19.12.01 · eco-evo-bio / biogeography

Biogeography

shipped3 tiersLean: nonepending prereqs

Anchor (Master): MacArthur & Wilson — The Theory of Island Biogeography (1967); Hubbell — The Unified Neutral Theory of Biodiversity and Biogeography (2001); relevant primary literature

Intuition Beginner

Why are kangaroos found only in Australia, polar bears only in the Arctic, and toucans only in the American tropics? Biogeography is the study of the geographic distribution of species and the processes that produce those patterns. It asks: where do species live, why do they live there, and how have their distributions changed over time?

Historical biogeography examines how geological events (continental drift, mountain building, sea level changes) and evolutionary processes (speciation, extinction, dispersal) have shaped current distributions over millions of years. The key insight is that Earth's continents have moved. Approximately 250 million years ago, all land was united in a single supercontinent (Pangaea). Its breakup, starting about 200 million years ago, separated populations that then evolved in isolation. This is why South America and Africa, once joined, share some groups (ratite birds, cichlid fishes) that diversified independently after the continents separated.

Ecological biogeography focuses on how current environmental conditions -- climate, habitat, resources, and species interactions -- determine where species live today. A fundamental pattern is the latitudinal diversity gradient: species richness is highest in the tropics and declines toward the poles. Tropical rainforests contain approximately 50% of Earth's species on only about 7% of its land area. This gradient holds for most groups of organisms, from trees to insects to birds to mammals.

One of the most influential ideas in ecological biogeography is island biogeography theory, developed by Robert MacArthur and E. O. Wilson in 1967. They proposed that the number of species on an island represents a dynamic equilibrium between two opposing processes: immigration (new species arriving) and extinction (existing species dying out). The equilibrium is determined by island size (larger islands have lower extinction rates and more habitat diversity, supporting more species) and isolation (more remote islands receive fewer immigrants, supporting fewer species).

Wallace's line, identified by Alfred Russel Wallace in 1859, is a biogeographic boundary running between Bali and Lombok and between Borneo and Sulawesi in Indonesia. West of the line, the fauna is Asian (tigers, elephants, primates); east of the line, the fauna is Australian (marsupials, cockatoos, birds of paradise). This sharp transition corresponds to a deep ocean trench that remained a barrier even when sea levels dropped during ice ages, preventing overland dispersal.

Visual Beginner

Factors affecting species richness on islands:

Factor	Effect on species richness	Reason
Larger island area	More species	Lower extinction rate, more habitat diversity
Smaller isolation distance	More species	Higher immigration rate
Higher habitat diversity	More species	More ecological niches

The equilibrium model of island biogeography:

Rate
 |    \        Extinction rate (increases with # species)
 |     \      /
 |      \    /
 |       \  /
 |        \/  Equilibrium point (S*)
 |        /\
 |       /  \
 |      /    Immigration rate (decreases with # species)
 |     /
 |    /
 |___________________
  0              Number of species on island

At equilibrium, immigration rate equals extinction rate, and the number of species remains stable (though the particular species present may change -- this is species turnover).

Latitudinal diversity gradient:

Latitude	Example region	Approximate tree species per hectare
Tropical (0-23 degrees)	Amazon rainforest	200-300
Subtropical (23-35 degrees)	Southeastern US	30-50
Temperate (35-55 degrees)	Northern Europe	5-15
Boreal (55-70 degrees)	Siberian taiga	1-5

Worked example Beginner

The species-area relationship is one of the most robust patterns in ecology. It is described by the equation:

$S = c A^{z}$

where $S$ is the number of species, $A$ is the area, $c$ is a constant that depends on the taxonomic group and region, and $z$ is the slope of the log-log relationship (typically 0.2-0.35 for islands, 0.1-0.2 for mainland habitats).

If a 1 km^2 island has 50 species, and a 100 km^2 island has 200 species, we can estimate $z$ :

$lo g (200) = lo g (c) + z \cdot lo g (100)$ $lo g (50) = lo g (c) + z \cdot lo g (1)$

Subtracting: $lo g (200) - lo g (50) = z \cdot lo g (100)$

$lo g (4) = z \cdot lo g (100) = z \cdot 2$

$z = lo g (4) /2 = 0.602/2 \approx 0.30$

This $z$ value of 0.30 is within the typical range for islands. The practical implication: if habitat area is reduced by 90% (from 100 km^2 to 10 km^2), the predicted number of species is:

$S_{10} = S_{100} \times (10/100)^{0.30} = 200 \times 0.1 0^{0.30} = 200 \times 0.501 \approx 100$ species.

A 90% reduction in area leads to approximately a 50% loss of species. This species-area relationship is central to predicting extinction from habitat loss in conservation biology.

A second worked example applies the MacArthur-Wilson equilibrium model. Consider two islands off the coast of a mainland with a species pool of $P = 200$ species. Island 1 is large (10 km^2) and near the mainland (1 km away). Island 2 is small (1 km^2) and far from the mainland (50 km away). Assume a maximum immigration rate $I_{0}$ that depends on distance: $I_{0} = 10$ species/year for the near island and $I_{0} = 2$ species/year for the far island. Assume an extinction rate constant $e = 1$ species/year.

For Island 1: $S^{*} = P \cdot I_{0} \cdot A / (I_{0} \cdot A + e \cdot P) = 200 \times 10 \times 10/ (10 \times 10 + 1 \times 200) = 20, 000/300 \approx 67$ species.

For Island 2: $S^{*} = 200 \times 2 \times 1/ (2 \times 1 + 1 \times 200) = 400/202 \approx 2$ species.

