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Biodiversity and ecological surveying
Using quadrats and transects,
measuring the abundance and distribution of organisms - plants or animals - data calculations, how to use a key
and draw a kite diagram, use of indicator species - investigating pollution -
monitoring abiotic factors
Doc Brown's
school biology revision notes: GCSE biology, IGCSE biology, O level
biology, ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
students of biology
This page will help you answer questions
such as ... What is a quadrat? How do
you do a quadrat survey? How do you do calculations from quadrat
measurements? What is a transect? How do you do
a survey using a transect? What are indicator species? How do you draw kite
diagrams? How to use an identification key
Sub-index for this page
(a)
Introduction
to investigating distribution and abundance
(b)
Surveying using quadrats
(c)
Example of quadrat calculations based on
sampling data
(d)
Surveying using transects
(e)
Estimating the percentage cover (distribution) of a species from a quadrat
(f)
Estimating a population size by using a capture-recapture technique
(g)
Three ways of trapping animals
(h)
Kite diagrams to show abundance and distribution of
organisms
(i)
Using keys to identify organisms
(j)
Monitoring techniques, instruments for measuring abiotic
factors - pollution, living organisms as indicators of environmental changes
- indicator species
(a)
Introduction to investigating distribution and
abundance
Some definitions
The distribution of animal and plant species is
important to scientists to understand the ecology of a particular
habitat.
The distribution of an organism is
where you find it in its habitat e.g. part or the whole of a river,
stream, field, heathland etc.
The abundance or population size
of an organism is how many individuals are present in a given area.
Some general points
Where an organism is found depends on several
environmental factors e.g. dry sandy soil or damp marshy ground,
brighter light in the open or shaded by trees or bushes.
Each species of plant or animal is adapted to live
in its particular habitat, but one patch of ground might be better
suited than another.
This means the distribution of any species can
vary even within the same habitat area.
Methodology
You need to know the methods of how to
investigate the distribution and abundance of organisms in a given
habitat.
Most habitats are relatively large areas and
it would be too time consuming to count all the numbers of
individual animals/plants of every species over the whole area.
Therefore you have to adopt a sampling
strategy, and from the data, scale up the numbers to
estimate the whole population of selected animal or plant
species.
Abundances can be estimated by
counting the number of individuals (e.g. identified plant/animal) or
the percentage cover (e.g. lichen on a stone wall) for selected
small areas chosen at random.
From these 'counts' you can then scale
up to allow for the total area of the habitat.
You can survey a habitat in two ways: Using
(1) a quadrat or using (2) a transect. - both methods
described in detail below, but there other points to make before
looking at them.
You can measure the number of an organism
in two or more sample areas of a habitat using a quadrat (e.g.
counting within a 1 m x 1m square frame) and compare the
results.
You might choose quite different
locations, but within the same habitat.
You can study how a distribution changes
across a wider area by surveying with quadrats along a transect
- basically following a linear path across a habitat.
You can lay out a long line or tape
measure and systematically lay the quadrat down every one or
more metres, but keeping the sampling intervals the same
distance apart.
There are also capture-recapture
techniques to estimate the size of a population.
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(b) Surveying: (1)
Surveying using quadrats
 A
quadrat is defined as a frame, traditionally square, used
in ecology and geography to isolate a standard unit of area for study of
the distribution of an item over a large area.
You can measure how common an organism is in two
or more sampled areas of a habitat using a small quadrat and
comparing the distribution numbers of species of plants or animals in
each location in a much larger area.
For a plant in the same habitat (e.g. same field)
you might choose dry/damp areas or bright light/shaded areas or any
permutation of conditions (here 4 possibilities, yes?).
Suppose you are surveying a field, you can place
the 1 m2 quadrat in specific locations or choose some places
at random over a wide area.
The frame of the quadrat can be made of wood or
metal. Illustrated is 1 m x 1 m quadrat and wire strung across at 10 cm
intervals. In this case there 100 10x10 cm square possibilities for
sampling, each has x,y coordinates of 1-10,1-10. You do NOT count all 100
mini-squares, instead you can use a random number function on your
calculator to select e.g. 10 of them. The square with x,y co-ordinates
of 7,4 is shown on the quadrat diagram. This 'mesh' size is ok for very
small organisms e.g. tiny flowers.
