Indirect competition causes widespread displacement of one naturalized parasitoid of imported fire ants (Diptera: Phoridae: Pseudacteon) by another

Species abundances in natural systems are usually close to some equilibrium making mechanisms that maintain or prevent species co-existence difficult to discern. Biological control projects provide an opportunity to observe systems transition between equilibriums as a result of the influence of the newly introduced species. In the southeastern United States and Texas, species of phorid fly parasitoids are being sequentially introduced as biological control agents for imported fire ants. The first two species introduced, Pseudacteon tricuspis and P. curvatus, partition the host niche based upon body size, and co-exist broadly in their native range in Argentina, indicating they would form a co-existing and complementary suite of parasitoids in North America. This study examines the interaction between these parasitoids at multiple temporal and spatial scales. Surprisingly, data at all scales reveal that as P. curvatus establishes at a site it competitively displaces P. tricuspis. Experiments reveal the operation of a strong, trait-mediated, indirect effect allowing locally common species to preempt reproductive opportunities from rarer species. This is the first documentation of this type of indirect competition leading to population displacement in a two consumer, shared resource system. Finally, a re-examination of published data from their native range reveals that a previously overlooked negative relationship between the densities of these two species also exists there, suggesting that the same processes as those reported here also operate in South America. Despite obvious niche partitioning, the data indicate that dynamic processes allow for these species to co-exist in both their introduced and native ranges.