15.1: Introduction and Types of Competition
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Competition
Competition is an interaction between organisms or species in which both require a resource that is in limited supply (such as food, water, or territory) (Begon et al. 1996). Competition lowers the fitness of both organisms involved, since the presence of one of the organisms always reduces the amount of the resource available to the other (Lang & Benbow 2013).
Figure \(\PageIndex{1}\) Sea anemones compete for the territory in tide pools.
In the study of community ecology, competition within and between members of a species is an important biological interaction. Competition is one of many interacting biotic and abiotic factors that affect community structure, species diversity, and population dynamics (shifts in a population over time) (Lang & Benbow 2013). Competition within and between species for resources is important in natural selection.
There are three major mechanisms of competition: interference, exploitation, and apparent competition (in order from most direct to least direct). Interference and exploitation competition can be classed as "real" forms of competition, while apparent competition is not, as organisms do not share a resource, but instead share a predator (Lang & Benbow 2013). Competition among members of the same species is known as intraspecific competition, while competition between individuals of different species is known as interspecific competition. Diffuse competition refers to the summed effects of all competitors.
Studies show that intraspecific competition can regulate population dynamics (changes in population size over time). This occurs because individuals become crowded as a population grows. Since individuals within a population require the same resources, crowding causes resources to become more limited. Some individuals (typically small juveniles) eventually do not acquire enough resources and die or do not reproduce. This reduces population size and slows population growth. The logistic growth model, which includes a term for the species’ carrying capacity, is a model of intraspecific competition.
Species also interact with other species that require the same resources. Consequently, interspecific competition can alter the sizes of many species' populations at the same time. Experiments demonstrate that when species compete for a limited resource, one species eventually drives the populations of other species extinct. According to the competitive exclusion principle, species less suited to compete for resources must either adapt or die out, although competitive exclusion is rarely found in natural ecosystems (Lang & Benbow 2013). Ecologists use a modified version of coupled logistic growth models for competing species, known as the Lotka-Volterra Competition model, to model interspecific competition.
Figure \(\PageIndex{2}\): The three different mechanisms of competition. Solid arrows indicate direct relationships, dashed lines represent indirect relationships. Developed by N. Gownaris.
15.1 Types of Competition
15.1.1 Interference Competition
Figure \(\PageIndex{3}\): Male-male competition in red deer during rut is an example of interference competition within a species.
During interference competition, also called contest competition, organisms of the same species or of two or more different species interact directly by competing for scarce resources. For example, large aphids defend feeding sites on cottonwood leaves by ejecting smaller aphids from better sites. Male-male competition in red deer during rut is an example of interference competition that occurs within a species (intraspecific competition).
Interference competition occurs directly between individuals via aggression when the individuals interfere with foraging, survival, reproduction of others, or by directly preventing their physical establishment in a portion of the habitat. An example of this can be seen between the ant Novomessor cockerelli and red harvester ants, where the former interferes with the ability of the latter to forage by plugging the entrances to their colonies with small rocks (Barton et al. 2002). Male bowerbirds, who create elaborate structures called bowers to attract potential mates, may reduce the fitness of their neighbors directly by stealing decorations from their structures (Le Bourlot et al. 2014).
In animals, interference competition is a strategy mainly adopted by larger and stronger organisms within a habitat. As such, populations with high interference competition have adult-driven generation cycles. At first, the growth of juveniles is stunted by larger adult competitors. However, once the juveniles reach adulthood, they experience a secondary growth cycle (Le Bourlot et al. 2014). Plants, on the other hand, primarily engage in interference competition with their neighbors through allelopathy, or the production of biochemicals (Schenk 2006).
Interference competition can be seen as a strategy that has a clear cost (injury or death) and benefit (obtaining resources that would have gone to other organisms) (Case & Gilpin 1975). In order to cope with strong interference competition, other organisms often either do the same or engage in exploitation competition. For example, depending on the season, larger ungulate red deer males are competitively dominant due to interference competition. However, does and fawns have dealt with this through temporal resource partitioning — foraging for food only when adult males are not present (Stone et al. 2018).
