Overcompensation and population cycles in an ungulate

ALTHOUGH theoretical studies show that overcompensatory density-dependent mechanisms can potentially generate regular or chaotic fluctuations in animal numbers, the majority of realistic single-species models of invertebrate populations are not overcompensatory enough to cause sustained population cycles^1-3. The possibility that Overcompensation may generate cycles or chaos in vertebrate populations has seldom been considered. Here we show that highly overcompensating density-dependent mortality can generate recurrent population crashes consistent with those observed in a naturally limited population of Soay sheep. The observed interval of three or more years between crashes points to sharp 'focusing' of mortality over a narrow range of population density.