Interspecific Competition

We have seen in the previous sections how a species' geographic range and occupation of particular habitats is limited by the species' adaptations to environmental conditions, resource levels, and life-history traits. The effects of abiotic factors on survival or growth rate of a population could be plotted with each axis corresponding to one factor. If we imagine many axes, each defining one dimension of an n-dimensional space, the region of this space suitable for growth of a species is what Hutchinson (1957) envisioned as the species' "fundamental niche." The fundamental niche of a species is all combinations of environmental conditions that are acceptable for the persistence of a population. Because of predation and competition with other species, populations of a species will not be present at all habitats that satisfy the species' fundamental niche. The reduced hypervolume corresponding to the conditions that a species is actually able to occupy is called its "realized niche." Interactions between species are fundamental processes in defining which species will be present in a given location.

Interspecific competition can operate according to the same mechanism as intraspecific competition, except that the individuals competing are from different species. A finite pool of resources is available at any given time, and if resources are consumed faster than they are replenished, growth rates decline. The strongest competitors are able to maintain higher growth rates despite lower levels of resources, and they drive resources yet lower. This form of interspecific competition is known as "resource-based competition" or "exploitative competition." Tilman (1982) suggested that if resources are replenished at a constant rate in a stable environment, the population of each species would reach an equilibrium point at which death rates equal reproduction rates. Hence, n is equal to 0 at this equilibrium population density. According to the Monod model above, the equilibrium resource concentration (R*) at the equilibrium population density for a given species is

At resource concentrations below R*, a population's reproduction rate is lower than its death rate. When multiple species use the same pool of resources, all populations will attempt to grow to their equilibrium levels. The species with the lowest value for R* will determine the equilibrium resource concentration, resulting in the local elimination of other species because of resource levels too low to support them.

Given this description of interspecific competition, how do similar species coexist? One key component of the resource-based model is the presence of constant resource supply rates in the environment. Spatial and temporal variability in resources leads to growth being limited by different resources in different times and places. Tilman (1982) suggested that the number of similar species that can coexist in a habitat should be equal to the number of potentially limiting resources used in that habitat, since species that are superior competitors for one resource are typically not as competitive for others. Another important assumption of the model is constancy of death rates. Discussion of density-independent mortality factors has suggested that this is a departure from reality. Species that are r-selected may be outcompeted under normal conditions, but flourish when mortality spikes for the dominant species. "Fugitive species" avoid competition by dispersing into habitat patches where the dominant species has become locally extinct. Mortality rates can also be altered through "interference competition," in which one competing species impacts another through direct aggressive action rather than resource use. Examples include human efforts to control agricultural pests and physical attacks between animals. In soil, the degree of competition that is mediated by interference mechanisms is unknown, but there is the potential for interference competition to play an important role. This type of competition would be likely in systems containing antibiotic-producing organisms. Davelos et al. (2004) confirmed Waksman's much earlier observations when they found a wide variety of antibiotic production and resistant phenotypes present in soil streptomycetes at one location, suggesting many organisms capable of competitive interference. It also suggests that organisms have developed mechanisms to avoid this type of interference.

Similar species evolve to use different subtypes of the same resource, or their niches can shift in other ways. This is known as resource partitioning and was taken as some of the first evidence of competition and natural selection. Considering the mechanisms of species coexistence to be components of the niche, we arrive at the conclusion of Hanski et al. (1995) that the niche is equivalent to an ecological species concept. The concept that no two species having identical niches can coexist is the "competitive exclusion principle." So, how similar can two species be and still coexist? This has been explored theoretically to a limited extent (beginning with MacArthur and Levins, 1967). In reality, extinctions due to competition have been documented, but evolution can allow an environment to be partitioned into an astonishing array of niches. For example, Rozen and Lenski (2000) showed that a pure culture of Escherichia coli developed spontaneously into distinct subtypes that coexisted because of physiological (niche) differences. Niches can shift in terms of environmental tolerances as well, resulting essentially in species living in different habitats.

In soil, competition has been exploited as a mechanism for biocontrol, but it has also been blamed for the failure of many soil inoculation programs. Fluorescent pseudomonads have been shown to suppress a variety of plant pathogens by secretion of antibiotics (interference competition) and siderophores, which sequester iron (resource competition). Strains of Fusarium oxysporum that are nonpathogenic can be superior competitors for carbon and root colonization sites (Alabouvette et al., 1996). Organisms introduced into sterilized soil often survive, while populations decline rapidly in nonsterile soil. The relatively short half-life of introduced populations has been observed for a variety of groups, including some biocontrol agents, rhizobia, fecal organisms, and genetically modified microbes. This has been attributed to competition, but could also be a result of trophic interactions, the type of biological interaction described next. In rare cases, inoculated populations have survived (in reduced numbers compared to the inoculum size) if the environment is modified to match their niche requirements or they are naturally strong competitors.

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