Figure 16.18 Apparent competition between sage and grasses in California. (a) Researchers originally hypothesized that the bare zone surrounding the purple sage shrubs (Salvia leucophylla) occurred because of allelopathic chemicals the shrubs produced. However, subsequent research discovered that herbivores used the shrubs as a refuge and only fed on grasses that were close to the safety of the shrub. To demonstrate that the effect was due to mice eating the plants surrounding the sage shrubs, researchers placed open-sided and closed cages in the bare zone, as shown here in a photo of the researchers in the Santa Ynez Valley, California, in 1968. (b) One year later they found nearly 20 times more vegetation growing in the closed cages, which confirmed that herbivores were causing the bare zone of vegetation around the shrubs. Error bars are standard errors.
Photo courtesy Richard W. Halsey. Data from B. Bartholomew, Bare zone between California shrub and grassland communities: The role of animals, Science 170 (1970): 1210–1212.