How Tree Diversity Regulates Invading Forest Pests

A national-scale study of U.S. forests found strong relationships between the diversity of native tree species and the number of nonnative pests that pose economic and ecological threats to the nation’s forests.

“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,” says Kevin Potter, a North Carolina State University research associate professor in the Department of Forestry and Environmental Sciences and co-author of an article about the research in Proceedings of the National Academy of Sciences.

“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.”

To better understand how nonnative insects and diseases invade U.S.
forests, researchers tested conflicting ideas about biodiversity. The first is
that having more tree species can facilitate the diversity of pests by providing
more places for them to gain a toehold. Another possibility is that tree biodiversity
can have protective effects for forests, such as by diluting the pool of host trees
and making it harder for pests to become established.  

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

Combining two national county-level data sets, researchers found that
relationships between tree diversity and pest diversity follow a hump-shaped

“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,” Potter says. “Then you have a decreasing number of pest species as the number of host tree species increases.”

Overall, counties where forests have 30 to 40 different host tree
species tend to have the most nonnative pests. But the effects depend on
whether the invader is a specialist that can infest only a single tree species
or whether it’s a generalist, like the gypsy moth, which can spread to more
than 60 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,” Potter says.

“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

Researchers with the U.S. Forest Service, Purdue, NC State, Czech
University of Life Sciences and Duke collaborated on the study, which used two
large datasets. The U.S. Forest Inventory and Analysis, a national forest
census, contains information from 135,000 forested plots across the U.S. where
crews regularly measure trees and check environmental conditions. For this
study, the FIA data were used to compile counts of tree species for each of
2,098 counties. The Alien Forest Pest Explorer database offers a county-level record
of the presence or absence of nonnative insects and diseases, including 66 used
for this study.

Bare trunks of redbay trees in a Georgia forest
A combination of two nonnative pests, laurel wilt disease and redbay ambrosia beetle, has killed hundreds of millions of redbay trees in coastal areas of the Southeast, including Evans County, Georgia. Photo by Kevin M. Potter, NC State University

Researchers also examined other factors that could affect pest
invasions, such as human population density and environmental conditions,
including precipitation, elevation and average temperature. Tree biodiversity
was a better predictor of nonnative pests, Potter says.

Results could help prioritize monitoring efforts for forests most at
risk for future pest invasions, he says.

“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.” 

This work was supported in part by
National Science Foundation Macrosystems Biology grants DEB-1241932 and

– ford –

Note: An
abstract of the paper follows.

“Tree diversity
regulates forest pest invasion”

Authors: Qinfeng
Guo, Songlin Fei, Kevin M. Potter, Andrew Liebhold, Jun Wen

Published: Proceedings of the National Academy of Sciences


Abstract: Nonnative
pests often cause cascading ecological impacts, leading to detrimental
socio-economic consequences. However, it is still unclear how plant diversity
may influence insect and disease invasions. High species diversity in host
communities may promote pest invasions by providing more niches (i.e.,
facilitation), but can also diminish invasion success because low host
dominance may make it more difficult for pests to establish (i.e., dilution).
Most studies to date have focused on small-scale, experimental, or individual
pest/disease species, while large-scale empirical studies, especially in natural
ecosystems, are extremely rare. Using subcontinental-level data, we examined
the role of tree diversity on pest invasion across the conterminous USA. We find
that the tree-pest relationships are hump-shaped. Pest diversity increases with
tree diversity at low tree diversity (because of facility or amplification),
and is reduced at higher tree diversity (the 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. Our findings suggest the role of native species diversity in
regulating nonnative pest invasions.    

This post was originally published in NC State News.