Students find less moss on walls where pollutant levels are higher. Those walls also receive less water.
Which comparison would provide the strongest evidence about whether the pollutant affects moss abundance?
Compare walls with high pollutant levels and little water with walls with low pollutant levels and more water.
Count the moss on many more walls with high pollutant levels and little water.
Compare walls with the same water availability but different pollutant levels.
Compare walls with the same pollutant levels but different water availability.
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.