Organisation of an Ecosystem¶

Part of Ecology.

An ecosystem is more than a list of organisms. It is a system in which matter is recycled, energy is transferred and environmental changes alter the balance between populations. Understanding these cycles explains why ecosystems can sustain themselves over millions of years.

What You Need to Learn¶

Follow feeding relationships and the recycling of carbon and water through an ecosystem. Learn how sampling reveals where species live and how abundant they are, and use results to calculate averages and interpret graphs. Explain decay, compost and biogas, then evaluate how changing conditions alter species distributions.

Levels of Organisation¶

A food chain or food web shows who eats whom. A food web is a more realistic picture than a single food chain because most organisms eat (and are eaten by) more than one species.

• Producers — photosynthetic organisms (plants, algae) that convert light energy into biomass.
• Primary consumers — herbivores that eat producers.
• Secondary consumers — carnivores that eat primary consumers.
• Tertiary consumers — carnivores that eat secondary consumers.
• Decomposers — bacteria and fungi that break down dead organisms and waste, returning minerals to the soil.

Because organisms are linked in food webs, removing one species can affect many others. For example, removing a keystone predator may allow a prey species to over-reproduce, consuming all available vegetation and collapsing the ecosystem.

Sampling Distribution and Abundance¶

A quadrat marks a known area for counting plants or slow-moving organisms. Select positions randomly to avoid favouring places with unusually many or few individuals, use a consistent counting rule, and sample enough quadrats to represent the habitat. Mean count per quadrat is total count divided by number of quadrats. If five equal quadrats contain 2, 4, 4, 5 and 10 plants, the mean is 5, the median is 4 and the mode is 4. Mean density multiplied by habitat area estimates total abundance, provided the sample is representative.

A transect is a line across an environmental change, such as a shore or a gradient of shade. Sample at regular intervals along it to see how species distribution changes with the conditions. Put distance or the measured environmental factor on the horizontal axis and abundance on the vertical axis, using labelled units and a scale that shows the pattern.

Predator–prey graphs often show repeated rises and falls, with the predator peak following the prey peak. More prey supports predator reproduction; increased predation reduces prey numbers, after which predator numbers fall through food shortage. Food-chain arrows point from the food to its consumer, showing the direction of transfer.

The Carbon Cycle¶

Carbon is constantly transferred between the atmosphere, living organisms, and the ground. The key processes are:

Carbon enters living organisms:

• Photosynthesis — plants absorb CO₂ from the atmosphere and incorporate carbon into glucose and other organic molecules.

Carbon passes between organisms:

• Feeding — animals eat plants, transferring organic carbon along food chains.

Carbon returns to the atmosphere:

• Respiration — all living organisms release CO₂ during aerobic respiration.
• Decomposition — decomposers (bacteria and fungi) break down dead organisms and waste, releasing CO₂ through their own respiration.
• Combustion — burning of wood, fossil fuels and organic matter releases stored carbon as CO₂.

Long-term carbon storage:

• When organisms die in conditions that prevent decomposition (e.g. compressed underwater sediment), their carbon is stored as fossil fuels (coal, oil, natural gas) over millions of years. Burning these fuels releases carbon that has been locked away for a very long time, adding extra CO₂ to the atmosphere.

The Water Cycle¶

Water is recycled continuously through the environment, supplying fresh water that land organisms need for chemical reactions and transport. The main stages are:

1. Evaporation — solar energy causes water to evaporate from oceans, lakes and rivers, forming water vapour.
2. Transpiration — plants release water vapour through their leaves (transpiration stream).
3. Condensation — water vapour cools as it rises and forms clouds (liquid water droplets).
4. Precipitation — water falls as rain, snow or hail onto land and into water bodies.
5. Surface runoff — water flows across land surface into streams and rivers, returning to the sea.
6. Infiltration — some water soaks into the ground and is stored in underground rocks (aquifers).

