Niche Partitioning

Tilman's resource competition theory (1982) explains coexistence through niche differentiation. Each species is defined by its Zero Net Growth Isocline (ZNGI) — the resource combination at which it exactly replaces itself. In a 2-resource system, ZNGIs are L-shaped lines. Competitive exclusion occurs when one species' ZNGI falls entirely below the other's — it wins everywhere. Coexistence requires that each species limits its own growth more than the other species does: the ZNGI intersection falls in the supply region, with each species consuming relatively more of the resource that limits it. This is a mechanistic basis for the Lotka-Volterra coexistence condition.