Density-dependent effects of an invasive wasp on the morphology of an endemic New Zealand ant

Invasive species can cause extinction, but perhaps more commonly they compete for resources, resulting in subtle life history modification in native species. Changes in food quantity and quality as a result of biological invasions have the potential to influence the size and morphology of native species. The invasive wasp Vespula vulgaris reaches its highest known population densities in the honeydew beech forests of New Zealand, where it competes with native ants (Prolasius advenus) for food. We tested the hypothesis that this competition would reduce ant worker size in areas of high wasp density. A significant reduction in worker size was found with increasing invasive wasp density, after accounting for site and environmental covariate effects. However, this significant average reduction in size was small. For example, an increase of 10 wasp nests ha−1 was associated with an average decrease of only 1.2 % in the Weber’s length of native ant workers. We also examined scaling of morphological relationships in ants to test the hypothesis that scaling would vary between ant populations at differing wasp densities. Scaling relationships for six out of eight measured morphological characters significantly differed in ants when subject to high wasp densities, relative to areas of low wasp abundance. These results indicate that invasive wasps can potentially reduce native ant size, and alter scaling relationships, likely by limiting resource availability during ant development. Such subtle effects of an invasive species may be common in situations where invasive species exert strong competitive pressure.