Cedar Creek LTER

Long-term Biodiversity Experiments Reveal Insights to Ecosystem Function

Our data on primary productivity and plant species abundances, collected annually since 1982, demonstrates that grasslands with greater plant biodiversity were both more stable when faced with a major drought and had less year-to-year variability in their productivity in response to normal variation in climate.

These findings inspired research on the potential effects of biodiversity on population, community and ecosystem processes. These are now the oldest and largest scale experimental studies of the population, community and ecosystem effects of diversity in existence, and have shown that greater plant biodiversity leads to greater community biomass, to lower incidence of plant disease, to lower rates of invasion by other plant species, and to greater insect diversity. These results and results of other studies have generated considerable debate. Our ongoing experiments are allowing us to test many alternative hypotheses.

In recent analyses, we have found that plant diversity and niche complement-arity have had progressively stronger effects on ecosystem functioning . More than half of the higher diversity plots outperformed even the single best monoculture plot. These results help resolve debate over biodiversity and ecosystem functioning, show effects at higher than expected diversity levels, and demonstrate, for these ecosystems, that even the best-chosen monocultures cannot achieve greater productivity or carbon stores than higher-diversity sites. In both our biodiversity experiment and in a study in native oak savannah we found that higher local plant diversity led tolower rates of invasion by exotic, non-native plant species. Analyses suggest that plant diversity had this effect because higher plant diversity leads to lower levels of limiting resources, and it was these lower resource levels that caused reduced invasion rates. We have also found that plant diversity has effects that ramify up the food web, influencing the diversity of herbivorous, predatory and parasitic arthropods.

Identifying the long-term effects of pollution on the environment

Long-term nitrogen addition experiments have shown that high diversity ecosystems dominated by native warm season grasses shifted to low diversity mixtures dominated by cool-season grasses within 15 years at all but the lowest rates of N deposition. This shift was associated with decreased biomass carbon-to-nitrogen ratios (C:N), increased N mineralization, increased soil nitrate losses, and low C storage. Thus, grasslands with high N retention and C storage rates were the most vulnerable to species losses and major shifts in C and N cycling in response to elevated nitrogen deposition.

Another major experiment combines our interests in the effects of biodiversity and of nitrogen deposition with an interest in the direct (nonclimatic) effects of elevated atmospheric CO2. This major grassland field experiment, now in its fifth year, tests the hypothesis that plant diversity and composition influence the enhancement of biomass and carbon acquisition in ecosystems subjected to elevated atmospheric CO2 concentrations and nitrogen deposition. We found that the enhanced biomass accumulation in response to elevated levels of CO2 or nitrogen, or their combination, is less in species-poor than in species-rich assemblages.