Allee effect due to predator functional response: Effects on population growth rate and extinction in an experimental zooplankton system

Background/Question/Methods Allee effects are increasingly recognized to play an important role in the population dynamics of low density populations.  Mate limitation and obligate cooperation are the mechanisms most commonly associated with Allee effects, but an increasing number of both theoretical and empirical studies have shown positive density dependence in low density populations to arise from predation, as a result of the predator’s functional response.  With a Type II functional response, predators are expected to cause higher per capita mortality in lower density prey populations, leading to positive density dependence in prey survival.  If this effect on fitness is not offset in low density populations by other mechanisms (e.g. lower intraspecific competition), the ubiquity of predator-prey interactions and the commonness of the Type II functional response suggests that a wide range of species may be expected to exhibit Allee effects.  To test the population-scale impact of this mechanism we exposed Daphnia magna populations to a constant level of predation from an invertebrate predator, Chaoborus larvae, expected to have a Type II functional response.  Fifty-six D. magna populations were established along a gradient of population density (0.7 – 46 L^-1) and monitored daily for population size and persistence. 

Results/Conclusions
In the presence of the predator, per capita growth rate was positively related to initial population size (Allee effect), while control populations exhibited negative density dependence.  The predator increased extinction by increasing the probability of extinction in D. magna populations with low initial population size.  We confirm that an Allee effect resulting from a Type II functional response can outweigh negative density dependence at the population level and increase the rate of extinction.  This may have important implications for population dynamics and an increased risk of extinction in small/low density populations, especially when a generalist predator may not respond numerically to the rare species.