The outcome of competition between species can be changed by a broad suite of factors, including features of the physical environment, disturbance, and interactions with other species.

For example, a difference in abiotic conditions— as might occur from one place to another—can cause a competitive reversal, in which the species that was the inferior competitor in one habitat becomes the superior competitor in another.

Interactions with other species can have similar effects on the outcome of competition between species. The presence of herbivores has been shown to reverse the outcome of competition between species of encrusting marine algae (Steneck et al. 1991) and between ragwort (Senecio jacobaea) and other plant species (FIGURE 14.15). Herbivores can have this effect if they prefer to feed on the superior competitor, thereby reducing the growth, reproduction, or survival of that species. What is true of herbivores is also true of predators, pathogens, and mutualists: an increase or decrease in the abundance of such species can change the outcome of competition among the species with which they interact.

FIGURE 14.15 Herbivores Can Alter the Outcome of Competition Ragwortfleabeetles are herbivores that feed on ragwort (Senecio jacobaea), an invasive plant species. The graph tracks the biomasses of ragwort, grasses, and forbs (broad-leaved herbaceous plants) at a site in western Oregon after the flea beetle was introduced there in 1980. The results show that in the absence of the flea beetle, ragwort was a superior competitor, but it declined precipitously when the beetle was introduced. (After P. McEvoy et al. 1991. Ecol Appl 1: 430-432.) View larger image

In later chapters, we'll explore many examples in which species interactions alter competitive outcomes—sometimes preventing a superior competitor from driving other species to extinction. Here, we'll focus on the effects of the physical environment and disturbance.