Interspecific Competition, Exclusion and Coexistence

Competition involves a cost, not merely proximity

Competition occurs when the presence or activity of one organism reduces the resource access, growth, survival or reproductive success of another. Interspecific competition involves different species; intraspecific competition occurs within a species. Two plants growing beside one another are not automatically demonstrated competitors merely because they are close, but if each reduces the other's access to limiting light, water or soil nutrients the interaction is competitive. Population ecologists compare performance with and without the other species to establish the cost.

The chapter's sign notation assigns (−,−) to competition because both species are negatively affected relative to a no-competition baseline, although one may suffer much more than the other. Stronger competition can reduce per-capita growth, breeding output or the amount of habitat occupied. Similar species often compete strongly when their resource requirements overlap, but relatedness alone does not determine competition. Distinctly related organisms can compete for the same food or space.

Exploitative and interference competition

In exploitative competition, organisms use a shared resource so that less remains available to others. For example, flamingoes and some resident fish in shallow South American lakes can both consume zooplankton. Their different body plans do not prevent them from being competitors if feeding by one reduces the food available to the other. The degree of harm depends on resource availability, feeding rates and alternatives. Competition need not produce physical contact between the organisms.

In interference competition, one species directly obstructs access or successful use of a resource, for example through territorial behaviour, aggressive exclusion or chemical inhibition. An organism's feeding efficiency can be reduced by another's presence even where the total resource supply is not strongly depleted. Competition is therefore broader than the claim that two species always fight over scarce food. In practice, researchers must distinguish resource removal from direct obstruction and identify an appropriate measure of fitness or growth.

Evidence from controlled and field experiments

Laboratory experiments permit comparisons under controlled food, temperature and space conditions. In classical studies, Gause investigated populations of protozoans such as Paramecium in separate and mixed cultures. Under particular restricted resource conditions, one species performed worse in joint culture and could be eliminated locally. The difference between a species grown alone and the same species grown with a competitor is central evidence for interspecific competition. Results depend on the precise organisms, culture conditions and resource supply.

Field experiments can be equally informative when a competitor is removed or excluded from marked plots. On rocky shores of Scotland, the larger barnacle Balanus, now often treated under updated nomenclature, limits the lower distribution of the smaller barnacle Chthamalus through competition for attachment space. When the larger competitor is experimentally removed, smaller barnacles can occupy areas from which they were previously excluded. This expansion is called competitive release. It directly connects the competitor's presence to a previously constrained distribution.

What competitive exclusion actually predicts

Competitive exclusion predicts that two populations dependent on essentially the same limiting resources and occupying the same niche cannot indefinitely maintain a stable balance if one consistently gains a competitive advantage under fixed conditions. Eventually the inferior competitor may decline or be eliminated locally. This is a conditional statement, not a law that any two related species anywhere cannot coexist. Habitat variation, disturbance, temporal changes, predators, dispersal and niche differences can all influence the result.

The source mentions introduced goats as a possible contributor to the decline of the Abingdon Island tortoise lineage in the Galápagos. Such historical examples demonstrate how introduced herbivores can alter vegetation and resources, but extinction cannot be attributed definitively to one competitive mechanism without broader evidence about historical hunting, habitat damage and demographic vulnerability. A persuasive competition claim identifies the shared resource, measures its effects and evaluates alternative explanations.

Resource partitioning allows competing populations to persist

Resource partitioning occurs when species use different components of a resource, different feeding locations or different times of activity, reducing overlap. Two birds that consume similar insects may forage in different sections of a tree canopy; fish may feed at different depths; and herbivores may prefer different plant parts. Partitioning changes the extent to which one population's consumption limits another. It can arise through inherited differences, behaviour, learning or ecological sorting, depending on the system.

Robert MacArthur's work on closely related warblers provides an influential case: several species foraged at different heights and positions within conifer trees, allowing spatial and behavioural differences in food use. Their coexistence does not mean no competition occurs; rather, reduced overlap can lower the intensity of exclusion. Niche differences and partitioning can also change as population sizes or resource supplies change. The ecological insight is that species coexistence can depend on how, when and where resources are used.

Studying competition rigorously

To test competition, measure a population's response to the presence of the other species while controlling as far as possible for habitat differences. A removal design compares plots from which a competitor is excluded with plots in which it remains. Differences in survival, feeding rate, reproduction or occupied space can provide evidence. Replication is important because two plots may differ for reasons unrelated to the treatment. Resource measurements help determine whether exploitation or interference explains the outcome.

An increase in one species after another disappears is consistent with release from competition, but could also result from simultaneous changes in predators, climate or disease. Competition should therefore be treated as a specific causal hypothesis rather than a label assigned to every overlapping pair of species. Its ecological effects can range from subtle reductions in efficiency to major range restriction or local exclusion.

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