Levels of organisation

Ecologists describe living things at different scales:

• Population — all the organisms of one species in an area.
• Community — all the populations of different species in an area.
• Ecosystem — the community plus the non-living (abiotic) parts of the environment, interacting together.

Within an ecosystem, factors that affect organisms are abiotic (non-living: light, temperature, water, pollutants, pH, mineral ions) or biotic (living: food, predators, disease, competition).

Interdependence: species depend on each other for food, shelter, pollination and seed dispersal. If one species is removed, others can be affected — the whole ecosystem is connected. Some species depend closely on one other species: a parasite (e.g. a tapeworm or flea) lives in or on a host and harms it; in mutualism both species benefit (e.g. nitrogen-fixing bacteria in the root nodules of legumes).

Population or community?

Competition & predator–prey cycles

Organisms compete for the resources they need. Animals compete for food, water, mates and territory; plants compete for light, water, space and mineral ions.

Where a predator eats a prey species, their numbers rise and fall in a linked predator–prey cycle:

Key pattern: as prey increase, predators have more food and increase too. More predators eat more prey, so prey fall, then predators fall — and the cycle repeats. The predator curve always lags behind the prey.

Reading the cycle

Sampling with quadrats

We can't count every organism, so we sample and scale up. A quadrat is a square frame (often 1 m²) placed on the ground to count organisms.

• Random sampling — place quadrats at random (e.g. using coordinates) to fairly estimate the mean number per quadrat, then scale up to the whole area. This avoids bias.
• Belt transects — a line (tape) across an area; you place quadrats at intervals along it to see how a species changes with distance (e.g. up a beach) — good for studying a gradient in abiotic factors.

Random vs transect: use random quadrats to estimate total numbers fairly; use a transect when you want to see how distribution changes across a gradient.

Your turn — estimate the population

Why do predator and prey numbers cycle?

Biodiversity

Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem. A high biodiversity makes an ecosystem more stable, because species depend less on any single other species.

Biodiversity matters because it keeps ecosystems healthy and provides us with food, medicines and clean air and water. Human activities are reducing it, so we try to maintain it by:

• Protecting and restoring habitats (nature reserves, reforestation) and breeding programmes for endangered species.
• Replanting hedgerows and creating field margins on farmland.
• Recycling resources rather than dumping waste in landfill.

Watch out: biodiversity is about the number of different species, not just the number of individuals. A field of one crop has low biodiversity even if it has millions of plants.

How can fertiliser kill the fish in a lake?

The water & carbon cycles

Materials are constantly recycled through ecosystems so life can keep reusing them.

Water cycle: the Sun's energy evaporates water; it condenses into clouds and falls as precipitation (rain/snow); water returns to the sea via rivers, and plants return water vapour to the air by transpiration. It provides fresh water for life on land. In areas of drought, potable (drinking) water can be made by desalination — removing the salt from sea water, e.g. by distillation or reverse osmosis.

Carbon cycle:

• Photosynthesis removes CO₂ from the air, locking carbon into plants.
• Respiration (by plants, animals and microbes) returns CO₂ to the air.
• Feeding passes carbon along food chains.
• Decomposition and combustion (burning fuels) return CO₂ to the atmosphere.

Balance: photosynthesis takes carbon out of the air; respiration, decomposition and combustion put it back. Burning fossil fuels adds extra CO₂ faster than it is removed.

Removing carbon dioxide

The nitrogen cycle

Plants need nitrogen to make proteins, but they can't use the nitrogen gas that makes up most of the air. Bacteria do the vital work:

• Nitrogen-fixing bacteria — turn nitrogen gas into ammonia/nitrogen compounds (some live in root nodules of legumes).
• Decomposers — break down dead organisms and waste, releasing ammonia.
• Nitrifying bacteria — convert ammonia into nitrates, which plants absorb through their roots.
• Denitrifying bacteria — convert nitrates back into nitrogen gas (this returns N₂ to the air; usually in waterlogged soils).

Farmers can add nitrates to the soil with fertilisers, or use crop rotation: growing a legume crop (e.g. peas, beans or clover) in some years, whose nitrogen-fixing bacteria add nitrogen compounds to the soil.

Nail the direction: nitrifying bacteria make nitrates (useful to plants); denitrifying bacteria remove them, returning nitrogen gas to the air. Nitrogen-fixing bacteria do the opposite of denitrifying.

Which bacteria do what?

Tap a job, then tap the bacteria that do it.

Decomposition

Decomposers (bacteria and fungi) break down dead organisms and waste, releasing mineral ions back into the soil so plants can reuse them. The rate of decomposition is faster when:

• Warmer (up to an optimum) — faster enzyme activity in the microbes.
• More oxygen — most decomposers respire aerobically.
• More water/moisture — microbes need water to grow.

Gardeners use these ideas to make compost quickly, and it explains why food is preserved by cooling, drying or removing oxygen.

Watch out: at very high temperatures the rate falls again because the decomposers' enzymes denature. The graph rises to an optimum, then drops.

Speeding up compost

Human impacts on ecosystems

A growing human population and rising standard of living put pressure on ecosystems and reduce biodiversity:

• Eutrophication — fertiliser (nitrates) washed into rivers/lakes makes algae bloom; when they die, decomposers use up the oxygen, so fish and other organisms suffocate.
• Pollution — of water, air and land from waste, chemicals and sewage kills organisms and reduces biodiversity.
• Deforestation — cutting down forests destroys habitats, reduces biodiversity, and increases CO₂ in the air (less photosynthesis, plus burning).
• Fish farming — reduces overfishing of wild stocks, but waste and disease from the farms can spread to wild fish. Non-indigenous species (e.g. grey squirrels in the UK) can out-compete native species and reduce biodiversity.

Conservation works the other way: protecting habitats, replanting forests and hedgerows, breeding endangered species and recycling all help maintain biodiversity.

How can fertiliser kill the fish in a lake?

Match each description to the term

Tap a description on the left, then its matching term on the right.

The big ideas to know

You've covered the core of Edexcel Topic 9 — ecosystems and material cycles. Press Finish to see your score.