'ALLEE' EFFECT

The ecologist W. C. Allee (1931) wrote on aggregations of animals and its ecological significance. He proposed a model where there is a positive relationship between population density and the reproduction and survival of individuals. An Allee effect occurs when the per-capita birth rate increases with population density to some maximum value (the reproductive potential of the species) with the per capita death rate remaining constant. Where the two rates intersect there is an unstable equilibrium where little change in the ratio of births equalling deaths will result in either population growth or an inevitable decline to extinction. The intersection point (threshold) chaotically generates these two different dynamics.

Smaller populations are more subject to stochastic events. Therefore both drops in the overall population and increased fragmentation of the population may result in extinctions.

Allee, W. C. 1931. Animal Aggregations. A study in General Sociology. University of Chicago Press, Chicago.

An Allee effect can also be provided by positive relationship between population growth and resilience and the local density of conspecifics. With the aggregation of individuals examples of increased reproduction efficiency are shown in all vertebrate classes. Common examples are the calling aggregations of male frogs, where an individual males chances of breeding increase with the size of the calling group.

1) The Allee effect and conspecific attraction.

Conspecific attraction occurs where animal density is higher than expected by the quality of habitat. The degree of conspecific attraction is the degree of the Allee effect. Conspecific attraction may increase the search efficiency of both males and females for suitable breeding areas. The calling by the first pioneer at a new site would provide a target for other frogs. This could reduce mortality during migration to targeted sites. When animals are at low densities it would also increase the chance of breeding success.

2) Conservation. At low population densities populations relying on conspecific attraction to build certain densities may be particularly prone to extinction. This trend becomes stronger with increases in the Allee effect. Within metapopulation dynamics models conspecific attraction gives a lower threshold of patch occupancy at which all subpopulations will reach extinction. The chance of extinction is greater with increases in the Allee effect or decreases in the population.