Intense selection on isolated populations can cause loss of genetic diversity, which if persistent, reduces adaptive potential and increases extinction probability. Phenotypic evidence of inherited tolerance suggests that polychlorinated biphenyls (PCBs), have acted as strong selective agents on populations of a non-migratory fish, Fundulus heteroclitus, indigenous to heavily contaminated sites. To evaluate population genetic structure and test for effects of intense, multi-generational PCB contamination on genetic diversity, we used AFLP analysis on fish collected from six sites along the east coast of North America that varied widely in PCB contamination. The sites included a heavily contaminated urban harbor (New Bedford, MA), an adjacent moderately contaminated sub-estuary (Buzzards Bay, MA), and an uncontaminated estuary 60 km away (Narragansett, RI). AFLP markers distinguished populations at moderate and small scales, suggesting genetic differentiation at distances of 2 km or less. Genetic diversity did not differ across the study sites. Genome-wide diversity may have been preserved because of large effective population sizes and/or because the mechanism for genetic adaptation to these contaminants affected only a small number of loci. Alternatively, loss in diversity may have been restored with moderate levels of migration and relatively short generation time for this species.

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Invasive species generally occur and thrive in human-disturbed ecosystems, but Brachyponera chinensis (Asian needle ant, formerly 'Pachycondyla chinensis') also invades intact forests. The invasion into native habitats potentially puts B. chinensis in direct competition with the keystone seed-dispersing ants in the genus Aphaenogaster. We observed B. chinensis colonizing artificial nests placed in deciduous forest of the north Georgia Piedmont (US). Their presence appeared to displace existing Aphaenogaster rudis and Reticulitermes flavipes (subterranean termite) colonies. We subsequently mapped the B. chinensis invasion as well as co-existing A. rudis and R. flavipes colonies by examining coarse woody material (CWM) for nesting colonies. We tested whether the B. chinensis invasion changed with forest microclimates, covaried with A. rudis and/or R. flavipes occurrence, and whether it was associated with failed dispersal of a dominant understory herb. Our results and observations suggest that B. chinensis shares ecological niche requirements (temperature, moisture and CWM as nesting habitat) with A. rudis, severely diminishing the abundance of this native ant. In supplanting A. rudis, B. chinensis appears to play an equivalent role to A. rudis as a termite predator, but fails as a seed disperser. Essentially, the invader substitutes for the negative but not the positive species interactions, thereby apparently shifting ecological dynamics in the invaded system.