Intraspecific competition

Intraspecific competition is an interaction in population ecology, whereby members of the same species compete for limited resources. This leads to a reduction in fitness for both individuals.[1] By contrast, interspecific competition occurs when members of different species compete for a shared resource. Members of the same species have very similar resources requirements whereas different species have a smaller contested resource overlap, resulting in intraspecific competition generally being a stronger force than interspecific competition.[2]

Individuals can compete for food, water, space, light, mates or any other resource which is required for survival or reproduction. The resource must be limited for competition to occur; if every member of the species can obtain a sufficient amount of every resource then individuals do not compete and the population grows exponentially.[1] Exponential growth is very rare in nature because resources are finite and so not every individual in a population can survive, leading to intraspecific competition for the scarce resources.

When resources are limited, an increase in population size reduces the quantity of resources available for each individual, reducing the per capita fitness in the population. As a result, the growth rate of a population slows as intraspecific competition becomes more intense, making it a negatively density dependent process. The falling population growth rate as population increases can be modelled effectively with the logistic growth model.[3] The rate of change of population density eventually falls to zero, the point ecologists have termed the carrying capacity (K). The carrying capacity of a population is the maximum number of individuals that can live in a population stably; numbers larger than this will suffer a negative population growth until eventually reaching the carrying capacity, whereas populations smaller than the carrying capacity will grow until they reach it.

Intraspecific competition doesn't just involve direct interactions between members of the same species (such as male deer locking horns when competing for mates) but can also include indirect interactions where an individual depletes a shared resource (such as a grizzly bear catching a salmon that can then no longer be eaten by bears at different points along a river).

The way in which resources are partitioned by organisms also varies and can be split into scramble and contest competition. Scramble competition involves a relatively even distribution of resources among a population as all individuals exploit a common resource pool. In contrast, contest competition is the uneven distribution of resources and occurs when hierarchies in a population influence the amount of resource each individual receives. Organisms in the most prized territories or at the top of the hierarchies obtain a sufficient quantity of the resources, whereas individuals without a territory don’t obtain any of the resource.[1]

Male hartebeest locking horns and fiercely defending their territories. An example of direct competition.

Mechanisms

Direct

Interference competition is the process by which individuals directly compete with one another in pursuit of a resource. It can involve fighting, stealing or ritualised combat. Direct intraspecific competition also includes animals claiming a territory which then excludes other animals from entering the area. There may not be an actual conflict between the two competitors, but the animal excluded from the territory suffers a fitness loss due to a reduced foraging area and is unable to enter the area as it risks confrontation from a more dominant member of the population. As organisms are encountering each other during interference competition, they are able to evolve behavioural strategies and morphologies to out-compete rivals in their population.[4]

Flamingos competing via interference competition, potentially for territories, mates or food.

For example, different populations of the northern slimy salamander (Plethodon glutinosus) have evolved varying levels of aggression depending on the intensity of intraspecific competition. In populations where the resources are scarcer, more aggressive behaviours are likely to evolve. It is a more effective strategy to fight rivals within the species harder instead of searching for other options due to the lack of available food.[5] More aggressive salamanders are more likely obtain the resources they require to reproduce whereas timid salamanders may starve before reproducing, so aggression can spread through the population.

In addition, a study on Chilean flamingos (Phoenicopterus chilensis) found that birds in a bond were much more aggressive than single birds. The paired birds were significantly more likely to start an agonistic encounter in defense of their mate or young whereas single birds were typically non-breeding and less likely to fight.[6] Not all flamingos can mate in the population because of an unsuitable sex ratio or some dominant flamingos mating with multiple partners. Mates are a fiercely contested resource in many species as the production of offspring is essential for an individual to propagate its genes.

Indirect

Organisms can compete indirectly, either via exploitative or apparent competition. Exploitative competition involves individuals depleting a shared resource and both suffering a loss in fitness as a result. The organisms may not actually come into contact and only interact via the shared resource indirectly.

For instance, exploitative competition has been shown experimentally between juvenile wolf spiders (Schizocosa ocreata). Both increasing the density of young spiders and reducing the available food supply lowered the growth of individual spiders. Food is clearly a limiting resource for the wolf spiders but there was no direct competition between juveniles for food, just a reduction in fitness due to the increased population density.[7] The negative density dependence in young wolf spiders is evident: as the population density increases further, growth rates continues to fall and could potentially reach zero (as predicted by the logistic growth model). This is also seen in Viviparous lizard, or Lacerta vivipara, where the existence of color morphs within a population depends on the density and intraspecific competition.

