Photosynthetic organisms, usually green plants and algae, are the producers of biomass for life on Earth. They make glucose by photosynthesis, and every food chain begins with a producer.
Producers are eaten by primary consumers, which may in turn be eaten by secondary consumers and then tertiary consumers. Consumers that kill and eat other animals are called predators, and the animals they eat are called prey. In a stable community the numbers of predators and prey rise and fall in cycles. When prey numbers rise, predators have more food and their numbers rise a little later. More predators then eat more prey, so prey numbers fall, and the predators go short of food.
Ecologists use quadrats and transects to find the distribution and abundance of a species. A quadrat is a square frame placed on the ground so the organisms inside can be counted. A transect is a line laid across a habitat, with quadrats placed along it, to show how the species changes with a factor such as light. In required practical 9 you place quadrats at random to estimate the population size, and you place them along a transect to see how a factor affects distribution.
To summarise abundance, the mean is the total divided by the number of quadrats, the median is the middle value when the results are in order, and the mode is the most common value.