The Thorny Economics of Preventing Exotic Species Introductions

American chestnut
American chestnut was once a keystone species. Non-native pathogens erased it from the landscape by the early 1900s. USFS image.

What if we lose tree species we know, love, and need? It has happened before.

“Look at what happened to the American chestnut,” says U.S. Forest Service research forester Thomas Holmes. “Look at what’s happening right now to hemlock, redbay, and ash trees.” All three species, as well as many more, are threatened by non-native insects or pathogens.

Non-native insects, pathogens, plants, and animals have been arriving for hundreds of years. Most fail to become established, but some thrive and rampage through their new homes. These non-native species cause immense ecological and economic damage.

“Currently, homeowners and municipalities shoulder most of the costs,” says Holmes.

Local governments treat, cut down, and replace ailing street trees. Municipalities spend an average of $1.7 billion each year cleaning up after wood-boring insects such as the emerald ash borer.

Homeowners face similar challenges, and when trees die, property values decline. Each year, tree death from non-native insects and pathogens costs homeowners an estimated $1.5 billion in lost property values.

About 70 percent of non-native insects and diseases in the U.S. hitchhiked on live plants. Untreated wood packing material can also carry an unintended cargo – exotic wood-boring insects.

“International trade is like a global experiment that samples biotas,” says Holmes. “Differences in historical trading patterns and biogeography suggest that not all trading partners present equivalent risks in biological invasions.”

Holmes recently led a research team that reviewed the costs and benefits of forest phytosanitary security. The research began as part of the IUFRO working group on the Social Dimensions of Forest Health, and the study was published in the journal Current Forestry Reports.

Although the consequences of biological invasions can be dire, the cost of preventing them can seem prohibitive. And there is more than one way to err.

emerald ash borer
Sick trees are expensive. Every year, non-native pests cost homeowners an estimated $1.5 billion in lost property values. Photo courtesy of Max Pixel.

“If we err on the side of caution – that is, over-regulation – we’ll spend money on unnecessary prevention programs,” says Holmes. “However, if we err on the under-regulation side, we run the risk of continuing and costly invasions.”

Holmes and his colleagues related the choices to type I and type II errors. Type I errors are false positives. In the context of the study, type I errors are equivalent to over-regulation. Type II errors are false negatives, and correspond to under-regulation.

“We want to help decision-makers balance the outcomes posed by this dilemma,” says Holmes. “Do we spend more money to prevent biological invasions, or do we live with the consequences?” Both routes are costly and potentially irreversible. Once money has been spent on new phytosanitary policies, costs are sunk and policies are difficult to reverse.   Similarly, once a non-native species becomes established, control costs and economic losses may continue for decades or longer.

In the past, most decision-makers have preferred a wait-and-see approach. The strategy lowers some costs but can lead to unpleasant surprises.

“Many invasive species have long lag times between establishment and the moment when economic impacts are apparent,” says Holmes.

For example, the hemlock woolly adelgid was detected in Virginia in 1951. For about the next 40 years, the miniscule insect lurked in forests. Eventually it became destructive and has now killed millions of eastern and Carolina hemlock trees.

“Forest ecology, trade, and economics are connected by a web of complex, long-term, and uncertain relationships,” says Holmes. “The future ecological and economic risks of new biological invasions are largely unknown, and it is the role of science to reduce uncertainty so that better decisions can be made.”

The study suggests a fuller accounting of the costs associated with the loss of forest ecosystem services from biological invasions is needed.

hemlock woolly adelgid
Some non-native species, such as the hemlock woolly adelgid, have long time lags between introduction and invasion. Photo courtesy of Nicholas Tonelli.

Holmes and his colleagues outlined several measures for trading partners to consider:

  • Using a systems approach, which has multiple steps. For example, logs could be required to be free of visible pests before shipment, shipped during a low risk period, unloaded and stored in a zone that is free of suitable plant hosts, and fumigated within days of entry.
  • Improving quarantine and control practices.
  • Establishing sentinel tree programs.
  • Increasing cooperation between the research community and the nursery industry.
  • Developing risk-based border inspection procedures.

“Preventing biological invasion is expensive,” says Holmes. “However, the potential for irreversible ecological and economic damage argues for precautionary measures.”

Read the full text of the article.

For more information, email Thomas Holmes at

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