Today sees the start of London zoo’s annual stock-take where the numbers of each species are counted. This task maybe a quick job for the keepers of the big cats or large reptiles but spare a thought for the invertebrate keepers, counting every butterfly could keep them occupied for up to 4 weeks.
This story could form the basis to a nice introduction when teaching about how scientists estimate population numbers.
You could show this image of fish in an aquarium and ask the students how they would count the population of each species of fish in the tank.
What problems would they face and how would they overcome them to produce an accurate number?
This could lead into discussions about how they could identify the species of each fish (maybe by using a key for those that look very similar) and why some species may be more difficult to count (some will live in the crevices in the rocks, others are very fast moving).
There is also scope here for some ‘how science works’ questioning: The number has to be accurate –so how would they do this? The method they use has to eliminate possible errors and it would be a good idea to get someone else to repeat the count and then compare the repeatability of the results. Both of these apply to any type of data collection.
You could then show this image of an ant colony and ask the students how they would count this population. The answer is that this is nigh on impossible so the keepers estimate the size instead. This is a nice introduction to why scientists can only estimate numbers of wild populations – the size is usually too big.
During my time as a science teacher I came up with numerous ways of helping students learn about estimating populations. The most common field-technique that comes up in exam specifications is using quadrats. I found that instead of showing the class a quadrat and explaining how it was used, it was more beneficial to their understanding if they figured out that a good way of estimating the population of stationary organisms was to use a grid technique.
I developed this resource:
- Start off by showing the class the image of a field of daisies on the whiteboard (which is only shown for a few seconds) and ask them to write down an estimate of the population. (Slide 1)
- Ask them to explain what technique they used to make this estimation. Discuss how to make this more accurate (slide 2).
- Show the same image but this time with a grid overlay and ask them how this would help them (slide 3). If they counted the number in one square and multiplied it by the total number would this be an accurate estimation? How could they improve on this method? They should realise that the more squares you use, and a mean calculated, the more accurate the answer would be. Discuss that the distribution of daisies is not even – does this affect which squares you would use?
- Ask students to work in pairs and estimate the number of daisies on Handout 1 using a ‘quadrat’ cut out from Handout 2. Ask pairs to use these to estimate the number of daisies. The pairs might decide to place them in a grid like fashion or simply throw them onto the paper – both methods are valid but the placement should be random.
- Ask pairs to feedback on what technique they used and what their estimated number was. Discuss the methods and then reveal the real number (103).
If you are feeling brave you may then want to take the class outside to use real quadrats on the school field but hopefully they will get the idea from this simple paper exercise.
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