Mechanisms altering exotic-native proportions in plant communities, and impacts on biodiversity and ecosystems

Understanding the causes and consequences of variation in species composition and diversity across space and time are basic questions in ecology that have strong implications for conservation and restoration. However, exotic-dominated ecosystems, in which species with potentially novel interactions occur, are altering the context in which we ask these questions. The work presented here tested for the first time whether 1) experimental community assembly history manipulations of the seeds of native species could alter species composition, diversity, and ecosystem measures in grasslands dominated by exotic species, and 2) whether exotic-dominated grasslands differ from native-dominated grasslands for multiple aspects of diversity and ecosystems across a latitudinal gradient and over a growing season. Differences in community assembly history resulted in either a more diverse, native state when native seeds were added in spring and without priority effects, or a low-diversity, exotic-dominated state when seeds were added in summer or with priority effects. The exotic-dominated state persisted for eight years, even with an experimental seed addition of native species, and the differences resulted in altered productivity and fire temperatures. Across the latitudinal gradient, exotic-dominated grasslands had lower levels of species diversity, but had higher levels of tissue N concentrations. Productivity in exotic grasslands was higher in the south but it was lower in the north, where biomass peaked earlier in the growing season than in native grasslands. Exotic-dominated grasslands were more strongly dominated by C 4 species in the south and C3 species in the north, with a strong shift at 34-36 degrees, implying that an important measure of functional diversity was altered. Experimental results from a common garden experiment indicated that the difference in functional diversity was at least partially caused by species themselves. Pollinator communities did not differ between exotic and native grasslands, implying that other factors may be important in structuring pollinator communities. Overall, results from this work imply that novel ecosystems should be considered when testing ecological theory in the field and when considering impacts of species on ecosystems. Managing grasslands and restorations for high levels of species diversity will require understanding the impacts of multiple exotic species.