Emergent Allee effects in top predators feeding on structured prey populations

Abstract

Top predators that forage in a purely exploitative manner on smaller stages of a size-structured prey population have been shown to exhibit an Allee effect. This Allee effect emerges from the changes that predators induce in the prey-population size distribution and represents a feedback of predator density on its own performance, in which the feedback operates through and is modified by the life history of the prey. We demonstrate that these emergent Allee effects will occur only if the prey, in the absence of predators, is regulated by density dependence in development through one of its juvenile stages, as opposed to regulation through adult fecundity. In particular, for an emergent Allee effect to occur, overcompensation is required in the maturation rate out of the regulating juvenile stage, such that a decrease in juvenile density will increase the total maturation rate to larger/older stages. If this condition is satisfied, predators with negative size selection, which forage on small prey, exhibit an emergent Allee effect, as do predators with positive size selection, which forage on large adult prey. By contrast, predators that forage on juveniles in the regulating stage never exhibit emergent Allee effects. We conclude that the basic lifehistory characteristics of many species make them prone to exhibiting emergent Allee effects, resulting in an increased likelihood that communities possess alternative stable states or exhibit catastrophic shifts in structure and dynamics.

How cod shapes its world

“…By feeding size selectively on different sprat stages cod may hence change the structure of its main prey population, exert a positive feedback on its own food availability and thus shape its feeding environment to its own benefit. This positive feedback of cod on its own food environment is possible, as long as predation does not primarily target the small-adult sprat themselves (De Roos et al, 2003). In the Gulf of St. Lawrence cod predation has indeed been shown to be highest for prey individuals of 0.5-2 g (Duplisea, 2005).…”

The recent invasion of Rutilus rutilus (L.) (Pisces: Cyprinidae) in a large South- Alpine lake: Lago Maggiore

“…Predator stocking and prey culling can be used as management tools to counteract low levels of top predators. It has been shown (de Roos & Persson 2002;de Roos et al 2003) that piscivore stocking must be very intense to be successful and hence is not a practical management strategy in large lakes. Prey culling may be a more realistic approach to improve the status of fish stock in a sustainable way and promote predator recovery (Persson et al 2007b).…”

Recruitment facilitation can drive alternative states on temperate reefs

“…The resulting increased predation on the superior competitor (macroalgae) in turn benefits the inferior competitor, leading to an indirect mutualism (sensu Boucher et al 1982) that benefits both the inferior competitor and predator. Furthermore, this positive interaction is at the root of an emergent Allee effect (sensu de Roos et al 2003) for the grazers, where a minimum threshold herbivore population density is necessary for them to reach a dominant state. This Allee effect emerges with greater dependence of herbivore recruitment on CCA (greater f H ; Fig.…”