Population dynamics of obligate cooperators

Obligate cooperative breeding species demonstrate a high rate of group exti nction, which may be due to the existence of a critical number of helpers b elow which the group cannot subsist. Through a simple model, we study the p opulation dynamics of obligate cooperative breeding species, taking into ac count the existence of a lower threshold below which the instantaneous grow th rate becomes negative. The model successively incorporates (i) a distinc tion between species that need helpers for reproduction, survival or both, (ii) the existence of a migration rate accounting for dispersal, and (iii) stochastic mortality to simulate the effects of random catastrophic events. Our results suggest that the need for a minimum number of helpers increase s the risk of extinction for obligate cooperative breeding species. The con straint imposed by this threshold is higher when helpers are needed for rep roduction only or for both reproduction and survival. By driving them below this lower threshold, stochastic mortality of lower amplitude and/or lower frequency than for non-cooperative breeders may be sufficient to cause the extinction of obligate cooperative breeding groups. Migration may have a b uffering effect only for groups where immigration is higher than emigration ; otherwise (when immigrants from nearby groups are not available) it lower s the difference between actual group size and critical threshold, thereby constituting a higher constraint.