A population of 300 whooping cranes is reduced to 40 by habitat destruction. Conservation managers debate whether to intervene immediately or wait to see if the population recovers naturally. What does the extinction vortex model most strongly suggest?
AWait and observe — populations have evolved resilience mechanisms and small populations can often recover if the habitat pressure is removed
BIntervene immediately — at 40 individuals, genetic drift and inbreeding are already accelerating, and recovery becomes exponentially harder as size decreases further
CWait until the population drops below 10 before intervening, as that is the threshold where Allee effects become significant
DIntervene only if a genetic survey confirms loss of heterozygosity, since drift alone at 40 individuals is insufficient to trigger the vortex
The extinction vortex's defining feature is positive feedback — each decline makes the next decline more likely and faster. At 40 individuals, drift is already rapidly eliminating alleles, inbreeding is difficult to avoid, and demographic stochasticity can eliminate breeding adults in a single bad year. Waiting for the population to drop further means intervening against a stronger vortex with fewer individuals to work with. Genetic rescue (introducing unrelated individuals), captive breeding, or habitat restoration must precede further decline, not wait for it. Option C inverts the logic — Allee effects begin operating well above 10 individuals.
Question 2 Multiple Choice
A species of cooperative predator normally hunts in packs of 8–12. Its population is reduced to 3 isolated individuals in a nature reserve. Even with abundant prey and no predators, the population fails to recover. What ecological mechanism best explains this?
AGenetic drift has eliminated all fitness-related alleles, making reproduction impossible
BAn Allee effect: cooperative hunting requires a minimum group size, so below this threshold per-capita growth rate becomes negative even in otherwise favorable conditions
CInbreeding depression from three generations of close breeding has reduced reproductive rates below replacement
DDemographic stochasticity has eliminated all females, making breeding impossible regardless of group size
This is a textbook demographic Allee effect. The species requires pack coordination for successful hunting — a biological function that cannot be performed below a minimum group size. With only 3 individuals, the pack is too small to hunt effectively, individuals fail to meet caloric needs, and reproductive success plummets. Per-capita growth becomes negative not because of inbreeding or drift (which are slower-acting) but because the basic cooperative function is impaired. This is distinct from the extinction vortex (which involves genetic feedback); Allee effects can drive a population to extinction in ecologically favorable conditions.
Question 3 True / False
Genetic diversity lost through drift in a small population cannot be recovered quickly through new mutations alone.
TTrue
FFalse
Answer: True
Mutation rates in most organisms are on the order of 10⁻⁸ to 10⁻⁹ per base pair per generation. The number of new mutations per generation in a small population is far too low to compensate for alleles lost through drift, which eliminates variants at a rate proportional to 1/(2Ne). A population that has bottlenecked to 20 individuals may lose a substantial fraction of its genetic variation within 5–10 generations; recovering equivalent diversity through mutation alone would take tens of thousands of generations. This is why genetic rescue — introducing immigrants from other populations — is the only rapid solution, and why the extinction vortex is difficult to escape without external intervention.
Question 4 True / False
Allee effects are driven by increased competition for resources at low population densities, making individuals worse off when there are fewer competitors.
TTrue
FFalse
Answer: False
This reverses the mechanism. Allee effects occur because some aspects of individual fitness depend on having enough conspecifics around — not because competition decreases. Examples include: finding a mate in a sparse population, cooperative defense against predators, group hunting efficiency, pollination success when flowers are rare, and shoaling behavior that reduces predation risk. The standard density-dependent logic (fewer individuals = less competition = higher per-capita growth) is inverted by Allee effects, which say: fewer individuals = impaired cooperation = lower per-capita fitness. The effect is about interdependence, not competition.
Question 5 Short Answer
Why does the extinction vortex accelerate as population size decreases, rather than stabilizing or slowing down?
Think about your answer, then reveal below.
Model answer: Because the feedback is positive: each of the forces that shrink the population becomes stronger as the population gets smaller. Genetic drift eliminates alleles faster in smaller populations (rate ∝ 1/2Ne). Inbreeding increases as the pool of unrelated mates shrinks, compounding inbreeding depression. Demographic stochasticity — random variation in births and deaths — has proportionally larger effects in smaller groups. Environmental perturbations that a large population absorbs can eliminate a large fraction of a small one. Each round of decline intensifies all these pressures simultaneously, making the next round of decline faster and larger. This is positive feedback: the system is self-amplifying, not self-correcting.
The contrast with negative feedback helps clarify the concept. In standard population regulation (logistic growth), declining population size reduces competition and increases per-capita growth, pulling the population back toward carrying capacity — negative feedback. The extinction vortex replaces this correcting force with an amplifying one: declining size reduces fitness, which reduces size further. Once in the vortex, recovery requires breaking the feedback loop from outside the system.