biology

Foster's rule

Foster's rule is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Foster's rule rather than just read about it. In short: Foster's rule, also known as the island rule or the island effect, is an ecogeographical rule in evolutionary biology stating that members of a species get smaller or bigger depending on the resources available in the environment. For example, it is known that pygmy mammoths evolved from normal mammoths on small islands.

Key takeaways

Foster's rule belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Foster's rule to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Foster's rule from memory before moving on to harder problems.

Reference excerpt

Foster's rule, also known as the island rule or the island effect, is an ecogeographical rule in evolutionary biology stating that members of a species get smaller or bigger depending on the resources available in the environment. For example, it is known that pygmy mammoths evolved from normal mammoths on small islands. Similar evolutionary paths have been observed in elephants, hippopotamuses, boas, sloths, deer (such as Key deer) and humans. It is part of the more general phenomenon of island syndrome which describes the differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts. The rule was first formulated by Leigh Van Valen in 1973 based on the study by mammalogist J. Bristol Foster in 1964. In it, Foster compared 116 island species to their mainland varieties. Foster proposed that certain island creatures evolved larger body size (insular gigantism) while others became smaller (insular dwarfism). Foster proposed the simple explanation that smaller creatures get larger when predation pressure is relaxed because of the absence of some of the predators of the mainland, and larger creatures become smaller when food resources are limited because of land area constraints. The idea was expanded upon in The Theory of Island Biogeography, by Robert MacArthur and Edward O. Wilson. In 1978, Ted J. Case published a longer paper on the topic in the journal Ecology. Recent literature has also applied the island rule to plants. There are some cases that do not neatly fit the rule; for example, artiodactyls have on several islands evolved into both dwarf and giant forms. The Island Rule is a contested topic in evolutionary biology. Some argue that, since body size is a trait that is affected by multiple factors, and not just by organisms moving to an island, genetic variations across all populations could also cause the body mass differences between mainland and island populations.

References

External links "Why Do Islands Breed Giants (And Sometimes Dwarfs)?", episode of PBS's Nova

Illustrations

Worked examples

Example 1 — a first encounter with Foster's rule

Start with the simplest possible case. Write down what Foster's rule claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Foster's rule before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Foster's rule ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Foster's rule

In research: Foster's rule appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Foster's rule in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Foster's rule is common in secondary-school and first-year university syllabi. It links to neighbouring topics Animal size, Ecogeographic rules, Ecology terminology, so understanding it makes those chapters shorter.
In everyday life: Look for Foster's rule outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Foster's rule in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Foster's rule means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Foster's rule out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Foster's rule in simple terms?

Why does Foster's rule matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Foster's rule?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Foster's rule.