I wrote myself a quick computer programme in BBC
basic (above left) to generate 10 random x,y coordinates (above
right).
Link to the above programme
(it might work on Microsoft platforms after querying it, probably won't
work on other platforms?)
After placing the quadrat at selected locations
you e.g. count the flowers in each 10 x 10 cm2 square or the
total in the whole1 m2 of the quadrat - the whole quadrat is
1 m x 1m.
Here the yellow flowers are quite large and best
counted per 1 m2, giving you quantitative data e.g.
species of flower/m2.
To count the population using 10 x 10 cm squares
it needs to be a very small flower or insect.
Photo from the Cornfield wild flower project
Hutton-le-Hole - Ryedale Folk
Museum
Again, I've superimposed a 1 m2
quadrat, sub-divided into 20 cm x 20 cm smaller quadrats.
Here you could count each species of flower per m2
or choose a smaller are of 20 x 20 cm2 (0.04 m2
quadrat) or 40 x 40 cm2 (0.16 m2 quadrat)
areas - you just have to make a sensible decision.
Example
of quadrat calculations based on
sampling data
Example
1. Calculating a population
density
Suppose you did a count of some very small
species of flower in 10 of 10 cm2 mini-quadrats (10 cm x
10 cm) of a 1 m2
quadrat placed in a sunny location. The mini-quadrats can be
selected using the random number generator.
Data counts 1-10: 7, 8, 12, 9, 9, 10, 11,
10, 9, and 8 flowers
Total count = 93 flowers
Average per 10 cm2 = 93/10 =
9.3 flowers/mini-quadrat
Now there are 100 10 cm2 squares in
the full 1 m2 quadrat.
Therefore total in 1 m2 quadrat =
9.3 x 100 = 930 flowers.
The 'flower density' =
930 per m2
If you repeated the measurements in a more
shaded spot, you might find a much lower population density of
the same flower.
If you know the total area, call it
A m2,
you just multiply the 930
x A = total population in that area (see next example).
This is just a scaling up exercise from several small sample areas
chose at random.
Example 2.
Calculating a population
size (abundance)
Suppose you counted the abundance of a
relatively rare flower using a 1 m2 quadrat placed
8
times at random across a piece of land (its habitat) measuring 80 m
x 120 m.
Flower data counts 1-8: 2, 5, 0, 1, 2, 0, 1
and 4
(a) Calculate the average density of
the rare flower per metre2
Total flower count = 15
Flower density = 15/8 =
1.875/m2
(no need to round up at this stage)
(b) Calculate the whole population size
of the flower in this particular habitat
Total area of habitat = 80 x 120 = 9600 m2
Total population = density x total area
Population size = 1.875 x 9600 =
18,000 flowers
(maybe its not that rare in this made-up
calculation!)
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(d) Surveying: (2)
Surveying using transects
A belt transect is a path/gradient along which one counts and records occurrences
of the species of study.
You might wish to study how the distribution of
organisms changes by sampling across a transect.
A transect is used to survey a wider area in a
more systematic way than just doing a few quadrats.
e.g you can use a sequence of quadrats along a
transect to find out how organisms are distributed across a change in
habitat due to an abiotic factor - bright light to shade, damp to
dry ground, change in soil composition (due to underlying differences in
geology e.g. limestone and sandstone)
Below is a photograph of a field of wild flowers.
I've drawn on the photograph how you use a 1 m2
quadrat along the line of a transect to survey the species of
wild flowers from the hedge at the top to the bottom of the field.
You can visually see that the distribution
(concentration) of white and yellow flowers changes as you come down the
field and these can be accurately counted to give you quantitative
data.
Photo of the Cornfield wild flower project
Hutton-le-Hole - Ryedale Folk
Museum (August 2019) well worth a visit.
20 cm x 20 cm squares in the 1 m2 quadrat
You can count the number of each species in 1 m2
(100 x 100 = 10,000 cm2) areas or randomly sample the smaller
0.04 m2 (20 x 20 = 400 cm2) areas.
Doing a transect survey
In the preceding section I've described how to
use a quadrat.
Here you lay out a long string line from the
starting point to the end point.
Using a long tape measure you measure out 1,
2, 3 m etc. and place a 1 m2 quadrat at these points.
Count the organisms e.g. plant species you are
interested in and then move the quadrat on 1 m further down the
line.
You can survey every metre or 2 or 3 metres if
its a very long transect.
