(5A) Ecological surveying Method 4.
Estimating a population size by using a capture-recapture technique
See also next section on trapping animals.
capture (1st number) ==> mark ==> release ==> recapture
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
number in 1st sample x number in 2nd sample
| number in 2nd
sample previously marked in 1st sample
The calculation is quite simple (unlike the capturing
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!
(5B) Three ways
of trapping animals
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
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.
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.
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
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
Like with the sweep net, stand in your 1st
location and sweep the net along the bottom of the pond and
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.
Keywords, phrases and learning objectives for this part on ecological surveying
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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