Decomposition¶

Decomposers (bacteria and fungi) are essential for recycling — they break down dead organisms and waste products, releasing mineral ions back into the soil where plants can reabsorb them.

Decomposers break down molecules by secreting enzymes outside their cells. Soluble products are then absorbed.

Factors affecting the rate of decomposition:¶

Working with Decay Data¶

Calculate mean rate as the change measured divided by elapsed time. If a sample loses 12 g over 4 days, its mean mass-loss rate is 3 g per day. Compare rates using the same units and method, and graph temperature against rate to show increases up to an optimum and declines when enzymes are damaged. Repeat measurements and control other variables before attributing a difference to temperature, moisture or oxygen.

Practical applications:¶

Compost heaps — garden waste decomposes aerobically to produce compost, a nutrient-rich material that improves soil structure and fertility. Turning a compost heap adds oxygen and speeds decomposition.

Biogas generators — organic waste (animal dung, crop waste) is decomposed anaerobically by bacteria. This produces biogas, a mixture mainly of methane (CH₄) and CO₂. Methane can be burned as a fuel for cooking, heating, or generating electricity. Biogas is a renewable energy source.

Food preservation — decomposition can be slowed by:

• Refrigeration/freezing — low temperature slows enzyme activity in decomposers.
• Drying — removes water, preventing microbial growth.
• Airtight packaging — limits entry of oxygen and microorganisms. It does not itself sterilise food; canning combines heating with sealing, and some microorganisms can grow without oxygen.

Impact of Environmental Change¶

Changes to abiotic conditions can shift where organisms live. These changes may be seasonal, differ between geographical locations or result from human activities:

• Temperature change — species distributions shift towards cooler regions or higher altitudes. Many British bird species now breed earlier in the year because spring arrives sooner. Tropical insects are extending their range northward.
• Water availability — drought reduces plant populations; flooding creates new wetland habitats. Migrating animals (e.g. wildebeest in the Serengeti) follow seasonal rainfall.
• Atmospheric composition — air pollution (e.g. sulphur dioxide) can kill sensitive species such as lichens, which are used as pollution indicators. Rising CO₂ affects photosynthesis rates and is linked to climate change.

Environmental change can have indirect effects through the food web: if one species declines, its predators may also decline while its prey may increase — cascading through the entire ecosystem.

Common Confusions¶

• Decomposers and detritivores: Decomposers (bacteria, fungi) break down organic matter by releasing enzymes. Detritivores (e.g. earthworms, woodlice) eat dead matter and physically break it into smaller pieces, speeding up decomposition — but they are not the same thing.
• The carbon cycle and combustion: Students sometimes omit combustion from the carbon cycle. It is a major pathway for carbon to return to the atmosphere, especially since the Industrial Revolution.
• Aerobic vs anaerobic decomposition: Most decomposers work best with oxygen (aerobic). Biogas generators deliberately use anaerobic conditions to produce methane. Waterlogged soils are anaerobic, which is why peat bogs preserve bodies for thousands of years.

Key Terms¶

• Ecosystem: a community of organisms interacting with the non-living components of its environment.
• Decomposer: a microorganism (bacterium or fungus) that breaks down dead organic matter and waste, releasing minerals.
• Decay: the breakdown of dead material by decomposers.
• Compost: decomposed organic material used to enrich soil.
• Biogas generator: a sealed vessel in which organic waste is broken down anaerobically by bacteria to produce methane-rich biogas.
• Transpiration: the loss of water vapour from plant leaves.
• Precipitation: water falling from the atmosphere as rain, snow, hail or sleet.
• Detritivore: an organism (e.g. earthworm, woodlouse) that feeds on dead organic material, physically breaking it into smaller pieces.

Connected Pages¶

• Adaptations, Interdependence and Competition
• Biodiversity and the Effect of Human Interaction on Ecosystems
• Biology Key Terms Glossary
• Required Practical Reference