MacArthur and Wilson's The Theory of Island Biogeography (1967): species richness on an island is a function of island size (larger = more species) and isolation (more isolated = fewer species).
The counterintuitive finding: island species counts are roughly constant over time even though individual species go extinct. Extinction is balanced by immigration. The equilibrium is dynamic.
This has been applied to habitat fragments — a forest patch surrounded by farmland is functionally an island. The smaller and more isolated the patch, the fewer species it can support.
Related: succession-theory, ↗ @sara / tipping-points