Up to this point in the book we have largely focused on the physical factors that control the structure and function of wetland communities: flooding, fertility, and disturbance. It is now time to consider biological factors. We begin here with competition. We will define competition as the negative effects that one organism has upon another by consuming, or controlling access to, a resource that is limited in availability. That is, it is an interaction in which both organisms experience a negative effect. Competition is widespread and important, although its importance depends upon the species or the habitat being considered. As just one of many possible examples consider the effects of competition upon common marsh plants (Figure 5.1). The effects of competition were measured by moving six species of plants into two sets of conditions: clearings (no competition) and intact vegetation (competition). In every case, the plants in clearings grew significantly better than the plants with neighbors. The difference in the height of each pair of histograms gives one measure of how important competition was for that species – in this case, Pontederia cordata seemed to be the weakest competitor, since it showed the greatest reduction in vegetated plots.

The basis of competition is lack of resources. All living organisms require a rather limited number of elements to make up their bodies (Table 5.1). Some organisms will accumulate these resources at the expense of others, thereby reducing growth, survival, or reproduction of their neighbors.