our focal native species (Lestes sponsa) was unaffected by increasing competitive complexity as larvae, showing no fitness benefits in multispecies treatments compared to intraspecific or even interspecific scenarios in the presence of the range shifter. However, multispecies competition with both native and range-shifting species improved adult survival of our focal native species at higher temperatures, compared to interspecific competition with just the range shifter. For our focal range-shifting species (Ischnura elegans), larval growth rate was significantly reduced in multispecies treatments compared to intraspecific or two-species interspecific scenarios, yet adult range shifter survival showed no decrease in multispecies treatments. Furthermore, range shifter larvae displayed improved survival and growth in colder temperatures, compared to a lack of any temperature effects on adult survival. These results suggest that biotic resistance may alleviate the competitive impacts of range shifters on native communities by providing a life stage-dependent benefit to native species while simultaneously decreasing the fitness of range shifters. However, shifting temperatures can cause this interaction to swap between competition and facilitation, creating an environmentally dependent scenario that may benefit both range shifters and resident species, promoting the maintenance of diversity in high latitude communities.