Effects of plant community phylogenetic diversity and site characteristics on the success of invasive species: Evidence from Minnesota’s deciduous forest

Background/Question/Methods

The uneven density of invasive plant species across the landscape suggests some communities are more receptive to invasion than others. Since post invasion control is difficult and expensive, understanding site characteristics that affect susceptibility might inform management decisions that reduce the level of invasion. Ecological theory predicts that increasing diversity decreases the likelihood of invasion. However, empirical studies provide no clear consensus on the role of diversity in reducing invasion. Phylogenetic distances between invaders and recipient communities are also predicted to affect invasion. When alien species are closely related to native species they may possess pre-adaptations allowing them to be successful invaders. Alternatively, alien species that are distantly related to natives might be able to exploit unfilled ecological niches.

We investigated how native plant phylogenetic diversity, phylogenetic distance between invaders and natives, and abiotic site characteristics affected the abundance of European buckthorn (Rhamnus cathartica). We surveyed 41 oak forests in Minnesota and estimated a community phylogeny of 276 species to ask whether (1) species richness and phylogenetic diversity of resident species affected the abundance of buckthorn, (2) the phylogenetic distance between buckthorn and members of the resident community affected its abundance, and (3) what site characteristics explained overall abundance of buckthorn. 

Results/Conclusions

Stands with high woody plant richness harbored high levels of buckthorn suggesting these sites are generally conducive to woody species. By contrast, stands with high phylogenetic diversity harbored low levels of buckthorn indicating that high phylogenetic diversity may limit niche space for invaders. Furthermore, buckthorn was most abundant when it was more closely related to species in the resident community suggesting buckthorn possess pre-adaptations to local environmental conditions (assuming some degree of niche conservatism). Multiple regression models incorporating abiotic site characteristics indicated the most important predictor of buckthorn abundance was its propagule pressure, followed by soil pH, and community phylogenetic diversity. Even though phylogenetic diversity is a significant predictor of buckthorn abundance, buckthorn propagule pressure appears to trump other factors determining the level of buckthorn invasion.

These results suggest successful control of buckthorn should begin with an assessment of propagule availability in the surrounding area. A collaborative effort with neighboring landowners to reduce the abundance of mature buckthorn specimens along roadsides and in residential areas is the first step in controlling buckthorn invasion at the stand scale. Failing to account for propagule pressure will likely nullify efforts to reduce buckthorn invasion even when management strategies maximize native species diversity.