Forests, Competition and Succession David A. Perry Oregon State University Competition Succession Competition, the struggle for limited resources, than would be possible if all were competing for and succession, the sequence of change in dominant the same set of resources. organisms following colonization, have long been This article first discusses competition: why it key concepts employed by ecologists to understand occurs, why it does not occur, and how it shapes and organize the patterns of nature. Although com- the structure of communities. It then turns to the petition and succession are distinct processes, they patterns and mechanisms of succession, many of are closely related for at least two reasons. First, which turn on the nature of both competitive and successional trajectories are largely driven by inter- cooperative interactions among species. actions among organisms, including (but not re- stricted to) competition. Second, both are inti- mately related to the degree of equilibrium or I. COMPETITION disequilibrium in ecosystems and landscapes. Ecol- ogists once believed that succession led inexorably "The inhabitants of the world at each suc- to a stable equilibrium within a given community cessive period in its history have beaten their of organisms, the composition of which was deter- predecessors in the race for life." (Charles mined in large part by who won the struggle for Darwin, "The Origin of Species," 1859) limited resources. Although that view has not been totally discarded, most ecologists now recognize One of the oldest ideas in ecology is that individ- that change is the rule rather than the exception in uals utilizing the same resource will compete if nature, with few if any ecological communities that resource is in short supply. For many years achieving a long-lasting equilibrium in species ecologists assumed that the sizes of all populations composition. Disturbances at many spatial and within a given community were ultimately limited temporal scales create "shifting mosaics" of com- by resources, hence competition was believed to munities in different stages of succession, resulting be an inevitable consequence of making a living in diverse niches that allow more species to coexist and the major determinant of community structure (i.e., the number of species and size of each popula- tion). Constant struggle is not necessarily implied; over many generations species may evolve ways Portions excerpted from D. A. Perry (1995). "Forest Ecosys- tems. - Copyright Johns Hopkins University Press, with per- to avoid competition through allocating resources. mission. Nevertheless, communities are ultimately struc- Encyclopedia of Environmental Biology Copyright © 1995 by Academic Press, Inc. Volume 2 135 All rights of reproduction in any form reserved. 136 FORESTS, COMPETITION AND SUCCESSION tured by competition, be it ongoing or, as a pop- alternates up the trophic ladder: plants compete; ular phrase puts it, the "ghost of competition herbivores are held down by predation, hence do past." not compete; and carnivores compete because they Although competition for resources undoubt- are at the top of the food chain and therefore have edly occurs (at least in some trophic levels), the no predators. Evidence in support of this idea is notion that it is the major organizing force in nature equivocal. However, there is little doubt that pre- is overly simplistic. Most ecologists now recognize dation, disturbance, or climatic fluctuations can and that ecological communities are complicated and frequently do act to maintain populations below variable, with their structure shaped by many inter- what their food supply would permit. This is often acting environmental and biotic factors in addition the case with herbivorous insects and its can also to resource competition. Species may not compete occur in plant communities when herbivores and for resources at all or they may simultaneously pathogens increase the species richness of plant compete and cooperate. For example, multispecies communities by reducing the ability of any one flocks of insectivorous birds commonly follow ant species to competitively dominate others. Higher swarms in the tropics, feeding on insects flushed order interactions other than predation may come by the ants. In 1983, F. Bourliere listed two possible into play to reduce competition. Such is the case advantages to such mixed flocking: more efficient with at least some types of mycorrhizal fungi, hunting in a patchy environment and more eyes which by mediating a more equitable distribution and ears for protection against predators. Different of resources among individual plants reduce the species of monkeys also frequently forage together, ability of one or more species to dominate a com- even when they are quite similar in body size and munity. diet. Once again, Bourliere (1983, p. 87): ". 2. Species may compete for the same resources the main advantage for mixed groups (of monkeys) but also benefit one another in some way that tends appears to be to make easier the location of patchily to dilute or negate their negative effects. This un- distributed food sources and the detection of preda- doubtedly occurs in ecosystems (as exemplified by tors. A monospecific group of similar size might the mixed foraging groups discussed earlier), but well offer the same benefits to its members, but at may not be readily apparent from casual observa- the cost of a stringent social hierarchy which would tion or even from experiments unless conducted imply a greater energy expenditure for its enforce- over many years (which seldom happens). It has ment." A. F. Hunter and L. W. Aarssen list the been hypothesized that plant species participate in following ways in which coexisting plant species defense guilds, i.e., either directly or indirectly re- have been demonstrated to help one another: ". duce herbivory and/or pathogenesis within the improving the soil or microclimate, providing community. For example, flowering plants are physical support, transferring nutrients, distracting common in young conifer forests, where they or deterring predators or parasites, reducing the probably compete with the conifers for various impact of other competitors, encouraging benefi- resources. However, nectar produced by flowers cial rhizosphere components or discouraging detri- of these plants is important in the diet of at least mental ones, and attracting pollinators or dispersal some insects that prey on defoliating insects. It has agents." [See F ORAGING STRATEGIES.] been documented that 148 species of parasitoids At least three things come into play to modify (important predators of defoliating insects) are as- the importance of competition within ecological sociated with flowering plants in forests of northern communities. Germany. What is the net effect of these plants on conifers? If they were not in the ecosystem would 1. Species within a given trophic level may be faster conifer growth eventually be negated by limited by factors other than resources, hence larger populations of defoliating insects made pos- seldom or never have to compete. It has been sible by lower populations of their predators? suggested that the importance of competition Questions such as these are seldom entertained in FORESTS, COMPETITION AND SUCCESSION 137 competition studies. Potentially competing species of the lifetime of a stand these may compete with have also been hypothesized to form cooperative conifers, but during wildfire they protect the coni- guilds based on protecting and stabilizing ecologi- fers. Species-diverse grasslands are more stable dur- cal commons such as shared mutualists (e.g., polli- ing a severe drought than species-simple grass- nators, mycorrhizal fungi) and soils. "Facilitation," lands. [See FIRE ECOLOGY. in which one plant species has some effect that Interactions among species requiring the same benefits others, is commonly observed during suc- resources are complex, varying over time and with cession. It does not follow that a "facilitator" does environmental conditions. In contrast, much of the not also compete for resources; however, the net traditional ecological thinking about the role of effect is positive rather than negative. On the other competition in structuring communities has been hand, there are clear instances in which one plant shaped by mathematical models that treat two in- species has a net negative effect on others, and other teracting species as if they were in a constant envi- cases in which the net effect of one species on an- ronment and isolated from other species in the sys- other varies with time and environment. These tem. L. Stone and A. Roberts developed a more points are followed in the section on succession. realistic model that evaluates the interactions be- 3. Complexity at scales ranging from landscapes tween any two species ". within the framework through individual trees to soil aggregates creates of the community to which they belong." Their diverse niches that allow species to avoid competi- approach, which they refer to as the "inverse tion through specializing. method," deals strictly with interactions within a given trophic level. In other words, these are inter- One of the primary criticisms leveled against the actions in which the participants potentially com- idea that communities are structured primarily by pete for resources. The criterion they use to deter- competition is that few experiments have really mine whether a species benefits or suffers in