The intensity of competition is a physiological concept, related directly to the well-being of individual organisms but only indirectly and conditionally to their fitness, and even more indirectly to the evolution of populations and the structure of communities. The importance of competition is primarily an ecological and evolutionary concept, related directly to the ecology and fitness of individuals but only indirectly to their physiological states. The intensity of competition is not necessarily correlated with the intensities of predation, disturbance, abiotic stress, or other ecological processes. The importance of competition is necessarily relative to the importances of other processes. Intensity refers primarily to the process of present competition, whereas importance refers primarily to the products of past competition. The distinction between the intensity and the importance of competition clarifies two long-standing ecological debates. Some ecologists have proposed that competition is greater in more stressful habitats, others the opposite, and still others that no such relationship exists. Evidence cited to refute or support these positions often confuses intensity and importance. Distinguishing between them focuses questions more sharply and indicates what sorts of new evidence should be sought. The more widely known debate over the prevalence of competition as an agent of community structure is a debate about the importance of competition, but evidence about the intensity of competition has often been used by both sides. We argue that intensity and importance need not be correlated, and so measurements of the intensity of competition are not directly relevant to this debate. This distinction also generates testable hypotheses and suggests directions for research. For example, we hypothesize that competition can be unimportant even if it is very intense: no such hypothesis is possible unless importance is distinguished from intensity. We discuss the application of these ideas to methods and theories used to study competition, ecological communities, and the evolution of competitive ability. We advocate a research approach that presumes multiple, interacting causes, including competition, affecting community structure, and we show how the distinction between intensity and importance helps to make this feasible.

Treefall gaps are through to contribute to the diversity of plants in tropical forests by providing opportunities for niche differentiation in modes of regeneration. To examine this hypothesis, we studied the survival, diameter growth, and recruitment of saplings in ≥100 species of woody plants in a 50—ha permanent plot of moist tropical forest on Barro Colorado Island, Panama, from 1982 to 1985. The performance of saplings in low—canopy sites (<10 m) was compared to that of saplings in high—canopy sites (°>10 m), and performance of common species was compared to that of rare species. Of the 108 species for which all three parameters of performance were measured, 104 fell into four response groups, each with characteristic patterns of survival, growth, recruitment, and response to canopy height. Pioneers (six species) survived poorly in both canopy—height categories, and survivors grew rapidly in low—canopy sites. Sapling recruitment was skewed toward low—canopy sites. Understory specialists (three species) survived well in high—canopy sites and poorly in low—canopy sites. They grew slowly and recruited poorly in both situations. Generalists (79 species) survived well and grew slowly in both canopy—height categories. Per—adult recruitment was usually low, and often skewed toward low—canopy sites. Poorly performing species (16 species) survived poorly, grew slowly, and recruited infrequently in both canopy—height categories. Most of the common (>10 saplings/ha) species appeared to be generalists. Many rare (<1 sapling/ha) or occasional (1—10 saplings/ha) species survived significantly (P ≤ .05) less well than the average survivorship of saplings, while many common species survived significantly better than average. Some rare or occasional species grew rapidly, either in low—canopy sites or in both canopy—height categories, while most common species grew slowly in both situations. Rare and occasional species had significantly more recruits per adult than did common species, but often this did not balance their higher mortality. Large differences in survival, growth, and recruitment between canopy—height categories were found only among rare and occasional species.

We tested several hypotheses about the relationship of competition to abiotic stress, using the vegetation of the semiarid Piceance Basin of northwestern Colorado. We studied competition among the shrubs Amelanchier utahensis, Artemisia tridentata, and Symphoricarpos oreophilus, and between the trees Pinus edulis and Juniperus osteosperma, in 10 sites. We calculated several indices of biotic moisture stress, based on the slope, aspect, and evaluation of each site. Competition was measured by regression of the distance separating neighboring plants vs. the sum of their canopy areas. The slope of such a regression (if significant and positive) measures the intensity of competition, and its coefficient of determination (r2) measures the importance of competition, between the plants involved. We detected competition among these plants in all but one combination of species and in most sites. No significant differences in the intensity of competition were found within species combinations. Significant differences in the importance of competition were found in one of three interspecific combinations of shrub species, and in two of three combinations of tree species. Neither the intensity nor the importance of competition showed any consistent relationship with any index of abiotic moisture stress. Thus, no hypothesized relationship between abiotic stress and competition is supported. Our data also show no consistent relationship between the importance of competition and its intensity, supporting our hypothesis that the intensity and the importance of competition are independent.

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Several aspects of the spatial pattern of plants were investigated in six very different stands in the alpine tundra of the Colorado Front Range. Pattern was related to the internal structure of the stands, to interactions among the species, and to microhabitats within the stands. Evidence from species‐species associations and species ordinations clearly divided the stands into two groups. The first was characterized by thin, coarse, unstable soils, low plant cover, and nurse‐plant establishment. The second group was characterized by deeper, more mature, and more stable soils, higher plant cover, and more competition between species. Pattern evidence allowed some species to be categorized as competitors, ruderals, or stress‐tolerators in the context of the alpine vegetation. The two groups of stands also differed in the relative importance of species with these “strategies.”