Allee effects in plant and animal populations
Ecologists commonly work with the concepts of competition and negative density dependence. Negative density dependence means that with increasing population size Darwinian fitness of each its member declines. Competition for common resources is one of the mechanisms causing such dependence. It appears, however, that especially at low population sizes where competition is weak this does not need to hold.
Ecologists commonly work with the concepts of competition and negative density dependence. Negative density dependence means that with increasing population size Darwinian fitness of each its member declines. Competition for common resources is one of the mechanisms causing such dependence. It appears, however, that especially at low population sizes where competition is weak this does not need to hold. In such a situation one can observe the opposite phenomenon, namely that with increasing population size Darwinian fitness of each its member increases. This phenomenon has been termed the Allee effect (after the U.S. ecologist W. C. Allee) and can even lead to the appearance of a critical population size necessary for the entire population to survive: if population size falls below that value, the population will go extinct with high probability. There are a variety of mechanisms that can create Allee effects, including the need to find mates, avoid predators or modify unfavorable (e.g. toxic) environment. Although the first studies on Allee effects go back at least to 1920’s, until recently, this phenomenon eked out on the margin of ecological theory, in the shade of negative density dependence. The situation has changed dramatically in the last ten years, mainly due to human-caused habitat destruction and subsequent emphasis on species conservation. We can thus observe an ever increasing number of studies considering Allee effects from an ever increasing range of disciplines, including population and community ecology, population genetics, evolutionary biology, conservation biology and population management, parasitology, or even social sciences. The book entitled Allee effects in ecology and conservation (Courchamp et al. 2008) presents a detailed view on this currently very important phenomenon, including its history, variety of underlying mechanisms, and implications for population and community ecology, evolution and population management (including pest control, conservation of endangered species, and species exploitation). Actually, much of what we know about these implications comes from mathematical models. This is also the case of Berec et al. (2007) which not only reviews populations for which we have evidence of a simultaneous operation of two or more mechanisms underlying Allee effects, but also shows, by means of a mathematical model, that their interaction can take various forms. For example, even if none of two present mechanisms leads to the appearance of a critical population density when alone, their interaction can lead to a critical density. If, on the contrary, both mechanisms lead to the appearance of a critical population density when alone, their interaction does so, too, yet the resulting critical density can be disproportionately larger than any of the single ones. It becomes obvious that due to such interactions, failing to take all present Allee effect mechanisms into account could have adverse consequences for the management of threatened or exploited populations. Allee effect also strongly affects dynamics of predator-prey interactions; it generally destabilizes the coexistence equilibrium of the two species (Boukal et al. 2007).
Berec, L., Angulo, E., Courchamp, F. 2007. Multiple Allee effects and population management. Trends in Ecology & Evolution 22:185-191
Boukal, D. S., Sabelis, M. W., Berec, L. 2007. How predator functional responses and Allee effects in prey affect the paradox of enrichment and population collapses. Theoretical Population Biology 72: 136-147
Courchamp, F., Berec, L., Gascoigne, J. 2008. Allee effects in ecology and conservation. Oxford University Press.
