Island Biogeography - AP Environmental Science

An island 15 km from the mainland has area 10 km². A second island 15 km from the mainland has area 40 km². If $z=0.25$ , the ratio of predicted richness is $\frac{S_{40}}{S_{10}} = \left(\frac{40}{10}\right)^{0.25}$ . Which statement is correct?
Answer: The ratio is greater than 1, so the larger island has more species. Using the species-area relationship S= $cA^z$ with z=0.25, the ratio calculation gives $(40/10)^{0.25}$ = $4^{0.25}$ = √2 ≈ 1.41. Since this ratio is greater than 1, the larger island (40 km²) is predicted to have more species than the smaller island (10 km²). The positive exponent z indicates that species richness increases with area, which is consistent with island biogeography theory. Larger islands support more species due to lower extinction rates from larger population sizes and greater habitat diversity, following the well-established species-area relationship pattern observed across many island systems.
An island's equilibrium species richness is where immigration rate equals extinction rate. A storm destroys habitat on the island, reducing area by 50% while distance to the mainland is unchanged. Which outcome is most consistent with island biogeography theory?
Answer: Extinction rates increase, shifting equilibrium to lower species richness. Island biogeography theory establishes that smaller islands have higher extinction rates because they support smaller population sizes, have fewer resources, and less habitat diversity. When the storm reduces the island's area by 50%, extinction rates increase significantly while immigration rates remain unchanged (distance to mainland is constant). The equilibrium point shifts where the unchanged immigration curve intersects the now-higher extinction curve. This intersection occurs at a lower species richness than before the storm. The reduced area cannot support as many species at equilibrium, leading to local extinctions until a new, lower equilibrium is reached. This demonstrates how habitat loss directly reduces biodiversity capacity.
Two islands have identical area and distance, but Island P has more types of habitat (wetlands, forest, grassland) than Island Q (mostly forest). Under island biogeography and related species–area ideas, Island P is most likely to have:
Answer: higher species richness because more niches can support more species. Island biogeography theory, combined with niche theory, predicts that islands with greater habitat diversity can support more species at equilibrium. Island P's variety of habitats (wetlands, forest, grassland) provides more ecological niches and microenvironments than Island Q's predominantly forest habitat. This habitat heterogeneity allows more species to coexist by reducing direct competition and providing specialized niches for different ecological requirements. Even though both islands have identical area and distance (meaning similar basic immigration and extinction dynamics), the greater number of available niches on Island P enables it to support higher species richness at equilibrium.
An island's equilibrium richness is 60 species. A new predator is introduced, increasing extinction rates. If immigration remains unchanged, the new equilibrium richness will most likely be:
Answer: less than 60 species. Island biogeography theory predicts that introducing a predator will increase extinction rates across the island community, leading to lower equilibrium species richness. The predator adds a new source of mortality that wasn't present when the equilibrium was at 60 species. This shifts the extinction curve upward, meaning higher extinction rates at all levels of species richness. Since immigration rates remain unchanged, the new intersection of immigration and extinction curves occurs at a lower species number. The magnitude of the decrease depends on how severely the predator affects the various species, but the general prediction is a reduction below the original 60-species equilibrium.
A far island is large. Compared with a far island that is small, the large far island is expected to have:
Answer: lower extinction because larger islands support larger populations and more habitats. Island biogeography theory predicts that larger islands have lower extinction rates than smaller islands at equal distances from the mainland. The large far island can support larger, more stable populations than the small far island, reducing the probability of local extinctions. Larger islands also typically contain more habitat types and microenvironments, providing more ecological niches and refugia during environmental stress. Since both islands are equally far from the mainland, they receive similar immigration rates, but the large island's lower extinction rate allows it to maintain higher equilibrium species richness despite the challenging dispersal distance.
A researcher compares two islands with similar habitats: - Island A: 10 km², 8 km from mainland - Island B: 80 km², 8 km from mainland Using the species–area relationship $S=cA^z$ (with the same $c$ and $z$ for both islands), which statement best predicts the difference in equilibrium species richness?
