Allee effects driven by predation

Abstract

Summary1. In a population with Allee effects a positive relationship exists between fitness and population size or density. Allee effects may result in extinction thresholds and are therefore crucial in conservation and management. It has been shown theoretically that Allee effects can be driven by predation; however, there are few empirical data. Previous empirical work on Allee effects has emphasized that taxa with life-history characteristics such as co-operative breeding may be prone to such effects. Because predation is a general ecological mechanism, Allee effects may be more widespread than previously thought. 2. We used a series of simple heuristic models to develop a theoretical framework for understanding predation-driven Allee effects as a function of predator functional and aggregative responses. 3. Predators can create an Allee effect if they have a type I (linear) or type II (saturating) functional response without a type III (sigmoid) aggregative response, or vice versa. In addition, predation must be the main driver of prey dynamics, and prey must have little spatial or temporal refuge from predation. 4. We highlighted several, mainly unrecognized, examples of predation-driven Allee effects from the literature, the majority of which came from systems that had been perturbed by exploitation or introduced predators. 5. Synthesis and applications . Allee effects can arise from a general ecological process under a variety of different combinations of functional and aggregative responses. Allee effects may thus be present in a broad spectrum of different taxa with different types of life history, not only those taxa, such as broadcast spawners and co-operative breeders, on which empirical work has focused thus far. Conservation biologists and managers working with heavily exploited or threatened populations, or attempting reintroductions, should be aware of the possibility of a threshold population size or density below which extinction is likely. These thresholds can occur regardless of species life history, if predation is a major source of mortality and spatial and temporal predation refuges are limited.

Experimental demonstration of an Allee effect in microbial populations

“…A candidate mechanism is quorum sensing, which detects density, and is important in V. fischeri's symbiosis with bobtail squid, Euprymna scolopes. Many other species have similar interactions based on population density [9,17]. Populations that did not reach densities detectable by our methods were scored as failure to establish.…”

“…In contrast, for populations without a strong Allee effect, the probability of establishment will be a concave function of initial density owing to demographic stochasticity. Previously, Allee effects have been observed in experimental and natural populations of metazoans, with important implications for the management of vulnerable or invasive populations [9,10]. While microbes have been engineered to display an Allee effect [11,12], this is the first study, to the best of our knowledge, to explore their existence in an environmentally isolated microorganism.…”

Caught between two Allee effects: Trade-off between reproduction and predation risk

“…We use two alternative descriptions, F(N) = αN/(1+N/β) and F(N) = λN/(1+ λhN). The latter form is the standard formulation due to Holling (1959), in which λ scales the predator-prey encounter rate and h is the handling time of one prey individual, and represents an Allee effect due to predator dilution (Berec et al, 2007;Courchamp et al, 2008;Gascoigne and Lipcius, 2004). The former expression can be interpreted as a predation-driven Allee effect invoked by a behavioural response of the population (e.g.…”

“…Prey can actively defend themselves through predator mobbing (Krebs and Davies, 1993), more vigilant scanning for predators (Mooring et al, 2004) or confusion effect when schooling (Nottestad and Axelsen, 1999). Passive defence (the dilution effect) is characterized by per-capita predation probability declining with increasing prey density and corresponds to a type II functional response (Gascoigne and Lipcius, 2004). For example, woodland caribou face higher predation risk in smaller groups as predators usually kill one animal per group per attack irrespective of the group size (Wittmer et al, 2005).…”

Modelling the predation effects of invasive crayfish,Procambarus clarkii(Girard, 1852), on invasive zebra mussel,Dreissena polymorpha(Pallas, 1771), under laboratory conditions

“…The functional response is an essential component of predator-prey models (Jeschke et al 2002), but its estimation is rarely considered in an invasion context (Hooff & Bollens 2004;Radford et al 2007), which is surprising, given the known impacts of introduced predators (Wanless et al 2007). For instance, during the establishment stage of non-native invasive prey, the functional response of a predator can determine whether these prey are excluded from establishing, become established but do not proliferate and spread, or become overabundant and spread to new locations (Murdoch & Oaten 1975;Gascoigne & Lipcius 2004).…”