Irruptions

In ecology, an irruption is a rapid and widespread increase in the abundance of a species in a particular region, often following a period of low to moderate density. Irruptions can involve herbivores, omnivores, insects, or birds and are typically episodic, covering large geographic areas and lasting one or more years before a population crash or retreat occurs. They are driven by a combination of density-dependent factors, favorable environmental conditions, resource pulses, and, in some cases, reduced predation or increased dispersal.

Common drivers include abundant food supplies, favorable climate that supports reproduction and survival, and the buildup of individuals beyond the carrying capacity of the local habitat. Irruptions often begin with a high concentration of individuals in favorable patches, followed by rapid local expansion as resources allow, and later decline due to resource depletion, disease, predation, or adverse weather. The timing and magnitude of irruptions are variable and may be difficult to predict, frequently showing irregular intervals.

Examples of well-known irruptions include snowshoe hare populations in North American boreal forests, which can surge dramatically and influence predator dynamics; locust swarms that threaten agricultural areas in Africa and parts of Asia; and spruce budworm outbreaks in North American forests, leading to extensive defoliation. Irruptions can have substantial ecological and economic consequences, including altered vegetation communities, changes in predator–prey relationships, and damage to crops or timber.

Management focuses on monitoring population densities, forecasting potential outbreaks, and implementing pest-control or habitat-management strategies to mitigate impacts. Irruptions are distinct from regular seasonal migrations, though dispersal may accompany the population surge. See also outbreak and population dynamics.