In stationary organisms, such as plants, exploitative competition plays a much larger role than interference competition because individuals are rooted to a specific area and utilise resources in their immediate surroundings. Saplings will compete for light, most of which will be blocked and utilised by taller trees.[8] The saplings can be easily out-competed by larger members of their own species, which is one of the reasons why seed dispersal distances can be so large. Seeds that germinate in close proximity to the parents are very likely to be out-competed and die.

Apparent competition occurs in populations that are predated upon. An increase in population of the prey species will bring more predators to the area, which increases the risk of an individual being eaten and hence lowers its survivorship. Like exploitative competition, the individuals aren’t interacting directly but rather suffer a reduction in fitness as a consequence of the increasing population size. Apparent competition is generally associated with inter rather than intraspecific competition, whereby two different species share a common predator. An adaptation that makes one species less likely to be eaten results in a reduction in fitness for the other prey species because the predator species hunts more intensely as food has become more difficult to obtain. For example, native skinks (Oligosoma) in New Zealand suffered a large decline in population after the introduction of rabbits (Oryctolagus cuniculus).[9] Both species are eaten by ferrets (Mustela furo) so the introduction of rabbits resulted in immigration of ferrets to the area, which then depleted skink numbers.

Resource partitioning

Contest

Contest competition takes place when a resource is associated with a territory or hierarchical structure within the population. For instance: white-faced capuchin monkeys (Cebus capucinus) have different energy intakes based on their ranking within the group.[10] Both males and females compete for territories with the best access to food and the most successful monkeys are able to obtain a disproportionately large quantity of food and therefore have a higher fitness in comparison to the subordinate members of the group. In the case of Ctenophorus pictus lizards, males compete for territory. Among the polymorphic variants, red lizards have are more aggressive in defending their territory compared to their yellow counterparts.[11]

Aggressive encounters are potentially costly for individuals as they can get injured and be less able to reproduce. As a result, many species have evolved forms of ritualised combat to determine who wins access to a resource without having to undertake a dangerous fight. Male adders (Vipera berus) undertake complex ritualised confrontations when courting females. Generally, the larger male will win and fights rarely escalate to injury to either combatant.[12]

However, sometimes the resource may be so prized that potentially fatal confrontations can occur to acquire them. Male elephant seals, Mirounga augustirostris, engage in fierce competitive displays in an attempt to control a large harem of females with which to mate. The distribution of females and subsequent reproductive success is very uneven between males. The reproductive success of most males is zero; they die before breeding age or are prevented from mating by higher ranked males. In addition, just a few dominant males account for the majority of copulations.[13] The potential reproductive success for males is so great that many are killed before breeding age as they attempt to move up the hierarchy in their population.

Contest competition produces relatively stable population dynamics. The uneven distribution of resources results in some individuals dying off but helps to ensure that the members of the population that hold a territory can reproduce. As the number of territories in an area stays the same over time, the breeding population remains constant which produces a similar number of new individuals every breeding season.

Scramble

Scramble competition involves a more equal distribution of resources than contest competition and occurs when there is a common resource pool that an individual cannot be excluded from. For instance, grazing animals compete more strongly for grass as their population grows and food becomes a limiting resource. Each herbivore receives less food as more individuals compete for the same quantity of food.[4]

Scramble completion can lead to unstable population dynamics, the equal division of resources can result in very few of the organisms obtaining enough to survive and reproduce and this can cause population crashes. This phenomenon is called overcompensation. For instance, the caterpillars of cinnabar moths feed via scramble competition, and when there are too many caterpillars competing very few are able to pupate and there is a large population crash.[14] Subsequently, very few cinnabar moths are competing intraspecifically in the next generation so the population grows rapidly before crashing again.

Consequences of intraspecific competition

Slowed growth rates

Exponential human population growth in the last 1,000 years.

The major impact of intraspecific competition is reduced population growth rates as population density increases. When resources are infinite, intraspecific competition does not occur and populations can grow exponentially. Exponential population growth is exceedingly rare, but has been documented, most notably in humans since 1900. Elephant (Loxodonta africana) populations in Kruger National Park (South Africa) also grew exponentially in the mid-1900s after strict poaching controls were put in place.[15]

${\displaystyle {dN(t) \over dt}=rN(t)\left(1-{\frac {N(t)}{K}}\right)}$.

dN(t)/dt = rate of change of population density

N(t) = population size at time t

r = per capita growth rate

K = carrying capacity

Population growth against time in a population growing logistically. The steepest parts of the graph are where the population growth is most rapid.