Answer: Island B should have more species because larger islands support larger populations and lower extinction rates. The species-area relationship ( $S=cA^z$ ) predicts that larger islands support more species. Island B (80 km²) is 8 times larger than Island A (10 km²), and since the exponent z is positive, Island B will have more species than Island A. Larger islands have lower extinction rates because they can support larger, more stable populations and typically contain more diverse habitats and microenvironments. The greater habitat diversity provides more ecological niches, allowing more species to coexist. Additionally, larger populations are less vulnerable to random demographic fluctuations and local extinctions, contributing to higher species richness at equilibrium.
Four oceanic islands (all originally sterile after a volcanic eruption) differ in size and distance from the mainland species pool: - Island W: 2 km², 5 km from mainland - Island X: 2 km², 50 km from mainland - Island Y: 200 km², 5 km from mainland - Island Z: 200 km², 50 km from mainland Assuming island biogeography theory applies (species–area relationship; immigration decreases with distance; extinction decreases with area), which island is predicted to have the highest equilibrium species richness after many years?
Answer: Island Y Island biogeography theory predicts that species richness increases with island area (lower extinction rates due to larger populations and more habitats) and decreases with distance from the mainland (lower immigration rates). Island Y has the largest area (200 km²) and is closest to the mainland (5 km), giving it both the highest immigration rate and lowest extinction rate. This combination of high immigration and low extinction results in the highest equilibrium species richness. Islands with smaller areas or greater distances will have lower equilibrium richness due to higher extinction rates or lower immigration rates respectively.
Two islands are the same size (30 km $^2$ ). A storm destroys 70% of the vegetation on Island 1, reducing habitat diversity, while Island 2 is unaffected. Both islands are the same distance from the mainland. According to island biogeography principles, which change is most likely on Island 1 compared with Island 2 in the years following the storm?
Answer: Higher extinction rates leading to lower species richness due to reduced habitat and smaller effective area. Habitat destruction effectively reduces the functional area of an island by eliminating suitable living space and resources for many species. When 70% of vegetation is destroyed on Island 1, it experiences higher extinction rates because remaining populations are compressed into smaller areas with fewer resources, leading to stronger competition, reduced population sizes, and loss of specialist species requiring specific destroyed habitats. The effective area reduction means Island 1 now functions like a much smaller island in the biogeography model, shifting its extinction curve upward. Immigration rates may remain similar or even decrease slightly if arriving species cannot find suitable habitat. The new equilibrium will stabilize at lower species richness compared to undisturbed Island 2. This demonstrates how habitat quality and availability, not just total area, determine an island's capacity to support species.
Two islands are identical in size, but Island J is near the mainland and Island K is far from the mainland. In a standard island biogeography graph of immigration and extinction rates vs. number of species, which change best represents moving from Island J (near) to Island K (far), assuming extinction curves are otherwise similar?
Answer: The immigration curve shifts downward (lower immigration at all species numbers), lowering equilibrium richness. In island biogeography graphs, immigration and extinction rates are plotted against the number of species present. Distance from the mainland primarily affects immigration rates - far islands receive fewer colonists at any given species richness level. Moving from Island J (near) to Island K (far) causes the entire immigration curve to shift downward, meaning lower immigration rates at all species numbers. The extinction curve remains largely unchanged because it's primarily determined by island size, not distance. This downward shift in immigration results in a new equilibrium point at lower species richness, where the lowered immigration curve intersects with the unchanged extinction curve. Thus, distant islands support fewer species at equilibrium than near islands of the same size.
A near island and a far island have equal area. After a disease outbreak, extinction rates increase on both islands equally (at all richness values). What is the most likely effect on equilibrium richness for each island?
