Emery, S. E. 2005. Population and community approaches to understanding invasion in grasslands. Dissertation, Michigan State University, East Lansing, Michigan, USA.
Because of concern over the economic and ecological impacts of invasive species, ecologists are interested in determining the factors that regulate the establishment and spread of new species arriving in communities. My research focuses on understanding factors that regulate the invasibility of communities, as well as factors that regulate population growth o f invasive species, using a combination of field and mesocosm experiments in grassland plant communities of southwest Michigan.
In particular, I am interested in the role of dominant species in plant communities. In a field seed addition experiment at the W.K. Kellogg Biological Station, I investigated whether the identity and relative abundance o f dominant species could regulate invasibility. I measured invasibility of vegetation patches dominated by one of four different species by adding seeds of 19 species common to Midwest US grasslands. I found that identity, but not relative abundance, of these dominant species altered invasibility. I followed the field experiment with an outdoor mesocosm experiment where I was able to control initial soil conditions and species richness of communities, but could vary the identity and relative abundance of the dominant species. A seed addition quantified the invasibility of these constructed communities. I again found that the identity of dominants again had strong affects on invasion. Using a structural equation model, I showed that the abundance o f two species in particular had significant positive affects on community biomass and light reduction, which in turn had negative effects on invasibility in the first year of the experiment. Second year survival of invading seedlings was dependent on nitrogen availability, as well as biomass and light. There was some evidence that one dominant species in particular had additional negative effects on invasibility by some unknown mechanism.
I further explored the relationship between identity of dominant species and identity of successful invaders in both the field and mesocosm experiments by testing the hypothesis that successful invaders should be functionally dissimilar to the dominants. I used an indicator species analysis to test whether individual invader species were more closely associated with one dominant species over all others. Using broad functional groups o f C3 grasses, C4 grasses, and forbs, I found very little evidence that invaders are most successful in communities dominated by a species in a different functional group.
Additionally, I have used a prairie restoration experiment at the Fort Custer Training Center in southwest Michigan to examine factors that regulate population growth of established populations of an exotic species, Centaurea maculosa (spotted knapweed). I examined effects of timing and frequency of prescribed fire on growth rates of knapweed. I found that annual summer burning was most effective at reducing growth rates of C. maculosa.
Together, my research demonstrates the importance of considering multiple factors in attempts to understand species invasions. Disturbance, species identity, and community diversity can all have important roles in regulating the establishment and spread of exotic species.