Questions: Island Biogeography and the Species-Area Relationship

Question 1

A conservation manager monitors a forest fragment and finds it has maintained the same number of bird species for 10 years. She concludes the community is stable and no species are at risk. What does island biogeography theory suggest she may be overlooking?

• A. The species richness is artificially elevated by immigration from adjacent habitat, masking low reproductive success
• B. Even with constant species richness, individual species are going extinct and being replaced by new colonists — turnover may be high and populations may be small and fragile
• C. Stable species richness over 10 years proves the reserve has reached its maximum carrying capacity
• D. The species-area relationship predicts richness should increase over time, so flat richness indicates habitat degradation

Answer: (B)

Island biogeography predicts a dynamic equilibrium: species richness stays approximately constant because extinction and immigration rates balance, not because the community is frozen. Individual species turn over — some go locally extinct, others colonize. The manager is observing constant richness but not individual species fates. Small populations in habitat fragments may be extinction-prone even as the total count holds steady, particularly if immigration rates are low and cannot rescue declining populations.

Question 2

An oceanic island and a continental forest fragment of equal area are compared. The species-area z-value for the island is 0.30; for the fragment, it is 0.15. What best explains this difference?

• A. Forest fragments support inherently fewer species per unit area than oceanic islands due to lower habitat quality
• B. The surrounding matrix (farmland, roads) is less hostile than open ocean, allowing some dispersal across it — reducing effective isolation and lowering z
• C. The island's higher z-value indicates greater biodiversity per unit area caused by evolutionary isolation
• D. The species-area relationship has a different mathematical form for continental fragments and cannot be compared to oceanic islands

Answer: (B)

The z-value in S = cA^z captures how steeply species richness falls with decreasing area. Oceanic islands are surrounded by absolute barriers (open ocean), so small islands receive very few colonists and extinction is unrescued — species loss with decreasing area is steep (high z). Continental fragments are embedded in a traversable matrix; organisms can still disperse across farmland or roads, partially compensating for fragmentation. This reduces effective isolation, softens the extinction rate increase with decreasing area, and produces lower z-values (~0.15 vs. ~0.30).

Question 3

According to MacArthur and Wilson's island biogeography theory, a large island close to the mainland should support more species than a small island far from the mainland, because it has both lower extinction rates and higher immigration rates.

• T. True
• F. False

Answer: True

Correct. Island area depresses extinction rates: larger islands support larger populations with lower extinction risk. Proximity to the mainland elevates immigration rates: colonists reach nearby islands more frequently. Both factors push species richness upward. The equilibrium species count is therefore highest for large, close islands and lowest for small, remote islands — a testable prediction confirmed by empirical data from many archipelagos.

Question 4

Island biogeography theory predicts that once a habitat fragment reaches its equilibrium species richness, no individual species will go locally extinct as long as area and isolation remain constant.

• T. True
• F. False

Answer: False

The equilibrium is dynamic, not static. At equilibrium, the immigration rate and extinction rate are equal, meaning species continue to go extinct and arrive at matching rates. The richness count is stable, but the identities of the species present change over time — this is species turnover. MacArthur and Wilson's own field experiments on Florida mangrove islands confirmed rapid turnover: they defaunated islands and watched species counts recover to predicted equilibria, but the specific species differed from the originals.

Question 5

Explain the dynamic equilibrium at the heart of MacArthur and Wilson's island biogeography theory, and describe one implication this has for how we should interpret stable species richness in a nature reserve.

Model answer: MacArthur and Wilson proposed that species richness on islands is not a fixed property but a balance point between two opposing rates: immigration (new species arriving from a source pool) and local extinction (established species dying out). As richness increases, the immigration rate of new species falls (fewer unrepresented species left to arrive) while the extinction rate rises (more species competing for resources). Where these curves cross, richness reaches an equilibrium. Crucially, this equilibrium is dynamic — species continue to go extinct and be replaced by new colonists, so total count stays constant but individual species turn over. For conservation: stable species richness in a reserve does not mean the species are secure. If immigration from surrounding habitat is cut off, extinction events will not be rescued by recolonization, and richness will eventually decline even if no immediate change is visible.

The hotel analogy captures it well: constant occupancy doesn't mean the same guests — people check in and out continuously. For reserves, turnover matters because small populations going locally extinct may represent the loss of irreplaceable biodiversity even when the species count temporarily holds steady.