Summary
At low density, individual fitness declines, so small populations may fail to recover.
In depth
Mates are hard to find, group defence and cooperative hunting fail, and inbreeding rises at low density. Populations below a threshold can spiral to extinction.
Important in conservation and invasive species management.
Component Allee effects reduce individual fitness; demographic Allee effects reduce population growth. A threshold density exists below which decline is certain.
Remember: conservation of rare species must account for it.
Named after Warder Clyde Allee, who found that goldfish survive better in groups. A strong Allee effect has a critical threshold below which the population declines to extinction, a weak one reduces growth rate at low density but does not cause a threshold. Mechanisms include difficulty finding mates, loss of cooperative breeding (meerkats, African wild dogs), reduced predator dilution and failure to saturate predators.
The effect is a major concern in conservation and fishing: the passenger pigeon may have needed huge flocks to breed successfully, and some collapsed fish stocks fail to recover. For invasive species, Allee effects can slow spread, which allows eradication at low densities.
Remember: the typical per capita growth is highest at intermediate density if the effect occurs, unlike the usual logistic pattern.
Frequently asked questions
What is Allee Effect?
At low density, individual fitness declines, so small populations may fail to recover.
Which area of biology covers Allee Effect, and how advanced is it?
Allee Effect is one of the 54 topics in Ecology. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Exponential Growth, Metapopulations, Climate Change Ecology.
Can you give an example of Allee Effect?
African wild dogs need a minimum pack size to hunt and raise pups.
What should I learn before Allee Effect?
Work through the Beginner topics in Ecology first, such as Ecology, Ecosystem, Biotic and Abiotic Factors, then return to Allee Effect. Advanced topics assume the Intermediate vocabulary.
What topics are related to Allee Effect?
The closest topics to Allee Effect are Exponential Growth, Metapopulations, Climate Change Ecology, Trophic Cascade. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Allee Effect?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Allee Effect. To practise the whole of Ecology, use the Ecology quiz.
How is Allee Effect different from Exponential Growth?
Allee Effect: At low density, individual fitness declines, so small populations may fail to recover. Exponential Growth: Population growth at a constant per capita rate, producing a J-shaped curve in unlimited conditions. Read both side by side and note what each one explains.
How should I study Allee Effect step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Exponential Growth and explain the difference aloud. Finally, take the Ecology quiz and come back to this page for any question you miss.