A primary goal of grassland restoration ecology is to understand the factors that interact to generate variability in biological diversity and ecosystem services. We tested whether plant species diversity and milkweed abundance (host plant for monarch butterflies) in tallgrass prairie restorations is influenced by several aspects of establishment and management practices, including seeding methods, seed mix richness, disturbance from fire and mowing, soil characteristics (pH, bulk density, organic matter), and size, shape and age of the planting. Age of planting is predicted to be positively correlated with diversity, but this has been rarely tested. We sampled plant communities and soil at 100 restorations distributed throughout the state of Iowa. Five remnant sites were sampled as a reference. We also conducted stem counts of milkweed (Asclepias sp.) species and tested which site characteristics best predict milkweed abundance. Multiple regression and Structural Equation models were used to compare predictors.
Results/Conclusions
The strongest predictor of total plant species diversity was the degree of invasion, as measured by the proportion of biomass from exotic species. A structural equation model that accounted for 48% of variation in diversity and 47% of variation in exotic biomass, indicated that sites with higher organic matter and a more linear shape had an indirect positive effect on exotic biomass, which in turn decreased diversity. Fractal dimension also had a weak direct effect, positively related to diversity. Diversity was unrelated to age of planting. Significant predictors of milkweed stem abundance were soil pH, bulk density, and site shape, with the stems being most common in sites with higher pH, less compacted soil, and more linear habitat. Milkweeds were more abundant in roadsides than conservation plantings (p<0.01), suggesting that roadsides may be a better habitat for monarch host plants. Our results indicate that preventing the establishment of exotic species is essential for maximizing species diversity in tallgrass prairie restorations.