15.1.2 Exploitation Competition
Exploitation competition, or scramble competition, occurs indirectly when organisms both use a common limiting resource or shared food item. Instead of fighting or exhibiting aggressive behavior in order to win resources, exploitative competition occurs when resource use by one organism depletes the total amount available for the other organism. These organisms might never interact directly, but compete by responding to changes in resource levels. Very obvious examples of this phenomenon include a diurnal species and a nocturnal species that nevertheless share the same resources, or a plant that competes with neighboring plants for light, nutrients, and space for root growth (Jensen 1987).
This form of competition typically rewards those organisms who claim the resource first. As such, exploitation competition is often size-dependent and smaller organisms are favored since smaller organisms typically have higher foraging rates (Le Bourlot et al. 2014). Since smaller organisms have an advantage when exploitative competition is important in an ecosystem, this mechanism of competition might lead to a juvenile-driven generation cycle: individual juveniles succeed and grow fast, but once they mature they are outcompeted by smaller organisms (Le Bourlot et al. 2014).
In plants, exploitative competition can occur both above- and below-ground. Aboveground, plants reduce the fitness of their neighbors by vying for sunlight. Plants consume nitrogen by absorbing it into their roots, making nitrogen unavailable to nearby plants. Plants that produce many roots typically reduce soil nitrogen to very low levels, eventually killing neighboring plants.
Exploitative competition has also been shown to occur both within species (intraspecific) and between different species (interspecific). Furthermore, many competitive interactions between organisms are some combination of exploitative and interference competition, meaning the two mechanisms are far from mutually exclusive. For example, a recent 2019 study found that the native thrips species Frankliniella intonsa was competitively dominant over an invasive thrips species Frankliniella occidentalis because it not only exhibited greater time feeding (exploitative competition) but also greater time guarding its resources (interference competition) (Bhuyain & Lim 2019). Plants may also exhibit both forms of competition, not only scrambling for space for root growth but also directly inhibiting other plants' development through allelopathy.
15.1.3 Apparent Competition
Apparent competition occurs when two otherwise unrelated prey species indirectly compete for survival through a shared predator (Holt 1977). This form of competition typically manifests in new equilibrium abundances of each prey species. For example, suppose there are two species (species A and species B), which are preyed upon by food-limited predator species C. Scientists observe an increase in the abundance of species A and a decline in the abundance of species B. In an apparent competition model, this relationship is found to be mediated through predator C; a population explosion of species A increases the abundance of the predator species C due to a greater total food source. Since there are now more predators, species A and B would be hunted at higher rates than before. Thus, the success of species A was to the detriment of species B — not because they competed for resources, but because their increased numbers had indirect effects on the predator population.
References
Barton, K.E., Sanders, N.J., & Gordon, D.M. (2002). The effects of proximity and colony age on interspecific interference competition between the desert ants, Pogonomyrmex barbatus and Aphaenogaster cockerelli. The American Midland Naturalist, 148(2), pp. 376–382. doi:10.1674/0003-0031
Begon, M., Harper, J.L., & Townsend, C.R. (1996). Ecology: Individuals, populations and communities. Blackwell Science.
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Schenk, H.J. (2006). Root competition: Beyond resource depletion. Journal of Ecology, 94(4), pp. 725–739. doi:10.1111/j.1365-2745.2006.01124.x
Stone, D.B., Martin, J.A., Cohen, B.S., Prebyl, T.J., Killmaster, C., & Miller, K.V. (2018). Intraspecific temporal resource partitioning at white-tailed deer feeding sites. Current Zoology, 65(2), pp. 139–146. doi:10.1093/cz/zoy05
Contributors and Attributions
This chapter was written by N. Gownaris and T. Zallek, with text taken from the following CC-BY resources: