Biodiversity and growth asynchrony differences between native and novel exotic-dominated grasslands are associated with aboveground or belowground niche differences among species
Modeling with Lotka-Volterra equations suggests that species diversity can be reduced when there are large differences among species’ carrying capacities and when competition is asymmetrical. This reduction in diversity can be countered by equalizing mechanisms, or factors that reduce fitness differences among species, and stabilizing mechanisms, or factors that reduce interspecific relative to intraspecific competition. Stabilizing mechanisms can result in asynchronous growth among species over time. We tested whether growth differences or niche overlap among species: 1) maintain diversity and asynchronous growth, 2) are altered as exotic species and communities replace native ones, and 3) are effected by summer precipitation. A common garden experiment was established in Texas in 2008 using 36 widely-distributed native or exotic grassland species. Equal-sized transplants were planted into 208 1 x 1 m plots in monoculture or nine-species mixture of all exotic or all native species. Exotic-native treatments were crossed with summer irrigation treatments (128 or 0 mm). Plots have been sampled annually, including an unprecedented drought year in 2011. Temporal overlap was determined by estimating biomass non-destructively early and late each growing season. Spatial overlap was determined for canopies with point intercept estimates at 10-cm height increments, and for roots at 10-cm depth increments.
Peak biomass was only 8% of pre-drought values in 2011, but biomass drops were similar between native and exotic communities and between irrigated and non-irrigated plots. Species diversity and richness declined during the drought at a greater rate in exotic than native communities. Biodiversity maintenance was strongly related to variables associated with both equalizing and stabilizing mechanisms. A model that included niche overlap measures and a variable that quantified inherent species growth differences (CV of biomass across monocultures, CVmono) accounted for 56% of the variation in eventual species richness and 36% of the variation in growth asynchrony. Significant predictors, in order of importance, were temporal overlap, CVmono, and rooting depth overlap for species richness, and CVmono and canopy overlap for asynchrony. This represents one of the first attempts to test specific aspects of niche partitioning that underlie maintenance of species richness and growth asynchrony, and our results suggest that both equalizing and stabilizing mechanisms are operating. This maintenance is likely to be diminished as native communities are replaced by exotic ones.