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Foster’s Rule is a theory in evolutionary biology pertaining to the variation in the size of the body of species which may increase or decrease depending on the availability of environmental resources. The rule also states when any organism is isolated on an island with abundant resources, the organisms of a species might grow bigger in size when coupled with the absence of predator species. In the same way, the organisms of a species become smaller in size when they are isolated on an island where they have to compete for resources or when the resources are almost over coupled with predator species which are thriving in the same habitat, over a period of time.
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Key Terms: Foster’s Rule, Island Rule, Evolutionary Biology, Species, Insular Dwarfism, Insular Gigantism, Mammals, Isolated Islands
Read More: Predation
Bristol Foster’s Theory
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This theory was first stated by J. Bristol Foster in 1964. He published his theory in the journal called Nature in an article titled “Evolution Of Mammals On Islands”. He studied around 116 island species and then compared them to their mainland varieties.
He found out that the certain island creatures evolved into larger versions of themselves while others became smaller versions of themselves and on the basis of this, he proposed a simple explanation that smaller creatures get larger or become insular giants in the absence of the predators and also when there is the availability of resources as they are attracted to the mainland while the larger creatures become smaller or insular dwarfs when the food sources are absent.
Foster’s Rule
Insular Dwarfism
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The type of phyletic dwarfism in which we refer to the decrease in the average size of organisms of a species is called Insular dwarfism and can occur on the mainland. Insular dwarfism basically explains the process of why the growth of large animals shifts to reduced body sizes. This transformation occurs when the range of population is limited to a small environment, primarily islands or caves or isolated valleys and mountains.
- Insular dwarfism occurs naturally and is different from the intentional creation of dwarf breeds. Many times, this phenomenon took place throughout evolutionary history.
- It does not specifically occur on islands, but also in an isolated ecosystem which has been cut from external resources like caves, desert oases, isolated mountains or valleys, etc.
- The major cause of insular dwarfism is the limited range of the population which is thriving in a relatively smaller region. This dwarfism is often found in mainland animals that have a very good population on islands or isolated places. The reason for this is that in isolated places, the food supply is limited. Hence the requirement of the food supply is met by small animals hence they survive for long in such regions.
- When the food supply reloads, lives in these regions flourish.
- Insular dwarfs also have shorter gestation periods which leads to fewer generation times.
- The availability of food (prey) and their size contribute to being a major factor for carnivores.
Insular Dwarfism
Read More: Components of Ecosystem
Insular Gigantism
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The process in which the size of organisms of a species grows dramatically in an isolated island as compared to its mainland counterparts is called insular gigantism. Foster’s rule states that when the landmass animals populate islands, they launch into developing bigger and better versions of themselves.
Island endemics and other animals have become extinct due to the evolution of species over a period of time, which has also been observed in plants (insular plants).
Huge carnivorous animals fail to survive on islands due to oceanic dispersal. In the absence of these, huge predators, small carnivorous animals, reptiles and birds thrive and have an adequate supply of food and some carnivores like reptiles, birds and other species grow larger.
Decreased predator abundance on small animals allows them to become huge and also results in lesser competition for them. Moreover, expanded sizes of these living beings and declining weakness during food hardship situations as they can make due and travel for longer spans without food.
Insular Gigantism
Things to Remember
- Foster’s rule is related to both ecological and geographical aspects of the environment. It elaborates upon 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 process of everything working out because of restricted food supply and more rivalry among the species.
- Insular gigantism is seen in animals that relocate to the islands from the central areas. This is basically a result of reduced predation and lesser competition.
- Because of their expanded size, they can get by for longer intervals with practically no food supply.
Read More: Heredity and Evolution
Sample Questions
Ques. Who postulated Foster's Rule? (3 Marks)
Ans. Foster’s Rule was postulated by Bristol Foster in 1964 in his theory titled ‘Evolution Of Mammals On Islands’. Bristol suggested in his theory that some organisms turn into insular dwarfs or smaller versions of themselves, whereas some species turn into insular giants or bigger versions of themselves depending on the availability of resources and the absence of predation pressure.
Ques. What is Foster’s Rule? (3 Marks)
Ans. Foster’s rule is also called the Island rule. It alludes to the theory relating to the variety in the size of the body of species which is influenced by the availability of environmental resources. Briefly, it expresses that little creatures might form into greater creatures over a period of time when isolated on an island with abundant resources in the absence of predators.
Ques. List the two factors that govern Foster’s Rule. (3 Marks)
Ans. There are presumably two factors governing Foster’s Rule, the first is the decrease and increased variability of resources in an island environment. The second is the removal of predation pressure, that of enormous hunters which makes it profitable for prey species to keep a huge body size.
Ques. What is understood by the term ‘Insular Dwarfism’? (3 Marks)
Ans. Insular dwarfism can be defined as a type of phyletic dwarfism which depicts the decrease in the average size of organisms of a species. Island dwarfism is the other name for Insular Dwarfism and it can occur on the mainland. The body size is reduced gradually when the population's range is limited to a small environment, primarily islands or caves or isolated valleys and mountains.
Ques. What is Insular Gigantism? (3 Marks)
Ans. The process in which the size of organisms of a species grows dramatically in an isolated island as compared to its mainland counterparts is called insular gigantism. Foster’s rule states that when the landmass animals populate islands, they launch into developing bigger and better versions of themselves.
Ques. Explain the reasons for insular dwarfism. (3 Marks)
Ans. The main cause of insular dwarfism is the limited range of the population to a rather smaller region. Insular dwarfism can be commonly observed in mainland animals who populate islands or isolated places. Since the food supply is limited in isolated islands and the requirement of this food supply is sufficed by the small animals hence they survive for long in such regions.
Ques. List down the causes for insular gigantism. (3 Marks)
Ans. The large carnivore animals fail to survive on islands owing to the oceanic dispersal. Thus, in the absence of these huge carnivores, small carnivorous animals, reptiles and birds have an adequate supply of food and they grow larger. Also, the reduced predatory pressure on small animals allows them to become huge.
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