The large, near island supports approximately 67 species at equilibrium, while the small, remote island supports only about 2 species. This dramatic difference illustrates the combined effects of area (extinction reduction) and isolation (immigration reduction). Conservation planners apply this same logic when designing nature reserves: a single large reserve maintains far more species than several small reserves of equivalent total area.

Check your understanding Beginner

Exercise (easy, multiple choice).

According to island biogeography theory, which island would have the highest number of species at equilibrium?

A. Small and near the mainland B. Small and far from the mainland C. Large and near the mainland D. Large and far from the mainland

Hint

Two factors increase equilibrium species number: large area (lower extinction) and close proximity to mainland (higher immigration). Which option maximizes both?

Answer

Option C. A large island near the mainland has the highest equilibrium species number. Large size provides more habitat diversity and supports larger populations (reducing extinction rate). Close proximity to the mainland increases the immigration rate. The equilibrium species number is highest when immigration rate is high and extinction rate is low, which is achieved by a large, near island.

Exercise (easy, multiple choice).

What is the latitudinal diversity gradient?

A. Species richness increases from tropics to poles B. Species richness is highest in the tropics and decreases toward the poles C. Species richness is uniform across latitudes D. Species richness is highest at mid-latitudes

Hint

Tropical rainforests are the most species-rich terrestrial ecosystems. Polar regions have relatively few species. What pattern does this describe?

Answer

Option B. The latitudinal diversity gradient is the pattern in which species richness is highest in the tropics and declines toward the poles. This is one of the most pervasive patterns in ecology, observed in trees, insects, birds, mammals, marine organisms, and most other groups. Multiple hypotheses have been proposed to explain it: greater energy input (solar radiation) in the tropics supporting more species, greater climatic stability allowing specialization, longer evolutionary time for diversification in tropical regions, and greater habitat complexity in tropical forests.

Exercise (easy, true/false).

Continental drift has no relevance to modern species distributions because it occurred too long ago to affect extant species.

Hint

Consider why marsupials are found primarily in Australia and the Americas, or why the southern continents share some plant families (like the Proteaceae). When did Pangaea break up?

Answer

False. Continental drift has profoundly shaped modern species distributions. The breakup of Pangaea (beginning ~200 Ma) separated lineages that then evolved in isolation on different continents. This explains many disjunct distributions: marsupials in Australia and the Americas (not in Africa or Asia), ratite birds (ostriches in Africa, emus in Australia, rheas in South America, kiwis in New Zealand) reflecting their origin on the southern supercontinent Gondwana, and the distribution of the plant family Proteaceae across Australia, South America, and South Africa. Plate tectonics continues to shape distributions through mountain building, island formation, and changing ocean circulation.

Exercise 6

Exercise 6 (medium, short answer).

Explain why the latitudinal diversity gradient (more species in the tropics) is one of the most robust patterns in ecology, and evaluate at least three hypotheses proposed to explain it.

Hint

Consider both ecological mechanisms (energy, stability, competition) and historical mechanisms (time for speciation, past climate stability) that could generate the pattern.

Answer

The latitudinal diversity gradient is the consistent pattern of declining species richness from the equator toward the poles, observed in trees, insects, birds, mammals, marine organisms, and most other groups. At least three major hypotheses have been proposed:

Energy hypothesis: Tropical regions receive more solar radiation, supporting higher primary productivity, which in turn supports more species at higher trophic levels. Greater energy input means more individuals can be supported, and larger total populations reduce extinction rates. This hypothesis is supported by the correlation between productivity and species richness within many taxonomic groups, but it struggles to explain why some very productive systems (e.g., nutrient-enriched lakes) have low diversity.
Climate stability hypothesis: Tropical climates have been relatively stable over geological time, allowing species to persist longer (lower extinction) and to specialize on narrow niches (because conditions are predictable). Stability also permits the evolution of complex species interactions (specialist pollinators, specialized predator-prey relationships) that promote coexistence. The fact that tropical lineages tend to be older (as measured by molecular phylogenies) supports this hypothesis.
Time-for-speciation hypothesis: Tropical biomes have existed for longer than temperate or polar biomes (which were repeatedly disrupted by Pleistocene glaciations). More evolutionary time has allowed more speciation events to accumulate in tropical regions. The recency of temperate habitats (deglaciated only within the last 10,000-15,000 years) means they have not had sufficient time for speciation to produce tropical-level diversity.

No single hypothesis fully explains the gradient, and most biogeographers now favor a multifactorial explanation in which energy, stability, time, and other factors (including biotic interactions and geographic area, since the tropics occupy a larger area than temperate zones due to Earth's geometry) jointly produce the pattern.

Exercise 7

Exercise 7 (hard, analysis).

Climate change is shifting species ranges toward higher latitudes and elevations. Using island biogeography theory as an analogy, explain why habitat fragments in a human-dominated landscape may function as "islands." What are the implications for species that cannot disperse fast enough to track their shifting climate envelope?

Hint

Think of a nature reserve surrounded by agricultural land as an island of habitat in a hostile matrix. How does the species-area relationship apply? What happens as climate shifts the suitable conditions away from the reserve?

Answer

In a human-dominated landscape, patches of natural habitat (forest fragments, wetlands, grassland remnants) are surrounded by modified environments (agriculture, urban areas, roads) that are inhospitable to many species. These habitat patches function as islands in several ways: their species richness is determined by area (larger fragments support more species) and isolation (fragments closer to other fragments or to source populations retain more species through rescue effects). The species-area relationship applies: when a continuous habitat is fragmented into smaller patches, each patch retains a subset of the original species, with smaller patches losing more species through the relaxation process predicted by island biogeography theory.

