Plant diversity and ecosystem process rates in native vs. exotic communities

The homogenization of the earth's biota is affecting nearly every region of the earth. This homogenization is expected to increase due to increased movement of people and goods between regions. Grasslands, which cover roughly 25% of the planet, contain perhaps the most homogenized communities. Many exotic grassland species have spread to become common or even the dominant species in many grasslands. We are currently testing whether 1) species diversity is lower in exotic communities compared to comparable native communities, and 2) whether any reduction in diversity is caused by changes in species interactions in mixture. Specifically, we are testing whether exotic communities have reduced stabilizing or equalizing effects (Chesson 2000) compared to native communities. Exotic communities are predicted to have a higher amount of the selection effect and a lower amount of complementary resource use than native communities. These hypotheses are being tested with 9-species mixtures of paired exotic and native communities. Native and exotic species are paired based on similar ecological growth forms and phylogeny, and all species have are being compared between mixtures and their corresponding monocultures.