Plant species‐richness and invasion by exotics in relation to disturbance of wetland communities on the Riverine Plain, NSW

Abstract

AbstractA combination of field surveys and seed‐bank measurements were used to describe the macrophytic vegetation of intermittent wetlands on the Riverine Plain, in New South Wales. Three habitat types were sampled, representing wetland vegetation subjected to varying intensities of exogenous (human‐induced) disturbance: swamps (least disturbed), roadside table drains (moderately disturbed), and rice crops (highly disturbed). Forty‐two plant species were common to all three habitats and 36 species occurred in two of the three habitats. Of the 82 species that were restricted to a single habitat type, 33 were recorded for swamps, 19 for roadsides and 30 for rice fields.Species‐richness (number of species per 50 m2) was found to be significantly lower in rice fields compared with either roadsides or swamps, which were not significantly different. Richness of native vascular species differed significantly among all habitat types, with swamps > roadsides > rice fields. Richness of exotic species also differed: roadsides > rice fields > swamps. Although the least disturbed habitat (swamps) supported the least number of exotic species, there was no further evidence of a positive relationship between degree of disturbance and degree of invasion. The results provided some support for the hypothesis that maximum species‐richness occurs under conditions of moderate disturbance.A small negative correlation (r=−0.178, P < 0.01) was found between the number of exotic species and the number of native species occurring at a site. This suggests that, in this case, species‐richness is not an important factor in the resistance of vegetation to invasion by exotic species. The vegetation of intermittent wetlands appears to have a relatively high degree of resistance to invasion. This is attributed to the high level of endogenous disturbance to which the native vegetation is adapted.