Grasses (Poaceae) are one of the world’s most ecologically and economically important plant families. Invasive introduced grass species are major drivers of change in many ecosystems around the world, and hundreds more grass species have naturalized in their introduced habitats with relatively minor impacts. Here, we consider the global patterns of establishment of exotic grass species and how they are influenced by climate, long-term climate stability, human influence, fire and the phylogenetic composition of the native grass assemblage. Finally, we ask whether particular species traits are associated with especially strong responses to any of these factors, for example testing the hypothesis that grasses that utilize the C4 photosynthetic pathway are less constrained to particular climates. To address these questions, we combined data on global native and exotic distributions of grass species with functional trait and phylogenetic data.
Globally, the strongest predictor of grass species’ introduced distributions is climate distance to the native distribution. Species rarely establish in climates that are very dissimilar from that experienced in their native range. Secondarily, species are less likely to establish in regions with higher overall human influence, higher GDP and higher fire frequency. However, there was substantial species-to-species variation in these overall responses, with some species showing patterns strongly opposed to these general tendencies. Species traits explained some of this variation. For example, C4 species are less strongly constrained by climate matching and more likely to be positively associated with high fire frequency. On the other hand, large-seeded species are particularly strongly constrained by climate matching. Long-term climate stability and the phylogenetic composition of the native grass assemblage had little effect on exotic distributions in most cases. This approach provides valuable insight into the factors that determine large-scale patterns of introduced grass establishment, and should be applicable to other groups as well.