Climate change adds a critical complication. As temperature and precipitation patterns shift, the climate conditions that define a species' niche move geographically. For a species whose suitable climate moves beyond the boundaries of its current habitat fragment, survival requires dispersal to a new fragment with suitable conditions. However, the hostile matrix between fragments may be impassable: a forest-dependent salamander cannot cross a sun-baked agricultural field, and a cold-water fish cannot traverse warm, polluted streams between habitats.

The implications are severe for species with limited dispersal ability. Species that cannot track their moving climate envelope face three options: (a) adapt in place to the new conditions (which requires sufficient genetic variation and time); (b) disperse to new habitat (which may be impossible due to fragmentation); or (c) go extinct. Island biogeography theory predicts that small, isolated fragments will lose species most rapidly, and climate change accelerates this loss by making current conditions increasingly unsuitable. Conservation responses include establishing climate corridors (connected habitat strips that facilitate range shifts), identifying and protecting climate refugia (areas where suitable conditions will persist), and assisted migration (deliberate translocation of species to new locations with suitable future climate).

Formal definition Intermediate+

Biogeography is the study of the spatial distribution of biodiversity and the processes that generate and maintain it.

Historical biogeography

Vicariance is the splitting of a species' range by a geographic barrier (e.g., mountain uplift, sea level rise, continental breakup), producing allopatric populations that evolve independently. Dispersal is the movement of organisms across pre-existing barriers. Distinguishing between vicariance and dispersal as explanations for a given distribution requires phylogenetic analysis combined with geological history: if the divergence time between populations matches the timing of the geological event, vicariance is supported; if the divergence postdates the barrier, dispersal is more likely.

Phylogeography uses molecular phylogenetics (typically mitochondrial DNA sequences) to reconstruct the geographic history of populations. Coalescent theory provides the statistical framework for inferring population history (population size changes, range expansions, bottlenecks) from gene genealogies. The spatial distribution of genetic lineages reveals past dispersal routes, refugia during glacial periods, and secondary contact zones.

Island biogeography theory

The MacArthur-Wilson model describes the equilibrium number of species on an island as the point where the immigration rate $I$ equals the extinction rate $E$ :

$I (S) = I_{0} (1 - \frac{S}{P})$

$E (S) = \frac{e \cdot S}{A}$

where $S$ is the number of species on the island, $P$ is the total species pool (mainland species), $I_{0}$ is the maximum immigration rate (dependent on isolation distance $D$ ), $e$ is the extinction rate constant, and $A$ is island area. At equilibrium:

$S^{*} = \frac{P \cdot I _{0} \cdot A}{I _{0} \cdot A + e \cdot P}$

The model predicts: (a) larger islands have more species at equilibrium; (b) less isolated islands have more species at equilibrium; (c) the species present change over time (turnover) even though the total number remains constant.

Species-area relationship

The species-area relationship (SAR) is:

$S = c A^{z} or equivalently lo g S = lo g c + z lo g A$

For islands, $z$ typically ranges from 0.25 to 0.35. For nested samples within a mainland area (province SAR), $z$ is typically 0.12-0.18. The species-area relationship is one of the few genuine "laws" in ecology.

Key results Intermediate+

Result 1 (MacArthur-Wilson equilibrium dynamics). The equilibrium number of species on an island can be derived analytically. For an island at distance $D$ from the mainland with area $A$ :

$S^{*} = \frac{P}{1 + \frac{e}{I _{0}} \cdot \frac{P}{A}}$

This predicts that $S^{*}$ increases with area (approaching $P$ for very large islands) and decreases with isolation (through $I_{0}$ ). The model has been supported by experiments: Simberloff and Wilson (1969) defaunated mangrove islands in Florida Bay by fumigation and observed recolonization. Species numbers returned to pre-defaunation levels, and the equilibrium was approximately the same, but the particular species composition differed, confirming turnover.

Result 2 (Distance decay of similarity). The similarity in species composition between two sites decays with geographic distance, approximately following an exponential or power-law relationship:

$β_{sim} = a \cdot e^{- b \cdot d}$

where $β_{sim}$ is a similarity index, $d$ is geographic distance, and $a$ and $b$ are fitted parameters. This distance decay reflects the combined effects of environmental gradients, dispersal limitation, and historical contingency.

Exercise 1

Exercise 1 (medium, numeric).

Using the species-area relationship $S = c A^{z}$ with $z = 0.30$ and $c = 40$ : a nature reserve of 1,000 km^2 currently protects 400 species. If a highway project reduces the effective habitat area by 50%, how many species are predicted to be lost?

Hint

Calculate the new area (500 km^2), then use the species-area relationship to find the new species number. The difference is the predicted loss.

Answer

First, verify the initial conditions: $S = 40 \times 100 0^{0.30} = 40 \times 7.94 = 317.6$ . This does not match 400 species, suggesting different $c$ values. Let me recalculate using the given data directly.

$400 = c \times 100 0^{0.30}$ , so $c = 400/100 0^{0.30} = 400/7.94 \approx 50.4$ .

After 50% area reduction: $A = 500$ km^2.

$S_{n e w} = 50.4 \times 50 0^{0.30} = 50.4 \times 6.50 = 327.6$

Species lost: $400 - 328 = 72$ species.

Equivalently, using the ratio: $S_{n e w} / S_{o l d} = (500/1000)^{0.30} = 0. 5^{0.30} = 0.812$ .

$S_{n e w} = 400 \times 0.812 = 325$ species. Lost: 75 species (approximately).

A 50% habitat reduction leads to approximately a 19% species loss (not 50%), because species richness scales with area to the power $z < 1$ .

Exercise 2

Exercise 2 (medium, short answer).

Explain the difference between endemic and cosmopolitan species, and discuss why islands have disproportionately high rates of endemism.

