A plant parasite is removed from half the trees in an orchard. The remaining trees are untreated. The treated trees are then watered more frequently than the untreated trees. Treated trees grow faster.
Why does this result not show that parasite removal alone increased growth?
The sample includes trees that can grow without the parasite.
The untreated trees provide a comparison for the treated trees.
Water supply and parasite presence both differ between groups.
Tree growth provides a measurement of the effect of treatment.
82 exam-style questions on Edexcel GCSE Biology 9.1 Ecosystems and interdependence, covering 9.1.1 Levels of organisation in ecosystems, 9.1.2 Abiotic and biotic factors, 9.1.3 Importance of interdependence, 9.1.4 Parasitism and mutualism, and 9.1.5 Determining number of organisms in an area. Each one has a worked solution and a mark scheme showing where the marks go.