Population interactions: competition
In competition (−, −) both species lose fitness. Gause's competitive exclusion principle says closely related species using the same limited resources cannot coexist indefinitely.
What is competition?
Darwin believed that interspecific competition, competition between different species, is a strong force in evolution. People often think competition happens only between closely related species for limited resources. This is not fully true. Unrelated species can also compete: in some shallow South American lakes, visiting flamingoes and resident fishes compete for the same food, zooplankton. Resources need not be limiting either. In interference competition, one species lowers the feeding efficiency of another just by its presence, even when food and space are plentiful. So competition is best defined this way: the fitness of one species is significantly lower in the presence of another species. Fitness here is measured by r, the intrinsic rate of increase.
Evidence for competition in nature
- Laboratory experiments: Gause and others showed that when resources are limited, the competitively superior species eventually eliminates the other. Proof of such exclusion in nature is not always conclusive.
- Abingdon tortoise: on the Galapagos Islands, it became extinct within a decade after goats were introduced, apparently because the goats browsed more efficiently.
- Competitive release: a species held to a small area by a superior competitor spreads widely when that competitor is experimentally removed.
- Connell's barnacles: on the rocky coasts of Scotland, the larger, superior barnacle Balanus dominates the intertidal zone and excludes the smaller barnacle Chthamalus from it.
- Herbivores and plants appear to be harmed by competition more than carnivores.
Gause's competitive exclusion principle
Gause's principle states that two closely related species competing for the same resources cannot coexist indefinitely, and the competitively inferior one is eliminated in the end. This may be true when resources are limiting, but not otherwise. More recent studies do not support such broad claims. They show that species facing competition may evolve ways to coexist rather than one excluding the other.
Competitive exclusion vs resource partitioning
Watch a fruiting banyan tree: different birds feed at different heights and at different times of day, a simple picture of sharing one resource.
Farmers weed their wheat and paddy fields because weeds compete with the crop for the same water, light and minerals. Invasive Lantana spreads through Indian forests by outcompeting native shrubs.
2 marks: “State Gause's competitive exclusion principle. Under what condition does it hold?” Two closely related species competing for the same resources cannot coexist indefinitely; the competitively inferior one is eliminated. It holds when resources are limiting.
3 marks: “How can species facing competition coexist? Explain with MacArthur's example.” By resource partitioning: using a shared resource differently, such as feeding at different times or with different foraging patterns. MacArthur found five closely related warbler species on one tree avoided competition through different foraging behaviour.
3 marks: “Give two pieces of evidence that competition occurs in nature.” The Abingdon tortoise on the Galapagos became extinct within a decade of goats being introduced. Competitive release: on Scottish coasts, Balanus excludes Chthamalus from the intertidal zone, and Chthamalus spreads when Balanus is removed (Connell).
Gause excludes, MacArthur shares: exclusion when resources are short; partitioning lets species live together.