The logistic growth equation is an effective tool for modelling intraspecific competition despite its simplicity, and has been used to model many real biological systems. At low population densities, N(t) is much smaller than K and so the may determinant for population growth is just the per capita growth rate. However, as N(t) approaches the carrying capacity the second term in the logistic equation becomes smaller, reducing the rate of change of population density.[16]

The logistic growth curve is initially very similar to the exponential growth curve. When population density is low, individuals are free from competition and can grow rapidly. However, as the population reaches its maximum (the carrying capacity), intraspecific competition becomes fiercer and the per capita growth rate slows until the population reaches a stable size. At the carrying capacity, the rate of change of population density is zero because the population is as large as possible based on the resources available.[4] Experiments on Daphnia growth rates showed a striking adherence to the logistic growth curve.[17] The inflexion point in the Daphnia population density graph occurred at half the carrying capacity, as predicted by the logistic growth model.

Gause’s 1930s lab experiments showed logistic growth in microorganisms. Populations of yeast grown in test tubes initially grew exponentially. But as resources became scarcer, their growth rates slowed until reaching the carrying capacity.[3] If the populations were moved to a larger container with more resources they would continue to grow until reaching their new carrying capacity. The shape of their growth can be modeled very effectively with the logistic growth model.

References

1. ^ a b c Townsend (2008). Essentials of Ecology. pp. 103–105. ISBN 978-1-4051-5658-5.
2. ^ Connell, Joseph (November 1983). "On the prevalence and relative importance of interspecific competition: evidence from field experiments" (PDF). American Naturalist. 122 (5): 661–696. doi:10.1086/284165. Archived from the original (PDF) on 2014-10-26.
3. ^ a b Gause, Georgy (October 1932). "Experimental studies on the struggle for existence". Journal of Experimental Biology. 9 (4): 389–402.
4. ^ a b c Keddy, Paul (2001). Competition. Dordrecht. ISBN 978-1402002298.
5. ^ Nishikawa, Kiisa (1985). "Competition and the evolution of aggressive behavior in two species of terrestrial salamanders" (PDF). Evolution. 39 (6): 1282–1294. doi:10.2307/2408785. JSTOR 2408785. PMID 28564270.
6. ^ Perdue, Bonnie M.; Gaalema, Diann E.; Martin, Allison L.; Dampier, Stephanie M.; Maple, Terry L. (2010-02-22). "Factors affecting aggression in a captive flock of Chilean flamingos (Phoenicopterus chilensis)". Zoo Biology. 30 (1): 59–64. doi:10.1002/zoo.20313. PMID 20186725.
7. ^ Wise, David; Wagner (August 1992). "Evidence of exploitative competition among young stages of the wolf spider Schizocosa ocreata". Oecologia. 91 (1): 7–13. doi:10.1007/BF00317234. PMID 28313367.
8. ^ Connell, Joseph (1990). Perspectives on Plant Competition. The Blackburn Press. pp. 9–23. ISBN 978-1930665859.
9. ^ Norbury, Grant (December 2001). "Conserving dryland lizards by reducing predator-mediated apparent competition and direct competition with introduced rabbits". Journal of Applied Ecology. 38 (6): 1350–1361. doi:10.1046/j.0021-8901.2001.00685.x.
10. ^ Vogel, Erin (August 2005). "Rank differences in energy intake rates in white-faced capuchin monkeys, Cebus capucinus: the effects of contest competition". Behavioral Ecology and Sociobiology. 58 (4): 333–344. doi:10.1007/s00265-005-0960-4. JSTOR 25063623.
11. ^ Olsson, Mats; Schwartz, Tonia; Uller, Tobias; Healey, Mo (February 2009). "Effects of sperm storage and male colour on probability of paternity in a polychromatic lizard". Animal Behaviour. 77 (2): 419–424. doi:10.1016/j.anbehav.2008.10.017.
12. ^ Madsen, Thomas; Shine, Richard (1993). "Temporal variability in sexual selection acting on reproductive tactics and body size in male snakes". The American Naturalist. 141 (1): 166–171. doi:10.1086/285467. JSTOR 2462769. PMID 19426025.
13. ^ Le Bouef, Burney (1974). "Male-male Competition and Reproductive Success in Elephant Seals". Integrative and Comparative Biology. 14 (1): 163–176. doi:10.1093/icb/14.1.163.
14. ^ Crawley, Mick; Gillman (April 1990). "A comparative evaluation of models of cinnabar moth dynamics". Oecologia. 82 (4): 437–445. doi:10.1007/BF00319783. PMID 28311465.
15. ^ Young, Kim; Ferreira, Van Aarde (March 2009). "The influence of increasing population size and vegetation productivity on elephant distribution in the Kruger National Park". Austral Ecology. 34 (3): 329–342. doi:10.1111/j.1442-9993.2009.01934.x.
16. ^ Hanson, Floyd (1981). "Logistic growth with random density independent disasters". Theoretical Population Biology. 19 (1): 1–18. doi:10.1016/0040-5809(81)90032-0.
17. ^ Schoener, Thomas (March 1973). "Population growth regulated by intraspecific competition for energy or time: Some simple representations". Theoretical Population Biology. 4 (1): 56–84. doi:10.1016/0040-5809(73)90006-3. PMID 4726010.
Autotoxicity

