An experiment is a controlled way of testing an hypothesis, usually under laboratory conditions. Much of the practical fieldwork that students do while studying in biology is not experimental but involves observing and recording the behaviour or distribution of organisms.
Observations and Experiments
Fieldwork that looks at the distribution of plants and animals on the sea shore or samples the types of invertebrates living in a stream are observations. So is looking at woodlice in a 'choice chamber' and recording what they do.
Conducting an experiment (as opposed to making observations) is a way of testing a hypothesis (guess) about something that might be influencing the distribution of these marine organisms, freshwater invertebrates, or the behaviour of the woodlice.
Careful observation will usually suggest a number of possible experiments to help work out what is going on.
Marine Zonation Studies
After looking at the distribution of a number of plants and animals on the rocky sea shore students will often be asked to write a report. In this case there are many possible hypotheses to explain the reasons for the location of each species observed.
Maybe the fact that Fucus vesiculosus is found further up the shore than Fucus serratus is related to its ability to withstand desiccation (drying out) in air? This is testable. Maybe hang similar-sized, weighed samples of each species on a line in the same location and weigh them at regular intervals for several hours. When this has been done compare the percentage weight loss over time for the two species. The results of this experiment will tell you how good the hypothesis is (but can never 'prove' it).
Freshwater Pollution Studies
Small freshwater animals can be used as indicators of water pollution, and some (stonefly larvae) are only found in best quality streams.
Maybe the fact that stonefly larvae are only found in unpolluted water, while caddis fly larvae can be found in worse conditions, is related to the oxygen requirements of these two types of insect? Again an number of experiments spring to mind.
Woodlouse Behaviour Studies
Woodlice are often put into small transparent 'choice chambers' so that their behaviour can be observed. A typical arrangement would be to have one half of the container darkened, the other left fully lit. After a period of time observation usually reveals that most of the wooldlice are in the dark area, with only a few wandering around in the light. This seems to reflect their distribution in the wild – woodlice are rarely found walking about in broad daylight and are often discovered in dark places. But how does this work?
An experiment designed to test the hypothesis that woodlice move towards the light (a 'tropism') can easily be devised. Similarly the hypothesis that woodlice are more active when illuminated (a 'kinesis') could be tested.
Both experiments would yield results, in the sense that numbers could be generated (how many moving towards the light compared with the number walking the other way – how much activity in the light compared with darker conditions), but it might be difficult to decide what the results mean. Biological experiments often yield results that are difficult to interpret without resorting to statistical analysis. (The Student 't' test or Chi-squared test are often used.)
Observations simply record what is discovered, while experiments test the validity of hypotheses.
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