Ecosystems
What you'll learn
- How organisms are organised from individuals up to whole ecosystems.
- How abiotic and biotic factors affect communities.
- Why microorganisms are vital for decomposition and material cycles.
- How biomass moves through food chains, pyramids of biomass, and transfer-efficiency calculations.
The big picture: living things are part of systems
An ecosystem is not just “a place with animals and plants”. It is a network of living organisms, non-living conditions, feeding relationships, competition, and recycling of materials.
Ecosystem
An ecosystem is a community of organisms interacting with each other and with the non-living parts of their environment.
A woodland, pond, rocky shore, compost heap, and school field can all be ecosystems. Some are large, some are tiny, but the same ideas apply.
Levels of organisation in an ecosystem
Ecology builds up in levels:
- An individual organism is one living thing, such as one rabbit.
- A population is all the organisms of one species in a habitat, such as all the rabbits in a field.
- A community is all the populations of different species living together, such as grass, rabbits, foxes, worms, bacteria, and fungi.
- An ecosystem is the community plus the non-living environment, such as soil, air, water, temperature, and light.
Habitat
A habitat is the place where an organism lives, such as a pond, hedge, soil, tree, or grassland.
A stable ecosystem is one where populations may go up and down, but the overall community stays balanced over time because resources and materials are continually recycled.
Abiotic and biotic factors
Factors that affect organisms can be non-living or living.
Abiotic and biotic factors
An abiotic factor is a non-living environmental factor, such as temperature, light intensity, moisture level, or soil pH. A biotic factor is a living factor, such as predators, food availability, parasites, or competitors.
Abiotic factors can change which species survive in an area. For example:
- Higher light intensity can increase photosynthesis in plants.
- Low moisture level can reduce plant growth.
- Extreme temperature can reduce enzyme activity or kill organisms.
- Soil pH affects which plants grow well and how easily roots absorb mineral ions.
Biotic factors also affect communities:
- More predators may reduce prey numbers.
- Less food may increase competition.
- More parasites may reduce survival or reproduction.
Predicting the effect of a predator being removed
In a simple food chain: grass → rabbit → fox, imagine the foxes are removed.
- Foxes eat rabbits, so removing foxes reduces predation on rabbits.
- More rabbits survive and reproduce, so the rabbit population is likely to increase at first.
- Rabbits eat grass, so increased rabbit numbers cause more grazing and the grass population may decrease.
- If grass becomes scarce, rabbit numbers may later fall because food becomes a limiting factor.
Only thinking about the direct effect
If a predator is removed, do not stop at “prey increases”. Follow the knock-on effects through the food chain or food web.
Interdependence and competition
Organisms in a community are interdependent, meaning they rely on each other. This can involve feeding, pollination, shelter, decomposition, or nutrient recycling.
Competition
Competition happens when organisms need the same limited resource, such as food, light, water, space, mates, or mineral ions.
Plants often compete for light, water, space, and mineral ions. Animals may compete for food, territory, mates, and shelter.
Types of interdependence
Three important relationships are:
- Predation: one organism, the predator, kills and eats another organism, the prey.
- Mutualism: both organisms benefit, such as bees getting nectar while pollinating flowers.
- Parasitism: one organism benefits while the host is harmed, such as a tapeworm living in an animal’s gut.
An adaptation is a feature that helps an organism survive and reproduce in its environment. Different adaptations allow different species to survive in the same ecosystem, often by using resources in slightly different ways.
Recycling materials in ecosystems
Materials move between the living and non-living parts of an ecosystem.
Biotic and abiotic components
The biotic components are the living parts of an ecosystem. The abiotic components are the non-living parts, such as air, water, soil, and mineral ions.
Many materials cycle through ecosystems, including carbon, nitrogen, and water. This is different from energy: materials are recycled, but energy is not recycled. Energy enters through sunlight and is eventually transferred to the surroundings, often as heat.
The carbon cycle
Carbon is needed to make important biological molecules, including carbohydrates, fats, proteins, and DNA.
The main stages are:
- Photosynthesis removes carbon dioxide from the atmosphere and turns carbon into glucose in plants and algae.
- Feeding transfers carbon compounds through food chains.
- Respiration releases carbon dioxide back into the atmosphere from plants, animals, and microorganisms.
- Death and waste pass carbon compounds to decomposers.
- Decomposition breaks down dead material and waste.
- Combustion of fuels or biomass releases carbon dioxide.
Carbon is recycled
Carbon atoms move between carbon dioxide in the air, biomass in living organisms, dead organic matter, and decomposers.
The water cycle
Water is essential because cells need it for chemical reactions, transport, and maintaining tissues. Ecosystems also need water to maintain habitats such as ponds, rivers, wetlands, and moist soils.
The main stages are:
- Evaporation: liquid water becomes water vapour.
- Transpiration: water vapour is lost from plant leaves.
- Condensation: water vapour cools and forms clouds.
- Precipitation: water falls as rain, snow, sleet, or hail.
- Collection and runoff: water returns to rivers, lakes, oceans, and groundwater.
- Uptake by roots: plants absorb water from soil.
Water also helps the flow of nutrients because many mineral ions dissolve in water and move through soil, rivers, and organisms.
