Unit 3: Island Biogeography & Habitat Fragmentation - Video 2

Lecture goal: Explore how habitat fragmentation alters movement of plants/animals and apply the Theory of Island Biogeography (TIB).
Fragmentation example already discussed: Eastern Deciduous Forest → higher Lyme disease due to housing projects breaking forest continuity.

Classic image: land mass surrounded by water.
Ecological definition: “Any area of habitat isolated from other similar habitat by a barrier to species dispersal.”
- Barrier can be water, mountains, different vegetation, urban matrix, elevation, etc.
- Barrier strength is probabilistic, not absolute: prevents most, not all, crossings.
Species‐specific perspective:
- Island status depends on the taxon considered (earthworm vs. bird vs. deer vs. fish).
- Fish may see land as a barrier; terrestrial mammals often see water or dense forest as a barrier.

Classic: Australia relative to North America (ocean barrier thousands of km).
Forest clear‐cut “gap” inside continuous forest acts as an island to open‐habitat species.
Pond inside clear‐cut: island for aquatic organisms relative to surrounding land.
Plants growing in a clogged roof gutter (~few feet horizontal but large vertical gap) → island for soil microbes & seedlings.

Government relocated Andean farmers to lowland tropical dry forest.
Pattern: central village square + radial spokes of individual plots.
Massive clear‐cutting created many small forest patches (islands) & large soybean fields.
Presence of corridors (narrow forest strips) connecting bigger patches allows limited movement → maintains species richness.
Lesson: human land subdivision can generate hundreds of forest “islands” within former continuous habitat.

General rule: Larger area ⇒ more species.
Empirical graphs:
- Galápagos plants: positive linear or power relationship.
- English flowering plants; North-American birds → same trend.
Mathematical shorthand: $S = cA^z$ where
- $S$ = species richness,
- $A$ = area,
- $c, z$ = fitted constants (typically $0.15 \le z \le 0.35$ for islands).

Oceanic volcanic islands (new lava → primary succession) start with low richness; must wait for soil formation & colonists.
Sea-level rise islands (e.g., Aleutian chain) inherit pre-existing biota on ridge tops → higher initial richness.

True oceanic islands are surrounded by inhospitable matrix → steeper SAR slope.
Terrestrial plot inside continuous habitat: easier immigration & emigration → flatter slope.
Cause: on true islands immigration > emigration (arrivals accidental; departures unlikely). In continuous habitat flows are symmetrical.

Immigration rate curve
- $y$ -axis: new species per unit time.
- Highest when island empty; declines to $0$ as island list matches source pool.
Extinction rate curve
- $y$ -axis: species lost per unit time (by die-off or emigration).
- Starts near $0$ when few species (little competition); rises with richness.
Equilibrium point (*S*_{eq})
- Intersection where immigration = extinction.
- Predicts long-term steady‐state number of species.

If external event raises richness right of *S*_{eq} → extinction > immigration → richness pushed back down.
If event lowers richness left of *S*_{eq} → immigration > extinction → richness increases.

“Large & Near” → greatest richness.
“Small & Far” → lowest richness.
Students asked to sketch remaining curve combinations:
1. Extinction curves for Near vs. Far.
2. Immigration curves for Large vs. Small.

Study system: tiny mangrove islets (≈ $5$ – $50$ ft across).
Pre-survey: regional arthropod species pool $\approx 500$ – $1000$ .
Each island hosted only $20$ – $40$ spp.
Procedure:
1. Count arthropods.
2. Enclose island in plastic “circus tent.”
3. Fumigate → reset richness to $0$ .
4. Track recolonization days 0–300.
Results:
- Near islands recovered richness faster and overshot pre-value before stabilizing.
- Far islands approached but did not reach original richness within study window (≈ $270$ days), then declined.
Supports immigration-extinction framework & distance effect.

Non-interactive phase
- Early arrival stage; interactions minimal.
- Richness changes solely via immigration.
Interactive phase
- Competition, predation, mutualism intensify.
- Richness/diversity shift via biotic interactions.
Assortative (Successional) phase
- Community succession filters species (facilitation, tolerance, inhibition).
Evolutionary phase
- In-situ speciation, adaptation, character displacement alter richness/diversity.

Context: Smallest U.S. National Park at designation (~early 1900s).
Western Washington mammal pool: $86$ spp ( $100\%$ baseline).
Habitat modeling predicted park capacity ≈ $68$ spp ( $79\%$ of pool).
Observed trends:
- 1920s survey: $50$ spp (~ $58\%$ ).
- 1930s–50s: stable.
- Late 1970s: $37$ spp.
- Early 1980s: $25$ spp (low point).
Cause: State sold surrounding land → agriculture/towns → park became true island ringed by inhospitable matrix.
Restoration (late ’80s–’90s): repurchase & reforest buffer land; current mammals ≈ $65$ spp (≈ model expectation).

Species richness on islands/habitat fragments = balance of immigration vs. extinction.
Immigration influenced by: distance to source, dispersal ability, size of species pool, corridor presence.
Extinction influenced by: island size, resource availability, competition/predation, demographic stochasticity.
Equilibrium Theory predicts a stable richness (*S*_{eq}); disturbances shift richness but system tends to return.
Colonization proceeds through non-interactive → interactive → successional/assortative → evolutionary phases.
Conservation design: prioritize large, contiguous, near, connected reserves; incorporate corridors; control surrounding matrix.