From your results you can plot graphs
of organism density (species/m2) versus
distance down the transect (m).
You could also measure the light intensity
down the transect using a light meter, at the same distance
intervals you were measuring the plant density.
You can then compare the two graphs and
see if there is any connection between the plant species density and
the intensity of light falling on the transect.
The calculations are just the same as I've shown
in the preceding (1)
Quadrats section.
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(e) Surveying (3)
Estimating the percentage cover (distribution) of a species from a quadrat
Another way to do a survey is to photograph the
area or habitat of an organism you are interested in.
This is done on a large scale to survey farmland
and monitor the distribution and growth of crops.
Above is a photograph of a section of a stone
wall on which two species of lichen are growing.
In order to help estimate the % cover of the
orange lichen and grey lichen a 10 x 10 grid (the quadrat) has been
drawn over the photograph - making 100 squares (or mini-quadrats).
If a mini-square is filled with half or
over half of the species it counts as 1/100 of the area.
If the mini-square is less than half-full a
species does not count.
My estimates of the distribution as measured by
the % cover (do you agree?)
You can think of the percentage cover as a
measure of the distribution or abundance.
Orange lichen: Only 3 squares present
(all in top left).
Therefore the
orange lichen cover estimate is 3%.
Grey lichen: I found it easier to count
the squares where it was absent, which I found to be 17.
Therefore the
grey lichen cover estimate is 83%
The
estimate of total lichen cover is 3 + 83 =
86%
If you surveyed another part of the stone wall
in e.g. different light or moisture conditions, you would find the %
cover might be different.
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(f) Surveying: (4)
Estimating a population size by using a capture-recapture technique
See next section on trapping animals.
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 |
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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(g) 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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(h) Kite diagrams to show
abundance and distribution of organisms
Kite diagrams are used to show abundance and distribution of
organisms along a transect.
The transect can be measured out in a linear way across
a habitat e.g. a field, a stretch of woodland, a rive bank - methods of
surveying and using quadrats has already been described in previous
sections.
You might counting the abundance of identified
species in 1 m2 quadrats.
A kite diagram could also be produced based on the depth
of an aquatic or marine environment, where the transect is simply a
vertical sampling line - you sample the water a fixed depth intervals.
You might be counting the number of identified
species in given volume of water e.g. 1 litre.
An example of a kite diagram is shown below involving
counting three species at 1 m intervals.
The vertical y axis represents the abundance
of the organism e.g.
the percentage cover in vegetation or lichen on a
stone surface,
It might represent the number of crustaceans per
volume of water,
or any other quantitative measure of the abundance
of an organism.
The abundance is plotted above and below the zero
base line to give a symmetrical shape (often 'kite looking').
The relative abundance of each organism at a
given distance along the transect is given by the thickness
of the 'kit shape'
The horizontal x axis is the distance
along the line of the transect.
Examples of interpreting a kite diagram (based on
the diagram above)
Species 1:
To the nearest m, it occurs twice between 1 and
24 m, and, 32 and 45 m along the transect.
The maximum abundance of 14% occurs at 5 m and
18.5 m along the transect.
Species 2:
To the nearest m, it occurs three times at 1-10
m, 11-36 and 38-55 m along the transect.
The maximum abundance of 14% occurs at 29 m and
43 m along the transect.
Species 3:
To the nearest m, it occurs once between 35 and
56 m along the transect.
The maximum abundance of 22% occurs at 51 m
along the transect.
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(i)
Using keys to identify organisms
An identification key is based on a series of
questions and descriptive statements to enable to identify a
type of organism, and if detailed enough, a particular species of plant
or animal.
Keys are useful in identifying organisms that you
capture in some
trapping technique.
As you work your way down a key you gradually
narrow the options as to what the organism may be.
A "Minibeast" is a term for a
variety of arthropods and other invertebrates, including spiders, ants,
butterflies, bees, wasps, flies, woodlice, and many others.
Part of a more complex key for minibeasts that you
find on the ground, or in the air, in a garden or woodland.
Note: Centipedes can have between 15 and 177 pairs of
legs depending on the species (average 35 pairs), and millipedes can
have up to 200 pairs. The main difference is that centipedes have one
pair of legs on each segment of their bodies and millipedes have two - I
thought you would really like to know this!
