Irruptive Growth - Irruptive Growth in Mammal Populations

Irruptive Growth in Mammal Populations

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 completion, 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.