Competition and the Struggle for Existence

When Darwin spoke of the 'struggle for existence' and the survival of the fittest, he was convinced that interspecific competition is a potent force in organic evolution. It is commonly believed that competition happens only between closely related species fighting over the same limiting resource — but that picture is not entirely true.

First, totally unrelated species can also compete for the same resource. In some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton of the lake. Competition is not confined to relatives.

Resources Need Not Be Limiting

Second, resources do not even have to be in short supply for competition to occur. In interference competition, the feeding efficiency of one species is reduced by the inhibitory presence of another, even when food and space are abundant.

For this reason, competition is best defined not by scarcity but by its effect on fitness: it is a process in which the fitness of one species — measured by its intrinsic rate of increase, r — is significantly lower in the presence of another species. Wherever one species drags down another's r, competition is at work.

Competitive Exclusion and Evidence from Nature

Gause's Competitive Exclusion Principle states that two closely related species competing for the same resources cannot coexist indefinitely, and the competitively inferior one is eventually eliminated. This holds when resources are limiting, though not in every case.

Evidence for competition in nature comes from several sources. The Abingdon tortoise in the Galapagos Islands became extinct within a decade after goats were introduced, apparently because the goats had greater browsing efficiency. A second line of evidence is competitive release: a species whose range is restricted by a superior competitor expands its distribution dramatically once that competitor is experimentally removed. Connell's elegant experiments on the rocky coasts of Scotland showed that the larger, competitively superior barnacle Balanus dominates the intertidal zone and excludes the smaller Chthamalus from it. In general, herbivores and plants appear to be more adversely affected by competition than carnivores.

Coexistence Through Resource Partitioning

More recent studies caution against sweeping generalisations. Species facing competition do not always exclude one another — many evolve mechanisms that promote coexistence instead. One such mechanism is resource partitioning. If two species would compete for the same resource, they can sidestep the clash by using it at different times or through different foraging patterns.

The classic demonstration is MacArthur's study of five closely related species of warblers living on the same tree. Rather than eliminating one another, they coexist by differing in their foraging behaviour, each exploiting the tree in its own way.

Quick Recap

  • Darwin saw interspecific competition as a potent force in evolution (the 'struggle for existence').
  • Competition is not limited to close relatives — flamingoes and fishes compete for zooplankton in South American lakes.
  • Resources need not be limiting; in interference competition one species lowers another's feeding efficiency.
  • Competition is defined as the fitness (r) of one species being significantly lower in the presence of another.
  • Gause's Competitive Exclusion Principle: closely related competitors for the same resource cannot coexist indefinitely; the inferior one is eliminated.
  • Evidence: Abingdon tortoise extinct after goats were introduced (Galapagos); competitive release; Connell's work — Balanus excludes Chthamalus.
  • Resource partitioning allows coexistence — MacArthur's five warbler species on one tree differ in foraging behaviour.

Solved Examples — Section 10

Q1. Give an example showing that unrelated species can compete.

Answer: In shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton.

Q2. What is interference competition?

Answer: Competition in which the feeding efficiency of one species is reduced by the inhibitory presence of another, even when resources are abundant.

Q3. How is competition best defined?

Answer: As a process in which the fitness of one species (measured by its intrinsic rate of increase, r) is significantly lower in the presence of another species.

Q4. State Gause's Competitive Exclusion Principle.

Answer: Two closely related species competing for the same resources cannot coexist indefinitely, and the competitively inferior species is eventually eliminated.

Q5. What did Connell's experiments on Scottish coasts demonstrate?

Answer: The larger, competitively superior barnacle Balanus dominates the intertidal zone and excludes the smaller Chthamalus from it.

Q6. How do MacArthur's five warbler species coexist on the same tree?

Answer: By resource partitioning — they differ in their foraging behaviour, avoiding competition and coexisting.