The invasion of forests by non-native insect pests and pathogens is usually portrayed as a straightforward process. Take an organism that’s new to an area, maybe add climate change, and — voila — outbreak. The full web of ecological relationships in which invasion takes place, and particularly the role of plant diversity, doesn’t get much attention.
Yet plant diversity — unlike climate change — is something that can easily be addressed at local scales. And according to a continent-scale analysis of the relationships between forest tree diversity and invasion, having more species can help protect forests.
“Our work emphasizes the critical importance of conserving native biodiversity in reducing the damages from pest invasions,” said Qinfeng Guo, an ecologist with the U.S. Forest Service and lead author of the study, in a press release accompanying its publication. The study appeared in the journal Proceedings of the National Academy of Sciences.
To be sure, discourses around “pests” and “invasions” can be slippery. A pest is simply a name for something people don’t like, and what’s branded as an invasion — such as outbreaks of spruce budworm in northern North America — may be normal, even life-promoting parts of long-term ecological cycles that just happen to be economically problematic.
But these invasions appear to be happening with historically unusual frequency, particularly those involving organisms that people have carried outside their historical range. Rather than providing a healthy dose of disturbance, they could diminish the long-term future of some forests. Understanding the dynamics of invasion is essential.
Unlike most such studies, say Guo and colleagues, which involve just a few species, theirs spanned 66 non-native invasive pests — 51 insects and 15 pathogens — in forests across the continental United States. The resulting relationship between tree diversity and pest diversity followed a hump-shaped curve: forests with just a few tee species had just a few species of pests, and pest richness increased in tandem with tree richness until hitting an inflection point around 35 tree species. After that, a greater variety of trees meant a lesser variety of pests, and forests with the most tree species had the fewest pests of all.
Guo’s team thinks that a so-called dilution effect occurs around that midpoint. When there are many different kinds of trees, they tend to be spread out; it’s harder for introduced organisms to become established and proliferate. Tree diversity even proved to be more important to pest proliferation than rainfall, temperature, elevation, or human population density.
The findings might help people better manage forests to resist invasion, says Guo. Homogeneous forests are not an answer: they might have low pest diversity, but they’re highly prone to catastrophic outbreaks. Rather, forest managers and concerned communities could consider in greater detail the variety of trees under their care and their role as potential pest hosts, and cultivate pest-reducing forest assemblages.
There’s also “a major knowledge gap,” Guo says, about the roles played by pest-eating insects and animals, and whether they too could be harnessed. Those studies remain to be conducted. In the meantime, the new findings underscore that invasion isn’t just about new species in environments unprepared for their depredations. It’s a function of community.
Source: Guo et al. “Tree diversity regulates forest pest invasion.” Proceedings of the National Academy of Sciences, 2019.
Image: Chris Waits / Flickr
About the author: Brandon Keim is a freelance journalist specializing in animals, nature and science, and the author of The Eye of the Sandpiper: Stories From the Living World. Connect with him on Twitter, Instagram and Facebook.
