Forest and Shade Tree Pathology

Invasive Species

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Nightmare or reality?

Imagine the perfect tree.

It is huge, majestic, graceful, and beautiful. It is abundant. Its fruit is a ready source of food and commerce for mountain families and a holiday treat for us all; it is also an important source of food for wild birds, squirrels, turkeys, deer, and bears. The wood is hard, straight-grained, easily worked and beautiful. The heartwood is almost magically resistant to decay. It is used for everything from fine furniture to utility poles. When the tree is harvested, it sprouts back vigorously from the roots. The tree grows fast so that the forest is readily renewed to provide its many benefits to future generations of people and wildlife.

It is an ideal tree. If you could custom design a tree with all the best features, you would get the American chestnut.

Now imagine the perfect disease.

The pathogen grows fast and is highly virulent. It can infect anywhere on the stem or branches and quickly kill the bark in a spreading canker. It reproduces rapidly with huge numbers of spores. The spores can be shot into the air and carried by wind, can be splashed by raindrops, and can be carried by insects and birds to new trees. The pathogen comes from another continent, but it causes little damage there because the trees there evolved with it for eons and have effective resistance mechanisms. Here in America, our perfect tree did not evolve with the pathogen and is helpless.

The perfect pathogen is brought to America and invades our forests. Within 40 years, the American chestnut is virtually gone from our landscape. The perfect disease is chestnut blight, brought in accidentally with some Asian nursery stock.

This is the story of invasive species. We may never be able to bring back the American chestnut the way it was, but we can prevent future invasions that can decimate our forests, our crops, our waters and wildlands.

Overview of the invasive species problem

Non-native, invasive species constitute one of the greatest economic and environmental challenges in the history of our natural and managed ecosystems. Nonnative plants and plant pests cost more than $100 billion per year in crop and timber losses plus the expense of herbicides and pesticides. And this figure does not include the costs of invasions in less intensively managed ecosystems such as wetlands. The cost of invasive species to the U.S. economy is estimated to exceed $138 billion per year. About 50,000 species have been introduced to the United States directly or indirectly by humans. Many have displaced and endangered native species in many ecosystems; nearly half of the species listed as threatened or endangered are at risk primarily because of exotic invaders. Biologist Edward Wilson has claimed that the introduction of alien species is second only to habitat loss as the leading cause of extinctions worldwide (Δ). Coastal and inland waters, forests and other ecosystems across the country are degraded by the invaders. It is difficult to find a landscape or waterscape that is not impacted to some extent in the contiguous U.S. Many ecosystems have been dramatically altered by exotic species.

A book from the National Academy of Sciences, Predicting Invasions of Nonindigenous Plants and Plant Pests, examines this growing problem.

What does this have to do with forest pathology?

A lot. Our most devastating and destructive forest diseases are mostly exotic. For examples, consider:

Some of these diseases have practically eliminated the dominant native species from some of our major forest ecosystems. At best, they have made forest management more complex and expensive.

What is the solution?

We need to be much more aggressive about controlling imports (and exports!) and insisting that items that carry invaders in international trade and travel be treated to kill pathogens. This conflicts with trade, of course, and is a very complex issue. But this is the number one solution and deserves the greatest emphasis.

We also need to be better organized and more aggressive in finding, attacking and eradicating invaders as soon as possible after their establishment on a new continent. In many cases, eradication will not be feasible, but early detection and rapid response will result in successful eradication of some.

Finally, continued action to suppress and contain established invaders is needed even when eradication is no longer feasible. Limiting their damage and spread will save money, protect uninvaded ecosystems, and buy time, during which more effective measures may be found.

In the United States, the National Invasive Species Council was created as part of a national initiative to address invasive species. Lets all wish them luck and help in any way we can.

New Zealand has one of the most proactive and effective programs for interdiction and early detection of potentially harmful, nonnative organisms. This is probably related to progressive leadership, but also to the fact that it is a relative small, remote island nation, increasing the effectiveness of measures to prevent the entrance of invasive organisms. See below for links to more information on the system and a major review of it.


Last modified 27 May, 2007