What factors lead to population crashes?

Population crashes are primarily caused by factors such as disease, predation, habitat destruction, and drastic environmental changes.

Population crashes, or population declines, are significant decreases in the size of a population of a species. This can occur due to a variety of factors, often interrelated, that negatively impact the survival and reproduction rates of the species.

One of the main causes of population crashes is disease. When a disease spreads through a population, it can significantly reduce the number of individuals, especially if the population lacks immunity to the disease. This can be seen in historical human population crashes, such as the Black Death in Europe, and in wildlife populations, such as the decline in Tasmanian devils due to a contagious cancer.

Predation is another key factor. If a predator population increases, or a new predator is introduced, it can lead to a rapid decline in the prey population. This is particularly the case if the prey species has not evolved defences against the predator. An example of this is the introduction of the brown tree snake to Guam, which led to the extinction of several bird species on the island.

Habitat destruction is a major cause of population crashes, particularly in the current era of rapid human-induced environmental change. When a species' habitat is destroyed or significantly altered, it can lead to a rapid decline in the population. This can be due to direct loss of individuals during the destruction, or because the remaining habitat is not suitable for the species to survive and reproduce. For example, deforestation has led to significant declines in orangutan populations in Borneo and Sumatra.

Finally, drastic environmental changes, such as climate change, can lead to population crashes. Changes in temperature, precipitation, or other environmental factors can make a habitat unsuitable for a species. This can lead to a decline in the population, either because individuals cannot survive the new conditions, or because they cannot reproduce successfully. For example, changes in sea ice conditions due to global warming have led to declines in polar bear populations.