Predicting Richness of Native, Rare, and Exotic Plants in Response to Habitat and Disturbance Variables across a Variegated Landscape

Species richness of native, rare native, and exotic understorey plants was recorded at 120 sites in temperate grassy vegetation in New South Wales. Linear models were used to predict the effects of environment and disturbance on the richness of each of these groups. Total native species and rare native species showed similar responses, with richness declining on sites of increasing natural fertility of parent material as well as declining under conditions of water enrichment (resulting from human-induced changes in drainage characteristics, leading to increased run-off), severe livestock grazing, and soil disturbance. The response of rare native species to water enrichment, however, was significantly greater than that of all native species. Exotic species richness varied in reverse to that of native species with positive responses to water enrichment and soil disturbance. The contrasting behaviors are attributed to differences in the evolutionary history of native and exotic assemblages and their resulting preadaptations to a landscape recently subjected to agricultural settlement. It would appear that for exogenous disturbances, the intermediate disturbance hypothesis is not supported by our data. In the sampled region, pastures represent the major land-use in terms of area, but have relatively low densities of native and rare species compared with more lightly grazed areas. However, their management is considered to be essential to the maintenance of diversity on a regional scale.