PATTERNS OF INVASION OF AN URBAN REMNANT OF A SPECIES-RICH GRASSLAND IN SOUTHEASTERN AUSTRALIA BY NONNATIVE PLANT-SPECIES

The invasion by non-native plant species of an urban remnant of a spec ies-rich Themeda triandra grassland in southeastern Australia was quan tified and related to abiotic influences. Richness and cover of non-na tive species were highest at the edges of the remnant and declined to relatively uniform levels within the remnant. Native species richness and cover were lowest at the edge adjoining a roadside but then showed little relation to distance from edge. Roadside edge quadrats were fl oristically distinct from most other quadrats when ordinated by Detren ded Correspondence Analysis. Soil phosphorus was significantly higher at the roadside edge but did not vary within the remnant itself. All o ther abiotic factors measured (NH4, NO3, S, pH and % organic carbon) s howed little variation across the remnant. Non-native species richness and cover were strongly correlated with soil phosphorus levels. Nativ e species were negatively correlated with soil phosphorus levels. Cano nical Correspondence Analysis identified the perennial non-native gras ses of high biomass as species most dependent on high soil nutrient le vels. Such species may be resource-limited in undisturbed soils. Three classes of non-native plants have invaded this species-rich grassland . (1) generalist species (greater than or equal to 50 % frequency), mo stly therophytes with non-specialized habitat or germination requireme nts; (2) resource-limited species comprising perennial species of high biomass that are dependent on nutrient increases and/or soil disturba nces before they can invade the community and; (3) species of intermed iate frequency (1-30 %), of low to high biomass potential, that appear to have non-specialized habitat requirements but are currently limite d by seed dispersal, seedling establishment or the current site manage ment. Native species richness and cover are most negatively affected b y increases in non-native cover. Declines are largely evident once the non-native cover exceeds 40 %. Widespread, generalist non-native spec ies are numerous in intact sites and will have to be considered a perm anent part of the flora of remnant grasslands. Management must aim to minimize increases in cover of any non-native species or the disturban ces that favour the establishment of competitive nonnative grasses if the native grassland flora is to be conserved in small, fragmented rem nants.