Population Ecology

Measuring population size

For plants or slow-moving organisms, ecologists use quadrat sampling: a square frame of known area is placed at random points in the habitat, the organisms inside each quadrat are counted, and the average count is scaled up to estimate the total population in the whole area.

For mobile animals, a mark-release-recapture method is used instead. A sample is caught, marked in a harmless way, and released.

After enough time has passed for the marked individuals to mix back into the population, a second sample is caught, and the proportion of marked individuals in it is used to estimate the total population size.

Once total population size has been estimated, ecologists often convert it into population density, the number of individuals per unit area or volume, calculated as population size divided by the area or volume sampled. Comparing density across habitats, or across different times of the year, reveals whether a population is becoming more crowded or more sparse, independently of the size of the area studied.

Placing each quadrat at a genuinely random position, for example using coordinates generated from a random number table laid over a grid map of the habitat, is essential, otherwise the sample becomes biased towards patches that happen to look more, or less, densely populated.

Factors that change population size

• Natality, the birth rate, which increases population size.
• Mortality, the death rate, which decreases population size.
• Immigration, individuals moving into the population, which increases population size.
• Emigration, individuals moving out of the population, which decreases population size.

The population growth curve and carrying capacity

When a small population enters a habitat with abundant resources, it typically grows slowly at first, then rapidly as births greatly exceed deaths, and finally levels off as the population approaches the carrying capacity, the maximum population size the habitat's resources can support. This produces an S-shaped, or sigmoid, growth curve.

Once a population is near its carrying capacity, limiting factors such as food supply, space, predators, and disease keep growth in check, and the population size fluctuates around a fairly stable level.

How this is examined

SPM questions commonly give quadrat count data and ask you to calculate the estimated population density or size, or give mark-recapture figures and ask you to apply the formula: estimated population size = (number marked in first catch × total number in second catch) ÷ number of marked individuals recaptured. You may also be asked to sketch or interpret a sigmoid growth curve and label its phases.

Density-dependent and density-independent factors

The factors that regulate population size close to the carrying capacity are usually grouped into two kinds:

• Density-dependent factors, food supply, disease, predation and competition, whose effect on the population becomes more severe as population density increases, tending to push the population back towards its carrying capacity.
• Density-independent factors, events such as floods, fires, drought and extreme temperature, which reduce a population by a similar proportion regardless of how dense the population already is.
• Near the carrying capacity, density-dependent factors usually dominate: as more individuals compete for the same limited food and space, birth rate falls and death rate rises until the two roughly balance, keeping the population fluctuating around a stable level.

Worked exam-style question

Question. A group of students wanted to estimate the population size of grasshoppers in a field. They caught 40 grasshoppers, marked each one with a small dot of non-toxic paint, and released them back into the field.

Two days later, they caught 50 grasshoppers, of which 8 were marked. (a) Name the method used to estimate the population size.

(b) Calculate the estimated population size of grasshoppers in the field, showing your working. (c) State one assumption this method makes about the marked grasshoppers.

(d) Suggest why the students waited two days before taking the second sample.

Model answer. (a) The mark-release-recapture method. (b) Estimated population size = (number marked in first catch × total caught in second catch) ÷ number of marked individuals recaptured = (40 × 50) ÷ 8 = 250 grasshoppers.

(c) The marked grasshoppers mix randomly back into the whole population, and marking neither harms them nor changes how easily they are caught or preyed upon; the population is also assumed to be closed, with no births, deaths, immigration or emigration between the two catches. (d) Waiting two days allows the marked grasshoppers time to disperse and mix evenly with the rest of the population, so the proportion marked in the second catch fairly represents the proportion marked in the whole population.

Practice question

Try this. Ten 1 m² quadrats were placed at random in a 200 m² field of daisies. The numbers of daisies counted in the ten quadrats were 4, 6, 5, 7, 3, 6, 5, 8, 4 and 6.

Calculate (a) the mean number of daisies per quadrat, (b) the population density of daisies, and (c) the estimated total number of daisies in the field.

Exam tip

Key terms

Population ecology sits within the wider study of ecosystems, these related terms give useful background:

• Ecosystem, the populations of different species in a habitat together with the physical environment they interact with.
• Food chain, a sequence showing how energy passes between the feeding populations in an ecosystem.
• Producer, an organism, such as a green plant, that makes its own food and supports the populations that feed on it.
• Decomposer, an organism that breaks down dead organisms, recycling nutrients that other populations depend on.

Source:SRC-DSKP-EN

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