Biodiversity maintenance mechanisms differ between native and novel exotic-dominated communities

Authors: WILSEY, BRIAN - IOWA STATE UNIVERSITY; TEASCHNER, TERRI - IOWA STATE UNIVERSITY; DANESHGAR, PEDRAM - IOWA STATE UNIVERSITY; ISBELL, FOREST - IOWA STATE UNIVERSITY; Polley, Herbert

Submitted to: Ecology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2009
Publication Date: 4/9/2009
Citation: Wilsey, B.J., Teaschner, T.B., Daneshgar, P.P., Isbell, F.A., Polley, H.W. 2009. Biodiversity maintenance mechanisms differ between native and novel exotic-dominated communities. Ecology Letters. 12:432-442.

Interpretive Summary: Many native grasslands have been converted to communities of exotic or introduced plant species. Species typically are fewer (species richness is lower) and exhibit greater disparities in relative abundances (species evennness is lower) on exotic than native grasslands. It is not know, however, whether these components of plant diversity are inherently lower in exotic than native plant communities or result from greater soil fertility, more frequent disturbances, or other factors often associated with exotic invasions. In order to determine where exotic species themselves cause lower diversity, we measured changes in diversity over time in experimental plant communities planted to either all exotic or all native perennial plants common to the Blackland Prairie region of central Texas. Species diversity declined more rapidly in exotic than native plant communities. During the year following planting, relative abundances of species in exotic communities diverged rapidly, with some species becoming very abundant while other species became rare. By the second year, these changes resulted in greater species loss from exotic than native communities. Diversity declined more rapidly in exotic than native communities because species that were most productive when grown alone dominated other species in communities of exotics, whereas less- productive species were favored in communities of native species. Perhaps because of their long history of association, native species appear to better partition available plant resources and thereby to co-exit than do their exotic counterparts.

Technical Abstract: The ongoing homogenization of the Earth’s biota is affecting nearly every region of Earth. We experimentally compared species diversity decline between nine-species grassland communities containing either all exotic (i.e. introduced) or all native species under controlled field conditions. Exotic and native species were paired to control for growth form, mode of photosynthesis and phylogeny. Experimental plots were planted with equal-mass seedlings on a common soil type and species diversity, net primary productivity, and yielding behavior were measured during the first two growing seasons. Aboveground biomass was greater in exotics than natives, and this difference was much larger in mixtures than in monocultures. Species diversity declined more in exotic than native communities with exotics having 40% lower Simpson’s diversity than natives by year 2. Differences in diversity were associated with changes in evenness and not richness in year 1, and these changes in evenness eventually led to drops in richness in year 2. Diversity declines were influenced by different mechanisms in exotic and native communities. In exotic communities, the selection effect was positive (i.e. overyielding occurred in species with high biomass in monoculture), and diversity declined linearly as the selection effect increased. In native communities, however, the selection effect was negative (i.e. overyielding occurred in species with low biomass in monoculture) and there was no relationship with diversity decline. This suggests that native communities have stronger stabilizing mechanisms than comparable exotic communities, and that native-exotic status is important to understanding relationships between biodiversity and ecosystem functioning and other related questions.