Montana State University

Monitoring better than EDRR for managing many invasive plant populations

May 22, 2009 -- Melynda Harrison, MSU News

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Land managers may benefit from spending more time monitoring invasive plants to identify those that pose the greatest threat, according to an article in the January-March 2009 issue of "Invasive Plant Science and Management."

The article, "The Rationale for Monitoring Invasive Plant Populations as a Crucial Step for Management," relates results from a study by Montana State University researchers Bruce Maxwell, Erik Lehnhoff and Lisa Rew.

Early detection and rapid response (EDRR) is a mantra many land managers use as the best strategy for managing invasive plants. The study shows there may be a better way.

"If you get to a population early enough, EDRR can really work," said Maxwell. "But the problem is we don't know what 'early enough' is."

EDRR can work under the right conditions, such as at ports of entry when a species moves from one continent to another, but as invasions move into a continent frequent introductions at the local and regional level are more likely. These populations are harder to detect and eradicate.

It may be more effective to monitor populations of a particular species to determine how invasive the population (patch) is likely to be. Knowing the invasion potential can help managers determine the best method to manage invasive plants.

Recently introduced species explore new habitat to find the most suitable environment. They invade the best habitat first, and if there isn't more suitable habitat, the invasive plant population probably won't expand further.

"Therefore the habitat becomes the defining feature and we should be able to monitor and figure out what is and isn't a source population," Maxwell said.

Source populations (populations that will expand their habitat) need to be the focus of management efforts, according to the authors. Non-source populations can be left alone.

"If we eliminate a source population on preferred habitat, we are effectively managing the invasive species," Maxwell said. "We don't need to focus herbicides or other management tools on populations that aren't going to expand."

Managers are concerned that if they spend too much time monitoring there will be less time and money to spend in directly managing the invasive plants, according to the authors.

"Most populations are not invading and monitoring can really pay off," said Maxwell.

The researchers estimate that managers could dedicate 50 percent of their management time to monitoring populations without risk of accelerating invasions or reducing the impact of their weed management program.

The researchers used a model to evaluate four plant management strategies over 20 years: (1) managing a fixed number of populations at random each year, (2) managing an equivalent number of populations along a road each year, (3) managing half of the fixed populations that were determined by monitoring to be sources of new populations, and (4) managing an equivalent set of source populations only on even years, leaving the odd years for monitoring.

They found that monitoring half of the time and managing half of the time (strategies 1 and 3) was most effective. Using one of these strategies, managers would be killing half as many invasive plants. But, if the management was directed at source populations (as determined through monitoring), the end result would be better than only managing.

"It takes seven to 10 years to pay off, but then it has great potential for slowing down, and in some cases, stopping the invasion," Maxwell said.

According to the authors, "Land managers may be wise to adopt a new mantra: 'early detection, rapid monitoring, and thorough management.'"

Bruce Maxwell at (406) 994-7060 or