The figure above shows elk browsing (i.e. evidence of them eating the tree) and

Question

The figure above shows elk browsing (i.e. evidence of them eating the tree) and height of cottonwood trees in two sites (Soda Butte Creek and Lamar River Islands) in Yellowstone National Park. At each site, observations were made in areas where there was low risk of wolves hunting and areas where there was high risk of wolves hunting elk. Explain what these data suggest about how wolves indirectly influence cottonwood trees.

Low Risk Sites High Risk Sites Soda Butte Creek 100 80 m 60 2 40 20 250 200 150 5 100 Lamar River Islands 100 60 DD 40 250 S 200 150 100 , 1998 1999 2000 2001 1998 1999 2000 2001 Year

Explanation / Answer

Wolves have a positive effect on the growth of cottonwood trees.

At the region of Soda Butte Creek, low-risk sites show a high percentage of browsed stems and the resulting stem height is less compared to high-risk sites. Since wolves keep herbivores away from the plants, they exert a positive effect on vegetation growth in high predation areas for herbivores.

At the region of Lamar river islands, low-risk sites show almost completely browsed stems and the resulting stem height is less compared to high-risk sites. At the high-risk areas, the percentage of the browsed stem is low and the resulting stem height is increased comparatively.

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