Trends in Ecology & Evolution
PerspectivesInverse density dependence and the Allee effect
Section snippets
Causes of inverse density dependence
The Allee effect strictly refers to inverse density dependence at low density. Factors involved in generating inverse density dependence are numerous and have been described for most major animal taxa3. These factors can be classified into three main categories4. The first is genetic inbreeding and loss of heterozygosity, leading to decreased fitness (such as demonstrated for plants5, 6). The second is demographic stochasticity (including sex-ratio fluctuations)4, as illustrated by the failure
Implications for population dynamics
For all these phenomena, and others, the major consequence of the Allee effect is the existence of a critical density below which the aggregation unit considered (e.g. population, colony or social group) is likely to go extinct. This has been shown theoretically both with deterministic and stochastic mathematical models4, 13, 14, and although still scarce, empirical evidence exists from very diverse ecological systems (Box 2). Because the mechanism is quite straightforward, very basic
Repercussions across trophic levels
Although it is an intraspecific phenomenon, some interspecific relationships are strongly influenced by the Allee effect. It is well known that smaller groups of prey may be more exposed to predation than larger groups11. It has also been shown, both theoretically and empirically, that if a species is a secondary prey item of a predator with a type II functional response, its death rate from predation will be inverse density dependent19. Similarly, the rate of infection for many parasitoid
Implications for applied ecology
Species subjected to a strong Allee effect might be more susceptible to catastrophic population collapses with only a slight increase in mortality, resulting either from harvesting or ‘natural’ causes. Two generally conflicting interests in ecosystem management, long-term optimal harvesting and biodiversity preservation, which both aim to prevent these collapses, have much to gain from acknowledging potential Allee effects. In fisheries, for example, the existence of multiple equilibria has
What prospects for the Allee effect?
The early studies of population biology have been largely dominated by the importance of (negative) density dependence at high densities. In a time when anthropogenic disturbance has driven many populations to small sizes and/or low densities, we must now focus on the other extreme of population sizes. Indeed, studies demonstrating Allee effects and determining their causal mechanisms, either theoretically or empirically, ought to be more numerous in the future. There is a range of logistical
Unlinked references
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Acknowledgements
The authors would like to thank Jan Lindström, Mads Forchammer, Rosie Cooney, Phil Stephen, Ilkka Hanski, Hamish McCallum and an anonymous referee for critical comments on the manuscript. Financial support was provided by a TMR 30 Marie Curie Fellowship from the European Community.
References (51)
Mate location, population growth and species extinction
Biol. Conserv.
(1998)- et al.
Consequences of the Allee effect for behaviour, ecology and conservation
Trends Ecol. Evol.
(1999) Cooperative hunting and group size: assumptions and currencies
Anim. Behav.
(1997)- et al.
(Meta)population dynamics of infectious diseases
Trends Ecol. Evol.
(1997) - et al.
Competition mediated by parasites: biological and theoretical progress
Trends Ecol. Evol.
(1998) - et al.
Allee dynamics and the spread of invading organisms
Theor. Popul. Biol.
(1993) - et al.
Choice of colony size in birds
Trends Ecol. Evol.
(1990) - et al.
Factors influencing the hunting success of an african wild dog pack
Anim. Behav.
(1993) Immunocontraception for wildlife population-control
Trends Ecol. Evol.
(1996)Principles of Animal Ecology
(1949)
Fundamentals of Ecology
A review of animal population dynamics at extremely reduced population levels
Report to the International Whaling Commission
Demographic stochasticity and Allee effect on a scale with isotropic noise
Oikos
Population fragmentation may reduce fertility to zero in Banksia goodii – a demonstration of the Allee effect
Oecologia
RAPD variation in relation to population size and plant fitness in the rare Gentianella germanica (Gentianaceae)
Am. J. Bot.
Saving the Kakapo: the conservation of the world’s most peculiar parrot
Bird Cons. Int.
Cost of cooperative behaviour in suricates (Suricata suricatta)
Proc. R. Soc. London Ser. B
Harvest refugia in marine invertebrate fisheries – models and applications to the Red Sea urchin
Strongylocentrotus franciscanus
Am. Zool.
Predation, group size and mortality in a cooperative mongoose
Suricata suricatta
J. Anim. Ecol.
Population dynamics of obligate cooperators
Proc. R. Soc. London Ser. B
Allee effects: population growth, critical density and the chance of extinction
Nat. Res. Model.
Allee effects in metapopulation dynamics
Am. Nat.
Extinction thresholds in demographic models of territorial populations
Am. Nat.
Allee effect and population dynamics in the Glanville fritillary butterfly
Oikos
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