Biology 12

468 MHR • Unit 5 Population Dynamics

Similar methods are used in studies of aquatic

ecosystems to estimate the size of various

populations. Often, samples of a known volume of

water are collected. The water is then passed through

a net or sieve, and the number of organisms in each

sample is counted. As with quadrats, the average

density (number of individuals per unit volume)

can be used to estimate the size of the population

contained in the entire body of water.

When using any sampling technique, it is

important to take randomsamples — that is, samples

in which all individuals in the population have an

equal chance of being represented. To have the best

chance of taking such a sample, it is important to

know something about how the individuals in the

population are distributed — in other words, to

understand the population’s dispersion. In the

MiniLab on the next page, you will practise one

technique used for estimating populations of

mobile species.

Population Dispersion

Imagine walking along a transect and making a note

on your data sheet each time you see a member of

the population in which you are interested. Do you

think you would be counting individuals at regular

intervals — perhaps one every 10 m? Or do you

think it more likely that you would come across

several individuals at once, and then none, until

you reach another group? This, of course, depends

on how the members of the population are dispersed.

To simplify discussion, ecologists tend to

recognize three general patterns of dispersion:

uniform, random, and clumped (see Figure 14.5).

It is important to realize, however, that there is

actually a continuum of dispersion patterns. Many

species typically display patterns that fall between

two of these three types. Factors important in

determining dispersion include the distribution of

Ecologists are often more interested in knowing the relative

densities of two or more populations (that is, whether one

is more or less dense than another) rather than the actual

or absolutedensity of each one. Relative densities can be

determined by looking for signs of activity along transects.

For example, the relative density of bears in an area is often

estimated by looking for footprints, claw marks on trees,

clumps of hair caught on bushes, and piles of droppings

(usually called “scats” by biologists). Since it is difficult to

distinguish between grizzly (Ursus horribilis) and black

bears (U. americanus) using some of these signs, samples

of hair or scats are brought back to the lab for DNA analysis.

BIO FACT

Figure 14.4When sampling the density of certain plants in

a quadrat, it is often too difficult to count the number of

individuals of a species. In this case, the proportion of the

quadrat covered or shaded by one or more plant species is

recorded. The area sampled is usually square or circular.