Hint

Endemic species are found nowhere else. Cosmopolitan species have worldwide distributions. What factors make islands evolutionary laboratories for unique species?

Answer

Endemic species are restricted to a specific geographic area and found nowhere else (e.g., lemurs in Madagascar, kiwis in New Zealand). Cosmopolitan species have broad distributions across multiple continents or ocean basins (e.g., the barn owl, found on every continent except Antarctica).

Islands have disproportionately high rates of endemism for several reasons:

Isolation: The ocean barrier prevents gene flow with mainland populations, allowing island populations to diverge genetically and morphologically from their ancestors.
Small population sizes: Genetic drift is stronger in small populations, rapidly fixing novel alleles that differentiate island populations.
Novel selective environments: Islands often lack the predators, competitors, and parasites found on the mainland, creating relaxed selection on some traits and strong selection on others. This drives ecological release and niche shifts.
Founder effects: The small number of colonizers that establish island populations carry only a subset of the genetic variation in the source population, creating immediate genetic differentiation.
Adaptive radiation: In the absence of competitors, colonizing species can diversify to fill multiple ecological niches (e.g., Hawaiian honeycreepers, Galapagos finches).

Madagascar, for example, has an endemism rate of approximately 90% for plants, 95% for reptiles, and 100% for lemurs -- virtually all its native species are found nowhere else.

Exercise 3

Exercise 3 (medium, short answer).

Explain how the breakup of Gondwana produced the modern distribution of ratite birds (ostrich, emu, rhea, kiwi, cassowary). Is this pattern best explained by vicariance or dispersal, and what evidence supports your conclusion?

Hint

Consider the timing of ratite divergences relative to the timing of Gondwanan breakup. Molecular clock estimates and the fossil record are relevant.

Answer

The ratite distribution is primarily explained by vicariance associated with the breakup of Gondwana. The ancestral ratite population was widely distributed across Gondwana during the late Cretaceous. As Gondwana fragmented -- first separating Africa (~~105 Ma), then India-Madagascar (~~88 Ma), then South America and Australia-Antarctica (~~80 Ma), and finally Australia and Antarctica (~~35 Ma) -- ratite populations on each continental fragment became isolated and evolved independently.

Molecular clock analyses place the major ratite divergences at approximately 80-90 Ma, broadly consistent with the sequence of Gondwanan breakup. The shared presence of ratites on all former Gondwanan continents (except India, where ratites are now extinct but fossil evidence confirms their past presence) matches the prediction of vicariance: related lineages on fragments of a once-continuous landmass.

Some complications exist. New Zealand kiwis are most closely related to the extinct Madagascar elephant bird (rather than to the Australian emu), a relationship that is difficult to explain by simple vicariance and may require some over-water dispersal. Additionally, the ancestor of ratites was likely capable of flight, meaning that some ratite lineages may have dispersed between separating continents before losing the ability to fly. The pattern is therefore best described as primarily vicariant with secondary dispersal modifying the details.

Exercise 4

Exercise 4 (hard, analysis).

Compare and contrast the predictions of MacArthur-Wilson island biogeography theory and Hubbell's neutral theory for the species-area relationship. Under what conditions would each theory make similar predictions, and when would they diverge?

Hint

MacArthur-Wilson predicts equilibrium species number based on immigration-extinction balance. Hubbell predicts species abundance distributions from birth-death-dispersal stochasticity. Consider what determines the slope parameter z in each model.

Answer

Both theories predict species-area relationships, but from different mechanisms:

MacArthur-Wilson derives the SAR from an equilibrium between immigration (dependent on isolation) and extinction (dependent on island area and population size). The z parameter emerges from the interaction between these rates. Larger islands have lower extinction rates (supporting larger populations), and less isolated islands have higher immigration rates. The model explicitly incorporates dispersal limitation (through the immigration rate parameter $I_{0}$ ) and demographic stochasticity (through the extinction rate).

Hubbell's neutral theory derives the SAR from a metacommunity model in which all species are demographically identical. The local community is a sample from the metacommunity, and the number of species in the local sample depends on the sample size (which scales with area). The theory predicts $z \approx 0.25$ for islands as an emergent property of the zero-sum multinomial distribution of species abundances. No niche differences or species-specific traits are required.

The theories make similar predictions when: (a) the community is at or near equilibrium; (b) the species pool is large relative to local diversity; and (c) dispersal limitation operates uniformly across species. Both predict that larger areas contain more species and that the relationship follows a power law.

They diverge when: (a) the community is not at equilibrium (e.g., recently disturbed islands where MacArthur-Wilson's transient dynamics differ from neutral theory's relaxation dynamics); (b) strong niche structure exists, which would cause deviations from neutral predictions (e.g., habitat filtering might produce a steeper SAR than neutral theory predicts); (c) specific species traits affect colonization or extinction differentially (which MacArthur-Wilson can partially accommodate but neutral theory excludes by assumption). Empirical tests suggest that neutral theory fits SARs well for many systems, but that adding niche structure improves predictions in environmentally heterogeneous landscapes.

Exercise 8

Exercise 8 (medium, short answer).

Describe how molecular phylogenetics (phylogeography) can be used to distinguish between vicariance and dispersal as explanations for a disjunct species distribution. Provide a specific example.

Hint

Vicariance predicts that the divergence time between populations should match the timing of the geological event that separated them. Dispersal predicts that divergence postdates the barrier. Molecular clocks date the divergence.

Answer

Molecular phylogenetics can distinguish vicariance from dispersal by estimating the timing of population divergence using molecular clocks and comparing this timing to the known geological history of the barrier:

If vicariance: The divergence time between populations on either side of the barrier should coincide with the formation of the barrier. For example, if a mountain range uplifted 5 million years ago and split a species into eastern and western populations, molecular clock analysis should date the divergence of these populations to approximately 5 Ma.

