"Population estimates are usually based on demographic data alone with no consideration of exogenous factors. This is unrealistic. For living organisms, the fact of their own existence ensures that no environment or habitat remains ideal for long. As the subject population expands, it will invariably use up any crucial resource in fixed supply. Even renewable resources can be depleted once the population goes into ‘overshoot’, a situation in which aggregate consumption exceeds food species’ recovery rates or waste accumulation exceeds natural assimilative capacity. The rise and fall of reindeer populations introduced to two previously unoccupied (by reindeer) Pribilof Islands in the early 20th century is a classic example. Collapse was attributed to overgrazed food sources (primarily lichen) abetted by the stress of exceptionally cold winter."