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Forests with higher tree diversity also have higher pest diversity

A study focused on how non-native insects and diseases invade U.S. forests has identified strong relationships between the diversity of native tree species and the number of invasive pests, which threaten the nation’s forests both economically and ecologically.

Study co-author Kevin Potter is a professor in the Department of Forestry and Environmental Sciences at North Carolina State University.

“Every few years we get a new exotic insect or disease that comes in and is able to do a number on our native forests,” said Professor Potter.

“Emerald ash borer is clobbering a number of ash species in the Midwest and increasingly in the South. The chestnut, a magnificent tree that had immense ecosystem value as well as economic value in the South and North, is pretty much gone because of a pathogen. And hemlocks are under attack by the hemlock woolly adelgid from the Northeast along the Appalachian Mountains into the South.”

The researchers tested two conflicting ideas about biodiversity. One theory is that having more tree species can facilitate the diversity of pests by providing more places for them to become established. Another idea is that tree biodiversity can have protective effects for forests by diluting the concentration of host trees and making it harder for pests to flourish.

“We found that both facilitation and dilution seem to be happening at the same time,” said Professor Potter. “What we found is that native tree biodiversity really is important, but it’s important in different ways at different times.”

The study revealed that relationships between tree diversity and pest diversity followed a hump-shaped curve.

“As you have an increasing number of tree species, you have an increasing number of pest species, up to an inflection point where that relationship changes,” explained Professor Potter. “Then you have a decreasing number of pest species as the number of host tree species increases.”

According to the researchers, forests that have 30 to 40 different host tree species tend to have the most non-native pests. The extent of this effect depends on whether the invader is a specialist that can infest only a single tree species or a generalist that can spread to many different hosts.

“What we see is that forests in the Midwest and up into New England are at the middle part of that hump-shaped curve in terms of the number of host tree species, and those are places where there have been a lot of insect and disease problems,” said Professor Potter.

“Out West we have fewer insect and disease pests, but in some cases they still do a lot of damage because the forests are not diverse. If you have a specialist pest come in and knock back one of the major components of your biodiversity, then that can have a greater impact. An example of how that works would be Sudden Oak Death, a disease in California that’s affecting oaks there.”

Professor Potter said that tree biodiversity was a better predictor of non-native pests compared to other factors such as human population density and environmental conditions. “The unfortunate reality is that a lot of times we don’t notice these exotic pests and diseases until they’ve gotten established and start having an impact on our native species, when it’s almost too late.”

The study is published in the journal Proceedings of the National Academy of Sciences.

By Chrissy Sexton, Staff Writer

Image Credit: Kevin M. Potter, North Carolina State University

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