Figure 18.6 Independent species distribution. (a) The Great Smoky Mountains exhibit change in plant species composition with changes in elevation and soil moisture. (b) When a tree survey was conducted across a moisture gradient within an elevation of 1,067 to 1,372 m, researchers found that each species has its greatest abundance at different points along the moisture gradient. Moreover, each species appears and disappears at different points along the moisture gradient. This led researchers to reject the hypothesis that plant communities are distinct entities in which the existence of one species depends on the presence of one or more other species.

Data from R. H. Whittaker, Vegetation of the Great Smoky Mountains, Ecological Monographs 26 (1956): 1–80. Photo by Chuck Summers/Contemplative Images.