Island Biogeography

Picture two curves on a graph, species number on the x-axis. The immigration rate curve starts high and slopes downward: when few species live on an island, almost any new arrival is a species not yet there, so immigration counts fast. As species pile up, fewer newcomers are actually new, so the curve falls. The extinction rate curve starts low and slopes upward: with few species and lots of open resources, few die out, but as species crowd in, competition and limited space push extinctions higher. Where the two curves cross is the equilibrium number of species, the point where new arrivals exactly replace losses.

Distance from the mainland controls the immigration curve. Near islands get a steady stream of colonizers, so their immigration curve sits high, crossing extinction at a larger equilibrium number. Far islands get fewer arrivals, so their curve sits low, crossing at a smaller number. Island size controls the extinction curve. Large islands offer more area, more habitat variety, and bigger populations that are harder to wipe out, so their extinction curve sits low. Small islands crowd species together with fewer resources, pushing extinction up and equilibrium down. Big and near beats small and far every time.

This idea reaches past real islands. Forest fragments surrounded by farmland or highways act like islands in a sea of unsuitable habitat, and cutting one large forest into several small, more isolated pieces lowers the total species the land can support. This connects directly to succession and habitat fragmentation later in the unit, where shrinking and isolating habitat becomes a major driver of biodiversity loss.