Island Biogeography

• A. The geological formation of volcanic islands
• B. The distribution of species across islands and the processes that determine their diversity ✓
• C. The economic value of island tourism to local communities
• D. The navigation routes used by early seafaring explorers

Why: Island biogeography studies how species are distributed among islands and what processes, such as immigration, extinction, and speciation, shape the number and identity of species found there.

• A. Island color and average rainfall
• B. Island size and distance from the mainland source ✓
• C. Island population density and soil pH
• D. Wind speed and ocean depth

Why: MacArthur and Wilson proposed that the number of species on an island reflects a balance between immigration and extinction, with larger islands supporting more species and islands closer to a source pool receiving more immigrants.

• A. Extinction rates are higher on larger islands because of greater competition
• B. Extinction rates are lower on larger islands because they support larger populations that are less vulnerable to chance events ✓
• C. Extinction rates are unrelated to island size
• D. Extinction rates are highest on medium-sized islands only

Why: Larger islands generally support bigger populations, which are less likely to die out from random demographic fluctuations, and they often contain more habitat diversity, reducing extinction risk.

• A. The observation that larger islands tend to have more species than smaller islands ✓
• B. The observation that islands with more species have higher land prices
• C. The observation that species on islands are always larger than their mainland relatives
• D. The observation that islands always have fewer species than continents of equal size

Why: The species-area relationship describes the widely documented pattern that species richness increases with island area, often following a power-law curve, and it is a foundational pattern in island biogeography.

• A. Conservation agencies relocating endangered species to islands
• B. Immigration from a source population reducing the chance that a small local population will go extinct ✓
• C. Islands recovering from volcanic eruptions through natural succession
• D. Predators eliminating invasive species from island ecosystems

Why: The rescue effect occurs when immigrants from nearby populations arrive and boost a declining local population, lowering its probability of extinction, which is especially important for populations on isolated islands.

• A. Islands with larger areas tend to support more species than smaller islands ✓
• B. Smaller islands always have higher endemism than larger islands
• C. Species richness is independent of the size of the habitat
• D. Islands closer to the mainland always have fewer species than distant islands

Why: The species-area relationship states that species richness increases with island area, because larger islands offer more habitat diversity, more resources, and larger populations that are less prone to extinction.

• A. S = cA^z, a power function ✓
• B. S = a + bA, a linear function with zero intercept
• C. S = e^(-kA), an exponential decay function
• D. S = 1/A, an inverse function

Why: The power function S = cA^z, where S is species number, A is area, c is a constant, and z is the slope on log-log axes, is the standard model. Typical z values range from about 0.2 to 0.35 for islands.

• A. The slope of the line ✓
• B. The total number of species on the largest island
• C. The distance from the mainland
• D. The rate of immigration

Why: When the power function is log-transformed, log S = log c + z log A, so z is the slope of the line on log-log axes. It indicates how rapidly species richness increases with area.

• A. Larger islands have greater habitat heterogeneity
• B. Larger islands support larger populations that resist extinction
• C. Larger islands have higher immigration rates because of their size alone ✓
• D. Larger islands can support more niches and resource types

Why: Immigration rates are driven mainly by isolation (distance from source populations), not by island area. Larger islands do have lower extinction rates and greater habitat diversity, which explain higher richness.

• A. It shows that small reserves can protect as many species as large reserves
• B. It suggests that reducing habitat area can lead to significant species loss ✓
• C. It proves that species richness is unaffected by habitat fragmentation
• D. It indicates that edge habitats always increase biodiversity

Why: Because species richness scales with area, habitat loss and fragmentation reduce the number of species an area can support. This principle underlies arguments for preserving large contiguous reserves.

• A. The island's total land area
• B. The island's distance from the mainland or source pool ✓
• C. The island's elevation above sea level
• D. The number of predators present on the island

Why: Immigration rate declines with increasing distance from the mainland or source pool because more distant islands are harder for colonizing organisms to reach. Island area mainly affects extinction rates, not immigration.

• A. Larger islands have higher extinction rates because they support more species
• B. Extinction rate is independent of island size
• C. Smaller islands have higher extinction rates because they support smaller populations that are more vulnerable to stochastic events ✓
• D. Extinction rate increases linearly with distance from the mainland

Why: Small islands support smaller populations, which are more susceptible to random demographic fluctuations, environmental disturbances, and chance events that can drive species to extinction.

