Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.


Measuring population sizes

It may not be easy, or even possible, to obtain a direct value for this number.

Possible problem situations:
>how big an area
>how to define
>(animals) may move in/out of area,die/reproduce
>(large ones) will run away/can see/hear/smell observers
>may be counted twice
>perhaps come out only at night/ at some other time
>may roam over a large area
>are difficult to id - many species/varieties
>may not be in flower
>how many leaves = 1 plant e.g. grass
>can be quite small

In most cases Biologists decide to measure a sample and use this to estimate the size of a population.

It is best to take the sample in a special way so as to avoid bias and end up with meaningful results. This might involve random sampling, or a systematic approach designed to cancel out some of the problems.

In this way it is possible to make comparisons between different areas, or to see how populations change in the same area over a period of time.
Try and think of some examples of each:
Comparisons between different areas
> effects of different treatments e.g. fertilisers, weedkillers
(applied to agricultural land?)

Changes in populations in the same area over a period of time
> Effects of changing environment, e.g. global warming
> effects of management practices, e.g. fishing quotas, agricultural or forestry activity etc.
> study of colonisation of a particular area

A standard technique involves the use of quadrats, which are usually square frames, used to define an area for special study. The results from the area sampled using quadrats is then scaled up to give a meaningful estimate of population size.
What is the area covered by a quadrat with the size 0.5m x 0.5m (50cm x 50cm)?
>0.25m2 - one quarter of a square metre - OR 2500cm2

Possible problems?

>Size of quadrats
>How many samples to take
>Identification of species
>How to randomise position of quadrats: random numbers: x-, y-co-ordinates

Sometimes quadrats are further subdivided ("gridded quadrats"), perhaps 5x5, or 10x10. This may assist in working out percentage cover, and systematic sampling of the frequency of distribution of individual species.

Actually not all quadrats are square. Circular frames may be used, but it may be difficult to position them, or to make comparisons with other techniques. Loops of string or wire may also be used.

Some quadrats have a series of points, like a comb.

Mark release recapture technique

In this procedure attempts are made to catch an animal (live!), then these are counted and marked in some (hopefully harmless) way, then released into their habitat. A little later, trapping is repeated, and the number of (previously) marked and unmarked individuals are recorded.

An estimate of the number of individuals in the population (N) is given by

N = n1 x n2 / m2

n1 = number of animals first marked and released.
n2 = number of animals captured in the second sample
m2 = number of marked animals in the second sample

This is known as the Lincoln index.

Possible problems:

>Trapping may cause injury or distress (to the animals, not the people doing the trapping) - shrews may not get enough food and die of exposure
>Similarly, marking may cause injury or distress (to the animals, not the people doing the trapping) - paint/ nail varnish may be toxic/ applied to the wrong part of animals - trimming fur may affect viability of animals
>Marks may wear away or wash off (between 2 sampling times)
>Marks may make animals more obvious to predators
>Animals may learn to avoid traps, or become more likely to go to traps (if there is food bait in the trap)
>Timing between samples must be right - enough time for released animals to mix with the rest of the population - depends on the organism involved!
>Population size may be affected by organisms entering or leaving the area, or births and deaths
>Efficiency of trapping must be more or less the same each time
>Predators may get used to traps and break them open
>If sample sizes are too small results will be unreliable - mathematical corrections may be necessary.

More indirect techniques of estimating numbers

Aerial surveys are useful for getting into inaccessible areas, but you feel the animals are rather disturbed by the aeroplane.
Nineteen (no actually I think it's twenty) beisa oryx antelope-previously thought to be extinct in Southern Sudan, appear to chase the shadow of a survey plane in Boma National Park.

Images of distinctively marked animals caught on photo traps can be used to count individuals. aeroplane.

Other indirect techniques include finding and counting pellets left by birds of prey and droppings such as otter spraint (!)

Web references

Lake Baikal From Wikipedia, the free encyclopedia

An interesting student site about Labrador Park Singapore and the seagrass ecosystem at the beach, Seagrass is a flowering plant, not seaweed!

The Lincoln Index

The first camera-trap photographs of the endangered Northwest African, or Saharan cheetah, are obtained in Algeria.

http://news.bbc.co.uk/1/hi/sci/tech/8036405.stm World's smallest pigs 'thriving'
World's smallest pigs 'thriving': The world's smallest and rarest pigs are "thriving" following their release into the wild last year, conservationists report. Camera-trap footage and surveys suggest that the captive-bred pygmy hogs have adapted well to their new home in the grasslands of Assam in India.

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