Ecological Effects on Eastern Chipmunk Behavior

Title, authors, journal, year, pages: The title of the article is “Influence of Canopy Closure and Shrub Coverage on Travel along Coarse Woody Debris by Eastern Chipmunks (Tamias striatus).” The authors are Patrick A. Zollner and Kevin J. Crane. The article is located in the journal The American Midland Naturalist from July, 2003 on pages 151-157.

Question: The authors want to discover whether the amount of canopy closure and shrub cover in the forests of the northern United States affects the use of coarse woody debris by eastern chipmunks in their struggles to avoid predators. Previous studies have documented that small mammals, such as chipmunks, mice, and voles, use physical cover to protect themselves against predators as they travel along the ground. Coarse woody debris, which includes fallen logs, pieces of wood and branches, can be used for this purpose. Logs provide a shelter for small mammals, they reduce the noise the animals make as they move, and they are used as landmarks for quick movement. Logs can also elevate the small mammals, providing an improved ability to see predators. The authors want to know whether an open or closed canopy and dense or sparse shrub cover in a forest influences the travel paths of the chipmunks.

Hypothesis: Zollner and Crane believe that coarse woody debris is more important to eastern chipmunks when they face a high risk from predators. Thus, the chipmunks will use the debris to reduce the risk. Previous studies have shown that chipmunks have a greater chance of being attacked at sites with open canopies and thick shrub cover, so coarse woody debris would be more useful at these sites. Without logs and branches for protection, an open canopy and dense shrub layers would probably leave a chipmunk more exposed, while giving a predator a place to hide and wait for its prey. Also, the more coarse woody debris found in a particular area, the more likely the chipmunks will travel throughout that area.

Predictions: The authors predict that there will be much greater movement along coarse woody debris for the chipmunks at sites with open canopies and thick shrub cover. This will give the chipmunks a larger amount of protection in these high-risk areas. Zollner and Crane also predict that the use of coarse woody debris will be negatively associated with canopy coverage and positively associated with shrub coverage. Therefore, as the amount of debris used increases, the amount of canopy coverage should decrease and the amount of shrub cover should increase. This would produce an open canopy and dense shrub layer.

Test: The predictions were tested by using three sites in a forest located in Wisconsin. Variables were kept as even as possible between each site. Fifty-two chipmunks were captured and tracking spools were attached to each one. Each chipmunk was then released and the tracking spools left trails of thread as they traveled. The next day, each trail was followed and flags were placed in the ground at 2 meter intervals and at points where the chipmunks changed direction. Distances were calculated using a tape measure between each flag. The distance from each flag to the original site of capture was also measured as well as the distance from each flag to the chipmunk’s burrow at the end of the trail. While following each trail, a note was made whenever the thread laid directly over logs and branches, which was considered movement along coarse woody debris. The authors estimated the percentage of ground covered by the debris in the area where each chipmunk was tracked. They also estimated the percentages of canopy closure and shrub cover at each site. Statistical analyses were used to determine which variables (age, sex, availability of logs, canopy closure, or shrub layers) influenced the use of coarse woody debris by the chipmunks. They also analyzed the results to see whether there was a correlation between canopy closure and shrub cover and a correlation between canopy closure and logs available.

Data: The results show that the chipmunks traveled an average of 107.2 meters from the initial capture site. An average of 4.6% of the movement was along coarse woody debris. The data reveals that there was a positive association between the use of coarse woody debris and the percent ground cover of logs and shrubs. There was a negative association between the use of coarse woody debris and the percent of canopy closure. The age and sex of the chipmunks were not factors in the results. The data also indicates that most of the variation in travel along the logs by the chipmunks was explained by the percent cover of available logs at a site, followed by the percentage of canopy closure, and thirdly, the percentage of shrub cover. There was a negative correlation between canopy coverage and shrub cover, meaning that as the canopy becomes less dense and more open, the shrubs become thicker.

Evaluation: The data supports the hypothesis by showing that eastern chipmunks travel more on coarse woody debris in areas where there is a greater availability of logs. Open canopies and dense shrubs, which create a higher risk from predators, force chipmunks to more often use the debris as travel pathways because it helps protect them. When the threat from predators is low, coarse woody debris is not as useful or important to the chipmunks. The results of the study generate new ideas about a human beings’ role in forest management. Our actions, such as cutting trees down to the ground and clearing away shrubs, can have far-reaching affects, and it could potentially affect the survival of eastern chipmunks. On the other hand, understanding the results of studies such as this one by Zollner and Crane can give us the knowledge to manage the forests in ways that benefit many species of animals.