• A. Immigration rate is zero and extinction rate is maximized
• B. Immigration rate equals extinction rate, so species richness remains roughly constant ✓
• C. Immigration rate exceeds extinction rate, causing continuous species accumulation
• D. Both rates are zero because the community is fully saturated

Why: At equilibrium, the rate at which new species arrive equals the rate at which species go extinct, so the number of species on the island fluctuates around a stable value rather than changing systematically.

• A. The process by which predators prevent prey species from going extinct
• B. Immigration from nearby source populations that reduces the likelihood of local extinction for small populations ✓
• C. The recovery of an island's vegetation after a volcanic eruption
• D. The increase in species richness caused by human introduction of species

Why: The rescue effect occurs when immigrants from nearby populations boost small local populations, lowering their extinction probability and maintaining species persistence on islands or habitat patches.

• A. Closer islands have higher immigration rates because colonists can reach them more easily ✓
• B. Closer islands have lower extinction rates because they are colder
• C. Closer islands have larger areas by definition
• D. Distant islands attract more predators that eliminate prey species

Why: Proximity to a source pool increases the frequency with which dispersing organisms arrive, raising the immigration rate and therefore the equilibrium species number, all else being equal.

• A. Greater island area
• B. Smaller island area ✓
• C. Greater distance from the mainland
• D. Higher habitat heterogeneity

Why: Smaller islands support smaller populations, which are more vulnerable to random demographic fluctuations and stochastic events, so they experience higher extinction rates. Island area is one of the main predictors of extinction in MacArthur and Wilson's equilibrium theory.

• A. The recolonization of an island by immigrants from a source population that prevents local extinction ✓
• B. The rapid evolution of species to tolerate new predators
• C. The removal of invasive species by natural predators
• D. The increase in population size following a catastrophic disturbance

Why: The rescue effect occurs when immigration from nearby source populations reduces the likelihood that a small local population will go extinct. Populations that receive immigrants are less likely to die out than isolated ones.

• A. They have higher birth rates that exhaust resources
• B. They are more susceptible to demographic and genetic stochasticity ✓
• C. They always compete more intensely with other species
• D. They are immune to disease because of isolation

Why: Small populations are vulnerable to demographic stochasticity (chance variation in births and deaths) and genetic stochasticity (loss of genetic diversity through drift and inbreeding), both of which raise the risk of extinction.

• A. Carrying capacity
• B. Minimum viable population ✓
• C. Effective population size
• D. Niche breadth

Why: The minimum viable population (MVP) is the smallest population size that has a specified high probability of persisting for a given time, accounting for demographic, genetic, and environmental risks.

• A. Species richness reaches a stable, dynamic equilibrium with ongoing turnover ✓
• B. Species richness steadily increases without limit
• C. Species richness drops to zero
• D. Species richness becomes fixed with no species changes ever occurring

Why: At equilibrium, the rate at which new species arrive equals the rate at which species go extinct. Species richness stays roughly constant, but the identity of the species continues to change through ongoing turnover.

• A. Island age and soil fertility
• B. Immigration rate and extinction rate ✓
• C. Average rainfall and temperature
• D. Predator density and prey diversity

Why: MacArthur and Wilson proposed that the number of species on an island reaches a dynamic equilibrium where the rate of immigration of new species equals the rate of extinction of existing species.

• A. Islands farther from the mainland have higher species richness because they are more isolated.
• B. Islands farther from the mainland have lower species richness because immigration rates decline with distance. ✓
• C. Distance has no effect on species richness; only island size matters.
• D. Islands farther from the mainland have higher extinction rates but equal immigration rates.

Why: Greater distance from a source pool reduces the likelihood that colonizing organisms will reach the island, so immigration rates fall and the equilibrium number of species is lower.

• A. Larger islands have higher extinction rates because they support more competition.
• B. Smaller islands have lower extinction rates because they are more stable.
• C. Larger islands have lower extinction rates because they can support larger populations that are less vulnerable to chance extinction. ✓
• D. Island size has no effect on extinction rates.

