Species-composition distributions under competition, ecological drift and immigration

The composition of species within an ecosystem is a topic of great interest in community ecology. Multiple factors such as competition, ecological drift, and immigration all play a role in shaping the distribution of species within a community. These factors not only determine the average composition of the community, but also the likelihood of observing various community states.

The researchers in this study aimed to understand how competition impacts the neutral distribution of community composition, which is generated by both drift and immigration. To do this, they analyzed a stochastic two-species competition model that incorporates immigration and derived the stationary distribution of relative species composition near neutrality. The findings of their analysis reveal the conditions under which communities are usually balanced, versus when one species dominates the community.

Interestingly, even when the deterministic model predicts stable coexistence of both species, species dominance can become common in smaller communities. Furthermore, both balanced and dominant states can occur together, resulting in a trimodal distribution. Additionally, competitive asymmetry further biases the distribution towards the competitively favored species.

In summary, the researchers' results demonstrate that the interplay between competition, ecological drift, and immigration determines not only the variation in community composition, but also the likelihood of observing specific community states. Their findings provide an analytical link between neutral and competition theories and offer a framework for interpreting variation in community composition.