Competition (zoology)

Competition is the struggle between individuals of different species (interspecific competition) or between individuals of the same species (intraspecific competition) for food, territories, and mates to survive. It is a major driving force in evolution, the process by which living organisms change over time, with better-adapted species surviving and less well-adapted species dying. Evolution begins with mutation, changes in the nucleotide sequence of a gene or genes, resulting in the production of slightly altered genes called alleles, which encode slightly different proteins. These altered proteins are the expressed traits of an organism and may give the organism an advantage over its competitors. The organism outcompetes its rivals in the environment, and hence, the environment favors the better-adapted, fitter organism, a process called natural selection. A mutation may help an organism in one environment but may hurt the same organism in a different environment. For example, an albino squirrel may flourish in snowy regions but may die in warm regions. Mutations can be caused by chemicals called mutagens or by ionizing radiation such as ultraviolet light, X-rays, and gamma radiation.

The Struggle to Survive

The science of ecology is the experimental analysis of the distribution and abundance of organisms. Natural selection influences the distribution and abundance of organisms from place to place. The possible selecting factors include physical factors like temperature and light, chemical factors such as water and salt, and species interactions. Any of these factors can influence the survivability of organisms in any environment. According to ecologist Charles Krebs, species interactions include four principal types: mutualism, which is the living together of two species that benefit each other (for example, humans and their pets); commensalism, which is the living together of two species that results in distinct benefits to one species while the other remains unhurt (like the relationship of birds and trees); predation, which is the hunting, killing, and eating of one species by another (examples: cats and mice; dogs and deer); and competition, which is defined as an active struggle for survival among all the species in a given environment.

This struggle involves the acquisition of various resources: food, territory, and mates. Food is an obvious target of competition. All organisms must have energy to conduct the cellular chemical reactions (such as respiration) that keep them alive. Photoautotrophic organisms (plants, phytoplankton, photobacteria) obtain this energy by converting sunlight, carbon dioxide, and water into sugar, a process called photosynthesis. Photoautotrophs, also called producers, compete for light and water. For example, oak and hickory trees grow taller than most pines, thereby shading out smaller species and eventually dominating a forest. All other organisms—animals, zooplankton, and fungi—are heterotrophs; they must consume other organisms to obtain energy. Heterotrophs include herbivores, carnivores, omnivores, and saprotrophs. Herbivores (plant eaters such as rabbits and cattle) obtain the sugar manufactured by plants. Carnivores, or meat-eating animals, eat other heterotrophs to get the sugar that these heterotrophs received from other organisms. Omnivores, such as humans, eat plants and animals for the same reason. Saprotrophs (such as fungi and bacteria) decompose dead organisms for the same reason. Life on Earth functions by intricately complex food chains in which organisms consume other organisms to obtain energy. Each human being is composed of molecules that were once part of other living organisms, even other humans. Ultimately, Earth’s energy comes from the sun.

Competition Between and Within Species

Territoriality is important for two reasons: An organism needs a place to live, and this place must contain adequate food and water reserves. The territoriality instinct occurs in birds, many fish, some reptiles, and most mammals, as well as dragonflies and fiddler crabs. These animals mark their territory using sounds, aggressive behaviors, and scents. A strong, well-adapted organism will fight and drive away weaker individuals of the same or different species to maintain exclusive rights to an area containing a large food and water supply. Species that are less well-adapted will be relegated to areas where food and water are scarce. The stronger species will have more food and will tend to produce more offspring since they will easily attract mates. Being stronger or more adapted does not necessarily mean being physically stronger. A physically strong organism can be overwhelmed easily by numerous weak individuals. In general, adaptability is defined by an organism’s ability to prosper in a hostile environment and leave many viable offspring.

Within a species, males attempt to attract females to their territory, or vice versa, using courtship dances and displays, often including bright colors such as red and blue and exaggerated body size. To attract mates, the hooded seal inflates its hood, the mantis shrimp dances to make his patches of reflective shell sparkle, the red-caped manakin does a moonwalk dance, and the jumping spider poses with his ultraviolet-light reflecting patch toward the female. In other species, like the crested grebe and the bald eagle, both sexes participate in the mating ritual. Mating displays are very similar to the threat displays used to drive away competitors, although there is no hostility involved. Generally, females are attracted to dominant males who have the best, but not necessarily the largest, territories.