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(j) More on
monitoring and analysis techniques - measuring abiotic factors - pollution
and using living organisms as indicators of environmental changes
- indicator species
-
1. Surveying using indicator
species
-
Despite the presence of
pollutants, some species of plants/animals can live in polluted air or
water, but other organisms need clean air or clean water to survive and
prosper.
-
Some organisms are particularly
sensitive to changes in their environment and can be studied to
monitor the effect of human activities on the environment - such
organisms are called
indicator species.
-
The absence or presence of
these indicator species can be monitored and used as indicators of
pollution e.g. so you
can say much about whether a particular atmospheric or aquatic
environment is relatively polluted or unpolluted.
-
So, these indicator specie, being quite sensitive to their environment, can used in environmental monitoring and
hopefully control things to improve matters.
-
These pollution
indicators may live ...
-
... on surface exposed to
air e.g. lichen on rocks/stone walls, blackspot fungus on roses,
-
... live in water e.g.
mayfly larvae, stonefly larvae, freshwater shrimps, bloodworms,
sludgeworms.
-
Methods of surveying using indicator species
-
(i) A simple survey might just
consist of just seeing whether certain species are present or not.
-
(ii) You can employ some
observational-catching technique to actually count the population of
a species in a given area of land or volume of water.
-
Pros and cons of using
indicator species
-
Using indicator species is a
relatively quick and simple way of indicating whether land, water or
air is polluted.
-
Unfortunately, you cannot get an
accurate value for the concentration of a pollutant e.g. this might
need specialist chemical analysis.
-
The observations may be biased in
a negative way due competition between species for he same food
resources.
-
Therefore, sometimes it is better
to use some non-living indicator methods - see section 4.
-
Below, in sections 2. and 3. I've
described the use of a variety of living indicator species.
-
2. Lichens can be used as
air pollution
indicators, particularly of the concentration of sulfur dioxide in the
atmosphere.
-
Lichen
-
The cleaner the air in
the environment, the more varied species, and the greater numbers of
an individual species of lichen colonies are seen on rocks and stone
walls. You would observe the 'cleaner air' effect if you surveyed
walls all the way from a polluted town or city centre to some rural
location away from roads well beyond the town or city boundary, and
no doubt note the greater the numbers and variety of lichen growing
on the walls the further you where from the town/city centre.
-
Therefore, lichen
species can be used as quite a sensitive air pollution indicator
i.e. low populations of a limited number of lichen species indicates
polluted air, usually from sulphur dioxide (SO2).
-
Particular lichens are
sensitive to poisonous sulfur dioxide (even in very low
concentrations of SO2) from fossil fuel burning -
road vehicle exhausts, power station chimneys etc.
-
Blackspot fungus readily
grows on roses in relatively clean unpolluted air, but does not grow
as readily in polluted air - the fungus is killed by the polluting
sulfur dioxide. One advantage an urban gardener has over a country
gardener!
-
3. Invertebrate animals can be used as
water pollution indicators
and are used as indicators of the concentration of dissolved oxygen in
water.
-
Lakes that are stagnant
from overgrowth of algae (eutrophication from fertiliser run-off) become devoid of oxygen at
lower levels because the decay bacteria use up the oxygen. This
decreases invertebrate populations and animals that feed on them,
like fish, also decline - so whole food-chains and complex
ecosystems are disrupted.
-
If rivers become
polluted from raw sewage spills or silage spills, the concentration of
pathogens rise (extra food for them e.g. nitrate nutrients) and these microorganisms use up the
oxygen, so all species needing oxygen decline - which is nearly
everything!
-
Certain bacteria will
thrive in these conditions and consume oxygen in the process.
-
Some aquatic invertebrate
species actually thrive in low oxygen polluted water e.g. a high
population of blood worms, rat-tailed maggots and sludge worms indicates very polluted
water.
-
Particular invertebrate
animals like the mayfly larvae and stonefly nymphs are
sensitive to pollution, so their population size is a very good
indicator of the purity of the water.
-
Mayfly larvae and freshwater
mussels can tolerate slightly polluted water - just
sufficient oxygen for them to survive.
-
Alderfly larvae cannot survive
in polluted water - not enough oxygen for them to respire.
-
The above describes the effect of
three levels of pollution - high, medium and low.
