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GCSE biology notes: Ecology surveying: measurements using quadrats & transects

Biodiversity and ecological surveying - using quadrats and transects

Measuring the abundance and distribution of organisms - plants or animals - calculations - how to use a key and draw a kite diagram

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 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?


Sub-index for this page

Introduction to investigating distribution and abundance

Surveying using quadrats

Example of quadrat calculations based on sampling data

Surveying using transects

Estimating the percentage cover (distribution) of a species from a quadrat

Three ways of trapping animals

Kite diagrams to show abundance and distribution of organisms

Using keys to identify organisms

More on monitoring and using living organisms as indicators of environmental changes



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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(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 flowers 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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(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).

The calculations are just the same as I've shown in the preceding (1) Quadrats section.

 

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(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.

 


(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.

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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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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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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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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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.

      • You can mark out the transect with two sticks and a long piece of string.

    • 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?


More on monitoring and using living organisms as indicators of environmental changes

  • 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.

    • The absence or presence of these indicator species e.g. from monitored population counts, can say much about whether a particular atmospheric or aquatic environment is relatively polluted or unpolluted.

    • These indicator species can be quite sensitive to their environment and we can put their sensitivity to their surroundings to good use 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

  • Lichens can be used as air pollution indicators, particularly of the concentration of sulfur dioxide in the atmosphere.

    • 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!

  • 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) 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 invertebrate species actually thrive in low oxygen polluted water e.g. a high population of blood worms and sludge worms indicates very polluted water.

    • Particular invertebrate animals like the mayfly larvae and stonefly nymphs are particularly sensitive to pollution, so their population size is a very good indicator of the purity of the water. 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!

      • Knowledge and understanding of the process of eutrophication is not required.

  • Environmental changes can be measured using non-living indicators (usually sensors) to monitor factors such as oxygen levels in water, temperature and rainfall.

    • You should understand the use of equipment to measure oxygen levels, temperature and rainfall, all of which are important indicators of environment change on land or in water and the bigger picture of global climate change.

      • Special meter probes can be dipped into water to measure oxygen levels, a bit like pH meter probes that measure pH (which is also an important indicator of relative acidity-alkalinity). A decline in aquatic oxygen levels as measured by an oxygen probe gives an immediate warning of pollution.

      • Temperature can be measured directly and very accurately with a mercury thermometer (being replaces on health and safety grounds), or, electronically using a thermocouple system. Average temperatures for the year, or seasonal averages, are important indicators of climate change. Both air and sea temperatures are monitored.

      • Specialised electronic instruments can automatically and continuously monitor air pollution levels of carbon monoxide, sulphur dioxide and ozone levels in the atmosphere.

        • The data can be continuously fed, stored and analysed in computer systems for detailed analysis of air pollution patterns on a long-term basis, so a decline or an improvement in environmental conditions can be seen and its progress monitored.

        • You can do the same with pH, oxygen level and temperature probes continually monitoring water systems like rivers.

      • Rainfall is easily monitored with a rain gauge, manually with a calibrated glass container (a bit like a measuring cylinder), or automatically by weighing the water collected with a sensitive balance. Like temperature, rainfall is an important aspect of regional climate data.

        • All of these monitoring systems can be fully automated these days and so 'automatic weather stations' can be set up in remote locations and data sent by radio to a weather centre or laboratory.

        • 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.


  • Practical work you may have encountered?

    • investigative fieldwork involving sampling techniques and the use of quadrats and transects; which might include, on a local scale, the:

      • patterns of grass growth under trees,

      • distribution of daisy and dandelion plants in a field,

      • distribution of lichens or moss on trees, walls and other surfaces,

      • e.g. distribution of the alga Pleurococcus on trees, walls and other surfaces,

      • leaf size in plants growing on or climbing against walls, including height and effect of aspect.

    • analysing the measurement of specific abiotic factors in relation to the distribution of organisms

    • the study of hay infusions

    • the use of sensors to measure environmental conditions in a fieldwork context.

    • investigations of environmental conditions and organisms in a habitat such as a pond,

    • ‘hunt the cocktail stick’ using red and green cocktail sticks on a green background,

    • investigating the distribution of European banded snails,

    • investigating the behaviour of woodlice using choice chambers,

    • investigating the effect on plant growth of varying their environmental conditions, eg degrees of shade, density of sowing, supply of nutrients,

    • investigating particulate levels, eg with the use of sensors to measure environmental conditions,

    • the use of maximum–minimum thermometers, rainfall gauges and oxygen meters,

    • investigating the effect of phosphate on oxygen levels in water using jars with algae, water and varying numbers of drops of phosphate, then monitor oxygen using a meter,

    • computer simulations to model the effect on organisms of changes to the environment.

 


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