(5A) Ecological surveying Method 4. Estimating a population size by using a capture-recapture technique and subsequent calculation for biodiversity surveys

See also next section on trapping animals.

capture (1st number)  ==> mark ==> release ==> recapture (2nd number)

to estimate the size of the population of the animal.

You set a trap of some sort that is likely to capture, without harm to them, the animal whose population you wish to estimate. (see next section for methods of trapping)

After capturing your 1st sample of the population, count them and mark them in some harmless way and release them back into their habitat - their local environment.

You then set the same trap in the same place, same time of day and leave for the same time as the first experiment to get a 2nd sample of the animal.

Therefore you have recaptured a 2nd sample of the population.

You then count how many of them are marked from the first sample.

 

The population size is estimated from the formula

		number in 1st sample x number in 2nd sample
Population size	=	----------------------------------------------------------------------------------
		number in 2nd sample previously marked in 1st sample

The calculation is quite simple (unlike the capturing and marking!)

Suppose 40 of a specific animal were caught in a trap in 1 hour.

The next day, at the same time, 35 of the same animal were caught in 1 hour for the 2nd sample.

If 7 of the 2nd sample were marked from the 1st sample, calculate the size of the population.

population size = 40 x 35 / 7 = 400

 

This is not very accurate because of several assumptions made:

There has been no change in population size - best done in consecutive days with the same weather conditions - less time for births/deaths between counts.

The markings haven't affected the chance of the animals survival - bright colours not recommended - makes them more visible to predators!

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(5B) Three ways of trapping animals

1. A pooter for catching insects

A pooter is a simple device for collecting insects on the ground - no good if they are flying around!  The pooter consist of a bottle/tall beaker with the sealed with a larger rubber bung.  Two plastic/glass tubes pass through the bung.  The idea is to put the longer tube end over an insect and suck on the shorter tube to capture the insect.  The shorter tube contains a fine mesh so that you don't suck the insect into your mouth.

You can investigate several contrasting areas and suck in as many insects as you can in a given time e.g. 5 minutes. Count the number of insects caught and then repeat in another selected area of the same size - which could be the area in a 1 m² quadrat.

 

2. A pitfall trap to catch insects

A pitfall trap consists of a steep-sided container like a tall jam jar, which is sunk into a hole in the ground of the habitat you are investigating e.g. part of a field or your own garden!  The top of the container is covered with a raised cover which allows for the trap to be partly open. Any creature wandering in, falls down into the trap, but can't escape because of the steep sides of the container.  The cover also protects the trap from the weather.

You leave the pitfall trap overnight in the first selected area and in the morning you can count the number of insects trapped.  The following night you can select another area to sample and compare the results. You leave the traps in place for the same time. You could leave the trap in the same place and compare daytime and nighttime or at the same time period of daytime in different weather conditions.

 

3. Using nets to catch animals (e.g. insects or fish)

There are also sorts of nets depending on what you want to catch and where.

A sweep net is a made of a strong cloth mesh that can be swept through long grass, nettles or reeds to catch weevils, plant bugs, leafhoppers, beetles, spiders, wood wasps and even snails!

You stand still in your chosen sample area and sweep the net once from left to right through vegetation.

You then have to quickly sweep the net up and turn the contents of the net into a container to count the insects - or whatever else you catch.

You then repeat the sweep in a second location and compare the numbers of the two catches.
 

For aquatic locations you can use a simple pond net, usually made of a plastic mesh - the size of the mesh can be varied depending on what you want to catch.

With a pond net you can catch insects, small fish, water snails and other animals from ponds and rivers.

Like with the sweep net, stand in your 1st location and sweep the net along the bottom of the pond and river.

Turn the contents of the net out into a white dish and count the organisms you have caught.

Repeat the pond net sweep in another location and repeat the count via the white dish.

This allows you to compare several different locations in the same habitat e.g. near the bank of further out in deeper water (take care!).

Most moth traps use a light source to attract moths into a trap at night. Pheromone traps are also used. All moth traps have the same basic design using a powerful lamp light to attract the moths and a box (not a net) in which the moths get trapped for later examination.

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Key points -

Based on the syllabus-specifications for students taking the UK AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10 biology).

Revision notes for biology students on Estimating Population Size Using the Capture-Recapture Method, plus common animal capture techniques like pooters, pitfall traps, and nets.

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Estimating Population Size – Capture-Recapture Method

This method is used for animals that move and are difficult to count directly (e.g. insects, small mammals).

How Does It Work?

1. Capture a sample of animals.

2. Mark each one harmlessly (e.g. waterproof paint, tags) and release them back into the wild.

3. Wait a set amount of time to allow the marked individuals to mix back into the population.

4. Recapture another sample and count:

   • How many are already marked.

   • Total number caught in second sample.

Population Size Formula:

Estimated Population Size =

(Number in 1st sample) x (Number in 2nd sample) / (Number marked in 2nd sample that were also marked in 1st sample)

Example:

• 40 beetles captured and marked in round 1.

• 30 captured in round 2.

• 10 of these were marked.

Estimated population = (40 x 30) / {10} = 120

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Assumptions of the Method

• No immigration/emigration between samples.

• No births or deaths affecting the population.

• Marking doesn’t affect the animal’s chances of survival.

• Enough time is given for marked animals to mix randomly.

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Methods of Capturing Animals

1. Pooter (for small insects)

• A small jar with two tubes:

  • One tube in your mouth (with a filter).

  • One tube sucks in the insect.

• Used to safely collect tiny, flying, or crawling insects (e.g. ants, beetles).

• Common in grassy or leafy habitats.

2. Pitfall Trap (for ground-dwelling invertebrates)

• A cup or container buried with its rim at soil level.

• Creatures fall in and cannot climb out.

• Covered with leaves/stones to prevent rain and predators.

• Used for slugs, beetles, ants, etc.

3. Sweep Net (for taller grasses and shrubs)

• A sturdy net used to sweep through long grass or bushes.

• Good for catching flying insects and small invertebrates.

• Be gentle - capture, observe, then release!

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Exam Tips

• Make sure you know the formula for population estimation and how to apply it.

• Understand why marking should not affect the animal (e.g. doesn't hurt or change behavior).

• Know which tool is suitable for which type of organism/habitat.

• Be ready to evaluate reliability: "This method assumes a closed population."

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Keywords, phrases and learning objectives for this part on ecological surveying and biodiversity

In the context of surveying in ecology, know how to estimate population size using a capture-recapture, and describe various methods of capturing animals e.g. using a pooter, a pitfall and net capturing techniques.

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