Fragmentation & Island Biogeography Lecture

Forest fragmentation (breaking continuous forests into smaller patches)
- Previously linked to rise in Lyme disease in eastern deciduous forests.
- Housing developments act as fragmentation drivers.

Conventional image: land surrounded by water.
Ecological definition
- Island = Any patch of habitat isolated from similar habitat by a barrier that significantly restricts dispersal.
- Barriers can be:
- $\text{Water}$ (for terrestrial organisms)
- $\text{Land}$ (for aquatic organisms)
- Mountains, inhospitable habitat, elevation gradients, etc.
- Species–specific: what is an island for an earthworm may not be for a bird.
Examples
- Forest patch inside a clear-cut = island for shade-obligate plants & soil invertebrates.
- Pond in a meadow = island for aquatic taxa.
- Gutter full of soil on a roof: vertical distance creates an island for microbes & seedlings.

Australia
- Terrestrial mammals: true island (water is barrier).
- Marine fish: not an island (water continuous habitat).
Same habitat patch can be island or continuous matrix depending on organism.

Government relocated Andean farmers to lowlands.
Created radial “spoke” communities with soccer field, church, café, etc.
Soybean export fields: large diagonal strips.
Results
- Vast tropical dry forest cleared → many tiny forest islands.
- Presence of corridors (narrow strips of contiguous forest) between some fragments allows limited movement & maintains diversity.
- Lesson: leaving connected habitat strips mitigates some fragmentation impacts.

General empirical rule: $S = cA^{z}$
- $S$ = species richness, $A$ = area, $c$ & $z$ = constants (z ≈ $0.15\text{–}0.35$ for most taxa).
Data sets presented
- Galápagos plants: larger islands → more species.
- English flowering plants, North-American birds show identical positive trends.
Applies to true islands and terrestrial plots.

Volcanic islands
- Begin sterile; rely on primary succession → lower richness.
Land-bridge / sea-level islands (e.g., Aleutians; former peninsulas high & dry during glacial maxima)
- Start with pre-existing communities → higher richness.

True oceanic islands (isolated by hostile matrix) accumulate species faster with added area than patches embedded in similar mainland habitat.
Because immigration outweighs emigration on isolated islands; on continental patches, immigration ≈ emigration.

Species richness (not “diversity”) determined by balance of:
- Immigration rate (new species $\text{time}^{-1}$ ).
- Extinction rate (species lost $\text{time}^{-1}$ ).
Immigration curve
- Highest when $S \approx 0$ .
- Declines to $0$ when island contains complete species pool.
Extinction curve
- $0$ when $S = 0$ .
- Increases with $S$ due to competition, resource limits.
Equilibrium point (*E*)
- Intersection of curves → expected long-term $S_{eq}$ .
- Disturbances that push $S$ left (losses) trigger high immigration → recovery toward $S_{eq}$ .
- Push $S$ right (additions) trigger extinctions → decline toward $S_{eq}$ .

Near vs. far islands (same size)
- Near: higher immigration → higher $S_{eq}$ .
- Far: lower immigration → lower $S_{eq}$ .

Large vs. small islands (same distance)
- Small: higher extinction, lower $S_{eq}$ .
- Large: lower extinction, higher $S_{eq}$ .

Mangrove islets (diameter $\approx 5\text{–}50\,\text{ft}$ ).
Steps
1. Survey arthropods (species pool $\approx$ $500\text{–}1000$ spp.).
2. Cover islands with tents; fumigate (reset $S=0$ ).
3. Monitor recolonisation.
Findings
- Islands held $\approx 20\text{–}40$ species at equilibrium.
- Near islands re-established pre-defaunation richness (~ $250\text{–}270$ days) then oscillated.
- Far islands lagged, never fully reached prior richness within study window.
- Species composition changed (turnover) even when richness stabilised.

Non-interactive phase
- Early colonisers; minimal biotic interactions; richness driven solely by immigration.
Interactive phase
- Competition, predation, mutualism develop; richness/diversity reshaped by interactions.
Assortative (successional) phase
- Habitat modification & succession mechanisms (facilitation, tolerance, inhibition) dominate.
Evolutionary phase
- In situ speciation/adaptation alter richness; important on long-isolated, large islands.

Smallest U.S. national park at designation (~early $1910$ s).
Expected to hold $\approx 68$ of state’s $86$ mammal spp.
Actual counts
- $1920$ s: $\approx 50$ spp.
- $1970$ s: $\approx 37$ spp.
- $1980$ s: $\approx 25$ spp.
Cause: state land sales converted surrounding similar habitat into residential/industrial matrix → park became true island.
Restoration: 1980s–90s corridor & re-forestation purchases raised current richness to $\approx 65$ spp. (near theoretical maximum).
Demonstrates reversible effects when barriers removed.

Island species richness = dynamic equilibrium of immigration vs. extinction.
Immigration influenced by
- Size of species pool
- Dispersal ability
- Distance (near vs. far)
Extinction influenced by
- Island area (small vs. large)
- Resource availability & competition
- Presence/absence of predators, mutualists, etc.
Four colonisation phases (non-interactive, interactive, assortative/successional, evolutionary) govern temporal patterns.
Reserve design should heed island principles: large, clustered, connected preserves sustain higher richness.