Some plant and animal communities appear to resist invasions of introduced species; however, the intrinsic susceptibility of an area to invasion, otherwise known as invasibility, has been shown to differ between biomes. It is unclear whether the abiotic properties of the ecosystem, biotic properties of the ecosystem, number of individual invaders, or some combination of these factors determine whether a community will be invaded or not.

“ My research goals are to continue to investigate questions relating to habitat invasibility and impacts of non-indigenous species, and to relate these studies to ecological restoration and reserve design. ”

There has been a disproportionate emphasis on "biotic resistance" (ways in which the resident species repel invaders) in invasion resistance studies. This was the first experiment to test three hypothesized components of invasibility simultaneously: biotic and abiotic properties of the recipient ecosystem, and propagule pressure, which is the number of invaders introduced into the system.

The discovery that propagule pressure overwhelmed both the abiotic and biotic resistance properties of the ecosystem to invasion was novel and the way that we tested it was an original experimental approach. Our nested experimental design crossed propagule pressure of invaders with abiotic and biotic resistance factors. This was done by manipulating the richness of plots within a natural forested habitat that experienced different flooding regimes. We then arranged for these plots to be invaded with two propagule pressure treatments: a one-time and two-time invasion of the plots with ten adults and/or seedlings (<1/2 m) of individual invader species randomly chosen from widespread native and introduced species commonly found throughout the floodplains of our study site.

This is the first experimental study to demonstrate the dominance of propagule pressure (the number of invaders introduced into the system) in determining habitat invasibility in comparison with other candidate controlling factors. Compared to propagule pressure, the physical environment (flood regime) and the number of established resident species had negligible impact on net native and nonnative invasion success, despite manipulations that forced a significant reduction in resident richness, and a flooding regime that varied drastically among plots.

An enormous amount of literature states that biotic properties of ecosystems are the most important controlling factors for biological invasion of natural habitats—the idea was elevated to the status of a dogma. However, there was little direct evidence that this was so and very little consideration of other possible factors. We thought it was time to take a step back and consider multiple hypotheses for the observations that some communities and ecosystems are invaded more than others.

The field work was a major challenge. We conducted an extensive search across the southeastern United States in order to find an appropriate site so that these three hypotheses could be tested simultaneously. Additionally, the plot richness treatment involved transplanting small plants into plots. In order to find these transplants, we had to establish many transects through rough terrain. After the plots received the transplants, they were watered for two weeks, which involved carrying heavy watering cans through the forest from the river to the plots. The research team and I spilled a lot of water, bumped into many trees, and got a lot of scratches and cuts during those four field seasons!

My research goals are to continue to investigate questions relating to habitat invasibility and impacts of non-indigenous species, and to relate these studies to ecological restoration and reserve design.

The policy implications of this research are clear: any reduction in the rate at which numbers of nonindigenous species arrive at sites is likely to reduce greatly the probability that invasions will succeed.