Fundamental to the phenomenon of biodiversity are the mechanisms of species coexistence. Thus, aspects of my work focus on basic and applied questions regarding mechanisms that contribute to species invasions, and either promote species coexistence or conversely lead to alternative stable states.
In California grasslands, the replacement of native perennial grasses by Mediterranean annual grasses has been assumed to arise from the superior competitive abilities of the annuals. Yet this is a paradox. Annual and perennial life histories differ in important allocation tradeoffs that should make perennials competitively superior for below ground resources as has been demonstrated in other systems. Using a large-scale restoration experiment, we found that native perennials were indeed better competitors for limiting resources, as predicted, but were extremely recruitment limited. These results show that observed dominance of an invasive species is not sufficient evidence to infer competitive hierarchies, and that experiments that examine both niche and dispersal assembly mechanisms are required (Seabloom & Harpole et al. 2003, PNAS).
The long-term dominance of exotic annual grasses in California is very suggestive of the presence of alternative stable states. Although many lines of theory predict the existence of alternative stable states, empirical evidence is scarce. We have initiated large-scale, NSF and USDA funded projects in northern and southern California to test the roles of grazing, soil-microorganism-mediated positive-feedbacks, nitrogen deposition, and dispersal limitation as drivers of alternative vegetation states. Exotic plant species in California grasslands have been shown to alter soil microbial communities so that they foster conditions that promote their own growth while inhibiting the growth of native species. Alternatively, it has been suggested that dominance of exotic annual grasses in California grasslands is due to cattle grazing. We hypothesize that a combination of above- and below-ground trophic interactions results in threshold behavior and alternative vegetation states.