Population Size and Its Status

The size of a population tells us a great deal about its status in a habitat. Whatever ecological process we want to investigate — the outcome of competition with another species, the impact of a predator, or the effect of a pesticide — we always evaluate it in terms of some change in population size. Population size, technically called population density (N), is therefore the single most useful thing to keep track of.

In nature this size can span an enormous range. It can be as low as fewer than 10 individuals — the Siberian cranes that visit the Bharatpur wetlands in a given year — or it can run into millions, as with Chlamydomonas in a pond.

Why Numbers Are Not Always the Right Measure

Although the total number of individuals is usually the most appropriate measure of density, in some cases it is either meaningless or difficult to determine. Imagine an area with 200 carrot grass (Parthenium hysterophorus) plants and a single huge banyan tree with a vast canopy. Saying that the banyan's density is 'low' compared with the carrot grass badly underestimates the enormous role the banyan plays in that community. In such cases the percent cover or biomass is a far more meaningful measure of population size.

When Counting Is Simply Not Possible

Total number is also awkward when a population is huge and counting becomes impossible or hopelessly time-consuming. If you had a dense laboratory culture of bacteria in a petri dish, counting every cell would be out of the question, so some other measure of density has to be used.

For many ecological investigations we do not even need the absolute density — a relative density serves the purpose equally well. The number of fish caught per trap, for instance, is a good enough measure of the fish density in a lake.

Estimating Density Indirectly

More often than not, ecologists are obliged to estimate population sizes indirectly, without actually counting or even seeing the individuals. A familiar example is the tiger census carried out in national parks and tiger reserves, which is often based on pug marks and fecal pellets rather than direct sightings.

The measure chosen — numbers, percent cover, biomass, catch per trap or indirect signs — depends entirely on the species and the situation being studied.

Quick Recap

  • Population size reflects a population's status; ecological effects are judged by changes in it.
  • Population size is technically called population density (N).
  • It need not be measured in numbers alone — sometimes total number is meaningless or impractical.
  • For a lone huge banyan tree among 200 carrot grass plants, percent cover or biomass is a better measure than number.
  • For huge populations (e.g. bacteria in a petri dish), counting is impossible, so other measures are used.
  • Relative density (e.g. number of fish caught per trap) is often enough.
  • Densities are frequently estimated indirectly — the tiger census uses pug marks and fecal pellets.
  • Sizes range from very low (fewer than 10 Siberian cranes at Bharatpur) to millions (Chlamydomonas in a pond).

Solved Examples — Section 3

Q1. What is population density?

Answer: The size of a population, technically designated N — the number of individuals of a species in a defined area, or another suitable measure of that abundance.

Q2. Why is total number sometimes a poor measure of density?

Answer: Because it can be meaningless or hard to determine — for example, a single huge banyan tree among 200 small carrot grass plants.

Q3. What is a better measure for the lone banyan in that example?

Answer: Percent cover or biomass, which reflects its enormous role in the community.

Q4. Give an example of a relative density measure.

Answer: The number of fish caught per trap, which indicates the fish density in a lake.

Q5. How is the tiger census in national parks usually carried out?

Answer: Indirectly, using pug marks and fecal pellets rather than counting or seeing every tiger.

Q6. Give an example of a very low and a very high population size.

Answer: Very low — fewer than 10 Siberian cranes at the Bharatpur wetlands; very high — millions of Chlamydomonas in a pond.