Size-asymmetric competition

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Size-asymmetric competition

What it means
- In size-asymmetric competition, bigger individuals grab a larger share of limited resources than smaller ones during competition.
- This is common in plants, but can also occur in animals. Often it happens because large individuals can “pre-empt” resources before smaller ones.

How scientists describe it (the theta idea)
- Resources are divided among n individuals with sizes B1, B2, ..., Bn.
- The amount of resource an individual i gets is proportional to (Bi)θ, where θ is a parameter that measures how strongly size matters.
- If θ = 1, competition is perfectly size-symmetric: a bigger individual gets proportionally more but not disproportionately more.
- If θ > 1, competition is size-asymmetric: bigger individuals get disproportionately more resources.
- As θ increases, large individuals gain more resources per unit of biomass than smaller ones.

Differences among resources in plant communities
- Light: competition is highly size-asymmetric because light comes from above. Taller, bigger leaves shade smaller ones, giving big plants a big edge.
- Nutrients: tends to be more size-symmetric, though patchy soils can create some size effects in roots.
- Water: less understood; the degree of size asymmetry is not well established.

Implications for plant communities
- Size asymmetry affects many ecological outcomes: how communities develop over time (succession), how biomass is distributed, grazing responses, population growth, ecosystem functioning, and how many species can coexist.
- When light becomes limiting due to dense growth or eutrophication (high nutrients), larger plants can outcompete smaller ones, often reducing species diversity.
- There are two main theories about competition:
- R* theory: competition is largely size-symmetric; success comes from withstanding low resource levels.
- CSR theory: competition is size-asymmetric; success comes from rapid growth and achieving large size.
- Evolution and traits:
- Plant height is strongly affected by asymmetric light competition.
- Under asymmetric light competition, plants may invest more in height and wood (instead of leaves or reproduction).
- More water availability can drive greater height, increasing the role of size-asymmetric competition.
- Larger seeds (producing larger seedlings) can be advantageous because they start bigger and can outcompete smaller young plants.
- Practical use: size asymmetry can be used in managing plant communities, such as suppressing weeds in crop fields. Weeds are harder to control in dry environments because crops can suppress them more effectively when light competition is strong and size-asymmetric.

See also
- Competition (biology)
- Asymmetric competition
- Resource (biology)
- Resource partitioning
- Plant ecology
- Jacob Weiner

Note: This is a simplified overview of the topic.