Interspecific competition
Interspecific competition, in ecology, is a form of competition in which individuals of different species compete for the same resources in an ecosystem. This can be contrasted with mutualism, a type of symbiosis. Competition between members of the same species is called intraspecific competition.
If a tree species in a dense forest grows taller than surrounding tree species, it is able to absorb more of the incoming sunlight. However, less sunlight is then available for the trees that are shaded by the taller tree, thus interspecific competition. Leopards and lions can also be in interspecific competition, since both species feed on the same prey, and can be negatively impacted by the presence of the other because they will have less food.
Competition is only one of many interacting biotic and abiotic factors that affect community structure. Moreover, competition is not always a straightforward, direct, interaction. Interspecific competition may occur when individuals of two separate species share a limiting resource in the same area. If the resource cannot support both populations, then lowered fecundity, growth, or survival may result in at least one species. Interspecific competition has the potential to alter populations, communities and the evolution of interacting species. On an individual organism level, competition can occur as interference or exploitative competition.
Types
All of the types described here can also apply to intraspecific competition, that is, competition among individuals within a species. Also, any specific example of interspecific competition can be described in terms of both a mechanism and an outcome.Based on mechanism
Exploitative competition, also referred to as resource competition, is a form of competition in which one species consumes and either reduces or more efficiently uses a shared limiting resource and therefore depletes the availability of the resource for the other species. Thus, it is an indirect interaction because the competing species interact via a shared resource.Interference competition is a form of competition in which individuals of one species interacts directly with individuals of another species via antagonistic displays or more aggressive behavior.
In a review and synthesis of experimental evidence regarding interspecific competition, Schoener described six specific types of mechanisms by which competition occurs, including consumptive, preemptive, overgrowth, chemical, territorial, and encounter. Consumption competition is always resource competition, but the others cannot always be regarded as exclusively exploitative or interference.
Separating the effect of resource use from that of interference is not easy. A good example of exploitative competition is found in aphid species competing over the sap in plant phloem. Each aphid species that feeds on host plant sap uses some of the resource, leaving less for competing species. In one study, Fordinae geoica was observed to out-compete F. formicaria to the extent that the latter species exhibited a reduction in survival by 84%. Another example is the one of competition for calling space in amphibians, where the calling activity of a species prevents the other one from calling in an area as wide as it would in allopatry. A last example is driving of bisexual rock lizards of genus Darevskia from their natural habitats by a daughter unisexual form; interference competition can be ruled out in this case, because parthenogenetic forms of the lizards never demonstrate aggressive behavior.
This type of competition can also be observed in forests where large trees dominate the canopy and thus allow little light to reach smaller competitors living below. These interactions have important implications for the population dynamics and distribution of both species.
Based on outcome
Scramble and contest competition refer to the relative success of competitors. Scramble competition is said to occur when each competitor is equal suppressed, either through reduction in survival or birth rates. Contest competition is said to occur when one or a few competitors are unaffected by competition, but all others suffer greatly, either through reduction in survival or birth rates. Sometimes these types of competition are referred to as symmetric vs. asymmetric competition. Scramble and contest competition are two ends of a spectrum, of completely equal or completely unequal effects.Apparent competition
Apparent competition is actually an example of predation that alters the relative abundances of prey on the same trophic level. It occurs when two or more species in a habitat affect shared natural enemies in a higher trophic level. If two species share a common predator, for example, apparent competition can exist between the two prey items in which the presence of each prey species increases the abundance of the shared enemy, and thereby suppresses one or both prey species. This mechanism gets its name from experiments in which one prey species is removed and the second prey species increases in abundance. Investigators sometimes mistakenly attribute the increase in abundance in the second species as evidence for resource competition between prey species. It is "apparently" competition, but is in fact due to a shared predator, parasitoid, parasite, or pathogen. Notably, species competing for resources may often also share predators in nature. Interactions via resource competition and shared predation may thus often influence one another, thus making it difficult to study and predict their outcome by only studying one of them.Consequences
Many studies, including those cited previously, have shown major impacts on both individuals and populations from interspecific competition. Documentation of these impacts has been found in species from every major branch of organism. The effects of interspecific competition can also reach communities and can even influence the evolution of species as they adapt to avoid competition. This evolution may result in the exclusion of a species in the habitat, niche separation, and local extinction. The changes of these species over time can also change communities as other species must adapt.Competitive exclusion
The competitive exclusion principle, also called "Gause's law" which arose from mathematical analysis and simple competition models states that two species that use the same limiting resource in the same way in the same space and time cannot coexist and must diverge from each other over time in order for the two species to coexist. One species will often exhibit an advantage in resource use. This superior competitor will out-compete the other with more efficient use of the limiting resource. As a result, the inferior competitor will suffer a decline in population over time. It will be excluded from the area and replaced by the superior competitor.A well-documented example of competitive exclusion was observed to occur between Dolly Varden charr and white spotted char in Japan. Both of these species were morphologically similar but the former species was found primarily at higher elevations than the latter. Although there was a zone of overlap, each species excluded the other from its dominant region by becoming better adapted to its habitat over time. In some such cases, each species gets displaced into an exclusive segment of the original habitat. Because each species suffers from competition, natural selection favors the avoidance of competition in such a way.
Niche differentiation is a process by which competitive exclusion leads to differences in resource use. In some cases, niche differentiation results in spatial displacement, where species avoid direct competition by occupying different areas. However, niche differentiation can also cause other changes, such as altered behaviors or ecological roles, that help species avoid competition. If competition avoidance is possible, species may specialize in different areas of the niche, minimizing overlap and resource competition. For example, spotted hyenas and lions in Africa share similar habitats and prey but have different hunting strategies. Hyenas use stamina to chase prey over long distances, while lions rely on ambush hunting. This difference in hunting strategies helps reduce direct competition for food.
Another example of niche differentiation comes from birds, where species with similar ecological requirements shift their behavior to avoid competition. In the Galapagos Islands, finch species have been observed to change their feeding habits within a few generations, adapting to new dietary resources to minimize competition. This adaptation allowed different finch species to coexist despite overlapping habitats and food sources. Similarly, hyenas and lions may alter their roles in the ecosystem through spatial and behavioral differentiation, helping them avoid direct conflict and share resources.
In some ecosystems, niche differentiation is influenced by third-party species or predators. For example, a keystone predator can significantly alter the behavior of competing species. Hyenas, by preying on lions or scavenging their kills, can reduce the lions' ability to dominate a territory. This helps other predators and scavengers, like cheetahs, access resources they might otherwise be excluded from. Additionally, in bacterial ecosystems, phage parasites have been shown to mediate coexistence between competing bacterial species by reducing the dominance of one species. This kind of interaction helps maintain biodiversity in microbial communities, which can have important implications for both medical research and ecological theory.