Microorganisms and decomposition
Decomposers
Decomposers are organisms, mainly bacteria and fungi, that break down dead organisms and waste materials.
Decomposers secrete enzymes onto dead material. The enzymes digest large biological molecules into smaller soluble molecules, which decomposers absorb. As decomposers respire, they release carbon dioxide. They also return mineral ions to the soil, where plants can absorb them again.
This is why microorganisms are vital: without them, dead material and waste would build up, and useful minerals would not be returned to the ecosystem.
Conditions affecting decomposition
This decomposition-rate detail is part of separate Biology J247, so it is especially relevant for you.
Aerobic and anaerobic
Aerobic means “with oxygen”. Anaerobic means “without oxygen”.
Decomposition is usually fastest when conditions suit decomposers:
- Warm temperature increases enzyme activity and microorganism growth, up to an optimum.
- Moist conditions allow microorganisms to survive and dissolved substances to diffuse.
- Oxygen availability allows aerobic respiration, which releases more energy for decomposers.
If conditions are too cold, too dry, too acidic, or lacking oxygen, decomposition slows down. If it is too hot, enzymes in decomposers may denature.
Compost heap conditions
A good compost heap is warm, moist, and aerated. Turning the compost adds oxygen, helping aerobic decomposers work faster.
Calculating decomposition rate
A sample of dead leaves has a mass of 100 g on day 0, 70 g on day 5, and 58 g on day 10.
- For days 0 to 5, the mass lost is 100−70=30 g100 - 70 = 30\ \text{g}100−70=30 g, so the rate is 305=6 g per day\frac{30}{5} = 6\ \text{g per day}530=6 g per day.
- For days 5 to 10, the mass lost is 70−58=12 g70 - 58 = 12\ \text{g}70−58=12 g, so the rate is 125=2.4 g per day\frac{12}{5} = 2.4\ \text{g per day}512=2.4 g per day.
- Over the full 10 days, the percentage mass lost is 42100×100=42%\frac{42}{100} \times 100 = 42\%10042×100=42%.
- The rate has decreased from 6 g per day to 2.4 g per day, so decomposition was faster in the first five days.
Food chains and trophic levels
A food chain shows feeding relationships and biomass transfer.
Trophic level
A trophic level is the position of an organism in a food chain, food web, or pyramid of biomass.
The first trophic level is always a producer. Producers make their own food, usually by photosynthesis. Plants and algae are producers.
A consumer gets biomass by eating other organisms:
- A primary consumer eats producers.
- A secondary consumer eats primary consumers.
- A tertiary consumer eats secondary consumers.
Food chain arrows
The arrows show the direction of biomass transfer, not the direction of “who is eating”. Grass → rabbit means biomass moves from grass into the rabbit when the rabbit eats the grass.
Pyramids of biomass
Biomass
Biomass is the mass of living material. In pyramids of biomass, it usually means the dry mass of organisms at each trophic level.
A pyramid of biomass shows how much biomass is present at each trophic level. The producer level is usually the widest, and the pyramid usually gets narrower higher up.
Biomass is lost between trophic levels because:
- Not all parts of an organism are eaten.
- Some eaten material is not digested and is lost as faeces; this is egestion.
- Some biomass is lost in waste products such as urea; this is excretion.
- Respiration uses glucose to release energy, producing carbon dioxide and water that leave the organism.
Biomass decreases up a food chain
Only some biomass is transferred to the next trophic level, so there is less biomass available for organisms higher up the food chain.
This also explains why food chains usually have only a few trophic levels. After several transfers, there is too little biomass and energy left to support another level. It is also why eating producers directly, such as wheat, is generally more efficient than eating animals that were fed on producers.
Calculating biomass transfer efficiency
The efficiency of biomass transfer tells you what percentage of biomass passes from one trophic level to the next.
efficiency of biomass transfer=biomass transferred to next levelbiomass at previous trophic level×100\text{efficiency of biomass transfer} = \frac{\text{biomass transferred to next level}}{\text{biomass at previous trophic level}} \times 100efficiency of biomass transfer=biomass at previous trophic levelbiomass transferred to next level×100Calculating biomass transfer efficiency
A field contains 1200 kg of grass biomass. The rabbits that feed on the grass have 120 kg of biomass.
- Choose the biomass that moved to the next trophic level: rabbits have 120 kg.
- Choose the biomass available at the previous trophic level: grass has 1200 kg.
- Substitute into the formula: 1201200×100=10%\frac{120}{1200} \times 100 = 10\%1200120×100=10%.
- The transfer efficiency from grass to rabbits is 10%, so 90% of the grass biomass was not transferred into rabbit biomass.
Use the correct direction
Always divide by the biomass in the trophic level below. If you divide the wrong way round, you can get an impossible efficiency above 100%.
In the exam
- For food webs, follow the arrows as biomass transfer, then trace knock-on effects one step at a time.
- For cycles, name the process: photosynthesis removes carbon dioxide; respiration, decomposition, and combustion return it.
- For calculations, write the formula, substitute the numbers, and include units or a percentage where needed.
Check yourself
- Why does a warm, moist, oxygen-rich compost heap decompose quickly?
- In the chain grass → rabbit → fox, what might happen if fox numbers fall?
- Why do pyramids of biomass usually get smaller at higher trophic levels?