If dispersal: The divergence time should postdate the formation of the barrier, because the species crossed the barrier after it was already in place. A divergence time much younger than the barrier indicates that the disjunct distribution resulted from long-distance dispersal, not vicariance.

A classic example involves the plant genus Nothofagus (southern beech). Nothofagus is distributed across South America, Australia, New Zealand, New Caledonia, and New Guinea -- all former parts of Gondwana. Vicariance predicts that the divergence times of Nothofagus lineages on different continents should match the sequence of Gondwanan breakup (South America-Antarctica separated ~35 Ma, Australia-New Zealand ~80 Ma, etc.). Molecular dating broadly supports this prediction for several lineage splits, confirming vicariance. However, some Nothofagus lineages show divergence times younger than the relevant continental separations, suggesting occasional long-distance dispersal between landmasses via ocean currents or wind. This example illustrates that real biogeographic patterns often involve both vicariance and dispersal, and molecular phylogenetics is essential for teasing apart their relative contributions.

The increasing availability of genomic data has greatly improved the precision of divergence time estimates, making phylogeographic analysis one of the most powerful tools in modern biogeography. Whole-genome sequencing of co-distributed species can reveal whether multiple taxa responded concordantly to the same geological events (supporting a shared vicariant history) or whether each taxon has a unique history of dispersal and isolation.

Exercise 5

Exercise 5 (medium, short answer).

Why do islands have disproportionately high rates of endemism compared to mainland areas of similar size? Discuss at least three factors that contribute to island endemism.

Hint

Consider the processes that create and maintain genetic distinctness: isolation, small population size, novel environments, and founder effects.

Answer

Islands have high endemism rates because several processes combine to produce genetic and morphological divergence from mainland ancestors:

Geographic isolation prevents gene flow with mainland populations. Without the homogenizing effect of immigration, island populations accumulate genetic differences through mutation and drift, and morphological differences through adaptation to local conditions. Over evolutionary time, these differences become sufficient to warrant recognition as distinct species.
Founder effects create immediate genetic differentiation. The small number of individuals that colonize an island carry only a subset of the genetic variation present in the source population. This genetic bottleneck creates a distinct genetic composition from the outset, and the lost variation cannot be replenished without further immigration.
Novel selective environments on islands often lack the predators, competitors, parasites, and diseases found on the mainland. This "ecological release" allows colonists to exploit niches that were previously occupied by other species on the mainland, driving adaptive divergence. The absence of mammalian predators on oceanic islands, for example, has led to the evolution of flightlessness in many island birds (the dodo, the kakapo, the Galapagos cormorant).
Small population sizes enhance the effect of genetic drift, rapidly fixing novel alleles that would remain rare in large mainland populations. This drift-driven divergence can produce morphological and genetic distinctness even without strong selection.

Madagascar exemplifies these processes: it has been isolated from all other landmasses for approximately 88 million years, and approximately 90% of its plant species and 95% of its reptile species are endemic. The entire lemur radiation (over 100 species) is endemic to Madagascar, having evolved from a single colonizing ancestor.

Advanced treatment Master

The theoretical foundations of biogeography have evolved considerably since MacArthur and Wilson's equilibrium model. Hubbell's unified neutral theory (2001) challenged the niche-based perspective by proposing that the patterns of species abundance and distribution in ecological communities can be explained by a model in which all species are demographically identical (neutral) and patterns arise entirely from stochastic processes: birth, death, dispersal, and speciation.

The neutral theory makes specific, testable predictions about the species-abundance distribution (predicting a zero-sum multinomial distribution), the species-area relationship (predicting $z \approx 0.25$ for islands), and the relationship between local and regional species richness. The theory generated intense debate because it makes accurate predictions without invoking niche differences, suggesting that many macroecological patterns are consistent with both niche and neutral mechanisms.

The resolution of this debate has converged on a synthesis: both niche and neutral processes operate in all communities, but their relative importance varies with spatial scale, environmental heterogeneity, and taxonomic group. At large spatial scales (continents to global), neutral processes (dispersal limitation, stochastic extinction) often dominate. At local scales (within a forest plot), niche processes (habitat filtering, competitive exclusion) are more important. The challenge is to quantify the relative contributions of each.

Climate change and range shifts. Anthropogenic climate change is altering species distributions at an unprecedented rate. Observational data from the past 50 years show that species are shifting their ranges toward higher latitudes (poleward) and higher elevations at rates of approximately 1-17 km per decade (for latitudinal shifts) and 11-40 m per decade (for elevational shifts). These shifts lag behind the rate of temperature change, creating "climate debts" where species' ranges are not yet in equilibrium with current climate.

The velocity of climate change -- the rate at which isotherms move across the landscape -- determines the speed at which species must migrate to track their climatic niche. In flat terrain, this velocity is high (tens of km per year in some regions); in mountainous terrain, it is much lower because moving a short distance upslope achieves the same temperature change. This predicts that flat landscapes (grasslands, boreal forests) will experience the greatest climate-driven disruption, while mountainous regions may serve as climate refugia.

Phylogeography and comparative phylogeography. The integration of molecular phylogenetics with geographic information has given rise to phylogeography, which reconstructs the spatial history of genetic lineages. Comparative phylogeography examines whether co-distributed species show concordant phylogeographic breaks, which would indicate shared historical responses to geological or climatic events. The southeastern United States, for example, shows concordant phylogeographic breaks across multiple taxa (fishes, salamanders, turtles, plants) at the Apalachicola River and the Mississippi River, reflecting vicariance during Pleistocene sea level fluctuations.

