Theory 024/Life & Evolution /Community ecology
Coexistence Needs a Trait Gap
Limiting similarity says close competitors cannot coexist. The threshold may depend on how finely the resource is divided.
Species coexist only when their trait difference exceeds a threshold set by the granularity of resources, so coexistence should fail between species that are different but not different enough.
01The evidence
Competition experiments show that species with small trait differences often exclude one another, while larger differences permit coexistence, with the boundary varying between systems.
02The tension
Limiting similarity predicts exclusion at small differences but does not say how small, and the threshold varies enough that it has been treated as unfalsifiable.
03The mechanism
Resources occur in discrete packets. Two species can partition them only if their traits differ enough to exploit different packets. If the packets are coarse, a large trait difference is needed to divide them.
That makes the coexistence threshold a function of resource granularity, which is measurable and predicts that the same trait difference can permit coexistence in one system and exclusion in another.
04What this is not
Fixed thresholds treat the boundary as a constant. This makes it a function of resource granularity, which predicts system-specific boundaries.
05Predictions
- P1The coexistence threshold should scale with resource granularity.
- P2The same trait gap should permit coexistence in fine-grained systems.
- P3Coarse resources should require larger differences.
06Refutation
A constant threshold across resource granularities would leave the fixed rule adequate.
07What it changes
It would make coexistence predictions system-specific.