Measuring EcosystemsAQA GCSE Biology: Revision notes

Photosynthetic organisms (usually green plants or algae) are the producers of biomass for almost all life on Earth, because they make glucose by photosynthesis.

Feeding relationships in an ecosystem can be shown as a food chain, which always starts with a producer:

producer → primary consumer → secondary consumer → tertiary consumer

• Primary consumers eat producers (they are herbivores)
• Secondary consumers eat primary consumers
• Tertiary consumers eat secondary consumers
• Consumers that kill and eat other animals are predators; the animals they eat are prey

In a stable community, predator and prey numbers rise and fall in repeating cycles: as prey numbers rise, predators have more food so their numbers rise too; more predators then reduce the number of prey, so predator numbers fall again, and the cycle repeats.

Ecologists need reliable ways to find out how many organisms of a species live in a habitat, and how they are spread out.

• A quadrat is a square frame (often 0.5 m × 0.5 m or 1 m × 1 m) placed on the ground to sample a small area
• A transect is a line laid across a habitat, with quadrats placed at intervals along it, used to see how a species' abundance changes across an environmental gradient (e.g. moving away from a hedge)

Method for measuring the population size of a common species (Required Practical 9):

1. Mark out the habitat and decide how many quadrats give a representative sample
2. Place quadrats randomly (e.g. using random number coordinates) to avoid bias
3. Count the number of the chosen species in each quadrat
4. Calculate the mean number of organisms per quadrat
5. Multiply the mean by the total number of quadrats that would fit in the whole area, to estimate the total population

Materials such as carbon and water are not used up — they cycle repeatedly through the abiotic (non-living) and biotic (living) parts of an ecosystem.

The carbon cycle:

• Carbon dioxide is removed from the atmosphere by plants and algae during photosynthesis
• Carbon compounds pass along food chains as organisms feed on each other
• Carbon is returned to the atmosphere as carbon dioxide by respiration in plants, animals and microorganisms
• Combustion (burning fuels) also releases carbon dioxide

The water cycle provides fresh water for plants and animals: water evaporates from oceans and rivers, falls as precipitation, and drains through rivers back into the seas. You do not need to know the nitrogen cycle for this course.

Microorganisms (decomposers) play a key role: they break down dead plants and animals, returning carbon to the atmosphere as carbon dioxide (via respiration) and mineral ions to the soil, which plants then reuse.

Changes in abiotic conditions can change where species are found, because a species can only survive within its tolerance range for a factor.

Environmental changes that affect species distribution include:

• Temperature — rising temperatures can push species towards the poles or higher altitudes
• Availability of water — droughts or changed rainfall reduce the range of species needing reliable water
• Composition of atmospheric gases — changes in carbon dioxide or oxygen levels affect plant growth and aquatic life

Higher Tier students should be able to evaluate evidence linking a specific environmental change to a change in the distribution of a named species, considering whether that change is the only possible cause.