Invasion of exotic plants in nutrient-enriched urban bushland

Invasion of exotic plants into vegetation communities on low-nutrient soils in Sydney is often attributed to increased phosphorus in the soil. However, the composition of native vegetation in nearby national parks has been found to be more closely correlated to combinations of soil nutrients and other environmental variables than phosphorus alone. This study examined whether phosphorus or a range of variables better matched patterns of native and exotic plants in urban bushland. Sites in urban bushland and national parks were sampled, vegetation frequency recorded and soil samples collected. Soil samples were analysed for a range of chemical and physical attributes. A significantly greater number of exotic species were found at the urban sites, and significantly fewer native species than in the national parks. All measured soil nutrients were found to be of a significantly increased concentration in urban soil. Using regression analysis, an index of all measured soil nutrients was found to explain more of the variation in the percentage of exotic species at a site than phosphorus alone. Multivariate analysis showed a gradient of sites from minimal exotic invasion to heavy invasion. However, this gradient also corresponded to changes in native species. The gradient was found to match that of increasing soil nutrient levels. A combination of soil nutrients was found to correlate better with the multivariate species composition than was phosphorus alone. The results suggest that it is the increase of many soil nutrients, not phosphorus alone, that is contributing to the invasion of urban bushland by exotic plants and the alteration of the suite of native species.