Plant diversity and ecosystem process rates in native vs. exotic communities.NSF DEB0639417

The homogenization of the earth's biota is affecting nearly every region of the earth. This homogenization is expected to increase due to increased movement of people and goods between regions. Grasslands, which cover roughly 25% of the planet, contain perhaps the most homogenized communities. Many exotic grassland species have spread to become common or even the dominant species in many grasslands, and native grasslands are being replaced by exotic grasslands either by humans planting exotic species or by exotic species not giving way to natives over time. This has led to a patchwork of exotic and occasional native grasslands on modern landscapes. We are currently testing whether 1) species diversity is lower in exotic communities compared to comparable native communities, and 2) whether any reduction in diversity is caused by changes in species interactions in mixture and reductions in niche overlap. Specifically, we are testing whether exotic communities have reduced stabilizing or equalizing effects (Chesson 2000) compared to native communities. Exotic communities are predicted to have a higher amount of the selection effect and a lower amount of complementary resource use (Loreau and Hector 2001) than native communities, and results so far have supported this prediction. These hypotheses are being tested with 9-species mixtures of paired exotic and native communities. Native and exotic species are paired based on similar ecological growth forms and phylogeny, and all species have are being compared between mixtures and their corresponding monocultures (Wilsey et al. 2009). Results suggest that exotic communities develop lower diversity than do comparable native communities. In native communities, low biomass species overyield in mixture (negative selection) and this overyielding is largest when inherent differences in biomass among species is greatest, which stablizes diversity. However, in exotic communities, high biomass species overyield (positive selection) and this effect leads to greater biodiversity decline. This suggests that yielding behavior is fundamentally different between presumably co-evolved natives and coevolutionarily naive exotic species (Wilsey et al. 2009).

Global climate change models are predicting altered precipitation in the Southern Plains associated with global climate change (US Global Change Research Program 2000, National Assessment Synthesis Team). These changes are expected to be largest in summer due to an increased frequency of tropical storms off the Gulf of Mexico (Allan and Soden 2008). Changes in precipitation are expected to affect the mean and the variance, leading to larger rainfall events with longer gaps between events.

    change
    Figure 1. Predictions of precipitation change by the US Global Change Research Program 2000. Green indicates ~20% increases. The Canadian model does not include alterations to the hydrological cycle, whereas the Hadley model does.
    moisture
    Figure 2. Predictions of soil moisture in summer. The Canadian model does not include alterations to the hydrological cycle, whereas the Hadley model does.
In 2008, we started an experiment that varies native-exotic community status as described above, crossed with altered precipitation treatments (0 or 128 mm in summer). Summer precipition treatments are either no precipitation or 8 mm added twice per week from July 15-August 15 (128 mm total). This study is designed to test for interactions between global climate change and success of exotic vs. native species and mixtures (Wilsey et al. 2011, Isbell et al. 2011). We are currently sampling this experiment twice per year.

Wilsey, B.J., Teaschner, T.B., Daneshgar, P.P., Isbell, F.I. and H.W. Polley. 2009. Biodiversity maintenance mechanisms differ between native and novel exotic-dominated communities. Ecology Letters 12:432-442

Isbell, F., Calcagno, V., Hector, A., Connolly, J., Harpole, W.S., Reich, P.B., Scherer-Lorenzen, M., Schmid, B., Tilman, D., van Ruijven, J., Weigelt, A., Wilsey, B.J., Zavaleta, E.S. and M. Loreau. 2011. High plant diversity is needed to maintain ecosystem services. Nature 477:199-202

Brian J. Wilsey

Plots in second year of study.