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The number of visitors to California's Yosemite National Park has swelled to four million a year. Now scientists think the impact of so many human beings is rippling among the web of organisms in the park, from cougars to mule deer all the way down to the park's signature wildflower, the evening primrose.

Although many of the Yosemite's cougars were shot by park officials in the 1920's, some of these predators remain. Yet they seem to avoid Yosemite Valley, the picturesque main entrance to the park that is home to numerous roads and buildings. The deer recognize this safe haven, says Oregon State University forestry scientist William Ripple. "We think the deer are moving closer to where the humans are." The many mule deer browse heavily on young California black oak and primrose in Yosemite Valley, and the plants are often unable to reach reproductive age. Ripple and his colleagues noticed that oak trees away from the visitor center were taller and more abundant.

By pushing predatory cougars out of the valley, humans have indirectly increased mule deer and reduced oak and primrose populations. This is an example of a trophic cascade—one that appears to be reducing biodiversity in one of the nation's most spectacular landscapes.