Autotoxicity, meaning self-toxicity, is a biological phenomenon whereby a species inhibits growth or reproduction of other members of its same species through the production of chemicals released into the environment. Like allelopathy, it is a type of interference competition but it is technically different: autotoxicity and contributes to intraspecific competition, whereas allelopathic effects refer to interspecific competition. Furthermore, autotoxic effects are always inhibitory, whereas allelopathic effects are not necessarily inhibitory–they may stimulate other organisms.This mechanism will result in reduced exploitative competition between members of the same species and may contribute to natural thinning in established communities. Inhibition of the growth of young plants will increase the availability of nutrients to older, established plants.

In cultivation, autotoxicity can make it difficult or impossible to grow the same species after harvest of a crop. For example, this is known in alfalfa and the tree Cunninghamia lanceolata Other species displaying autotoxicity include the rush Juncus effusus and the grass Lolium rigidum.

Bacterivore

Bacterivores are free-living, generally heterotrophic organisms, exclusively microscopic, which obtain energy and nutrients primarily or entirely from the consumption of bacteria. Many species of amoeba are bacterivores, as well as other types of protozoans. Commonly, all species of bacteria will be prey, but spores of some species, such as Clostridium perfringens, will never be prey, because of their cellular attributes.

Biological specificity

In biology, biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species.

Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organisms or constituents of living organisms of being special or doing something special. Each animal or plant species is special. It differs in some way from all other species... biological specificity is the major problem about understanding life."

Collective animal behavior

Collective animal behavior is a form of social behavior involving the coordinated behavior of large groups of similar animals as well as emergent properties of these groups. This can include the costs and benefits of group membership, the transfer of information across the group, the group decision-making process, and group locomotion and synchronization. Studying the principles of collective animal behavior has relevance to human engineering problems through the philosophy of biomimetics. For instance, determining the rules by which an individual animal navigates relative to its neighbors in a group can lead to advances in the deployment and control of groups of swimming or flying micro-robots such as UAVs (Unmanned Aerial Vehicles).

Competition (biology)

Competition is an interaction between organisms or species in which both the organisms or species are harmed. Limited supply of at least one resource (such as food, water, and territory) used by both can be a factor. Competition both within and between species is an important topic in ecology, especially community ecology. Competition is one of many interacting biotic and abiotic factors that affect community structure. Competition among members of the same species is known as intraspecific competition, while competition between individuals of different species is known as interspecific competition. Competition is not always straightforward, and can occur in both a direct and indirect fashion.According to the competitive exclusion principle, species less suited to compete for resources should either adapt or die out, although competitive exclusion is rarely found in natural ecosystems. According to evolutionary theory, this competition within and between species for resources is important in natural selection. However, competition may play less of a role than expansion among larger clades; this is termed the 'Room to Roam' hypothesis.

Copiotroph

A copiotroph is an organism found in environments rich in nutrients, particularly carbon. They are the opposite to oligotrophs, which survive in much lower carbon concentrations.

Copiotrophic organisms tend to grow in high organic substrate conditions. For example, copiotrophic organisms grow in Sewage lagoons. They grow in organic substrate conditions up to 100x higher than oligotrophs.

Density dependence

In population ecology, density-dependent processes occur when population growth rates are regulated by the density of a population. This article will focus on density-dependence in the context of macroparasite life cycles.

Disruptive selection

Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups. In this more individuals acquire peripheral character value at both ends of the distribution curve.

Dominance (ecology)

Ecological dominance is the degree to which a taxon is more numerous than its competitors in an ecological community, or makes up more of the biomass.

Most ecological communities are defined by their dominant species.

In many examples of wet woodland in western Europe, the dominant tree is alder (Alnus glutinosa).

In temperate bogs, the dominant vegetation is usually species of Sphagnum moss.

Tidal swamps in the tropics are usually dominated by species of mangrove (Rhizophoraceae)

Some sea floor communities are dominated by brittle stars.

