Biotic and abiotic controls of exotic species richness and abundance within and across old-field communities

Biological invasions threaten biodiversity by homogenizing communities and contributing to the local extinction of native species. Understanding biotic and abiotic controls on both exotic species richness and abundance are therefore critical for the maintenance of biodiversity. In this study, we surveyed 17 old fields in three sites in eastern Tennessee to investigate determinants of exotic richness and abundance at local and landscape scales. In each field we installed between ten and thirty 1m2 quadrats and measured plant species richness, foliar cover, and aboveground biomass, photon flux density (PPFD), and percent volumetric water content. We also collected one mineral soil sample from each quadrat to assess soil bulk density, gravimetric water content, pH, and potential N-mineralization and nitrification. Preliminary results indicate that the controls on biological invasions are scale dependent in old-field communities. At local scales, no single set of factors consistently predicted richness, cover, or biomass of exotic species within fields. At landscape scales, across fields, native species richness was positively related, and soil moisture negatively related, to exotic species richness. Cover and biomass of exotic species were negatively associated with native species richness and positively associated with soil moisture at landscape scales. Together native species richness and soil moisture explained 71% of the variation in exotic species richness and 49% of the variation in exotic species cover at landscape scales. We conclude that determinants of exotic species richness and abundance are idiosyncratic at local scales. At landscape scales, however, native species richness and soil moisture were important predictors of both exotic species richness and abundance.