Rapid biodiversity declines in both ungrazed and intensely grazed exotic grasslands
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Exotic-dominated ecosystems with low diversity are becoming increasingly common. It remains unclear, though, whether differences between native and exotic species (driver model), or changes in disturbances or resources (passenger model), allow exotics to become competitive dominants. In our field experiment, plant species origin (native or exotic), cattle grazing (ungrazed or intensely grazed once), and species composition treatments were fully crossed and randomly assigned to four-species mixtures and monocultures of grassland plants. We found that biodiversity declined more rapidly in exotic than in native species mixtures, regardless of our grazing disturbance treatment. Early declines in species evenness (i.e., increases in dominance) led to subsequent declines in species richness (i.e., local extinctions) in exotic mixtures. Specifically, Simpson’s diversity was 29% lower after 1 year, and species richness was 15% lower after 3 years, in exotic than in native mixtures. These rapid biodiversity declines in exotic mixtures were partly explained by decreased complementarity (i.e., niche partitioning and facilitation), presumably because exotic species lack the coevolutionary history that can lead to complementarity and coexistence in native communities. Thus, our results suggest that exotic species can drive biodiversity declines in the presence or absence of a grazing disturbance, partly because exotic species interactions differ from native species interactions. This implies that restoring plant biodiversity in grasslands may require removal of exotic species, in addition to disturbance management.
KeywordsLand use change Novel ecosystems Coexistence Complementarity Conservation
We thank Dan Isbell, Steve Goepferd, Leah Isbell, Kathryn Yurkonis, Chris Johnson, Robert Handler, Leanne Martin, Adam Heathcote, Wayne Roush, and numerous gardeners for help with field work. This project was supported by Grants from the US National Science Foundation (DEB-0639417 to BJW) and the Leopold Center for Sustainable Agriculture (to BJW and FII).
- D’Antonio CM, Vitousek PM (1992) Biological invasions by exotic grasses, the grass fire cycle, and global change. Annu Rev Ecol Syst 23:63–87Google Scholar
- Harper JL (1977) Population biology of plants. Academic Press, LondonGoogle Scholar
- Hobbs RJ, Arico S, Aronson J, Baron JS, Bridgewater P, Cramer VA, Epstein PR, Ewel JJ, Klink CA, Lugo AE, Norton D, Ojima D, Richardson DM, Sanderson EW, Valladares F, Vila M, Zamora R, Zobel M (2006) Novel ecosystems: theoretical and management aspects of the new ecological world order. Glob Ecol Biogeogr 15:1–7CrossRefGoogle Scholar
- Wilsey BJ (2005) Importance of species replication in understanding plant invasions into North American grasslands. In: Inderjit S (ed) Invasive plants: ecological and agricultural aspects. Birkhauser, BaselGoogle Scholar