Continental drift and the distribution of life. The movement of tectonic plates has been one of the most powerful forces shaping the distribution of organisms over geological time. The supercontinent Pangaea, which assembled approximately 335 million years ago and began breaking apart approximately 200 million years ago, explains many of the most striking disjunct distributions in modern biogeography. The southern portion of Pangaea, called Gondwana, united what are now South America, Africa, Antarctica, India, and Australia. Its subsequent breakup, which occurred in stages between approximately 180 and 30 million years ago, created the conditions for independent evolutionary trajectories on each fragment. The ratite birds -- flightless running birds including the ostrich (Africa), emu and cassowary (Australia), rhea (South America), and kiwi (New Zealand) -- represent a Gondwanan radiation whose modern distribution directly maps onto the fragments of the former supercontinent. Molecular dating places the divergence of ratite lineages at approximately 80-90 million years ago, consistent with the timing of Gondwanan breakup, though some debate remains about whether flighted ancestors dispersed between fragments after separation.

The plant family Proteaceae provides a parallel example. This family, which includes the iconic Protea of South Africa, the macadamia of Australia, and the Chilean firebush (Embothrium), is distributed across all the former Gondwanan continents. Fossil pollen of Proteaceae from the late Cretaceous has been found in Antarctica, confirming that the family was present on Gondwana before its breakup. The current distribution is therefore a vicariance pattern: the ancestors occupied a continuous range across Gondwana, and continental fragmentation isolated populations that then diversified independently. Similar patterns are found in southern beech (Nothofagus), whose distribution across South America, Australia, New Zealand, New Caledonia, and New Guinea matches the sequence of Gondwanan breakup so precisely that Nothofagus fossils have been used to reconstruct the timing of continental separation.

The assembly of Laurasia (the northern portion of Pangaea) created different biogeographic conditions. The Bering Land Bridge, which connected Siberia and Alaska intermittently during periods of low sea level throughout the Cenozoic, allowed exchange between Asian and North American faunas. This explains why the mammalian faunas of eastern Asia and North America share many groups (deer, bears, wolves, cats, beavers) that are absent from South America and Africa. The Great American Biotic Interchange, triggered by the formation of the Isthmus of Panama approximately 3 million years ago, allowed the mixing of previously isolated North and South American faunas. North American mammals (armadillos, opossums, and porcupines moved north; horses, tapirs, llamas, and cats moved south) invaded South America, with asymmetric results: North American-derived groups dominated many niches, contributing to the extinction of most large native South American mammals.

Island biogeography in action: empirical tests. The MacArthur-Wilson equilibrium model has been tested experimentally and observationally in a wide range of systems. The most famous experimental test was conducted by Daniel Simberloff and Edward O. Wilson in 1969-1970 on small mangrove islands in Florida Bay. They surveyed the arthropod fauna of each island, then encased selected islands in tenting and fumigated them with methyl bromide to eliminate all arthropods. The defaunated islands were then monitored for recolonization. The key results confirmed the model: species numbers returned to approximately pre-defaunation levels, and the equilibrium was roughly the same, but the particular species composition differed from the original community, confirming the prediction of species turnover. Larger islands accumulated species faster and supported more species at equilibrium, and nearer islands were colonized faster than remote ones, exactly as predicted.

The model has also been tested at larger spatial and temporal scales. The volcanic island of Surtsey, which emerged from the sea off Iceland in 1963, has been continuously monitored for colonization. The first vascular plant was recorded in 1965, and by 2020 approximately 75 species had established. Colonization rates were initially high and declined over time, consistent with the MacArthur-Wilson immigration curve. Similarly, Krakatau, devastated by a massive volcanic eruption in 1883, has been recolonized over the past 140 years, providing a natural experiment in island assembly. Birds, plants, and insects have colonized in predictable sequences: plants arrived first (via sea drift, wind, and bird transport), followed by insects and birds that depend on plant resources. The trajectory shows a pattern of rapid initial accumulation followed by slowing, consistent with the equilibrium prediction.

Wallace's line and biogeographic boundaries. Wallace's line, identified by Alfred Russel Wallace during his travels in the Malay Archipelago (1854-1862), remains one of the most striking biogeographic boundaries on Earth. Running between Bali and Lombok (separated by only 35 km of open ocean) and between Borneo and Sulawesi, the line marks a sharp transition from Asian fauna (tigers, rhinoceroses, primates, woodpeckers) to Australian fauna (marsupials, cockatoos, birds of paradise). The biogeographic significance of the line was not fully understood until the development of plate tectonics: the line corresponds to a deep ocean trench (the Wallace Trench) that remained a marine barrier even during Pleistocene sea level lows, when the shallow Sunda and Sahul continental shelves were exposed as dry land, connecting mainland Asia to Borneo, Java, and Sumatra (Sundaland) and connecting Australia to New Guinea (Sahul). The deep water between these continental shelves prevented overland dispersal of terrestrial organisms, maintaining the faunal boundary.

The region between Wallace's Line and Lydekker's Line (which marks the edge of the Australian continental shelf) is known as Wallacea, a zone of mixed Asian and Australian faunal elements. This transitional zone contains many endemic species, including the Komodo dragon, the babirusa (a pig-like suid with upward-curving tusks), and numerous bird species found nowhere else. The complex geography of Wallacea -- numerous small islands separated by deep-water channels -- created conditions for extensive allopatric speciation, making the region one of the world's most important biodiversity hotspots.

