1. Food availability and quality are important determinants of mammalian reproductive success, and long-term changes in food availability were assessed for their impact on diets and reproduction of three adjacent groups of vervet monkeys in Amboseli, Kenya in two periods spanning an interval of 9 years.
2. Diets were largely restricted to the products of two species of acacia trees (Acacia xanthophloea and Acacia tortilis), with food selection primarily determined by availability (tree density, size and seasonal production of foods).
3. Over this period the overall abundance of major foods, measured through absolute species density, declined while territory size increased.
4. Despite significant changes in food plant densities, diets remained relatively stable, suggesting a component of consistency in diet choice. Limited options or high costs for incorporation of novel foods are suggested as factors maintaining this stability, with deleterious consequences in the face of very long-term habitat changes.
5. This study suggests that the habitat deterioration, assessed by reduction in food densities, initiated local group extinction. An increased energy expenditure in foraging, high mortality and low reproductive rates ultimately led to a population crash under conditions of reduced food availability.