HOW ENVIRONMENTAL AND DISTURBANCE FACTORS INFLUENCE SPECIES COMPOSITION IN TEMPERATE AUSTRALIAN GRASSLANDS

A distinctive feature of Australian vegetational history is the abrupt ness of change since European settlement, involving the influx of exot ic species and the imposition of exogenous disturbances which are nove l in both intensity and character. This can produce two sources of hab itat variability: the natural patterns arising from environmental vari ation, as well as an overlying effect of disturbance. The relative imp ortance of these two types of variables were compared in temperate her baceous vegetation. Canonical Correspondence Analysis showed that envi ronment and disturbance had similar contributions to floristic variabi lity. Individually, lithology, altitude and soil disturbance were the strongest variables while slope position, grazing and water enrichment were slightly less important. Despite generally low levels of site sp ecificity, groups of species associated with lithology, slope position , altitude and different disturbance regimes were identified. Exotic s pecies were associated with higher levels of disturbance, but showed l evels of environmental specialization similar to the native component. Through combination of this analysis with a previous analysis of spec ies richness for the same data set, it became evident that environment al variation mostly resulted in species substitutions while disturbanc es led to losses of species, with partial replacement by exotics. Synt hesizing these results, we identified three broad groups in relation t o tolerance of levels of exogenous disturbance: (1) intolerant species -native taxa intolerant of severe disturbances and constituting the sp ecies-rich component of the vegetation; (2) tolerant species - exotic and native taxa occurring at both disturbed and undisturbed habitats a nd (3) disturbance specialists - predominantly exotic species, correla ted with high levels of disturbance.