Community perturbations: Introduced species and habitat fragmentation

The effect of perturbations on three ecological communities was studied using comparative and experimental methods. Fragmentation of chaparral habitat and its effect on chaparral specialist bird and rodent species was studied in coastal San Diego County, USA. The distribution of five bird species in isolated habitat fragments of known age (time since isolation) was compared to that in similar sized plots in continuous chaparral habitat. Fragments supported fewer species than plots, implying that population extinctions occur following isolation. Furthermore, bird species are lost in a predictable sequence which is highly correlated with density. Species which occur at lower density go extinct first. An analogous methodology was applied to chaparral specialist rodents. The results were similar. Plots in continuous habitat supported more species than fragments of similar size, and species which occur at low density occurred in fewer fragments than species of higher density.; The effect of introducing a new species to a community was investigated in a system of urban-dwelling geckos on islands in the tropical Pacific. A biogeographic pattern was documented in which the introduction of Hemidactylus frenatus, a sexual species, to an island is correlated with the decline of two previously resident asexual species, Lepidodactylus lugubris and Hemidactylus garnotii. On a local scale the interaction between L. lugubris and H. frenatus was studied experimentally in Fiji. The introduction of H. frenatus to buildings from which its was previously absent resulted in a significant decline in the density, body weight, and fecundity of L. lugubris, relative to controls. These experiments confirm that the biogeographic pattern is the result of interspecific competition. Behavioral experiments revealed that male H. frenatus were behaviorally dominant to asexual H. garnotii and L. lugubris. H. frenatus was more likely to approach, bite, and spatially displace the asexual species than vice versa. This indicates that at least part of the competitive advantage enjoyed by H. frenatus is due to behavioral interference. The structure of chaparral bird and rodent communities in habitat fragments is most significantly affected by the autecological variable density, while interspecific interactions are the predominant structuring force in the gecko community.