🏝️ Islands and Archipelagos: How Islands Work

What you’ll learn

1. Two DialsAdopt area and isolation as the two variables that predict nearly everything about an island, and see why islands are the only controlled experiment ecology has.You cannot experiment on a continent — there is no control and every variable moves at once. Islands are the same physics with two dials turned down, area and isolation, and they exist in thousands at every combination: a replicated experiment laid out by geology. Those two numbers predict how many species live there, which kinds, how strange they get, how catastrophically they collapse, and even how the humans behave. 'Island' is a position on those dials rather than a category, which is why Australia is one biologically and Britain barely is — and why a mountaintop, a lake and a forest fragment are too.
2. Three Ways to Build an IslandDistinguish volcanic, continental-fragment and coral islands by their starting conditions, and follow Darwin's subsidence theory of atolls.Volcanic islands (hotspot, ridge or arc) are born sterile, so their entire biota arrived. Continental fragments — Madagascar, New Zealand, Britain, Sri Lanka — start populated with whatever was standing on them when they detached, which is why Madagascar is an ancient continent left running in a sealed room. Coral islands are built by animals. Darwin reasoned in the 1830s that an atoll is a reef that outlived its island: fringing reef → the volcano subsides as the crust cools → barrier reef → the island vanishes and the ring remains. Drilling at Enewetak in 1952 hit basalt under more than a kilometre of coral limestone.
3. The EquationUnderstand the MacArthur–Wilson equilibrium, how the two dials shift it, and why S = cA^z became the design manual for nature reserves.Immigration falls as an island fills (arrivals are increasingly species already present) while extinction rises (more species means smaller populations each), so the rates cross at a dynamic equilibrium where arrivals and losses cancel. Isolation lowers the immigration curve — more isolated islands are less likely to receive immigrants — and area lowers the extinction curve, so big-and-near is richest and small-and-far poorest. The rescue effect and target effect refine it. The species–area relationship S = cA^z, or log(S) = log(c) + z·log(A), is a power law; z is generally lower for habitat fragments than true islands because fragments are leaky.
4. The FilterSee the ocean as a filter with a mesh rather than a barrier, and understand disharmony and its most consequential hole: no ground predators.S = cA^z says how many, not which — and an island biota is a heavily biased sample. Seeds, spores, birds, bats, wind-lofted insects and reptiles (which raft on vegetation for weeks without food or fresh water) cross; large land mammals and amphibians do not, since salt water kills a frog and its eggs. The result is disharmony: whole branches of the tree of life missing. The most consequential absence is ground predators — an island is a mainland with the predators deleted, so flight, toxins, thorns and vigilance become expensive subsidies to a threat that does not exist.
5. Why Island Species Get StrangeExplain Foster's rule and island flightlessness as budget reallocations, and see every island trait as a bet that the ocean will hold.Foster's rule: small species tend to evolve larger bodies and large species smaller ones. The big shrink under limited resources, no room for a large range, periodic food scarcity, shorter gestation and easier thermoregulation — Palaeoloxodon falconeri stood about 1 m and weighed about 250 kg, Magyarosaurus was six metres against mainland sauropods at 30+, and Homo floresiensis shows we are not exempt. The small grow: the dodo's ancestors were normal pigeons; Flores had giant rats. Flightlessness is a reallocation, not degeneration, and has evolved independently in rails, ducks, parrots, pigeons, cormorants and ibises.
6. The Bill Comes DueConnect island extinction rates directly to the adaptations of the previous two chapters rather than to vague fragility.Islands are 5.3% of the world's land, hold roughly 20% of Earth's biodiversity, and 21% of all plant species are island endemics — 63,280 of the 94,052 plant species native to islands. They also hold about 50% of threatened species and roughly 75% of the ~800 known extinctions since European expansion; for birds and mammals one analysis puts it at 95%. 176 island-endemic plants are already extinct — 55% of all known extinct plant species. The cause is exact: rats, cats and goats cancel every adaptive bet at once, and the area dial means the population is the species.
7. The Dials Apply to People TooApply isolation and area to humans — the Austronesian expansion, the rats it carried, and the EEZ arithmetic that makes tiny states maritime powers.Austronesian-speaking peoples crossed the emptiest surface on Earth in double-hulled canoes and found and re-found essentially every habitable Pacific island — and carried rats in every hull, so chapter six's ship arrived a millennium before any European, followed by waves of bird extinctions. On the other dial, an EEZ runs from the 12-nautical-mile territorial limit out to 200 nautical miles (370 km) from any coastline, regardless of the land behind it. France holds the largest at 10,186,624 km², owed overwhelmingly to its island territories; Australia's 8,148,250 km² exceeds its land territory; New Zealand's is roughly fifteen times its land area.
8. Turning the Dials by HandState the Maldives' risk precisely rather than confidently, and close the course by seeing both dials pass into human hands.The Maldives is 1,192 coral islands in 26 double-chain atolls, about 200 inhabited, holding some 601,269 people on 298 km² averaging around 1.5 m above sea level with a highest natural point of about 2.4 m. These are Darwin's atolls — the only landform with a built-in response to relative sea-level change — but a bleached reef cannot build, and the warming raising the sea is what bleaches it. The IPCC gave an upper limit of 59 cm by 2100 and has warned of uninhabitability; a 1988 Maldivian claim of full submergence by 2018 was wrong. Habitability fails first, via the freshwater lens and storm surge, not submergence.

Questions this course answers

Grounded in trusted sources

• Robert H. MacArthur and E. O. Wilson, 'The Theory of Island Biogeography' (Princeton University Press, 1967)
• Wikipedia, 'Insular biogeography' (https://en.wikipedia.org/wiki/Insular_biogeography)
• Wikipedia, 'Insular dwarfism' (https://en.wikipedia.org/wiki/Insular_dwarfism)
• Wikipedia, 'Exclusive economic zone' (https://en.wikipedia.org/wiki/Exclusive_economic_zone)
• Wikipedia, 'Maldives' (https://en.wikipedia.org/wiki/Maldives)
• Charles Darwin, 'The Structure and Distribution of Coral Reefs' (1842)
• Fernández-Palacios et al., 'Scientists' warning — The outstanding biodiversity of islands is in peril', Global Ecology and Conservation 31 (2021) (https://pmc.ncbi.nlm.nih.gov/articles/PMC8556160/)
• Loehle & Eschenbach, 'Historical bird and terrestrial mammal extinction rates and causes', Diversity and Distributions 18:84 (2012)

Every Wunder lesson is built from real, reputable sources — never invented.

Related History courses