Foster's Rule

What is Foster’s Rule?

Foster’s rule is also known as the Island rule. It refers to the theory pertaining to the variation in the size of the body of species which are influenced by the availability of environmental resources. Briefly, it states that small animals may develop into bigger animals over a period of time when isolated on an island with abundant resources in the absence of predators.

On the same lines, the theory propagates that, larger animals begin to shrink into smaller sizes when isolated on an island with very little to no resources available. The rule was proposed by Bristol Foster who published it in his paper titled ‘Evolution Of Mammals On Islands’ in the year 1964.

Foster compared 116 mainland varieties to their island species. He suggested that some creatures turned into smaller versions of themselves(insular dwarfism), while few others developed into bigger versions of self(insular gigantism). This proposal was governed by two factors –

  • Availability of resources and
  • Absence of predation pressure from the mainland

This pointed towards the hypothesis that various species grow differently under varying conditions. The theory stayed for longer as it conveyed, small prey grow bigger in the absence of predators and with plenty of food resources around whereas predators who underwent competition on islands became smaller with fewer resource requisites.

See also: Sustainability of Natural Resources

Insular Dwarfism

It is the shrinking of larger animals over a course of time. Also known as Island dwarfism, it can occur in the mainland, for example, it is observed in dwarf tamarins. One of the main causes is the limited range of the population to a rather smaller region, occurring often in islands. This process has occurred in history in some species of elephants and Dinosaurs. It can occur in segregated ecosystems, away from breeding and external influences such as deserts, caves, mountains etc.

It is often seen in mainland animals who populate islands. Inhabitants decrease as small regions mean limited food supply and requirement of this food supply is sufficed by small animals hence they survive for long in such regions. Once the food resources are reloaded, life flourishes. Smaller animals have fewer generation times and shorter gestation periods. Among flesh-eating animals, not a competition, but the availability of food (prey) and their size contributes to being a major factor.

Further Reading: Herbivores Facts

Insular Gigantism

Insular or Island gigantism is a process in which the size of an animal increases on an isolated island drastically when compared to its mainland counterparts. It is an aspect of Foster’s rule, which postulates that when mainland animals populate islands, they launch into developing bigger versions of themselves and larger animals tend to undergo insular dwarfism. Over time, due to the evolution of species, island endemics and other animals have become extinct, which has also been observed in plants (insular plants).

Huge carnivores animals fail to survive on islands due to oceanic dispersal, in whose absence, are occupied by other small carnivorous animals, reptiles and birds that eventually became huge. Reduced predatory pressure on small animals allows them to grow larger and also results in lesser competition to them. Increased sizes of such animals decrease vulnerability during food deprivation scenarios as they are able to survive and travel for longer intervals without food. For more information on Foster’s rule and adaptation of animals, please register at BYJU’S.

Also Read: Evolution

Key Points

  • Foster’s rule is an ecogeographical rule. It states how members of a species tend to be smaller or bigger depending upon the resource availability in the environment.
  • Insular dwarfism explains the shrinking of animals with the course of time due to limited food supply and more competition among the species.
  • Insular gigantism is seen in animals that migrate to the islands from the mainlands. This is mainly because of reduced predation and lesser competition.
  • Due to their increased size, they are able to survive for longer intervals without any food supply.

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