Answer: Both islands' equilibrium richness decreases. Island biogeography theory predicts that when extinction rates increase equally on both islands while immigration rates remain unchanged, both will experience reduced equilibrium species richness. The disease outbreak shifts both extinction curves upward, meaning higher extinction rates at all levels of species richness. Since immigration curves remain the same, the new intersection points of immigration and extinction occur at lower species richness values for both islands. This demonstrates how environmental disturbances that increase mortality or reduce population stability can lower the carrying capacity for species richness across an entire island system.
A large, near island and a small, far island are compared. Which combination of immigration and extinction rates is most likely for the large, near island?
Answer: High immigration and low extinction Island biogeography theory predicts that a large, near island will experience both high immigration (due to proximity to the mainland) and low extinction (due to large area supporting stable populations and diverse habitats). The combination of being close to the species source and having ample area creates optimal conditions for maintaining high species richness. High immigration ensures regular arrival of new colonists, while low extinction means established species persist longer. This combination results in the highest possible equilibrium species richness according to the theory, making large, near islands biodiversity hotspots.
A conservation planner wants to compare predicted equilibrium richness for four islands. Which ranking from highest to lowest richness best matches island biogeography theory? - Island A: large & near - Island B: large & far - Island C: small & near - Island D: small & far
Answer: A > C > B > D Island biogeography theory ranks islands by combining area and distance effects: larger islands and closer islands have higher equilibrium richness. Island A (large & near) has both advantages - high immigration due to proximity and low extinction due to large area. Island C (small & near) has high immigration but high extinction. Island B (large & far) has low extinction but low immigration. Island D (small & far) has both disadvantages - low immigration due to distance and high extinction due to small area. This creates the ranking A > C > B > D, with the large near island having highest richness and the small far island having lowest richness.
A small island (6 km $^2$ ) and a large island (60 km $^2$ ) are the same distance from the mainland. Both currently have 25 species. According to island biogeography theory, which statement is most likely true at this moment?
Answer: The small island likely has a higher extinction rate than the large island at 25 species. Island biogeography theory predicts that extinction rates are primarily determined by island area - smaller islands have higher extinction rates at any given species richness. Since both islands currently have 25 species but differ dramatically in size (6 km² vs 60 km²), the small island is operating above its typical equilibrium while the large island may be below its equilibrium. At this moment, the 25 species on the small island face more intense competition for limited resources, smaller population sizes, and less habitat diversity. These factors combine to create a higher extinction rate on the small island compared to the large island. The small island is likely losing species faster than the large island, even though they momentarily have equal richness.
A mainland species pool contains 200 potential colonist species. A near island and a far island are the same size. Which is the best prediction about their immigration curves (immigration rate vs. number of species already present)?
Answer: The near island's immigration curve lies above the far island's at all richness values. Island biogeography theory predicts that immigration rate decreases with distance from the mainland because dispersing organisms have lower success rates reaching more distant islands. Since both islands have access to the same mainland species pool of 200 potential colonists, the maximum possible immigration rate occurs when no species are yet present on the island. The near island will have consistently higher immigration rates at all levels of species richness because its proximity makes successful colonization more likely. The immigration curve for the near island lies above that of the far island, leading to higher equilibrium richness for the near island.
Four newly formed volcanic islands (A–D) are colonized from the same mainland species pool. Two islands are near the mainland (10 km) and two are far (100 km). Two islands are small (5 km $^2$ ) and two are large (50 km $^2$ ). Assume the species–area relationship holds (larger area supports more species) and that distance mainly affects immigration (closer = higher immigration). Which island is predicted to have the highest equilibrium species richness? Island A: near + small Island B: near + large Island C: far + small Island D: far + large
Answer: Island B (near + large) Island biogeography theory predicts that equilibrium species richness is determined by the balance between immigration and extinction rates. Immigration rates are higher for islands closer to the mainland (more colonizers can reach them), while extinction rates are lower on larger islands (more resources and space support larger populations). Island B combines both favorable conditions: it is near the mainland (10 km, high immigration) and large (50 km², low extinction). This combination results in the highest equilibrium species richness. Island A has high immigration but high extinction due to small size, Island D has low immigration despite low extinction, and Island C has both high extinction and low immigration, making it the least diverse.
