Population density thresholds or breakpoints are unstable equilibria that separate different kinds of dynamical behavior. Most population density thresholds are created by cooperative ⁺feedback processes such mating success, enemy functional responses, or group attack, but there are two main kinds:

1. Thresholds created by processes that manifest themselves at very low population densities, the most obvious being a reduction in the frequency of mating as densities get very low (Allee effect). Thresholds created under these circumstances usually separate extinction from growth behavior and are often called extinction or underpopulation thresholds.
2. Threshold created by processes that manifest themselves at relatively high population densities, such as saturation of natural enemy behavioral responses or overcoming the defenses of large and/or vigorous prey. These processes allow populations to escape the constraints placed upon them by enemies or food supplies and the unstable equilibria are therefore called escape thresholds.

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