Irruptive growth, sometimes called Malthusian growth, is a growth pattern over time, defined by population explosions and subsequent sharp population crashes, or diebacks. It is an extension of the Malthusian growth model, specifically the growth pattern that causes a Malthusian catastrophe, and can occur when populations overshoot their carrying capacity, a phenomenon typically associated with r-strategists.
An irruption is any sudden change in the population density of an organism. Populations which exhibit irruptive growth do not stabilize around their carrying capacity, a feature of logistic growth. Irruptive growth occurs when a species reproduces more rapidly than the environment is capable of supporting with the available resources. Irruptive growth is studied in population ecology.
Species that are r-strategist (species that evolve according to r-selection) are characterized by rapid development, early reproduction, small body size, and shorter lifespans, whereas K-strategist species (species that evolve according to K-selection) exhibit slow development, delayed reproduction, large body size, and longer lifespans. These are the main two evolutionary strategies for ensuring the continuation of the species by passing down its genome.
Species that are r-strategist have variable population sizes and are more likely to exhibit irruptive growth than K-strategist species, whose populations are usually constant and remain at or close to the carrying capacity of the environment. r-selection leads to high productivity, while K-selection leads to high efficiency. Productivity refers to the number of offspring produced, whereas efficiency refers to the quality and the probability of survival of individual offspring.
The human species is K-strategist; that is, each mating pair has a small number of offspring, of which the majority will survive to adulthood and reach reproductive age. Reproductive age is later in life for K-strategists. r-strategist species, such as some insects, have very large numbers of offspring, the majority of which will die before reaching physical maturity. If there is a change in their environment, more of these offspring may survive than is typical, leading to irruptive growth.
Because of these differences in the number of offspring produced between K- and r-strategists, K-strategist species are more likely to have stable populations and less likely to exhibit irruptive growth.
One exception to the general rule of K-strategists being less likely to exhibit irruptive growth is people. As Thomas Malthus described in his essay on the Principle of Population, human overpopulation is a serious issue, and one which can only become more pressing. Due to our advanced mental capacity, humans have succeeded in maximizing the carrying capacity of the environment through the invention of higher-yielding crops and transportation to get food from where it is produced to where it is needed for consumption However, the world's population continues to rise, and the capacity of the current food supply to adequately provide for all the people on earth is called ever more into doubt. Thus, we may become one of the few K-strategist species to experience a Malthusian catastrophe.
Populations of some species tend to initially show a lack of response to density-dependent factors that limit population size as it nears carrying capacity. The exhibition of Malthusian growth is dependent on a number of elements including resource availability, degree of both interspecific and intraspecific competition, and strength of predator-prey relationships.
Irruptive growth patterns are seen in a few mammal species, often in specific locations, and they are usually herbivores with relatively small body size. In cases where a single herbivore prey species is dominant in an ecosystem, there is likely to be a strong link with predator species, which serves to prevent un-checked population growth. Rabbits and house mice of Australia tend to show irruptive growth, likely because when a drought ends, they reproduce at a rapid rate and continue to breed at that rate while predator reproduction is still seasonal in occurrence. This allows for the population to explode and to be limited more by a return of dry conditions than by predators.
Larger herbivores like pronghorn and deer are also known to exhibit irruptive growth; this occurs in populations with high reproduction and delayed density dependent inhibition. The time that a species is most likely to irrupt in population growth is when a population is first inhabiting an area or when predators are first removed, and both weather and food supplies are in the species' favor. When an area is being colonized, populations of species can grow rapidly and predator species are often not present to limit growth. Resource availability is not an issue either for colonizing populations, which will also experience little intraspecific and interspecific competition in early settlement of a location.
Similar to white-tailed deer in North America, roe deer in European locations have shown increases in abundance even in the face of already extremely high densities. The deer are able to irrupt and continue to increase in density over their carrying capacity because in particular areas especially, populations show delayed response to density dependent factors. The mammal species that exhibit Malthusian growth are the ones that, under certain conditions, act more like r-strategists than K-strategists compared to surrounding populations.