Competition and Coexistence

One of the most fundamental ways in which the populations of species interact is by competing with one another. In an environment with just a single species, the population of this species grows exponentially unless limited by crowding and a short supply of resources. Individuals of the same species have very similar requirements from the environment for growth and reproduction, and once individuals become crowded, individuals must compete with each other for limited resources. In this single-species system, the term 'intraspecific competition' refers to the competitive interactions among individuals of the same species, where the growth, reproduction, and survival of some individuals are negatively affected. Grouping species (i.e., those species that aggregate for social or resource reasons) are particularly susceptible to crowding, and so may be expected to illustrate the most traits that enable coexistence. For instance, coral reef damsel-fishes can occupy small coral heads in groups of over 20 individuals, with a linear size-based dominance hierarchy (the largest fish is aggressively dominant over the next largest, and so forth; see Figure 1). Larger groups suffer slower growth overall, due to increased competition for zooplankton food, and smaller (lower-status) fish within groups grow more slowly. However, survivorship and hence maturity are higher in larger groups due to greater vigilance for predators and predator swamping, so that coexistence is beneficial overall.

The situation becomes more complex when the dynamics of two or more species and their interactions in an environment need to be considered. A competitive interaction between the populations of different species is termed 'interspecific competition', where growth, reproduction, and survival of one species is negatively affected either directly (e.g., interference competition) or indirectly (e.g., resource exploitation). For example, asymmetrical competition between two species can occur where one species has an advantage in resource exploitation or is able to interfere with another's ability to exploit resources. A great deal of effort has been invested in elucidating the role of interspecific competition in species coexistence.

Interestingly, however, a paucity of opportunities for interspecific competition in some communities can result in the coexistence of large numbers of species. This situation may arise when the individuals ofspecies are clumped (aggregated) in their spatial distribution. For example, populations of many shrubby plant species are spatially clumped in open-forest plant communities in southeastern Australia. Plant species that are better competitors for

Figure 1 Social group of the humbug damselfish (Dascyllus aruanus). Individuals coexist in dominance hierarchies. Photo by Peter Harrison.

resources would normally out-compete and eliminate inferior competitors, but competitively inferior species are able to coexist with competitively superior species where there is clumping of individuals. This is because individuals of competitively superior species are mostly competing with each other when clumped, and not with populations of inferior competitors, thus reducing the chance of weaker competitors being eliminated.

Competitive networks can promote coexistence in crowded communities by allowing no one species to be competitively dominant in all situations. For example, if species A is dominant over species B (i.e., A > B), B > C, and C > A, a simple three-species network will result. On coral reefs, such networks in corals apparently promote coexistence in the face of overgrowth.

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