Interspecific competition and resource partitioning
What interspecific competition is
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Interspecific competition is competition between individuals of different species.
- Compare intraspecific competition, which is between members of the same species.
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It occurs whenever two species require the same limited resource — food, water, light, nesting sites, territory or minerals.
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The relationship is –/–: both species are harmed, because each reduces the resource available to the other.
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Competition takes two forms:
- Exploitation competition — species compete indirectly by consuming the same resource. Neither needs to encounter the other; whoever gets there first depletes it.
- Interference competition — species compete directly, by fighting, excluding, or physically preventing access.
Fundamental and realised niche
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The fundamental niche is the full range of conditions and resources a species could use in the absence of competitors.
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The realised niche is the narrower range it actually occupies once competitors are present.
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The realised niche is always smaller than or equal to the fundamental niche, and the difference is the measure of competition's effect.
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This distinction is what makes competition experimentally testable: remove the competitor and the realised niche should expand towards the fundamental niche.
The competitive exclusion principle
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Two species cannot occupy exactly the same niche in the same place indefinitely.
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If two species use resources identically, the one that is even slightly more efficient will:
- obtain more of the resource
- convert it into more offspring
- increase in number while the other declines
- eventually eliminate the other locally.
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Small differences compound over generations, so exact ties do not persist. There are only three outcomes:
- Competitive exclusion — the weaker competitor is eliminated locally.
- Resource partitioning — the species diverge, each specialising on a different part of the resource.
- Local extinction or range restriction — the weaker competitor survives only where the stronger cannot.
Resource partitioning
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Resource partitioning is the division of a shared resource so that species use different parts of it and competition is reduced.
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It can be partitioned in several ways:
- Spatially — feeding at different heights in a tree, or different depths in a stream.
- Temporally — feeding at different times of day or seasons.
- By food type or size — taking different prey sizes or different plant parts.
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Character displacement is the evolutionary outcome: where two species overlap, natural selection favours individuals least like the competitor, so the species become more different in the zone of overlap than where either lives alone.
- This is powerful evidence for competition, because it is a pattern competition predicts and nothing else easily explains.
Competition in New Zealand
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New Zealand is an exceptionally clear case, because introduced species created competition that native species had never faced.
- Common wasps in South Island beech forest consume the honeydew produced by scale insects — the same energy-rich resource used by tūī, korimako and kākā. In peak season wasps remove the great majority of it, so native birds obtain less energy for breeding.
- Brushtail possums compete with native birds for fruit and flowers, and browse the canopy trees the birds depend on.
- Ship rats compete with native birds and invertebrates for seeds, fruit and invertebrate prey, as well as preying on them directly.
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The reason introduced competitors are so damaging is that native species evolved without them, so no resource partitioning has evolved and their niches overlap almost completely.
Selective advantage of responses to competition
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The responses to competition are what the standard is asking about, and each has a clear advantage:
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Resource partitioning / niche shift.
- An individual using the part of the resource its competitor uses least faces less competition and obtains more food per unit effort.
- More energy assimilated means more available for growth and gamete production, so it leaves more offspring.
- Over generations, selection shifts the whole population's niche away from the competitor's.
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Aggression and territorial exclusion (interference).
- Physically excluding a competitor secures exclusive access to a resource.
- The cost is the energy and injury risk of fighting, so this is favoured only where the resource is defensible and concentrated — worth defending a nectar-rich tree, not worth defending open grassland.
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Temporal shift.
- Feeding at a different time of day avoids direct encounter entirely, avoiding both the resource loss and the injury risk.
Worked Example
Worked Example
Two stream invertebrate species are studied in a South Island river.
- Where only species A is present, it grazes algae across the whole range of water speeds, from 0.1 to 0.9 m s−1.
- Where only species B is present, it also grazes across 0.1 to 0.9 m s−1.
- Where both are present, species A is found only in fast water (0.5–0.9 m s−1) and species B only in slow water (0.1–0.4 m s−1).
- Total numbers of each species are lower where both occur than where each occurs alone.
Explain what these results show, and explain why the response provides a selective advantage.
Answer:
What the results show. Each species alone uses the full range of water speeds — that range is its fundamental niche. Where both occur, each is restricted to part of that range — its realised niche. The reduction from fundamental to realised niche in the presence of the other species is direct evidence of interspecific competition, because the only variable that changed was the other species' presence.
That both species have lower total numbers where they co-occur confirms the relationship is –/–: neither benefits. Species A has not "won"; it is simply harmed less in fast water than species B is.
The outcome is resource partitioning rather than competitive exclusion — neither species has been eliminated. Instead the shared resource has been divided spatially, by water speed.
Why partitioning rather than exclusion has occurred. The two species must differ in where they are the better competitor. Species A is presumably better adapted to fast flow — perhaps a flatter body or stronger attachment — while species B performs better in slow water. Because each is superior in a different part of the gradient, neither can exclude the other everywhere, and the stable outcome is division of the gradient.
Why the response provides a selective advantage.
Consider an individual of species B in the zone of overlap.
- If it grazes in fast water, it competes directly with species A, which is the better competitor there. It obtains less algae per unit of foraging effort, and expends more energy maintaining position.
- If it grazes in slow water, it faces far less competition, so it obtains more algae for the same effort.
More energy assimilated and less expended means more energy available for growth and egg production, and better condition means a higher chance of surviving to reproduce. Individuals of species B that avoid fast water therefore leave more offspring than those that do not, and since the preference is heritable, its frequency rises in the population. The same argument in reverse applies to species A.
The wider consequence. Over generations, this selection pressure makes each species more specialised to its part of the gradient — character displacement. This reduces competition further, which is why partitioning is stable: it is self-reinforcing. Both species persist where, without partitioning, competitive exclusion would have eliminated one of them locally.