What is the Allee effect and how does it influence small populations?

• A. A positive correlation between population size and per-capita growth rate at low densities, increasing extinction risk for small populations.
• B. A negative correlation between population size and growth rate; helps small populations grow.
• C. An effect where population size does not affect growth rate.
• D. An environmental factor that only increases growth at high densities.

Answer: (B)

The Allee effect is when populations grow faster per individual as the number of individuals in the population increases, at low densities. In other words, small populations have a harder time growing because individuals benefit from the presence of others—finding mates, cooperative defense, group foraging, or other social interactions become less effective when there aren’t enough neighbors. As density increases, these benefits kick in and the per-capita growth rate rises.

This matters for small populations because if the per-capita growth rate is low at small sizes, the population can stall and even decline toward extinction, especially if there’s a threshold: below a certain size, growth cannot compensate for losses, while above it, growth can proceed toward the carrying capacity. If the effect is weak, growth is reduced but still positive at low densities; if it’s strong, there’s a clear critical minimum size below which declines are likely.

So the essence is positive density dependence at low densities: small populations struggle to grow, while larger populations reap growth advantages from having more conspecifics.