How environmental and disturbance factors influence species composition in temperate Australian grasslands

Abstract. A distinctive feature of Australian vegetational history is the abruptness of change since European settlement, involving the influx of exotic species and the imposition of exogenous disturbances which are novel in both intensity and character. This can produce two sources of habitat variability: the natural patterns arising from environmental variation, as well as an overlying effect of disturbance. The relative importance of these two types of variables were compared in temperate herbaceous vegetation. Canonical Correspondence Analysis showed that environment and disturbance had similar contributions to floristic variability. 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 specificity, groups of species associated with lithology, slope position, altitude and different disturbance regimes were identified. Exotic species were associated with higher levels of disturbance, but showed levels of environmental specialization similar to the native component. Through combination of this analysis with a previous analysis of species richness for the same data set, it became evident that environmental variation mostly resulted in species substitutions while disturbances led to losses of species, with partial replacement by exotics. Synthesizing these results, we identified three broad groups in relation to tolerance of levels of exogenous disturbance: (1) intolerant species ‐ native taxa intolerant of severe disturbances and constituting the species ‐ rich component of the vegetation; (2) tolerant species ‐ exotic and native taxa occurring at both disturbed and undisturbed habitats and (3) disturbance specialists ‐ predominantly exotic species, correlated with high levels of disturbance.