-
The less pollution in the lake
or river water, the less the growth of algae/bacteria etc. and the more
oxygen dissolve in the water (less used up), therefore the more mayflies
and stoneflies hatched out for the trout! and more trout for the
fisherman! BUT only in clean unpolluted water!
 |
 |
| You can measure the pH
of a solution very accurately using a pH meter and a glass membrane pH
probe. The pH meter is calibrated against
a standard buffer solution of accurately known pH. You can test rain
water, river water etc. or water shaken with soil (after filtration or
settling out) You can also use this instrument with a probe to
measure water temperature. |
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Typical learning objectives for this
page
-
Know that living organisms form
communities, and we need to understand the relationships within and between
these communities.
-
Know that these relationships are affected
by external influences.
-
You should be able to use your
skills, knowledge and understanding to:
-
suggest reasons for the
distribution of living organisms in a particular habitat,
-
evaluate methods
used to collect environmental data, and consider the validity of the method
and the reproducibility of the data as evidence for environmental change,
-
at the end of an investigation
and analysis, can you distinguish whether differences in distributions of an
organism are due to one or more environmental factors?
-
is it possible to control, or
allow for, different environmental factors?
-
and you should understand:
-
the terms mean (average
of all of a data set), median (middle value in a data set) and mode
(the most common value in a data set - could be more than one value)
-
that sample size is related to
both validity and reproducibility,
-
the larger the sample, random
sampling from many locations and the more times the experiment is repeated,
the more reliable will be the final analysis and conclusions,
-
reproducibility is the key to a
successful valid investigation,
-
Know and understand the physical factors that may
affect an organism in its habitat (area where the organism lives):
-
To study the distribution of a
species of animal or plant you must measure the population of the organism
in different sample areas.
-
A habitat is where an organism
lives (plant or animal) and its distribution is the areas where the
organisms live an this may depend on environmental factors such as ...
-
ambient temperature,
-
availability of nutrients in the
soil or water,
-
amount of
light falling on the habitat,
-
availability of water in soil
for plants,
-
availability of oxygen and carbon dioxide
in the air or water.
-
Organisms will be adapted by
evolution to fill a niche in a particular area of the environment,
-
but if there is a change in any
of the factors above, then organism populations will be affected, some will
increase and some will decrease,
-
in extreme cases, one species
might die out in an area and another species may take advantage and move in.
-
An organism will be more common
in an area, where environmental factors make conditions more suitable for
the organism to survive and reproduce e.g.
-
shade for plants that need
little sunlight, or out in the open for plants that need bright sunlight,
-
some creatures may prefer damp
conditions, others adapted to dry conditions,
-
greater density/availability of
the organism's specific food requirements
-
Know and understand that quantitative data on the
distribution of organisms can be obtained by:
-
 (i) random sampling with
quadrats to cover a large area without sampling all of it
-
A sampling quadrat is usually a
1m x 1m (1 m2) square frame of wood or plastic,
-
Therefore if you measure the
number of organisms in a quadrat you get the density in organisms per square
metre.
-
You can average the random
individual quadrat results to get the mean value for a particular organism/m2.
-
To work out the total population
of an organism in the area you have been randomly sampling, you multiply the
mean by the total area.
-
The more samples you take, the
more reliable is your data, and therefore any deductions made will also be
more reliable, but the data is only statistical, never completely precise,
but
-
In presenting data make sure you
know how to use the terms mean (average
of all of a data set), median (middle value in a data set) and mode
(the most common value in a data set - could be more than one value).
-
(ii) sampling with quadrats along a
linear transect to look for changes across an area of land e.g. to see how a
population changes across a wider area.
-
Note (iii) Whatever field work
you do, the only really reliable data, are data that are consistent, i.e.
always show the same pattern of organism distribution (plant or animal), and
in that way the data is repeatable and reproducible.
-
This involves random sampling
with samples using many quadrats and transects.
-
Does the data support the
question posed about organism distribution?
-
Are differences in population
due to environmental factors?
-
What are the variables?
-
Have the variables been
controlled properly in your survey design?
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Satellites are being
used to monitor several environmental factors eg decline of
forests by burning and replace with cattle or crops, the area of
ice/snow cover in arctic regions eg changes in the Greenland and
Antarctic ice sheets. Even individual remote glaciers can be
monitored - decline of some with temperature rise is concerning
climate scientists studying global warming.
pH paper you might get in a soil testing kit.
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