Competition (biology)
Competition is an
In the study of
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.[3] Competition among members of the same species is known as intraspecific competition, while competition between individuals of different species is known as interspecific competition.
According to the
Interference competition
During interference competition, also called contest competition, organisms interact directly by fighting 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.
Interference competition occurs directly between individuals via
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.[7] At first, the growth of juveniles is stunted by larger adult competitors. However, once the juveniles reach adulthood, they experience a secondary growth cycle.[7] Plants, on the other hand, primarily engage in interference competition with their neighbors through allelopathy, or the production of biochemicals.[8]
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).[9] 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.[10]
Exploitation competition
Exploitation competition, or
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.[7] 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.[7]
In plants, exploitative competition can occur both above- and below ground. Aboveground, plants reduce the fitness of their neighbors by vying for sunlight
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 thrip species Frankliniella intonsa was competitively dominant over an invasive thrip species Frankliniella occidentalis because it not only exhibited greater time feeding (exploitative competition) but also greater time guarding its resources (interference competition).[12] 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.
Apparent competition
Apparent competition occurs when two otherwise unrelated prey species indirectly compete for survival through a shared predator.[13] 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 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.
This one-predator/two-prey model has been explored by ecologists as early as 1925, but the term "apparent competition" was first coined by University of Florida ecologist Robert D. Holt in 1977.
Apparent competition and realized niche
Apparent competition can help shape a species'
Asymmetric apparent competition
Apparent competition can be symmetric or asymmetric.
Apparent competition in the human microbiome
Apparent competition has also been viewed in and on the human body. The human immune system can acts as the generalist predator, and a high abundance of a certain bacteria may induce an immune response, damaging all pathogens in the body. Another example of this is of two populations of bacteria that can both support a predatory bacteriophage. In most situations, the one that is most resistant to infection by the shared predator will replace the other.[17]
Apparent competition has also been suggested as an exploitable phenomenon for cancer treatments. Highly specialized viruses that are developed to target malignant cancer cells often go locally extinct prior to eradicating all cancer. However, if a virus were developed that targets both healthy and unhealthy host cells to some degree, the large number of healthy cells would support the predatory virus for long enough to eliminate all malignant cells.[17]
Size-asymmetric competition
Competition can be either complete symmetric (all individuals receive the same amount of resources, irrespective of their size), perfectly size symmetric (all individuals exploit the same amount of resource per unit biomass), or absolutely size-asymmetric (the largest individuals exploit all the available resource).
Among plants, size asymmetry is context-dependent and competition can be both asymmetric and symmetric depending on the most limiting resource. In forest stands, below-ground competition for nutrients and water is size-symmetric, because a tree's root system is typically proportionate to the biomass of the entire tree.[22] Conversely, above-ground competition for light is size-asymmetric — since light has directionality, the forest canopy is dominated entirely by the largest trees. These trees disproportionately exploit most of the resource for their biomass, making the interaction size asymmetric.[23] Whether above-ground or below-ground resources are more limiting can have major effects on the structure and diversity of ecological communities; in mixed beech stands, for example, size-asymmetric competition for light is a stronger predictor of growth compared with competition for soil resources.[24]
Within and between species
Competition can occur between individuals of the same species, called intraspecific competition, or between different species, called interspecific competition. Studies show that intraspecific competition can regulate population dynamics (changes in population size over time). This occurs because individuals become crowded as the 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.[citation needed]
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. These experiments suggest that competing species cannot coexist (they cannot live together in the same area) because the best competitor will exclude all other competing species.[citation needed]
Intraspecific
Intraspecific competition occurs when members of the same species compete for the same resources in an ecosystem.[25] A simple example is a stand of equally-spaced plants, which are all of the same age. The higher the density of plants, the more plants will be present per unit ground area, and the stronger the competition will be for resources such as light, water, or nutrients.
Interspecific
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. An example among animals could be the case of cheetahs and lions; since both species feed on similar prey, they are negatively impacted by the presence of the other because they will have less food, however, they still persist together, despite the prediction that under competition one will displace the other. In fact, lions sometimes steal prey items killed by cheetahs. Potential competitors can also kill each other, in so-called 'intraguild predation'. For example, in southern California coyotes often kill and eat gray foxes and bobcats, all three carnivores sharing the same stable prey (small mammals).[26]
An example among protozoa involves Paramecium aurelia and Paramecium caudatum. Russian ecologist, Georgy Gause, studied the competition between the two species of Paramecium that occurred as a result of their coexistence. Through his studies, Gause proposed the Competitive exclusion principle, observing the competition that occurred when their different ecological niches overlapped.[27]
Competition has been observed between individuals, populations, and species, but there is little evidence that competition has been the driving force in the evolution of large groups. For example, mammals lived beside reptiles for many millions of years of time but were unable to gain a competitive edge until dinosaurs were devastated by the Cretaceous–Paleogene extinction event.[4]
Evolutionary strategies
In evolutionary contexts, competition is related to the concept of r/K selection theory, which relates to the
In
where r is the
Competitive exclusion principle
To explain how species coexist, in 1934
Character displacement
Competition can cause species to evolve differences in traits. This occurs because the individuals of a species with traits similar to competing species always experience strong interspecific competition. These individuals have less reproduction and survival than individuals with traits that differ from their competitors. Consequently, they will not contribute many offspring to future generations. For example,
See also
- Biological interaction
- Character displacement
- Community
- Minimum viable population
- Scramble competition
- Resource (biology)
- Resource partitioning
References
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