Three explanations for species coexistence. One model that contains all of them.
Why do ecosystems sustain dozens or hundreds of competing species on limited resources? Ecology offers three narratives: competition-colonization trade-offs (better competitors are worse dispersers), environmental heterogeneity (different species prefer different patches), and ecological neutrality (species are functionally equivalent and drift randomly). Each narrative has its own mathematical model. Each model explains some coexistence patterns and fails on others.
A unified framework for propagule disperser communities (arXiv:2603.20707) embeds all three narratives in a single stochastic model. The key innovation isn't theoretical — it's computational. An algorithm determines which species coalitions can coexist at macroscopic equilibrium using matrix spectral properties, bypassing the expensive calculation of the full equilibrium itself.
The spectral shortcut works because coexistence at equilibrium depends on whether the invasion growth rates of excluded species are negative — whether an excluded species would shrink if introduced into the existing community. These growth rates are eigenvalue-adjacent: they depend on the spectral properties of the interaction matrix evaluated at the equilibrium. The algorithm checks the spectral condition directly, skipping the equilibrium calculation.
What emerges is that the three coexistence mechanisms interact synergistically. Competition-colonization trade-offs alone permit modest diversity. Heterogeneity alone permits modest diversity. Neutrality alone produces drift. Combined, they produce stable communities with far more species than any mechanism alone would support. The synergy isn't additive — combining two mechanisms produces more coexistence than the sum of their individual contributions.
The ecosystem doesn't choose one explanation. It uses all three simultaneously, and the combination is more than the sum of its parts.