Organisms cooperate during attack on large and vigorous or well-defended prey. For example, wolf packs can kill large mammals like moose while single wolves are restricted to eating small animals like beavers, rabbits and mice; bark beetles can kill vigorous trees when their numbers are high but are restricted to severely weakened or dead trees when numbers are low (Berryman et al. 1985, Raffa and Berryman 1983). Under these conditions the per-capita birth rate can rise with population density and/or the death rate can fall and this can create an R-function with an unstable Allee effect.

References:

• Berryman, A. A., B. Dennis, K. F. Raffa and N. C. Stenseth. 1985. Evolution of optimal group attack, with particular reference to bark beetles (Coleoptera: Scolytidae). Ecology 66: 898-903.
• Raffa, K. F. and A. A. Berryman. 1983. The role of host plant resistance in the colonization behavior and ecology of bark beetles (Coleoptera: Scolytidae). Ecological Monographs 53: 27-49.

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