Competition for food and territory is interspecific and intraspecific. Competition for mates is intraspecific. In an environment, the place where an organism lives (such as a eucalyptus tree or in rotting logs) is referred to as its habitat. Simultaneously, each species has its own unique niche, or occupation, in the environment (such as decomposer or carnivore). More than one species can occupy a habitat if they have different ecological niches. When two or more different species occupy the same habitat and niche, competition arises. One species will outcompete and dominate, while the population of the losing competitors may decrease, they may be driven away from the habitat, or they may adapt their behavior to accommodate a new niche.

Pecking Orders

In vertebrate organisms, intraspecific competition occurs between males as a group and between females as a group. Rarely is there male-versus-female competition, except in species having high social bonding—primates, for example. Competition begins when individuals are young. During play fighting, individuals nip or peck at each other while exhibiting threat displays. Dominant individuals exert their authority, while weaker individuals submit. The net result is a very ordered ranking of individuals from top to bottom, called a dominance hierarchy or pecking order. The top individual can threaten and force any individual below it to submit. The number two individual can threaten anyone except number one, and so on. The lowest-ranked individual can threaten no one and must submit to everyone. The lowest individual will have the least food, the worst territory, and the fewest (if any) mates. The number one individual will have the most food, the best territory, and the most mates. The pecking order changes over time because of continued group competition which is shown by challenges, aging, and accidents.

Pecking orders are evident in hens. A very dominant individual will peck other hens many times but will rarely be pecked. A less dominant individual will peck less but be pecked more. A correct ranking can be obtained easily by counting the pecking rate for each hen.

In the Netherlands, male black grouse contend with one another in an area called a “lek,” which may be occupied by as many as twenty males. The males establish their territories by pecking, wing-beating, and threat displays. The most dominant males occupy small territories (several hundred meters) at the center of the lek, where the food supply is greatest. Less dominant males occupy larger territories with fewer food reserves for the exterior of the lek. Established territories are maintained at measurable distances by crowing and flutter-jumping, with the home territory owner nearly always winning. Females, which nest in an adjoining meadow, are attracted to dominant males in the heavily contested small central territories.

A baboon troop can range in size from ten to two hundred members but usually averages about forty. Larger, dominant males and their many female mates move centrally within the troop. Less dominant males, with fewer females, lie toward the outside of the troop. Weak individuals at the troop periphery are more susceptible to predator attacks. Dominant males exert their authority by threat displays, such as the baring of the teeth or charging; weaker males submit by presenting their hindquarters. Conflicts are usually peacefully resolved.

Female lions maintain an organized pride with a single ruling male. Young males are expelled and wander alone in the wilderness. Upon reaching adulthood, males attempt to take over a pride to gain access to females. If a male is successful in capturing a pride and expelling his rival, he will often kill the cubs of the pride, simultaneously eliminating his rival’s descendants and stimulating the females to enter estrus for mating.

Competition Within Niches

Interspecific competition occurs between different species over food and water reserves and territories. Two or more species occupying the same niche and habitat will struggle for the available resources until either one species dominates and the others are excluded from the habitat, or the different species evolve into separate niches by targeting different food reserves, thus enabling all to survive in the same habitat. Numerous interspecific studies have been conducted, including crossbills, warblers, blackbirds, and insects, to mention a few.

Crossbills are small birds that live in Europe and Asia. Three crossbill species inhabit similar habitats and nearly similar niches. Each species has evolved a slightly modified beak to retrieve and eat seeds from three different cone-bearing (coniferous) trees. The white-winged crossbill has a slender beak for feeding from small larch cones, the common crossbill has a thicker beak for feeding from larger spruce cones, and the parrot crossbill has a very thick beak for feeding from pinecones. The evolution of different niches has enabled these three competitors to survive.

Another example of this phenomenon is shown by five species of warblers that inhabit the coniferous forests of the American Northeast. The myrtle warbler eats insects from all parts of trees up to seven meters high. The bay-breasted warbler eats insects from tree trunks six to twelve meters above the ground. The black-throated green, blackburnian, and Cape May warblers all feed near the treetops, according to elaborate studies by Robert H. MacArthur. The coexistence of five different species is probably the result of the warblers occupying different parts of the trees, with some warblers developing different feeding habits so that all survive.

G. H. Orians and G. Collier studied competitive exclusion between redwing and tricolored blackbirds. The introduction of tricolored blackbirds into redwing territories results in heavy redwing aggression, although the tricolored blackbirds nearly always prevail.

Two species of African ants, Anoplolepis longipes and Oecophylla longinoda, fight aggressively for territorial space. M. J. Way found that Anoplolepis prevails in sandy environments, whereas Oecophylla dominates in areas having thick vegetation.

