🌏 Biogeography: Why Species Live Where They Do

What you’ll learn

1. The Puzzle of the Empty HabitatGrasp why climate cannot explain which species live where, and why distribution is a historical record rather than an ecological one.Cacti and euphorbias converge on the same design in different deserts, showing climate sets form but not lineage. Hawaii's total lack of native ants — despite ants thriving there today as invaders — shows habitat suitability was never the constraint; reachability was. Distribution therefore records history: where a lineage got to, and what happened to the ground beneath it.
2. Wallace's LineUnderstand what Wallace observed between Bali and Lombok, why it was a discovery rather than a curiosity, and why the 'line' is really a zone.Crossing ~35 km from Bali to Lombok in 1856, Wallace found Asian barbets and woodpeckers replaced by Australasian cockatoos, honeyeaters and megapodes, despite identical climate, soil and volcanic origin. He traced the boundary north and published it in 1859; Huxley named it Wallace's Line in 1868. Later work showed the transition is a zone — Wallacea, between Wallace's and Lydekker's Lines — rather than a sharp edge.
3. The Ice-Age Map Beneath the LineExplain Wallace's Line through bathymetry and Pleistocene sea level, and see a distribution as a fossil of a vanished geography.Borneo, Java, Sumatra and Bali sit on the shallow Sunda Shelf, continental crust belonging to Asia; Australia and New Guinea share the Sahul Shelf. At glacial maxima, sea level fell as much as 120 m, exposing Sundaland and Sahul as dry continents that animals walked across. The deep Lombok Strait between them never closed, so Wallace's Line is an ice-age shoreline recorded in living fauna.
4. Vicariance: When the Ground Splits the RangeUnderstand vicariance — barriers arising to split a continuous range — and the dating prediction that makes it testable.Vicariance explains oceanic distributions without anyone crossing an ocean: the range stays put and the Earth breaks underneath it. Gondwana's breakup accounts for marsupials in Australia and the Americas — with fossil marsupials on Antarctica's Seymour Island confirming the missing link — and for southern beech across Chile, New Zealand, Tasmania and New Guinea. Crucially, vicariance predicts the biological split must match the geological date.
5. Dispersal and the Sweepstakes RouteUnderstand dispersal across pre-existing barriers, and how sweepstakes routes produce unbalanced, gap-filled faunas.Madagascar's native mammals — lemurs, tenrecs, some rodents and carnivores — are a lottery result, not a sample of Africa, with elephants, antelope, big cats and monkeys all absent. Simpson called such rarely-crossed barriers sweepstakes routes; rafting on vegetation mats filters for small, torpor-capable animals. Ali and Huber (2010) argued Paleogene ocean currents ran toward Madagascar, shortening the crossing during precisely the window lemurs needed.
6. How to Tell Them ApartApply the dating test that distinguishes dispersal from vicariance, and learn why a matching pattern is not proof.Vicariance predicts biological splits as old as the geological event; dispersal permits any younger age. The ratites — ostrich, rhea, emu, kiwi, moa, elephant bird — mapped Gondwana perfectly and were the flagship vicariance case. Molecular work placed the flying tinamous *inside* the ratite tree and made the elephant bird the kiwi's closest relative (Mitchell et al. 2014), showing flightlessness evolved repeatedly after flying ancestors dispersed.
7. Islands as EquationsUnderstand MacArthur and Wilson's equilibrium theory of island biogeography, and how area and distance generate the observed patterns.Species number was long known to scale with island area as S = cA^z. MacArthur and Wilson explained it as a dynamic equilibrium between immigration, which falls as an island fills, and extinction, which rises. Area lowers extinction and distance lowers immigration, predicting the large-and-near to small-and-far ranking — and implying continual turnover of species identity at a stable total.
8. Testing It: Kill an Island and WatchSee island biogeography theory experimentally tested, and understand precisely where the model stops working.Simberloff and Wilson fumigated tiny mangrove islets in the Florida Keys, reducing them to zero arthropod species, then tracked recolonisation. Counts returned to near their original values while species composition substantially changed — confirming the turnover prediction — with nearer islands refilling faster. Krakatau's post-1883 recolonisation provided a century-long uncontrolled parallel; the theory's limits include treating species as interchangeable and ignoring in-situ speciation.
9. The Island SyndromeUnderstand the island syndrome — dwarfism, gigantism, flightlessness and naivety — and why it turns lethal on contact.Islands lacking predators and competitors remodel their arrivals convergently. Insular dwarfism produced Sicily and Malta's metre-tall *Palaeoloxodon falconeri* at roughly 250 kg, Cyprus's and Crete's dwarf proboscideans, the Channel Islands mammoth, and — on Flores, within Wallacea — *Homo floresiensis*. Gigantism, flightlessness and lost fear are rational on an isolated island and catastrophic once rats, cats and pigs arrive, which is why islands carry a disproportionate share of recorded extinctions.
10. The Great American InterchangeUse the Great American Interchange as a natural experiment, including its genuinely contested dating and asymmetric outcome.South America evolved a strange endemic fauna in isolation — marsupial predators, ground sloths, glyptodonts, terror birds — until the Isthmus of Panama joined the continents. The traditional date is around 2.7–3 million years ago, though an earlier incomplete connection at 9–10 million years has been argued and contested. Traffic ran both ways but the outcome did not: North American lineages diversified in the south while South American emigrants largely failed in temperate North America.
11. Making Islands on PurposeApply island theory to habitat fragmentation, extinction debt, reserve design and the role of the matrix.A forest fragment is an island whose surrounding land its species cannot cross, so island theory's equations apply — and predict that richness inherited from the continuous forest must decay toward a lower equilibrium. That gap is extinction debt: species still present and already doomed. The SLOSS debate over reserve design never resolved into a rule, and the key correction was that the matrix, unlike ocean, varies in crossability — which is why corridors work.
12. The New DispersalSee humans as a new dispersal vector and climate as a moving barrier, and consolidate the course's argument.Barriers worked because crossing them was improbable; shipping and aviation have collapsed those odds within roughly five centuries, deleting the structure that organised terrestrial life. Meanwhile warming shifts tolerable conditions poleward and upslope, and mountain species climbing a cone face shrinking area with no 'up' at the summit. Panama's precedent predicts that removing barriers between long-separated faunas yields a few winners and many losers, not a merged fauna.

Questions this course answers

Grounded in trusted sources

• Alfred Russel Wallace — The Malay Archipelago (1869); The Geographical Distribution of Animals (1876)
• MacArthur, R.H. & Wilson, E.O. — The Theory of Island Biogeography (1967)
• Simberloff, D.S. & Wilson, E.O. — mangrove island defaunation experiments, Ecology (1969–1970)
• Ali, J.R. & Huber, M. (2010) 'Mammalian biodiversity on Madagascar controlled by ocean currents', Nature 463
• Mitchell, K.J. et al. (2014) 'Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution', Science 344
• Simberloff, D. & Abele, L.G. (1976) 'Island biogeography theory and conservation practice', Science
• Wikipedia — Wallace Line; Insular biogeography; Great American Interchange; Insular dwarfism; Ratite; Nothofagus
• Encyclopaedia Britannica — Biogeography; Gondwana; Island biogeography; Isthmus of Panama

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

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