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Isodar

Isodar 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 Isodar rather than just read about it. In short: Isodar is a theory of habitat selection in population biology proposed by Douglas W. Morris.

Isodar — main illustration
Isodar — illustration

Key takeaways

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

Reference excerpt

Isodar is a theory of habitat selection in population biology proposed by Douglas W. Morris. The theory underscores the importance of the abundance and thus competition between the members of the same species in selecting habitats. The name "isodar" stems from "iso" in Latin meaning same and "dar" from Darwin.

Background and theory An isodar, or habitat isodar, is a theory in evolutionary ecology developed in the late 1980s by Douglas W. Morris. Isodars model density-dependent habitat selection for one or two species in two habitats according to the ideal free and ideal despotic distributions. Isodar is a two-part word: "iso" meaning equal in Latin; "dar" for Darwinian evolution, and is defined as all combinations of population densities in habitats A and B such that both habitats offer the same fitness reward. Animals displaying an ideal free distribution distribute themselves among patches (sources of a resource) in such a way that each individual get the same amount of the resource. For example, if food is twice as abundant in habitat A compared to habitat B, the ideal free distribution-based model would predict that there will accordingly be twice as many animals competing for food in habitat A compared to habitat B. If the total number of animals is considered to be variable, there are two ways this can be plotted on a two-dimensional Cartesian graph. One way is to plot two lines on a graph of fitness vs. density of individuals. This graph can be used to predict the density of individuals at any given level of fitness. The second is to plot the density of individuals in habitat A vs. the density of individuals in habitat B when the fitness of all individuals is equal. The line in this second graph is an isodar line. Ideal free isodars predict that a species density in habitat A will increase linearly with its density in habitat B so that each individual in the species has the same fitness. If habitat A has higher quality resources than habitat B, then proportionately more individuals would be in habitat A then in habitat B. This can be shown on either a Fitness-Density graph (Figure 1) or a graph of density in two habitats (Figure 2).

Applications Isodars can be used to study density-dependent habitat selection between two species competing for two habitats. Species will equilibrate between the two habitats to maintain equal fitness within their own species and avoid competition with the other species. Isodars have also been used to show the effect of human habitat selection on biodiversity. They can also be employed to examine the cost and density dependence of habitat selection in a population. Criticism leveled against the method includes the fact that in attempting to condense a very complex combination of parameters into the single metric of population density, misleading conclusions about the underlying dynamics may be suggested or supported.

References

Illustrations

Isodar: Figure 1. Fitness of individuals by density in habitat A and habitat B. According to the ideal free distribution, individuals will equalize between habitat A and habitat B so that each individual has the same fitness. According to this example, habitat A will always have more individuals than habitat B.
Figure 1. Fitness of individuals by density in habitat A and habitat B. According to the ideal free distribution, individuals will equalize between habitat A and habitat B so that each individual has the same fitness. According to this example, habitat A will always have more individuals than habitat B.
Isodar: Figure 2. A habitat isodar for one species in two habitats. The species always prefers habitat A over habitat B. At low density, individuals will move into habitat A. As density increases, individuals will equalize fitness by dispersing to habitat B. However, habitat A will increase by double the amount as habitat B. The straight line represents equal fitness in both habitats.
Figure 2. A habitat isodar for one species in two habitats. The species always prefers habitat A over habitat B. At low density, individuals will move into habitat A. As density increases, individuals will equalize fitness by dispersing to habitat B. However, habitat A will increase by double the amount as habitat B. The straight line represents equal fitness in both habitats.

Worked examples

Example 1 — a first encounter with Isodar

Start with the simplest possible case. Write down what Isodar 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 Isodar 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 Isodar 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 Isodar

In research
Isodar 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 Isodar 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
Isodar is common in secondary-school and first-year university syllabi. It links to neighbouring topics Evolutionary ecology, Population ecology, so understanding it makes those chapters shorter.
In everyday life
Look for Isodar 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 Isodar in 20 minutes

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

Frequently asked questions

What is Isodar in simple terms?

Isodar is a theory of habitat selection in population biology proposed by Douglas W. Morris.

Why does Isodar 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 Isodar?

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 Isodar.

Tags

  • Evolutionary ecology
  • Population ecology

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