Intraspecific competition

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. 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. The rate of change of population density eventually falls to zero, the point ecologists have termed the carrying capacity (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 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.

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.

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 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. 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. 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). 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.