Think Like the Scientist: Scott Horn
Meet the Scientist | About the Study

As you read on the Meet the Scientist section, Scott Horn is interested in invasive plants (Chinese Privet) and their effect on pollinators. Below is a Natural Inquirer article about the impact of invasive plants and animals on deer mice. Use the links to see an example of developing a testable question (introduction), planning to test your question (method), analyzing your data (results), and explaining it all (discussion).

Visit Goll-ly! Don't Take a Knapweed! to see the whole article.

Introduction | Method | Results | Discussion

Don't Take a Knap-Weed!

Spotted Knapweed
Figure 1: Spotted Knapweed

Gall Fly
Figure 2: Gall Fly

Cross Section of Knapweed with Gall Fly Larva
Figure 3: Cross Section of Knapweed with Gall Fly Larva (on right)

Lolo Forest in Montana
Figure 4: The Lolo National Forest in Montana

A live trap in knapweed
Figure 5: A live trap in the knapweed

Average number of deer mice collected
Figure 6: The average number of deer mice trapped per site.

Graph of Average Number of Deer Mice Trapped by Site by Season
Figure 7: The average number of deer mice trapped by site by season of the year.


One risk to a native ecosystem is an invasion of nonnative plants or animals. In the Western United States, spotted knapweed is one of the most widely found nonnative plants (figure 1). Spotted knapweed was brought to the United States from Eastern Europe in the early 1900s. When spotted knapweed spreads into an area, it takes over and native plants cannot compete with it. This change reduces the amount of food available to animals that eat the native plants, such as livestock and other hoofed animals.

To control the spread of spotted knapweed, two types of gall flies have been released into areas with spotted knapweed (figure 2). Gall flies lay their eggs on the flowerhead of the spotted knapweed. When the eggs hatch and become larvae, the larvae burrow into the flowerhead to feed on the plant (figure 3). This feeding destroys the developing seeds and helps control the reproduction of spotted knapweed. Unfortunately, these gall flies are not native to the arid grassland ecosystem. The scientists in this study observed that deer mice prefer eating gall fly larvae more than they prefer native foods.

The scientists thought that this preference could cause a problem for the ecosystem overall. Gall fly larvae are available to deer mice from September through May of every year. They become unavailable in the summer when the adult gall flies emerge. This change means that deer mice, which usually do not reproduce in the winter when native food is scarce, might be reproducing into the winter months because gall fly larvae are available to eat. More importantly, the scientists thought that the availability of gall fly larvae in the winter might help more mice to survive the cold winters. Usually, deer mice reproduce during the summer when native foods are available. Because knapweed overtakes native plants, native foods are less available to the deer mice. You can see that the presence of spotted knapweed and gall flies are changing the habits of deer mice. Because gall fly larvae are available to eat throughout the winter, the population of deer mice might be growing beyond its normal size. The scientists wanted to study the effect of spotted knapweed and gall flies on the population of deer mice in arid grassland ecosystems. They predicted that they would find an overall increase in the population of deer mice within these ecosystems.

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The scientists selected two large areas to study in the Lolo National Forest in Montana (figure 1). The large areas were similar in many ways, except that one had mostly native plants and the other had been invaded by spotted knapweed. In the area with knapweed, gall flies had been released and were living throughout the area. Within the two large areas, the scientists selected four smaller areas. Then within each of those four areas, they identified four even smaller sites to study.

The scientists trapped deer mice using live traps at each of the 32 study areas (figure 2). Live traps enable scientists to trap animals unharmed and then release them back into the environment. They baited the traps with peanut butter and oats. The scientists checked each trap two times every morning for 4 days. When a mouse was trapped, the scientists weighed it and recorded its sex. They gave each mouse a different number and placed an ear tag on it. The scientists then released the mouse back into the environment.

The scientists repeated the trapping and recording of mice for 3 years. They trapped mice for 4 days each in May (spring), July (summer), and September (fall) of each year. The scientists then counted the number of different individual mice trapped at each site in each of the seasons. They compared the number of mice trapped in the spring with the number trapped the previous fall. The difference between the two numbers would give them an estimate of the number of mice that had lived throughout the winter.

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Throughout the 3 years, the scientists checked the traps 21,760 times. They trapped 583 individual deer mice. If they counted the deer mice they trapped more than once, in total they trapped deer mice 913 times. In all 3 years, the scientists trapped more deer mice in the knapweed sites than in the native grassland sites (figure 1). In 1999 and 2000, the number of deer mice was larger in every season, except for the summer of 2000 (figure 2).

Look closely at figure 2. Do you see a difference between 1999, 2000, and 2001? The 2001 numbers show a large decline in the number of mice found at knapweed sites. This decline probably happened because of the drought that occurred over the spring and summer of that year. Because the number of knapweed plants decreased, the scientists expect that the number of gall flies was reduced as well.

The scientists found that in the sites with knapweed plants, 80 percent of the spring population of deer mice trapped were adults. This finding told the scientists that the winter availability of gall fly larvae helped to keep deer mouse adults alive over the winter months.

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In this study, the invasive plant species knapweed was supposed to be controlled by the nonnative gall fly. Unfortunately, the knapweed population is still getting larger. In addition to not solving the knapweed problem, the introduction of gall flies has affected the population of other animals in the ecosystem.

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