A broad-scale study of U.S. forest data suggests a significant - but not simple - relationship between the number of native tree species and the number of nonnative forest pests.
"Invasive insects and diseases pose both ecological and economic threats to our forest ecosystems," says Qinfeng Guo, a research ecologist with the USDA Forest Service Southern Research Station and lead author of an article about the research in Proceedings of the National Academy of Sciences.
One example is the nonnative emerald ash borer, a beetle that has killed hundreds of millions of ash trees across the eastern U.S. Ash trees are popular street and landscaping trees, and the cost to treat, remove, and replace those infected with the beetle is estimated at more than $10 billion.
The study did not find a simple, linear relationship between tree diversity and pest diversity.
"When tree diversity is relatively low, the number of pest species increases with increasing tree diversity. This supports the facilitation hypothesis: greater tree diversity means more ecological niches for pests to exploit," says Guo. "However, when tree diversity continues to increase, we saw a threshold where pest species begin to decrease. This is dilution: high tree diversity usually means fewer individuals of each host tree species in the forest. This can suppress invasion, as certain pest species may no longer have enough resources, either food or habitat, to support their populations."
The study suggests that facilitation and dilution are both occurring in a forest community, but the importance of each shifts with overall tree diversity.
"There's a long standing hypothesis in invasion biology that high biodiversity can help to resist biotic invasions," says Guo. "Past experiments are mostly limited to single species at fine scales. They have shown evidence of dilution or facilitation but rarely both. Our study expands this inquiry and examines 66 important invasive pests found in natural forest ecosystems across the conterminous U.S."
The study examined records of tree species collected across public and private lands by the USDA Forest Service Forest Inventory and Analysis Program (FIA). Scientists combined this dataset with the Alien Forest Pest Explorer, a web tool developed by Northern Research Station co-author Andrew Liebhold. Its database includes spatial records of nonnative forest pests. The researchers focused on 66 of the top nonnative invasive species, including 51 insects and 15 pathogens. Together, these datasets describe where and how many tree species and invasive pest species co-exist.
The researchers looked at other factors, including temperature, precipitation, elevation, and human population density, as a proxy for pest arrival. Although these factors also seem important, tree diversity remains a strong predictor of nonnative pest invasions.
"Diverse plant communities may contain non-hosts as well as species that are natural enemies for potential nonnatives. Both of these can help land managers prevent the establishment of invasive pests - make them work harder to find resources," says Guo.
"There is a lag between a pest's arrival, its impact, and our detection of it. We know which native tree species are hosts for major invasives in the potential pest species pool. We know that the major pathways for pest introduction to new areas include trade or travel that involves moving infested wood materials. One way that we can slow or stop the spread of nonnative pests is by taking early action in prevention and detection. Our work emphasizes the critical importance of conserving native biodiversity in reducing the damages from pest invasions," adds Guo.
The research team also includes Songlin Fei of Purdue University, Kevin Potter of North Carolina State University, Andrew Liebhold of USDA Forest Service, and Jun Wen of Duke University. They hope that their results will inform monitoring efforts and help prioritize areas at greater risk for future pest invasions.