Allee effect

the benefit individuals gain from the presence of conspecifics, e.g. at low densities the per capita birth rate declines because of the difficulty of finding a member of the opposite sex. Also known in fisheries as depensation - mortality is depensatory when its rate (i.e. the proportion of population affected) increases as the size of the population decreases. Depensation may explain why marine fish populations like the Atlantic cod are slow to recover even when fishing is halted. Per capita mortality may increase because of changes in predator-prey interactions, mate availability may be reduced, fertilisation success may be lowered, operational sex ratios may change, and there may be a reduced intensity of social interactions during spawning. Compare compensatory mortality where the mortality rate decreases as the population size decreases