Survivors or reinvaders? Intraspecific priority effect masks reinvasion potential

Invasions of alien species on islands cause serious deleterious effects on native species through predation and competition often to the point of extinction. Where eradication is not possible ongoing control programs are the only alternative. Following control efforts there are risks of both recolonisation from survivors and reinvasion from neighbouring populations. Successful pest control efforts at such sites depend heavily on two rules of eradication: (1) individuals have to be removed faster than the growth rate and (2) reinvasion must be close to zero. We used a small near-shore island as a 'microcosm' to test whether both these rules could be met. We applied a molecular genetic approach to assess genetic differentiation of a ship rat (Rattus rattus) population on a nearshore island with the adjacent mainland population and investigated metapopulation dynamics and pest control success. Tissue samples from Goat Island, New Zealand, from three consecutive years were genotyped at 14 microsatellite loci and compared with the mainland populations. We showed moderate genetic differentiation between the two populations despite their close proximity and demonstrated that rats were neither being removed faster than they bred nor was reinvasion able to be managed close to zero. Furthermore, population reduction on the island counter-productively facilitated establishment by invading rats. These results have important implications for interpreting the relative roles of recolonization versus reinvasion following pest control operations. To properly manage invasive species at such sites, control must have the intensity of eradication efforts, and reinvasion must be managed both pre departure and post arrival.