Field Scientist

Field Scientist

How do you count every daisy in a field, or every limpet on a shore? You don't. You **sample** a bit and scale it up. This module is the ecologist's toolkit: the levels of life in an ecosystem, the quadrats and transects that turn a few counts into a whole population, and the awkward question of how far you can trust the number you end up with.

Levels of life

Ecologists organise living things into levels, and the boundary between the last two is where most marks are lost:

Where is the boundary?

A study records the fish, plants and insects living in a pond, together with the water temperature and the dissolved oxygen. Which level of organisation is being studied?

• An ecosystem, because the non-living conditions are included alongside the living things
• A community, because several different species are being recorded together
• A population, because everything recorded lives in the same pond
• A habitat, because it names the place the organisms live

Two ways to sample

Both use quadrats, and they answer different questions. Choosing the wrong one is a method error, not a small one:

RP9: the quadrat method

Put the steps of estimating a population with quadrats into the correct order.

• Choose random coordinates to avoid bias
• Place a quadrat at each coordinate
• Count the organisms inside each quadrat
• Calculate the mean number per quadrat
• Multiply by the total area to estimate the population

Find the mean

Five 1 m² quadrats gave these daisy counts: 4, 6, 5, 3, 7. What is the mean number of daisies per quadrat?

Scale it up

The mean is 5 daisies per 1 m² quadrat, and the whole field is 200 m². Estimate the total daisy population.

Mean, median and mode

Three averages, and quadrat data is exactly the kind that makes the difference between them matter. • The **mean** is the total divided by the number of values. It uses every reading, which is its strength and also its weakness: one freak count drags it a long way. • The **median** is the middle value once they are in order. One freak count barely moves it. • The **mode** is the value occurring most often. Some data sets have no mode at all, and that is not an error. So when a set of counts contains one reading far above the rest, the mean and the median can tell genuinely different stories, and you should say which you are using and why.

Which average, and why?

Five quadrats along a hedge gave these nettle counts: 3, 4, 5, 6, 22. Which average best represents a typical quadrat here?

• The median, 5, because the single count of 22 is far above the rest and pulls the mean away from what four of the five quadrats actually held
• The mean, 8, because it is the only average that uses every reading
• The mode, because it shows the count that came up most often
• The range, 19, because it captures the full spread of the data

Plot the transect

Walking a transect from shade into open field, you counted daisies per quadrat at each distance. The x-axis is distance in metres; the y-axis is the daisy count. Plot the five points: 0 m → 1, 1 m → 3, 2 m → 5, 3 m → 7, 4 m → 9.

Defend your fieldwork

Your results are challenged. Four questions, of the kind a six-mark method question asks.

• You used random quadrats for the field but a straight line of quadrats for the shore. Is that inconsistent?
• You placed five quadrats. A classmate placed fifty. What does her larger sample actually buy her?
• One of your quadrats landed on a path and contained nothing at all. What should you do with it?
• Your transect shows daisy numbers rising as you move out of the shade. How far does that go towards proving light is the cause?

Which are true?

Select ALL THREE statements that are TRUE.

• A transect is deliberately not random, and that is a feature of the method rather than a flaw in it
• A population figure from quadrat sampling is an estimate, and a larger sample makes it more reliable rather than making it exact
• A set of quadrat counts can legitimately have no mode at all
• A community includes the non-living conditions the organisms live in
• The mean is always the best average to quote, because it uses every reading
• A quadrat that lands on bare ground should be discarded, as it is unrepresentative

Fieldwork summary

All the organisms of one species in an area are a population; all the different species together are a _____; add the _____ conditions and you have an ecosystem. Quadrats are placed at _____ positions to avoid bias, and the population is estimated as the mean per quadrat multiplied by the total area. To show how a species is distributed across a gradient you use a _____ instead.

Plan the fieldwork

Exam practice. In about 60 words, describe how you would estimate the number of dandelions on a school field. Include:

• how you would decide where to place the quadrats, and why that matters
• what you would count and what you would calculate from the counts
• one thing you would do to make the estimate more reliable