Interspecific competition, therefore, results in the evolution of new traits and niches and the exclusion of certain species. Mathematical models of competition are based upon the work of A. J. Lotka and V. Volterra. The Lotka-Volterra equations attempt to measure competition between species for food and territory based on the population size of each species, the density of each species within the defined area, the rate of population increase of each species, and time.

Observing Competition

Studies of competition between individuals of the same or different species generally follow one basic method: observation. Interactions between organisms are observed and carefully measured to determine if the situation is competition, predation, parasitism, or mutualism. More detailed analyses of environmental chemical and physical conditions are used to determine the existence of additional influences. Observations of competition between organisms involve direct visual contact in the wild, mark-recapture experiments, transplant experiments, measurements of population sizes in given areas, and competition experiments in artificial environments.

Direct visual contact involves the scientist entering the field, finding a neutral, nonthreatening position, and watching and recording the actions of the subject organisms. The observer must be familiar with the habits of the subject organism and must be keen to detect subtle cues such as facial gestures, vocalizations, colors, and patterns of movement from individual to individual. Useful instruments include binoculars, telescopes, cameras, and sound recorders. The observer must track individuals over long distances so that territorial boundaries and all relevant actions are recorded, which may require living for long periods in uncomfortable conditions.

In transplant experiments, individuals of a given species are marked and released into a specific environmental situation, such as a new habitat or another species’ territory. The objective of the experiment is to see how well the introduced species fares in the new situation, as well as the responses of the various species that normally inhabit the area. The tricolored blackbird takeover of redwing blackbird territories is a prime example. Another example is the red wolf, a species that was extinct in the wild until several dozen captive wolves were released at the Alligator River Wildlife Refuge in eastern North Carolina. Their population increased through the early 2010s but declined steeply in the early 2020s. More remain in captivity than in the wild, but efforts to expand their wild population continue. Accidental transplants have had disastrous results for certain species. The African honeybee threatens the honey industry in Latin America and the southern United States because it is aggressive and produces poorly.

Scientists use several methods to measure animal population sizes. Some rely on the point-quarter technique, in which numerous rectangular areas of equal size are marked in the field. The number of organisms of each species in the habitat is counted for a given area, often using aerial photography or drones. The average of these areas is a relatively accurate population measure. This method is used in Alaska to count caribou. Other scientists use the capture-mark-recapture method to estimate animal populations. This involves trapping a sample of the animals in question, marking them with a tag or similar device, and releasing them. After a specified time interval, scientists recapture the animals and count the number of animals with markings. Another method, a line-transect survey, involves noting the number of animals seen when traveling in a straight line for a specific time period through a given habitat and then using this number in a set of mathematical formulas to estimate population size. This has been used for whales and some land animals. Population measurements can provide information for birth rates, death rates, immigration, and emigration over time for a given habitat.

Laboratory experiments involve confrontations between different species or individuals of the same species within an artificial environment. For example, male mouse territoriality can be studied by introducing an intruder into another male’s territory. Generally, the winner of the confrontation is the individual who nips its opponent more times. Usually, the individual who previously claimed the territory prevails and drives the intruder away. Similar studies have been performed with other mammalian, reptile, fish, insect, and bird species.

Interactions between species are subtle and intricate, and examining these interactions enables scientists to understand evolution, model various environments, and protect, reintroduce, and repopulate endangered species. Interspecific and intraspecific competition is a major driving force in evolution. The stronger species outcompete weaker species for the available ecological niches. Mutations in organisms create new traits and, therefore, new organisms (more species), which are selected by the environment for adaptability. The environment benefits because the most adapted, healthy species survive, whereas weaker species are excluded.

All environments consist of a complex array of species, each dependent on the others for survival. In each habitat, every species contributes in a unique and important way. Two species in a given habitat that share a niche will compete for essential resources like territory and food until one species drives the other away or until they adapt to each other and evolve different feeding habits and living arrangements.

Principal Terms

Evolution: Gradual changes in organisms over time, caused by mutation and selected by the environment, resulting in better-adapted organisms and new species

Habitat: The type of environment in which a particular organism prefers to live based on various physical and chemical conditions

Natural Selection: The ability of an organism or species to survive, compete, and reproduce in its habitat; success is dictated by the alleles (traits) that it possesses

Niche: An organism’s role in its habitat environment

Predation: A situation in which one animal species hunts and eats another species (examples: lynx versus hare; cheetah versus gazelle)

Territoriality: A phenomenon in animal behavior whereby individual organisms occupy and defend an area from other individuals of the same or different species

Threat Display: A territorial behavior exhibited by animals during defense of a territory, such as charging, showing bright colors, and exaggerating body size

Bibliography

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