Effects of land use and environment on alien and native macrophytes: lessons from a large-scale survey of Australian rivers

Aim We sampled riverine macrophyte communities and environmental conditions to compare drivers of alien and native abundance and to provide a general set of environmental correlates of invasion by aquatic macrophytes. Location Streams adjacent to three land-use types (intensive, agricultural and natural) across a large latitudinal gradient (approximately 27 degrees S-43 degrees S) in Australia. Sites were located near Brisbane (Queensland), Sydney (New South Wales), Canberra (Australian Capital Territory), Melbourne (Victoria) and Hobart (Tasmania). Methods Alien and native aquatic plant species cover, water quality, forest canopy and adjacent land use were measured in three catchment locations (low-, mid- and upper-catchment) in all cities. Mean richness and cover of native and alien macrophytes were compared in the five cities, three catchment locations, and three land-use types. Correlation tests examined relationships between alien and native richness at transect, site and city scales. Canonical correspondence analysis (CCA) determined the effects of environment on cover and richness of native and alien plant groups (emergents, floating, forbs/other, graminoids and submerged). Results Variation existed in the aquatic plant community at all scales, but strong patterns emerged with respect to land use and environmental gradients. Alien abundance was more responsive to anthropogenic disturbance (e.g. greater in intensive and agricultural land-use types, and greater where dissolved nutrients and conductivity were high) than natives, which were unaffected by land-use type and less responsive overall to environmental gradients. Native and alien richness were uncorrelated at all scales. Main conclusions Natives and aliens of the same life form did not respond similarly to the environment, suggesting inherent differences in their ability to capitalize on anthropogenic disturbance. Our results suggest invasion-susceptible habitats are those that receive nutrient pollutants and that occur in urban and agricultural areas low in the catchment. Our confidence in these patterns is strengthened by their consistency across a large latitudinal gradient.