EXAMPLES FROM NATURE

Examples that demonstrate the principle of competitive exclusion can be found in nature and studied, both by observation and by experiment. Such examples are directly related to the structure of the communities in which they occur, and they will be discussed more extensively. However, we include a few examples here, as we consider the contribution of competitive interactions to the structure of natural communities.
The competitive interactions between two species of barnacles that grew together on the same rocks along the coast of Scotland have been investigated by J.H. Connell of the University of California, Santa Barbara- Barnacles are marine animals (crustaceans) that have free-swimming larvae that settle down, cement themselves to rocks, and then remain permanently attached to that point. Of the two species Connell studied, Chthamalus stellatus lives in shallower water, where it is often exposed to air by tidal action, and Balanus balanoides occurs lower down, where it is rarely exposed to the atmosphere. In this deeper zone, Balanus could always outcompete Chthamalus by crowding it off the rocks, undercutting it, and replacing it even where it had begun to grow. When Connell removed Balanus from the area, however, Chlhamalus was easily able to occupy the deeper zone, indicating that no physiological or other general obstacles prevented it from becoming established there. In contrast, Balanus cannot survive in the shallow-water habitats where Chthamalus normally occurs. It evidently does not have the special physiological and morphological adaptations that allow Chthamalus to occupy this zone.
Viewing the same situation in evolutionary terms, one might suppose that Chthamalus evolved later than Balanus or that it reached the Scottish coast later than its rival. Unable to compete with Balanus in its main area of distribution, it nevertheless could survive in the relatively less favorable shallow-water habitats where Balanus did not occur. Conversely, it is possible that Balanus evolved more recently or reached the area more recently than Chlhamalus and possibly eliminated Chthamalus from most of its former range. In doing so it would have restricted Chthamalus to the less favorable sites where it was subject to periodic drying.
In another set of field observations, the late Princeton ecologist Robert Mac-Arthur studied five species of warblers, which are small, insect-eating birds that coexist part of the year in the forests of the northeastern United States and adjacent Canada. Although they all appeared to be competing for the same resources, MacArchur found that each species actually spent most of its time feeding in different parts of the trees, and so each ate different subsets of insects in those trees. Some of the species of warblers fed on insects near the ends of the branches, whereas others regularly penetrated well into the foliage; some stayed high on the trees, others fed on the lower branches; and these patterns were recombined in different ways characteristic of each of the warbler species. As a result of these different feeding habits, each species of warbler actually occupied a different niche; in other words, they had different ways of using the resources of the environment and thus were not really in direct competition with one another for limited resources.
Species of barnacles, warblers, and many other kinds of organisms coexist because the niches of the species involved differ.