Intra-population Interaction
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Intraspecific competition is an interaction in population ecology, whereby members of the same
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
compete for limited resources. This leads to a reduction in fitness for both individuals, but the more fit individual survives and is able to reproduce. By contrast,
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 mutualism, a type of symbiosis. ...
occurs when members of different species compete for a shared resource. Members of the same species have rather similar requirements for resources, whereas different species have a smaller contested resource overlap, resulting in intraspecific competition generally being a stronger force than interspecific competition. Individuals can compete for food, water, space, light, mates, or any other
resource Resource refers to all the materials available in our environment which are technologically accessible, economically feasible and culturally sustainable and help us to satisfy our needs and wants. Resources can broadly be classified upon their ...
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 Exponential growth is a process that increases quantity over time. It occurs when the instantaneous rate of change (that is, the derivative) of a quantity with respect to time is proportional to the quantity itself. Described as a function, a q ...
. Prolonged exponential growth is 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 A logistic function or logistic curve is a common S-shaped curve (sigmoid curve) with equation f(x) = \frac, where For values of x in the domain of real numbers from -\infty to +\infty, the S-curve shown on the right is obtained, with the ...
. The rate of change of population density eventually falls to zero, the point ecologists have termed the
carrying capacity The carrying capacity of an environment is the maximum population size of a biological species that can be sustained by that specific environment, given the food, habitat, water, and other resources available. The carrying capacity is defined as t ...
(''K''). However, a population can only grow to a very limited number within an environment. The carrying capacity, defined by the variable k, of an environment is the maximum number of individuals or species an environment can sustain and support over a longer period of time. The resources within an environment are limited, and are not endless. An environment can only support a certain number of individuals before its resources completely diminish. 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 does not 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 Scramble, Scrambled, or Scrambling may refer to: Arts, entertainment, and media Games * ''Scramble'' (video game), a 1981 arcade game Music Albums * ''Scramble'' (album), an album by Atlanta-based band the Coathangers * ''Scrambles'' (album) ...
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 A territory is an area of land, sea, or space, particularly belonging or connected to a country, person, or animal. In international politics, a territory is usually either the total area from which a state may extract power resources or an ...
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.


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 A territory is an area of land, sea, or space, particularly belonging or connected to a country, person, or animal. In international politics, a territory is usually either the total area from which a state may extract power resources or a ...
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. 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. 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. Not all flamingos can mate in the population because of an unsuitable
sex ratio The sex ratio (or gender ratio) is usually defined as the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. Many species devia ...
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 spider Wolf spiders are members of the family Lycosidae (). They are robust and agile hunters with excellent eyesight. They live mostly in solitude, hunt alone, and do not spin webs. Some are opportunistic hunters, pouncing upon prey as they find it or ...
s (''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 Population density (in agriculture: standing stock or plant density) is a measurement of population per unit land area. It is mostly applied to humans, but sometimes to other living organisms too. It is a key geographical term.Matt RosenberPopul ...
. 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 A logistic function or logistic curve is a common S-shaped curve (sigmoid curve) with equation f(x) = \frac, where For values of x in the domain of real numbers from -\infty to +\infty, the S-curve shown on the right is obtained, with the ...
). 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. 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 Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
. 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 ''Oligosoma'' is a genus of small to medium-sized skinks (family Scincidae) found only in New Zealand, Norfolk Island and Lord Howe Island. ''Oligosoma'' had previously been found to belong to the ''Eugongylus'' group of genera in the subfamily ...
'') in New Zealand suffered a large decline in population after the introduction of
rabbits Rabbits, also known as bunnies or bunny rabbits, are small mammals in the family Leporidae (which also contains the hares) of the order Lagomorpha (which also contains the pikas). ''Oryctolagus cuniculus'' includes the European rabbit specie ...
(''Oryctolagus cuniculus''). Both species are eaten by
ferret The ferret (''Mustela furo'') is a small, Domestication, domesticated species belonging to the family Mustelidae. The ferret is most likely a domesticated form of the wild European polecat (''Mustela putorius''), evidenced by their Hybrid (biol ...
s (''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. 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 ''Ctenophorus pictus'', commonly known as the painted ground-dragon or painted dragon,Waite ER (Editor). (1929). ''The Reptiles and Amphibians of South Australia''. Adelaide: Government Printer. 270 pp. is a species of lizard from the family Ag ...
lizards, males compete for territory. Among the polymorphic variants, red lizards have are more aggressive in defending their territory compared to their yellow counterparts. 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 ''Vipera berus'', the common European adderMallow D, Ludwig D, Nilson G. (2003). ''True Vipers: Natural History and Toxinology of Old World Vipers''. Malabar, Florida: Krieger Publishing Company. . or common European viper,Stidworthy J. (1974). ...
'') undertake complex ritualised confrontations when courting females. Generally, the larger male will win and fights rarely escalate to injury to either combatant. 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. 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 In ecology, scramble competition (or complete symmetric competition or exploitation 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, sinc ...
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. 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. 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

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 Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantidae an ...
(''Loxodonta africana'') populations in
Kruger National Park Kruger National Park is a South African National Park and one of the largest game reserves in Africa. It covers an area of in the provinces of Limpopo and Mpumalanga in northeastern South Africa, and extends from north to south and from ea ...
(South Africa) also grew exponentially in the mid-1900s after strict poaching controls were put in place. =r N(t) \left(1 - \frac\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 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 main 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. 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. Experiments on '' Daphnia'' growth rates showed a striking adherence to the logistic growth curve. 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 Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...
grown in test tubes initially grew exponentially. But as resources became scarcer, their growth rates slowed until reaching the carrying capacity. 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.


See also

*
Competition (biology) 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). Competition lowers the fitness of both organisms involved since the presence of one of th ...
*
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 mutualism, a type of symbiosis. ...
* Logistic model *
Plant density Plant density is the number of individual plants present per unit of ground area. It is most easily interpreted in the case of monospecific stands, where all plants belong to the same species and have germinated at the same time. However, it could ...
* Population ecology *
Sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
*
Sexual selection Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
*
Female intrasexual competition Female intrasexual competition is competition between women over a potential mate. Such competition might include self-promotion, derogation of other women, and direct and indirect aggression toward other women. Factors that influence female intr ...
* War – extreme result of intraspecific competition in
humans Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...


References

{{modelling ecosystems, expanded=other Biological interactions