Ecological dominance by Paratrechina longicornis (Hymenoptera : Formicidae), an invasive tramp ant, in biosphere 2

Tramp ante are invading disturbed ecosystems worldwide, exterminating untol d numbers of native species. They have even invaded Biosphere 2, a 1.28-hec tare closed greenhouse structure built in the Arizona desert as a microcosm for studying ecological interactions and global change. Invertebrate surve ys within Biosphere 2 from 1990 to 1997 have revealed dramatic changes in f aunal composition, including an almost complete replacement of the ant faun a by a single tramp ant species. In 1990-91, surveys in Biosphere 2 found no one ant species dominant. By 19 93, populations of the crazy ant, Paratrechina longicornis (Latreille), a t ramp species not found in 1990-91, had increased to extremely high levels. In 1996, virtually all ants (>99.9%) coming to bait were P. longicornis. We observed P. longicornis foragers feeding almost exclusively on the sugary excretions (honeydew) produced by vast numbers of Homoptera, primarily scal e insects and mealybugs, found on many of the plants. High densities of ant s were associated with high densities of homopterans. In 1997, soil and lit ter surveys found that the only invertebrates thriving in Biosphere 2, besi des P. longicornis and homopterans, were either species with effective defe nses against ants (well-armored isopods and millipedes) or tiny subterranea n species that can escape ant predation (mites, thief ants, and springtails ). A convergent pattern of biodiversity occurs in disturbed tropical and su btropical ecosystems dominated by tramp ants.