Human impacts on species distribution. Anthropogenic activities are reshaping species distributions at an unprecedented rate, creating novel biogeographic patterns that have no historical analogue. Intentional and accidental species introductions have broken down the geographic barriers that maintained distinct biotas for millions of years. The European colonization of the Americas, Australia, and islands worldwide introduced hundreds of species that have fundamentally altered the recipient ecosystems. The brown tree snake (Boiga irregularis), accidentally introduced to Guam after World War II, drove 10 of 13 native forest bird species to extinction through predation. The Nile perch, introduced into Lake Victoria in the 1950s to create a sport fishery, caused the extinction of approximately 200 endemic cichlid species through predation -- the largest documented vertebrate extinction caused by a single introduced species.

Climate change is shifting species ranges toward higher latitudes and elevations, as documented in hundreds of studies. The Edith's checkerspot butterfly (Euphydryas editha) has shifted its range northward and to higher elevations in western North America, with population extinctions at southern and low-elevation range margins occurring approximately four times faster than colonizations at northern margins, indicating that the range is contracting faster than it is expanding. Marine species are also shifting: fish communities in the North Sea have moved northward by approximately 100 km per decade in response to warming waters, with warm-water species replacing cold-water species. These climate-driven range shifts are creating novel species assemblages (no-analog communities) that have not existed previously, with unpredictable consequences for ecosystem function.

Connections Master

Macroevolution 19.08.01. Speciation and extinction, the macroevolutionary processes that generate and eliminate species, are the ultimate determinants of species distributions. Allopatric speciation is fundamentally a biogeographic process: geographic barriers create the isolation necessary for reproductive isolation to evolve. The timing and geography of speciation events, reconstructed from molecular phylogenies, provide the data for testing biogeographic hypotheses about the roles of vicariance and dispersal. Adaptive radiations on islands and archipelagos (Darwin's finches, Hawaiian honeycreepers, cichlid fishes in African rift lakes) are both macroevolutionary and biogeographic phenomena. The Cambrian explosion itself, discussed in macroevolution, created the initial body-plan diversity that was subsequently distributed across continents by tectonic processes.
Ecosystem ecology 19.11.01. The geographic distribution of ecosystems (biomes) is determined by climate (temperature and precipitation), and primary productivity varies predictably across biomes. Biogeography provides the spatial framework for understanding global patterns of energy flow and nutrient cycling. The latitudinal diversity gradient, a core biogeographic pattern, has implications for ecosystem function: the high species richness of tropical ecosystems may contribute to their high primary productivity and rapid nutrient cycling, though the direction of causation between diversity and ecosystem function remains debated.
Conservation biology 19.14.01. Island biogeography theory directly informs reserve design: larger reserves maintain more species, and corridors between reserves reduce effective isolation. The species-area relationship is used to predict extinction from habitat loss, and endemism hotspots are priorities for conservation investment. The concept of biodiversity hotspots, which combines species richness, endemism, and threat level to identify priority conservation areas, is fundamentally a biogeographic tool. Climate-driven range shifts add urgency to conservation planning: species must be able to track their moving climatic niches through connected habitats, or face extinction.
Coevolution 19.13.01. Geographic variation in species interactions (the geographic mosaic of coevolution) is fundamentally a biogeographic phenomenon. The strength and outcome of coevolutionary interactions vary across the landscape, producing a mosaic of adaptation and maladaptation. The distribution of coevolutionary hotspots and coldspots is shaped by the same processes of dispersal, vicariance, and environmental variation that structure species distributions. The geographic mosaic theory predicts that coevolved traits should show geographic structure (clinal variation, patchy distribution) that reflects the spatial pattern of selection, and this has been confirmed in systems like the newt-snake arms race described in 19.13.01.
Body plans 18.01.01. The distribution of animal body plans across continents and ocean basins reflects the combined effects of evolutionary history and continental drift. The concentration of marsupials in Australia resulted from the isolation of the Australian plate after the breakup of Gondwana, which prevented placental mammals from reaching the continent until relatively recently. The Cambrian explosion produced a diversity of body plans that were subsequently distributed across the globe by tectonic processes, with local extinction and radiation modifying the fauna on each continental fragment. Understanding the modern distribution of animal phyla requires integrating the developmental genetics of body plans with the tectonic history of continental movements.

Historical & philosophical context Master

Biogeography has always tied biology to Earth history. Early naturalists noticed that similar climates could hold different organisms, and that islands, mountains, rivers, and continents shaped distributions. Darwin and Wallace used geographic pattern as evidence for evolution; plate tectonics, phylogenetics, and paleoclimate later gave the field its modern causal machinery. The result is a discipline where place is not a backdrop. Geography, dispersal, extinction, adaptation, and historical accident jointly explain why life is where it is.

The intellectual history of biogeography can be traced through several transformative episodes. In the 18th century, Carl Linnaeus and Georges-Louis Leclerc, Comte de Buffon, noted that different regions of the world hosted different species, even under similar climatic conditions. Buffon's observation that the mammals of the Old and New World tropics were distinct -- despite similar climate -- became known as Buffon's Law and was one of the first formal biogeographic generalizations. This pattern could not be explained by climatic determinism alone and hinted at historical explanations that would not be fully articulated for another century.

Alfred Russel Wallace, working independently of Darwin, arrived at the theory of natural selection partly through his biogeographic observations. Wallace spent eight years (1854-1862) collecting specimens in the Malay Archipelago, during which he identified the sharp faunal boundary between Asian and Australian faunas that now bears his name. His 1855 paper "On the Law Which Has Regulated the Introduction of New Species" argued that new species arise in the same geographic area as their close relatives, a biogeographic observation that implied common descent with modification. This paper, published three years before the Darwin-Wallace joint papers of 1858, articulated a principle of biogeographic continuity that was a direct precursor to the theory of evolution. Wallace's two-volume work The Geographical Distribution of Animals (1876) established zoogeographic regions that are still used today and laid the foundation for historical biogeography as a scientific discipline.

