Intraspecific competition and population regulation
What intraspecific competition is
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Intraspecific competition is competition between individuals of the same species.
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It is a –/– relationship, like interspecific competition, but it is more intense — and understanding why is the key idea of this page.
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Members of the same species have identical requirements: the same food, the same nest sites, the same mates.
- Their niches overlap completely, not partially.
- So every individual is a competitor of every other individual for every resource it needs.
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By contrast, two different species almost always partition resources to some degree, so their competition is weaker.
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This means intraspecific competition is the strongest competitive force most organisms experience.
Density dependence
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Intraspecific competition is density-dependent: its intensity increases as population density increases.
- At low density there is enough resource for all, so competition is weak, individuals grow well and reproduce successfully.
- At high density the resource is divided among more individuals, so each gets less.
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The consequences at high density are:
- Reduced growth rate, because less energy is assimilated per individual.
- Reduced reproductive output — smaller clutches, fewer or smaller offspring.
- Increased mortality, especially among juveniles and the weakest competitors.
- Increased emigration, as individuals disperse to find less crowded areas.
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Together these push the birth rate down and the death rate up as density rises, so population growth slows.
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The density at which births equal deaths is the carrying capacity (K) — the maximum population the environment can sustain.
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Intraspecific competition is therefore the main mechanism regulating population size. It is a negative feedback: high density reduces growth, which reduces density.
Competition in plants
- Plants compete intraspecifically for light, water, nutrients and space.
- Because plants cannot move, the outcome is usually self-thinning.
- Seedlings germinate at very high density.
- Larger individuals shade smaller ones, capturing more light and growing faster still.
- Suppressed individuals receive too little light to maintain a positive carbon balance and die.
- Density falls while the survivors' individual mass rises.
- The result is a stand of fewer, larger plants — a predictable consequence of competition rather than of any external factor.
Scramble and contest competition
- Two patterns of outcome, and the difference matters for what happens to the population:
- Scramble competition — the resource is divided more or less equally among all competitors. At high density everyone gets too little, so all individuals do badly and the population may crash.
- Contest competition — some individuals obtain a full share and others obtain none. The winners survive and reproduce normally; the losers die or emigrate.
- Contest competition produces a more stable population, because a proportion of individuals always reproduces successfully. Territoriality and dominance hierarchies are both mechanisms of contest competition.
Selective advantage of responses to intraspecific competition
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The responses to competition are what the standard asks about:
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Dispersal / emigration.
- Leaving a crowded area for an unoccupied one gives access to undepleted resources.
- The cost is the risk of travelling and of arriving somewhere unsuitable; it is favoured when local competition is severe enough to outweigh that risk.
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Niche shift within the species.
- Taking slightly different food, or feeding at a different time, reduces direct competition with conspecifics — the same logic as resource partitioning between species, applied within one.
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Territoriality and dominance (next pages) convert scramble into contest competition, guaranteeing a share to some individuals.
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Altered reproductive investment.
- At high density, producing fewer, larger, better-provisioned offspring can be favoured, because small offspring would lose every contest. At low density, producing many small offspring is favoured, because most will find resources.
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In every case, the advantage is that the individual obtains more resource per unit effort than one that did not respond, so more energy goes into growth and reproduction and more offspring survive.
Worked Example
Worked Example
Seedlings of a native shrub are sown at four densities and grown for two years.
| Seeds sown per m2 | Plants surviving per m2 | Mean dry mass per plant (g) | Mean seeds per plant |
|---|---|---|---|
| 25 | 24 | 41 | 180 |
| 100 | 71 | 22 | 74 |
| 400 | 118 | 9 | 21 |
| 1600 | 131 | 7 | 14 |
Explain what these data show about intraspecific competition, and explain why a response to it would provide a selective advantage.
Answer:
What the data show. As sowing density rises, three things happen together:
- Survival falls sharply as a proportion. At 25 seeds m−2 almost all survive (24 of 25, 96%), but at 1600 only 131 survive — about 8%. So mortality is density-dependent: it increases as density increases.
- Mean mass per plant falls, from 41 g to 7 g. Each plant obtains a smaller share of the light, water and nutrients available.
- Mean seed production falls even more steeply, from 180 to 14 per plant — a reduction of over 90%.
Why this is intraspecific competition. All the plants are the same species, so their requirements are identical and their niches overlap completely. Every individual competes with every other for exactly the same light, water and soil nutrients. Because the resource supply per unit area is fixed, dividing it among more individuals necessarily gives each one less.
Why survivor numbers level off. Note that surviving plants per m2 rises only from 118 to 131 as sowing density quadruples from 400 to 1600. The area can support only a limited total mass of plant tissue, so beyond a point adding more seed produces almost no extra survivors — it simply produces more deaths. This levelling is the population approaching its carrying capacity, and it is the signature of self-thinning: larger seedlings shade smaller ones, the shaded individuals cannot maintain a positive carbon balance, and they die.
Why reproduction falls faster than mass. Mass falls about 6-fold but seed production falls about 13-fold. This is because a plant must meet its own maintenance and growth requirements first, and only surplus resources go into reproduction. When total resources are halved, the surplus is reduced by much more than half — which is why crowded plants are not merely smaller but disproportionately less fertile.
Why a response would provide a selective advantage.
Consider an individual seedling in the 1600 m−2 treatment.
- Growing where it germinated, it competes with many neighbours, obtains little light, reaches only 7 g and produces about 14 seeds.
- If its seed had instead been dispersed to an uncrowded site, it could reach 41 g and produce about 180 seeds — nearly 13 times as many offspring.
Any heritable feature that puts seed into less crowded conditions therefore leaves far more descendants. This is exactly why traits such as fleshy fruit attractive to birds, wind-dispersed seed and long-lived dormant seed are so widespread: each is a way of escaping competition with the parent and with siblings.
The same logic favours a plant producing fewer, larger, better-provisioned seeds where establishment is crowded, because a large seedling wins the shading contest, while many small seeds are favoured where sites are open.