Exposed rocky shorelines are dominated by sessile organisms such as barnacles and limpets.

Feeding frenzy

In ecology, a feeding frenzy occurs when predators are overwhelmed by the amount of prey available. For example, a large school of fish can cause nearby sharks, such as the lemon shark, to enter into a feeding frenzy. This can cause the sharks to go wild, biting anything that moves, including each other or anything else within biting range. Another functional explanation for feeding frenzy is competition amongst predators. This term is most often used when referring to sharks or piranhas. It has also been used as a term within journalism.

Griffon vulture

The griffon vulture (Gyps fulvus) is a large Old World vulture in the bird of prey family Accipitridae. It is also known as the Eurasian griffon. It is not to be confused with a different species, Rüppell's griffon vulture (Gyps rueppellii). It is closely related to the white-backed vulture (Gyps africanus).

Himalayan tahr

The Himalayan tahr (Hemitragus jemlahicus) is a large even-toed ungulate native to the Himalayas in southern Tibet, northern Pakistan, northern India and Nepal. It is listed as Near Threatened on the IUCN Red List, as the population is declining due to hunting and habitat loss.A recent phylogenetic analysis indicates that the genus Hemitragus is monospecific, and that the Himalayan tahr is a wild goat.The Himalayan tahr has been introduced to Argentina, New Zealand, South Africa and the United States.

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 (e.g. food or living space). This can be contrasted with interspecific cooperation, 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.

Direct 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, between amphibians, reptiles and mammals.

Organotroph

An organotroph is an organism that obtains hydrogen or electrons from organic substrates. This term is used in microbiology to classify and describe organisms based on how they obtain electrons for their respiration processes. Some organotrophs such as animals and many bacteria, are also heterotrophs. Organotrophs can be either anaerobic or aerobic.

Palaeosyops

Palaeosyops is a genus of small brontothere which lived during the early to middle Eocene. It was about the size of large cattle, with a weight of 600–800 kg depending on the species.

These animals are commonly found in Wyoming fossil beds primarily as fossilized teeth. From all of the species of this animal, it is concluded that P. major was the largest, reaching the size of a tapir. Its describer, Joseph Leidy, erroneously thought that Palaeosyops consumed both plants and animals after examining the fang-like canines. However, it is now known that all brontotheres were strict herbivores, and that many, if not most genera of hornless brontotheres had fang-like canines, possibly for both defense from predators, and intraspecific competition.

The paradox of the pesticides is a paradox that states that applying pesticide to a pest may end up increasing the abundance of the pest if the pesticide upsets natural predator–prey dynamics in the ecosystem.

Recruitment (biology)

In biology, especially marine biology, recruitment occurs when a juvenile organism joins a population, whether by birth or immigration, usually at a stage whereby the organisms are settled and able to be detected by an observer.There are two types of recruitment: closed and open.In the study of fisheries, recruitment is "the number of fish surviving to enter the fishery or to some life history stage such as settlement or maturity".

Scramble competition

In ecology, scramble competition (or complete symmetric competition) refers to a situation in which a resource is accessible to all competitors (that is, it is not monopolizable by an individual or group). However, since the particular resource is usually finite, scramble competition may lead to decreased survival rates for all competitors if the resource is used to its carrying capacity. Scramble competition is also defined as "[a] finite resource [that] is shared equally amongst the competitors so that the quantity of food per individual declines with increasing population density". A further description of scramble competition is "competition for a resource that is inadequate for the needs of all, but which is partitioned equally among contestants, so that no competitor obtains the amount it needs and all would die in extreme cases."

Storage effect

The storage effect is a coexistence mechanism proposed in the ecological theory of species coexistence, which tries to explain how such a wide variety of similar species are able to coexist within the same ecological community or guild. The storage effect was originally proposed in the 1980s to explain coexistence in diverse communities of coral reef fish, however it has since been generalized to cover a variety of ecological communities. The theory proposes one way for multiple species to coexist: in a changing environment, no species can be the best under all conditions. Instead, each species must have a unique response to varying environmental conditions, and a way of buffering against the effects of bad years. The storage effect gets its name because each population "stores" the gains in good years or microhabitats (patches) to help it survive population losses in bad years or patches. One strength of this theory is that, unlike most coexistence mechanisms, the storage effect can be measured and quantified, with units of per-capita growth rate (offspring per adult per generation).The storage effect can be caused by both temporal and spatial variation. The temporal storage effect (often referred to as simply "the storage effect") occurs when species benefit from changes in year-to-year environmental patterns, while the spatial storage effect occurs when species benefit from variation in microhabitats across a landscape.

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