Island biogeography

Concept

Island biogeography

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Statement

Species richness from area and isolation.

Why it matters

metapopulation-dynamics treats colonisation and extinction across habitat patches in the abstract; island biogeography theory applies exactly that same colonisation-extinction logic to real oceanic and habitat islands and yields a specific, testable quantitative prediction: an island's equilibrium species richness is set by the balance between immigration rate, which falls with isolation, and extinction rate, which falls with area. This gives conservation biology one of its clearest practical results, directly informing reserve design.

ecological-succession describes how a community's composition changes over time on a fixed patch; island biogeography adds the further dimension of how total species number itself depends on the size and isolation of that patch, at equilibrium.

Hypotheses

Immigration rate to an island declines as the number of species already present rises, and, at any richness, is higher for islands nearer a mainland source than for more isolated islands.Fewer new colonists are new to an already species-rich island (saturation), and isolation directly limits the rate at which potential colonists can reach the island at all. Extinction rate on an island rises as the number of species already present rises, and, at any richness, is higher on smaller islands than on larger ones.More species competing for a fixed resource base raises the average extinction risk per species, and smaller islands support smaller, more extinction-prone populations of each species present. The model treats species richness as a dynamic equilibrium, with species composition continually turning over even at constant total number, not a static, unchanging list — an island "at equilibrium" is still gaining and losing species at equal, offsetting rates.

Proof

I(S) \text{ decreasing in } S; \qquad E(S) \text{ increasing in } S

Immigration rate falls and extinction rate rises as current richness \(S\) increases, following the Hypotheses directly. A

I(\hat S) = E(\hat S) \ \Rightarrow\ \text{equilibrium richness } \hat S

Species gains and losses balance exactly where the two curves intersect; individual species continue turning over even though total richness stops changing at this point (Hypotheses, t3). A

\text{Near islands: higher } I(S) \text{ at every } S \ \Rightarrow\ \text{higher } \hat S, \text{ all else equal.}

Reduced isolation shifts the whole immigration curve upward, moving its intersection with the (unchanged) extinction curve to a higher equilibrium richness. A

\text{Large islands: lower } E(S) \text{ at every } S \ \Rightarrow\ \text{higher } \hat S, \text{ all else equal.}

Larger area supports larger, more extinction-resistant populations, shifting the extinction curve downward and, symmetrically to Step3, moving the equilibrium to a higher richness. A

S = cA^{z}

Combining Steps3–4's area and isolation effects into a compact empirical relationship: species number \(S\) scales with island area \(A\), with taxon- and region-specific constant \(c\) and exponent \(z\), typically found empirically in roughly the \(0.2\)–\(0.35\) range for true oceanic islands. B

Result

I(\hat S)=E(\hat S) \ \text{at equilibrium}; \qquad S=cA^{z}

Reading. Two independent axes set equilibrium richness — isolation controls immigration, area controls extinction — and their intersection is a dynamic, not static, balance.

Scope. Works best for true islands and comparably discrete habitat "islands" (mountaintops, lakes, forest fragments in a hostile matrix); the specific exponent \(z\) is empirical and varies by taxon and region rather than a single universal constant.

Corollaries & converses

metapopulation-dynamics' colonisation-extinction framework for patches in a landscape is the same logic generalised beyond literal islands to any discrete habitat patch, of which oceanic islands are the original, clearest test case.
Reserve design directly inherits this result's practical implication: a single large reserve typically supports higher equilibrium richness, and lower per-species extinction risk, than several small reserves of the same total area, and reserves placed nearer a source of colonists retain richness more readily than isolated ones.
Converse: observing species turnover, with different species present over time at roughly constant total richness, is itself evidence an island is near dynamic equilibrium, rather than evidence that no ecological change is occurring on it at all.

Fails without

Drop the declining-immigration / rising-extinction shape of the two rate curves (Hypotheses): without \(I(S)\) falling and \(E(S)\) rising, the two curves need not cross at a stable single equilibrium at all, and richness could instead increase without bound or collapse to zero.
Ignore habitat heterogeneity, treating isolation and area as the only drivers: an island richer in habitat diversity for its area (varied elevation, geology, or microclimate) can support higher richness than the plain \(S=cA^z\) relationship predicts from area alone, since habitat diversity itself supplies more distinct niches independent of raw area.

Common errors

Treating equilibrium richness as a fixed, unchanging list of the same species, rather than a constant total number maintained by ongoing turnover (Hypotheses, t3; Corollaries' converse).
Assuming \(z\) in \(S=cA^z\) is a universal constant; it is an empirically fit exponent that varies by taxonomic group and region.
Assuming isolation and area are the only two variables that matter, ignoring habitat heterogeneity as an independent contributor to richness (Fails without, second bullet).
Applying "one large reserve beats several small" as an absolute, universal rule, without considering that several smaller, well-connected reserves can sometimes better protect a broader range of habitat types or buffer against a single catastrophic event at one site.

Discussion

Robert MacArthur and E.O. Wilson proposed the equilibrium theory of island biogeography in 1967, reframing island species richness as a dynamic balance rather than a simple historical accident; the theory rapidly became a foundational tool in conservation biology once ecologists recognised that habitat fragments surrounded by human-altered land behave, ecologically, much like literal islands.

The theory's application to habitat fragments, rather than true oceanic islands, has attracted substantial debate, since the surrounding "matrix" (agricultural land, versus regrown secondary forest, for instance) is rarely as uniformly inhospitable to dispersal as open ocean is around a true island, weakening the clean isolation-immigration relationship the original theory assumes.

Common misconception: that island biogeography theory predicts which particular species will be present on an island. It predicts total species number at equilibrium, not species identity, which, per the turnover premise, can and does change over time even while richness itself remains roughly constant.

Worked examples

\text{Two islands, equal area, one near mainland and one far: near island has higher } I(S), \text{ same } E(S).

By Step3, the near island's higher immigration curve intersects the shared extinction curve at a higher equilibrium richness. A

\text{Two islands, equal distance from mainland, one large and one small: equal } I(S), \text{ small island has higher } E(S).

By Step4, the small island's higher extinction curve intersects the shared immigration curve at a lower equilibrium richness. A

\text{near-large: highest richness} \quad \cdots \quad \text{far-small: lowest richness}

Reading. The two independent axes combine additively in their qualitative effect: proximity and size each independently push equilibrium richness upward.

Scope. This four-way ordering (near-large, near-small, far-large, far-small) matches the general empirical pattern observed across real island systems.

Problems

Two reserves of equal total area are proposed, one as a single block and one split into four disconnected fragments. Using the Result, explain which is predicted to retain higher species richness over time.
Solution
The single large block presents a lower effective extinction rate \(E(S)\) at any given richness than the four smaller, disconnected fragments (Step4, Corollaries), so it is predicted to support a higher equilibrium richness for the same total area.
An island's measured species richness roughly doubles after a land bridge forms connecting it to the mainland during a period of low sea level. Explain this using the Result.
Solution
A land bridge sharply reduces effective isolation, raising the immigration curve \(I(S)\) at every richness (Step3) without changing the island's area or extinction curve; the intersection with \(E(S)\) therefore moves to a substantially higher equilibrium richness.
Explain why species turnover on an island at constant total richness is not evidence against the equilibrium model.
Solution
The model defines equilibrium as the point where immigration and extinction rates balance (Step2), not where they are individually zero; species continue arriving and going locally extinct at equal, offsetting rates even while total richness stays constant (Hypotheses, t3).