Interspecific competition

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 (e.g., resource or interference) and an outcome (symmetric or asymmetric).

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.[5] 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.[6] 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.

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.

Impacts on communities

Naturalised purple-loosestrife plants growing in the Cooper Marsh Conservation Area, near Cornwall Ontario.

As mentioned previously, interspecific competition has great impact on community composition and structure. Niche separation of species, local extinction and competitive exclusion are only some of the possible effects. In addition to these, interspecific competition can be the source of a cascade of effects that build on each other. An example of such an effect is the introduction of an invasive species to the United States, purple-loosestrife. This plant when introduced to wetland communities often outcompetes much of the native flora and decreases species richness, food and shelter to many other species at higher trophic levels. In this way, one species can influence the populations of many other species as well as through a myriad of other interactions. Because of the complicated web of interactions that make up every ecosystem and habitat, the results of interspecific competition are complex and site-specific.

The impacts of interspecific competition on populations have been formalized in a mathematical model called the Competitive Lotka–Volterra equations, which creates a theoretical prediction of interactions. It combines the effects of each species on the other. These effects are calculated separately for the first and second population respectively:

${\displaystyle {dN_{1} \over dt}=r_{1}N_{1}{K_{1}-N_{1}-\alpha N_{2} \over K_{1}}}$

${\displaystyle {dN_{2} \over dt}=r_{2}N_{2}{K_{2}-N_{2}-\beta N_{1} \over K_{2}}}$

In these formulae, N is the population size, t is time, K is the carrying capacity, r is the intrinsic rate of increase and α and β are the relative competition coefficients.[10] The results show the effect that the other species has on the species being calculated. The results can be graphed to show a trend and possible prediction for the future of the species. One problem with this model is that certain assumptions must be made for the calculation to work. These include the lack of migration and constancy of the carrying capacities and competition coefficients of both species. The complex nature of ecology determines that these assumptions are rarely true in the field but the model provides a basis for improved understanding of these important concepts.

An equivalent formulation of these models[11] is:

${\displaystyle {dN_{1} \over dt}=r_{1}N_{1}\left(1-\alpha _{11}N_{1}-\alpha _{12}N_{2}\right)}$

${\displaystyle {dN_{2} \over dt}=r_{2}N_{2}\left(1-\alpha _{21}N_{1}-\alpha _{22}N_{2}\right)}$

In these formulae, ${\displaystyle \alpha _{11}}$ is the effect that an individual of species 1 has on its own population growth rate. Similarly, ${\displaystyle \alpha _{12}}$ is the effect that an individual of species 2 has on the population growth rate of species 1. One can also read this as the effect on species 1 of species 2. In comparing this formulation to the one above, we note that ${\displaystyle \alpha _{11}=1/K_{1},~\alpha _{22}=1/K_{2}}$, and ${\displaystyle \alpha _{12}=\alpha /K_{1}}$.

Coexistence between competitors occurs when ${\displaystyle \alpha _{11}>\alpha _{12}}$ and ${\displaystyle \alpha _{22}>\alpha _{21}}$. We can translate this as coexistence occurs when the effect of each species on itself is greater the effect of the competitor.

There are other mathematical representations that model species competition, such as using non-polynomial functions.[12]

Interspecific competition is a major factor in macroevolution.[13] Darwin assumed that interspecific competition limits the number of species on Earth, as formulated in his wedge metaphor: "Nature may be compared to a surface covered with ten-thousand sharp wedges ... representing different species, all packed closely together and driven in by incessant blows, . . . sometimes a wedge of one form and sometimes another being struck; the one driven deeply in forcing out others; with the jar and shock often transmitted very far to other wedges in many lines of direction." (From Natural Selection - the "big book" from which Darwin abstracted the Origin).[14] The question whether interspecific competition limits global biodiversity is disputed today,[15] but analytical studies of the global Phanerozoic fossil record are in accordance with the existence of global (although not constant) carrying capacities for marine biodiversity.[16][17] Interspecific competition is also the basis for Van Valen's Red Queen hypothesis, and it may underlie the positive correlation between origination and extinction rates that is seen in almost all major taxa.[13]

In the previous examples, the macroevolutionary role of interspecific competition is that of a limiting factor of biodiversity, but interspecific competition also promotes niche differentiation and thus speciation and diversification.[18][19] The impact of interspecific competition may therefore change during phases of diversity build-up, from an initial phase where positive feedback mechanisms dominate to a later phase when niche-peremption limits further increase in the number of species; a possible example for this situation is the re-diversification of marine faunas after the end-Permian mass extinction event.[20]

• Minimum viable population
• Symbiosis
• Mutualism (biology)
• Macroevolution
• Red Queen hypothesis

• Competition for Territory: The Levins Model for Two Species Wolfram Demonstrations Project — requires CDF player (free)