A graph of immigration and extinction rates vs. number of species present shows the immigration curve shifted upward (higher immigration at all richness values) while the extinction curve remains the same. What happens to the equilibrium number of species?
Answer: It increases because the intersection occurs at a higher species richness. In island biogeography graphs, equilibrium occurs where immigration and extinction curves intersect. When the immigration curve shifts upward (higher immigration rates at all species richness levels) while extinction remains constant, the intersection point moves to a higher species richness value. This represents an increase in equilibrium species number because the island can now support more species before immigration and extinction rates balance. Higher immigration means more colonization events, allowing the island to maintain more species before extinction pressures become strong enough to balance the enhanced immigration rate.
A researcher uses $S=cA^z$ to compare two islands that differ only in area. Island M has area 10 km $^2$ and Island N has area 40 km $^2$ . If $z=0.30$ , approximately how many times greater is the predicted species richness on Island N than on Island M (assume same $c$ )?
Answer: $1.52$ times Using the species-area relationship S = $cA^z$ , we can find the ratio of species richness between two islands by dividing their equations: $S_N$ / $S_M$ = ( $cA_N^z$ )/( $cA_M^z$ ) = $(A_N/A_M)^z$ . With Island N having 40 km² and Island M having 10 km², the area ratio is 40/10 = 4. With z = 0.30, we calculate $4^{0.30}$ = 1.516, which rounds to approximately 1.52. This means Island N, with four times the area of Island M, supports about 1.52 times as many species. The z-value of 0.30 is typical for island systems and reflects the diminishing returns of increasing area - each doubling of area yields progressively smaller increases in species richness.
A near island and a far island have the same area. The mainland species pool shrinks due to habitat loss (fewer potential colonist species). What is the most likely effect on equilibrium richness on both islands?
Answer: Equilibrium richness decreases on both islands because the immigration curve is reduced by a smaller source pool. Island biogeography theory predicts that reducing the mainland species pool affects equilibrium richness on both islands by lowering their immigration curves. When the mainland loses species due to habitat destruction, fewer potential colonist species remain available to immigrate to either island. This reduces the maximum immigration rate (when islands are empty) and shifts the entire immigration curve downward. Since extinction curves remain unchanged on both islands, the new intersection points occur at lower species richness values. Both the near and far islands will experience reduced equilibrium richness, though the effect may be more pronounced on the far island if it was already receiving fewer immigrants.
A new volcanic island forms 5 km from the mainland. Over time, immigration and extinction rates change as the number of species on the island increases. In the equilibrium model of island biogeography, the equilibrium species richness occurs when:
Answer: immigration rate equals extinction rate Island biogeography theory describes species richness as a dynamic equilibrium between opposing processes of immigration and extinction. When a new volcanic island forms, it initially has no species, so immigration rate is at its maximum (many mainland species can colonize) while extinction rate is zero (no species present to go extinct). As species accumulate, immigration rate decreases (fewer new species available to colonize) while extinction rate increases (more species present that can go extinct due to competition, stochastic events, or small population sizes). The equilibrium species richness is reached when these two rates become equal - the number of new species arriving per unit time exactly balances the number going extinct. This creates a stable species number even though the actual species composition may continue to change through turnover.
An island's distance to the mainland increases after sea level rises, but its area stays the same. Which outcome is most consistent with island biogeography theory?
Answer: Immigration decreases and equilibrium richness decreases. Island biogeography theory predicts that increasing distance to the mainland reduces immigration rates while leaving extinction rates largely unchanged (since area remains constant). When sea level rises and increases the island's isolation, fewer organisms can successfully disperse across the greater water distance. This shifts the immigration curve downward while the extinction curve stays the same, causing the intersection (equilibrium) to occur at a lower species richness. The reduced colonization rate means the island can support fewer species at equilibrium, demonstrating how changing isolation affects biodiversity even without changes in habitat area.

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