Using data from 130,210 forest plots across the US, this study examines the effects of tree diversity on pest invasions. The authors found that tree diversity increases pest diversity by increasing the variety of host species available (i.e., facilitation), while also decreasing establishment of pests by increasing the number of non-hosts for any given pest species relative to the total number of trees (i.e., dilution). In other words:
The relative proportion of component tree species (hosts vs. nonhosts) plays a key role in determining pest invasions, as indicated by our evidence that host diversity may promote pest diversity while neighboring nonhost species could enhance the associational resistance of host species to nonnative pest invasions [Guo 2019].
More specifically, the study observed a hump-shaped relationship between tree and pest diversity.
Pest diversity increases with tree diversity at low tree diversity (because of facilitation or amplification) and is reduced at higher tree diversity (as a result of dilution). Thus, tree diversity likely regulates forest pest invasion through both facilitation and dilution that operate simultaneously, but their relative strengths vary with overall diversity [Guo 2019].
Other factors that influence pest invasions in forest ecosystems include: “climate, resource availability, spatial scale, and habitat fragmentation related to human disturbances.” Furthermore, “recent analyses indicate that pest species continue to be introduced and spread around the globe. Under climate and land use changes, many tree species could expand, contract, or undergo latitudinal/elevational shifts in their geographical ranges” [Guo 2019].
These findings underscore the importance of biodiversity in maintaining healthy and stable ecosystems, while also highlighting the complexity of ecosystems (given the non-linear relationship between tree and pest diversity) and the challenges that poses for restoration.
Guo, Qinfeng, et al., 2019, Tree diversity regulates forest pest invasion, PNAS 116:15, https://www.pnas.org/content/116/15/7382.short.