Organisation of an ecosystem

Photosynthetic organisms are the producers of biomass for life on Earth. Feeding relationships within a community can be represented by food chains, and all food chains begin with a producer which synthesises molecules — usually a green plant or alga making glucose by photosynthesis.

Producers are eaten by primary consumers, which in turn may be eaten by secondary consumers and then tertiary consumers. Consumers that kill and eat other animals are predators, and those eaten are prey.

In a stable community the numbers of predators and prey rise and fall in cycles, and you should be able to interpret graphs that model them.

Read a predator-prey graph in this order. The prey peak comes first. More prey means more food for predators, so the predator numbers rise after it. More predators eat more prey, so the prey numbers then fall. With less food the predators fall too, which lets the prey recover — and the cycle repeats. The predator curve always lags behind the prey curve, and spotting which line peaks first is how you tell them apart when they are unlabelled.

Ecologists use quadrats and transects to determine the distribution and abundance of species.

A quadrat is a square frame placed on the ground; you count what is inside it, repeat at random positions, and use the mean to estimate abundance across the whole area. A transect is a line across a habitat, with quadrats placed at intervals along it — used when you want to see how a species changes across a gradient, such as from the shore inland.

Random placement matters for abundance, because choosing where to put the quadrat biases the result. A transect is deliberately not random, because there you are studying the change with distance rather than the average.

Many different materials cycle through the abiotic and biotic components of an ecosystem. All materials in the living world are recycled to provide the building blocks for future organisms — the carbon atoms in you were in something else before, and will be again.

The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide, to be used by plants in photosynthesis.

The water cycle provides fresh water for plants and animals on land before draining into the seas. Water is continuously evaporated and precipitated.

You are not expected to study the nitrogen cycle.

The water cycle has four processes to name, and a question showing you a diagram will ask you to explain them rather than list them.

• Evaporation — energy from the Sun turns liquid water at the surface of the seas into water vapour. Plants add to this by transpiration, losing water vapour through their stomata. • Condensation — the vapour rises, cools, and condenses back into tiny droplets, forming clouds. • Precipitation — the droplets join until they are heavy enough to fall as rain, snow or hail, much of it onto land. • Runoff and collection — the water drains through the soil and along rivers, back into the seas, and the cycle repeats.

Why it matters to living organisms is the part most often left out: the cycle provides fresh water for plants and animals on land. Sea water is salty and unusable for most organisms, and evaporation leaves the salt behind — so the cycle is what turns an ocean into drinkable water, continuously, and delivers it inland.

The role of microorganisms is the part most often left out of an answer. Decomposers break down dead plants and animals and waste, returning carbon to the atmosphere as carbon dioxide through their own respiration, and returning mineral ions to the soil where plants can absorb them again. Without them the nutrients would stay locked in dead material and the cycle would stop.

Three factors affect the rate of decay of biological material, and each acts on the microorganisms doing the decaying:

• Temperature — warmth speeds up the enzymes in the decomposers, up to an optimum; too hot and those enzymes denature, so decay slows again. • Water — decomposers need moisture to survive and to secrete their enzymes; dry material barely decays, which is why food is preserved by drying. • Availability of oxygen — most decomposers respire aerobically, so more oxygen means faster decay.

Gardeners and farmers try to provide optimum conditions for rapid decay of waste biological material. The compost produced is used as a natural fertiliser for growing garden plants or crops — which is why a compost heap is kept moist, turned to let air in, and works faster in summer.

Anaerobic decay produces methane gas. Biogas generators can be used to produce methane as a fuel — decay without oxygen, deliberately, so the gas can be collected and burned.

Handling decay data is examined alongside the biology. Calculate rate changes as 1 ÷ time, then translate the information between numerical and graphical form: temperature on the x-axis, rate on the y-axis, a scale for the axes that fills at least half the grid, and a smooth curve rather than joined dots. Reading back the other way, quote the rate at a stated temperature with its units, and describe the curve as rising to an optimum and then falling — the same enzyme shape as every other rate graph in the course.

Environmental changes affect the distribution of species in an ecosystem. The changes include temperature, availability of water, and the composition of atmospheric gases, and they may be seasonal, geographic or caused by human interaction.

Seasonal change moves species temporarily — birds migrate, and return. Geographic change means conditions differ from place to place, so a species is found in some areas and not others. Human interaction is the one that changes distribution permanently: a warming climate shifts the range a species can live in, and a species already at the edge of its range has nowhere left to move to.

Evaluating an impact means using the data given, saying which direction the distribution shifts and why, and noticing what else depends on that species.