Release from intraspecific competition promotes dominance of a non-native invader

Abstract

Species can coexist through equalizing (similar fitness abilities) and stabilizing (unique niche requirements) mechanisms—assuming that intraspecific competition imposes more limitation than interspecific competition. Non-native species often de-stabilize coexistence, suggesting that they bring either a fitness advantage or a distinct niche requirement. We tested whether greater fitness or unique niche requirements best explained a successful North American invasion by the European Myrmica rubra ant. North American invaded-range M. rubra aggressively sting and occur in enormous numbers (suggesting a fitness advantage), yet our study site has a history of anthropogenic disturbance that might favor M. rubra (suggesting a unique niche). We compared M. rubra to native ants, principally the dominant North American woodland ant Aphaenogaster picea, using physiological health (lipids and size), monthly bait station surveys and aggression assays to assess fitness abilities, and we used nest surveys and isotope analysis to assess niche characteristics. We confirmed the field observations with laboratory experiments that tested colony aggression (direct competition) and food retrieval (indirect competition). In both the observational and experimental investigations, we found little evidence of M. rubra interspecific competitive advantage (aggression or food retrieval) or niche differentiation. Instead, M. rubra violated the basic assumption of coexistence theory: intraspecific competition consistently was less than interspecific competition. Freed up from the costs and limitations of territorial competition, some non-native species may outcompete native species by not competing with themselves. This ‘friendly release’ from intraspecific competition provides an ecological mechanism for some successful invasions.