Biodiversity, phenology and temporal niche differences between native- and novel exotic-dominated grasslands

Authors: WILSEY, BRIAN - Iowa State University; DANESHGAR, PEDRAM - Iowa State University; Polley, Herbert

Submitted to: Perspectives in Plant Ecology, Evolution and Systematics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2011
Publication Date: 9/1/2011
Citation: Wilsey, B.J., Daneshgar, P.P., Polley, H.W. 2011. Biodiversity, phenology and temporal niche differences between native- and novel exotic-dominated grasslands. Perspectives in Plant Ecology, Evolution and Systematics. 13:265-276.

Interpretive Summary: Many plant species have been introduced into areas outside their native ranges, often intentionally for use as ornamentals, food production, or forage for livestock, and have become common or even dominant components of plant communities. Grasslands dominated by non-native or exotic plants often contain relatively few plant species. The low diversity that is characteristic of exotic-dominated grasslands may result because species interactions are more antagonistic among exotic than native plants, such that exotic plants exclude other plant species more effectively than do native plants. Alternatively, diversity may be lower in exotic communities because factors that favor invasion by exotics, such as fertilization and frequent disturbances, also reduce species diversity. We measured diversity in exotic and native grasslands in central Texas and studied changes in species diversity in experimental mixtures of native and exotic species that were grown in field plots. Our objective was to determine whether interactions among plant species themselves contribute to the lower diversity often observed in exotic plant communities. Species diversity was consistently lower in Texas grasslands that were dominated by exotic than native plants. Results from the experimental study demonstrated that the lower diversity of exotic communities results partly from interactions among species. Exotic species strongly inhibited the growth of other exotic species, whereas native species had a smaller impact on neighboring plants. This trend was evident in plots exposed to ambient rainfall and those in which rainfall was supplemented by irrigation during the normally dry summer months. These results imply that species diversity is lower in grassland dominated by exotic than native plant species partly because species interactions are more antagonistic among exotic than native plants.

Technical Abstract: Exotic species are significant components of many ecosystems, and have become the dominant species in some situations. Many exotic plant species have been introduced or escaped into grasslands where they form low diversity ‘novel ecosystems’ of species with no evolutionary history of interaction. However, it is unknown whether exotic species are ‘passengers’ with diversity changes or ‘drivers’ of changes. We compared dynamics of species diversity in planted 9-species mixtures of all native or all exotic grassland vegetation. Exotics originated from several continents. Species diversity was lower in communities of exotic than native species, and the reduction in diversity among exotics was driven by interactions among species themselves. Reduced diversity in exotic communities resulted from lower complementarity and higher temporal niche overlap, occurred in plots that were both irrigated during summer and not irrigated, and was accompanied by early green-up during spring. These results indicate that exotic species may be ‘drivers’ of the lower diversity often observed in grasslands they dominate. Earlier green-up by exotics may complicate attempts to ascertain relationships between phenology and climate if the abundance of exotics increases within the time-frame in question. An increase in exotic species may cause earlier green-up irregardless of any climate change effects. Grasslands increasingly are conserved for biodiversity maintenance. Human selection for rapid and synchronous growth may increase niche overlap among exotic species and reduce local diversity in exotic-dominated communities. These costs and benefits should be considered in future management and restoration projects.