Intraspecific competition

Species members compete for resources

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, but the more fit individual survives and is able to reproduce. By contrast, interspecific competition 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 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. 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. 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.

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.

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 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) 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 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. Subsequently, very few cinnabar moths are competing intraspecifically in the next generation so the population grows rapidly before crashing again.