Competition for food between the exotic wasp Vespula germanica and the native ant assemblage of NW Patagonia: evidence of biotic resistance?

Abstract

The success of a biological invasion may depend on the interactions between the invader and the native biota. However, little experimental evidence demonstrates whether local species can successfully compete with exotics. We experimentally determined the existence of competition for food between the exotic wasp Vespula germanica, one of the most recent Patagonian invaders, and the native ant assemblage. Both wasps and ants are generalist predators and scavengers, sharing habitat and food resources. We selected 30 sites within scrubland habitats in where both ants and wasps were present. At each site, we placed containers with protein baits under three treatments: wasp exclusion, ant exclusion and control (i. e., free access for wasps and ants). Ant exclusion increased the number of wasps (with regard to a control), but wasp exclusion did not affect ant abundance. This result suggests that native ants affect the foraging activity of exotic wasps but not vice versa. Aggressive behaviors and worker aggregation may explain the competitive advantage of ants. Ants bite wasp legs and massively aggregate on food sources, physically limiting the landing of wasps on baits. If the outcome of interactions at baits reported here influence wasp population-level parameters, this competitive interaction could be one of the factors explaining the low abundance of this exotic wasp in NW Patagonia in comparison to other invaded regions.