Plant diversity and ecosystem process rates in native vs. exotic
communities. Funded by the U.S. National Science Foundation
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 increased precipitation in
the Southern Plains associated with global warming (US Global Change Research
Program 2000, National Assessment Synthesis Team). These increases are
expected to be largest in summer due to an
increased frequency of tropical storms off the Gulf of Mexico (Allan and Soden
2008). In the southern Plains, these increases in precipitation are expected
to lead to increased soil moisture (i.e. the precipitation effects will not be
cancelled out by increased temperature induced drying).
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.
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, and summer precipitation that mimics increased
tropical storm activity predicted by global climate change models. 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
Plots in second year of study.