6 ideas
The number of species on an island settles at a dynamic equilibrium where the rate of new species immigrating equals the rate of resident species going extinct. Immigration falls as an island gets farther from the mainland, and extinction rises as the island gets smaller. So small, remote islands hold fewer species even though individual species keep turning over.
When a continuous habitat is cut into isolated patches, each patch starts out holding more species than its area can support, which is called supersaturation. It then loses species over time until it reaches the lower count that its size and isolation allow. The losses are delayed, so a fragment that looks intact can already be committed to extinctions that have not happened yet.
Parks and reserves surrounded by farmland, roads and cities work like oceanic islands, because the surrounding landscape cuts off recolonization. Island extinction dynamics therefore now apply across continents. Nature reserves will lose species unless they are made large or connected.
The dodo evolved on Mauritius with no predators, so it became flightless and fearless. The case shows how island species adapted to isolation have no defenses against new threats, which makes islands the places where extinction happens first and fastest.
Small populations face four kinds of random risk that strengthen one another: demographic chance, environmental swings, catastrophes, and genetic problems such as inbreeding and drift. Each loss makes the next one more likely, pulling the population into a downward spiral called an extinction vortex. Population viability analysis models these risks to estimate the minimum population size that can persist for a set number of years.
Islands are isolated, simple and bounded, so they magnify evolutionary processes that are hard to see on mainlands. Examples include gigantism, dwarfism, flightlessness, adaptive radiation and the formation of new species. Darwin in the Galápagos and Wallace in the Malay Archipelago worked out natural selection by reading island patterns as experiments nature had already run.