Why: Larger islands hold bigger populations and more habitat diversity, which reduces the chance that random events will wipe out a species, thus lowering extinction rates.

• A. The point where the island first gets colonized by any species.
• B. The point where the number of species becomes zero.
• C. The point where immigration and extinction rates are equal, so species number remains roughly constant on average. ✓
• D. The point where all species on the island reach maximum population size.

Why: The equilibrium point is where the curves for immigration and extinction intersect, meaning gains and losses of species balance and the total species count fluctuates around a stable value.

• A. Species are permanently replaced by new species, so the identity of species changes while the total number stays roughly the same. ✓
• B. All species on the island go extinct and are replaced simultaneously every year.
• C. Species numbers increase steadily without limit over time.
• D. Species turnover occurs only on islands smaller than one square kilometer.

Why: Because immigration and extinction continue at equilibrium, species are constantly being lost and replaced by others, so the list of species changes even though the count remains stable.

• A. The gradual extinction of species on isolated islands
• B. The rapid diversification of a single ancestral species into many species adapted to different ecological niches ✓
• C. The movement of species from one island to another through ocean currents
• D. The uniform spread of a species across all available habitats

Why: Adaptive radiation describes how one ancestral lineage rapidly evolves into multiple species that occupy different ecological niches, often seen when colonizing new environments with few competitors, such as islands.

• A. Penguins
• B. Darwin's finches ✓
• C. Albatrosses
• D. Flamingos

Why: Darwin's finches descended from a common ancestor that colonized the Galápagos and evolved different beak shapes suited to eating seeds, insects, cactus, and other foods.

• A. Islands have more predators than mainland areas
• B. Islands offer abundant unused niches and reduced competition ✓
• C. Islands have constant, unchanging climates
• D. Islands prevent any genetic variation from arising

Why: When colonists arrive on islands, many ecological niches may be unoccupied and competition may be low, creating opportunities for populations to diverge and specialize.

• A. Convergent evolution ✓
• B. Genetic drift
• C. Allopatric isolation
• D. Vicariance

Why: Convergent evolution occurs when unrelated lineages develop similar traits because they face similar environmental pressures, such as similar body forms in island lizards or birds.

• A. Island species always become extinct within 100 years
• B. Small mammals on islands tend to evolve larger body sizes, while large mammals tend to evolve smaller body sizes ✓
• C. All island species lose their ability to fly
• D. Island species always have brighter coloration than mainland relatives

Why: The island rule describes a general pattern of body size evolution on islands: small species often grow larger when freed from predators and competitors, while large species often shrink in response to limited resources.

• A. Excessive rainfall
• B. Invasive alien species ✓
• C. Strong ocean currents
• D. High levels of endemic plant species

Why: Invasive alien species such as rats, cats, goats, and weedy plants have caused a large share of extinctions on islands, because island species often evolved without predators or competitors of these kinds and lack defenses against them.

• A. To increase the island's human population
• B. To allow native species to recover from predation and habitat damage ✓
• C. To change the island's climate
• D. To increase tourism revenue only

Why: Removing invasive mammals such as rats, goats, and cats allows native plants and animals, including ground-nesting birds and reptiles, to recover from predation, grazing, and habitat destruction.

• A. A species introduced by humans to an island
• B. A species found naturally on only one particular island or island group ✓
• C. A species that migrates between islands each year
• D. A species that is found on every continent

Why: An island endemic is a species that occurs naturally only on a specific island or island group and nowhere else, which makes such species especially vulnerable to extinction if their island's habitat is damaged.

• A. Islands have no sunlight
• B. Island populations are often small and isolated, with limited ability to recolonize after disturbances ✓
• C. Islands have too many predators
• D. Island species never reproduce

Why: Island populations tend to be small and geographically isolated, so a single disturbance such as a storm, disease, or invasive predator can wipe out a species, and natural recolonization from other areas is slow or impossible.

• A. Biosecurity measures such as inspecting cargo and visitors for stowaway species ✓
• B. Introducing more non-native species to compete with invaders
• C. Removing all native vegetation
• D. Allowing unrestricted travel between islands

Why: Biosecurity measures, including inspection of cargo, ships, aircraft, and travelers, are a key prevention tool because it is far cheaper and more effective to stop new invasive species from arriving than to eradicate them after they are established.