Leon Croizat's panbiogeography, developed in the mid-20th century, took a different approach. Croizat plotted the distributions of thousands of taxa on maps and connected the geographic centers of related taxa with lines he called "tracks." He found that tracks for unrelated groups often coincided, forming what he called "generalized tracks" that he interpreted as evidence of common Earth history affecting multiple lineages simultaneously. Croizat's methods were criticized as overly subjective, but his core insight -- that concordant distribution patterns across unrelated taxa indicate shared historical events -- was later vindicated by the integration of phylogenetics and plate tectonics. The modern discipline of cladistic biogeography, developed by Donn Rosen, Gareth Nelson, and Norman Platnick in the 1970s and 1980s, combined Croizat's track concept with phylogenetic systematic methods, providing a rigorous framework for testing biogeographic hypotheses.

The plate tectonics revolution of the 1960s provided the mechanism that had been missing from historical biogeography. Before plate tectonics, biogeographers invoked land bridges (now sunken) and continental foundering to explain disjunct distributions. The confirmation of continental drift by oceanographic evidence (seafloor spreading, magnetic striping, paleomagnetic data) transformed biogeographic explanation. Disjunct distributions of fossils and living organisms on separate continents -- Glossopteris ferns across all southern continents, Mesosaurus in South America and Africa, Lystrosaurus in Africa, India, and Antarctica -- were reinterpreted as evidence that these organisms had occupied continuous ranges on formerly contiguous continents. The match between the sequence of continental breakup (as dated by oceanic magnetic anomalies) and the divergence times of biotas on separate fragments (as estimated from molecular clocks) provides one of the most powerful tests of biogeographic hypotheses.

The philosophical dimension of biogeography centers on the tension between historical contingency and ecological determinism. To what extent are species distributions determined by current environmental conditions (climate, habitat, resources), and to what extent are they the product of historical accidents (where a lineage happened to originate, which barriers it happened to cross, which continents it happened to ride)? The answer is that both factors matter, but their relative importance varies with taxonomic group, spatial scale, and timescale. At large spatial and temporal scales, historical factors dominate: the absence of placental mammals in Australia prior to human introduction is a historical accident (the Australian continent separated before placentals arrived), not an ecological constraint. At local scales, ecological factors dominate: which plant species occupies a particular hillside is determined more by soil, moisture, and competition than by continental history. Biogeography, as a discipline, occupies the productive zone where these two explanatory frameworks meet.

Contemporary biogeography faces an ethical challenge that earlier practitioners did not anticipate. The discipline has documented the geographic patterns of biodiversity, and this knowledge now underpins conservation priority-setting. The identification of biodiversity hotspots, the mapping of endemic species distributions, and the prediction of climate-driven range shifts are all biogeographic exercises with direct conservation consequences. The field has thus shifted from a purely descriptive and explanatory enterprise to one with urgent practical implications: biogeographic knowledge is now a necessary input to decisions about which lands to protect, which species to prioritize, and how to design landscapes that allow species to persist under climate change. This normative dimension adds a layer of responsibility to biogeographic research that the field is still assimilating.

Bibliography Master

Campbell, N. A. & Reece, J. B. Biology, 12th ed. (Pearson, 2020). Ch. 52.
Lomolino, M. V., Riddle, B. R., Whittaker, R. J. & Brown, J. H. Biogeography, 5th ed. (Sinauer, 2017).
MacArthur, R. H. & Wilson, E. O. The Theory of Island Biogeography (Princeton University Press, 1967).
Hubbell, S. P. The Unified Neutral Theory of Biodiversity and Biogeography (Princeton University Press, 2001).
Wallace, A. R. The Geographical Distribution of Animals (Macmillan, 1876).
Simberloff, D. S. & Wilson, E. O. "Experimental zoogeography of islands: the colonization of empty islands." Ecology 50 (1969) 278-296.
Whittaker, R. J. & Fernandez-Palacios, J. M. Island Biogeography: Ecology, Evolution, and Conservation, 2nd ed. (Oxford University Press, 2007).
Croizat, L. Panbiogeography (Published by the author, Caracas, 1958).
Nelson, G. & Platnick, N. I. Systematics and Biogeography: Cladistics and Vicariance (Columbia University Press, 1981).
Losos, J. B. & Ricklefs, R. E. "Adaptation and diversification on islands." Nature 457 (2009) 830-836.

Prerequisites

19.08.01 pending
19.11.01 pending

Tier anchors

beginner: Campbell Biology, 12th ed. (2020), Ch. 52; Khan Academy Biology
intermediate: Lomolino, Riddle, Whittaker & Brown — Biogeography, 5th ed. (2017); Cox, Moore & Ladle — Biogeography: An Ecological and Evolutionary Approach, 9th ed.
master: MacArthur & Wilson — The Theory of Island Biogeography (1967); Hubbell — The Unified Neutral Theory of Biodiversity and Biogeography (2001); relevant primary literature

References

Campbell Biology, 12th ed. (Pearson, 2020) · Ch. 52 An Introduction to Ecology and the Biosphere
Lomolino, M. V., Riddle, B. R., Whittaker, R. J. & Brown, J. H. — Biogeography, 5th ed. (Sinauer, 2017) · Ch. 1-16
MacArthur, R. H. & Wilson, E. O. — The Theory of Island Biogeography (Princeton University Press, 1967) · The equilibrium model of island biogeography
Hubbell, S. P. — The Unified Neutral Theory of Biodiversity and Biogeography (Princeton University Press, 2001) · Monographs in Population Biology 32
Wallace, A. R. — The Geographical Distribution of Animals (Macmillan, 1876) · Wallace's line and zoogeographic regions

Estimated time

beginner: 15m
intermediate: 35m
master: 55m