PREDICTING RICHNESS OF NATIVE, RARE, AND EXOTIC PLANTS IN RESPONSE TOHABITAT AND DISTURBANCE VARIABLES ACROSS A VARIEGATED LANDSCAPE

Species richness o 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 a nd disturbance on the richness of each of these groups. Total native s pecies 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 inc reased run-off), severe livestock grazing, and soil disturbance. The r esponse of rare native species to water enrichment however, was signif icantly greater than that of all native species. Exotic species richne ss varied in reverse to that of native species with positive responses to water enrichment and soil disturbance, The contrasting behaviors a re 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 no t 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. Howev er, their management is considered to be essential to the maintenance of diversity on a regional scale.