Kyle Hemming, Richard Duncan and Elizabeth Wandrag
Institute for Applied Ecology, Faculty of Science and Technology, University of Canberra, Australia
Determining the drivers of species’ distributions is central to predicting the spread potential of exotic species. Most approaches rely on species distribution models, which estimate the potential distribution of an exotic species in a new region based on its distribution elsewhere. We use a new approach to determine the potential distribution of exotic grasses (family Poaceae) in Australia. We used gradients in native grass species richness (estimated using herbarium data) as a template for the potential distribution of exotic grasses across the continent. We identified the environmental variables that best explained native richness gradients and determined whether these also explained exotic richness gradients. If native and exotic grass species respond to environmental variables in similar ways, we would expect regions with high native richness but low exotic richness to be vulnerable to the spread of exotic grasses.
Initially, native and exotic grass species richness gradients and their relationships to environmental variables were dissimilar, suggesting that the native template concept was not supported. Native grasses had higher richness in warmer northern and coastal regions, while exotic grasses had higher richness in cooler temperate regions characterized by high levels of human impact. These differences, however, could be largely explained by photosynthetic pathways. Both exotic and native C3 grasses responded similarly to the environmental variables, and both groups exhibited higher species richness in southern temperate regions. This match in distributions suggests that exotic C3 grasses do not have a high spread potential. Conversely, while exotic and native C4 grasses again responded similarly to environmental variables, there was a large mismatch in their species distributions. Relative to native C4 grasses, exotic C4 grasses were underrepresented across much of Central and North-western Australia, suggesting that there is considerable potential for exotic C4 grasses to colonise these regions where native C4 richness is high.