Energy transfer, biodiversity and human impact
What you'll learn
- Why less energy is available at each feeding level in an ecosystem.
- How pyramids of biomass are shaped, and how to calculate transfer efficiency.
- How fish farming, introduced species and eutrophication affect biodiversity — the variety of living organisms.
- Why conserving biodiversity and improving food security — reliable access to food — matter.
Some of the energy-transfer calculations and food-security content are marked B in the Edexcel specification, so they are Separate GCSE Biology content; for 1BI0, you should know them.
Ecosystems: the starting point
Before we look at human impacts, you need the ecosystem vocabulary really secure.
Core ecosystem vocabulary
- An organism is one individual living thing.
- A species is a group of organisms that can reproduce to produce fertile offspring.
- A population is all the organisms of one species living in an area.
- A community is all the populations of different species living in an area.
- A habitat is the place where an organism lives.
- An ecosystem is a community of organisms interacting with each other and with the non-living environment.
- Biodiversity is the variety of living organisms in an area, or on Earth as a whole.
Energy transfer through food chains
A food chain shows how energy is transferred when one organism eats another. The arrows show the direction of energy transfer, not “what eats what”.
A producer is an organism, usually a green plant or alga, that makes its own food by photosynthesis. Photosynthesis is the process where plants and algae use light energy to make glucose. A consumer is an organism that gets energy by eating other organisms.
Each feeding position in a food chain is a trophic level. Producers are the first trophic level. Primary consumers eat producers. Secondary consumers eat primary consumers. Tertiary consumers eat secondary consumers.
Biomass is the mass of living material. A pyramid of biomass shows the total biomass at each trophic level, with producers at the bottom.
At each trophic level, organisms use some of their food in respiration, the chemical process that releases energy from glucose. Much of this energy is transferred to less useful forms, especially thermal energy transferred to the surroundings. Energy is also left in waste, dead material, and uneaten parts.
Only the energy stored in new biomass can be passed to the next trophic level. This is why there is usually less biomass higher up a food chain.
Why food chains are short
Energy transfer between trophic levels is inefficient, so higher trophic levels have less biomass available. This limits the number of organisms at each level and usually limits the length of a food chain.
Explaining a biomass pyramid
A food chain is: grass, then grasshoppers, then frogs, then herons. Explain why the heron level has the least biomass.
- Grass is the producer, so it stores energy in biomass made by photosynthesis.
- Grasshoppers only eat some of the grass, and they do not digest all the biomass they eat.
- Grasshoppers also use energy in respiration and lose material as waste, so only some biomass becomes grasshopper biomass.
- The same losses happen again from grasshoppers to frogs and from frogs to herons, so very little biomass is available at the top trophic level.
Biomass is not the same as number
A pyramid of biomass shows total mass, not the number of organisms. One large oak tree may have more biomass than thousands of insects feeding on it.
Calculating transfer efficiency
Efficiency is the percentage of energy that is usefully transferred from one trophic level to the next.
efficiency=energy transferred to next trophic levelenergy in previous trophic level×100\text{efficiency} = \frac{\text{energy transferred to next trophic level}}{\text{energy in previous trophic level}} \times 100efficiency=energy in previous trophic levelenergy transferred to next trophic level×100The same idea works for biomass:
percentage biomass transferred=biomass at next trophic levelbiomass at previous trophic level×100\text{percentage biomass transferred} = \frac{\text{biomass at next trophic level}}{\text{biomass at previous trophic level}} \times 100percentage biomass transferred=biomass at previous trophic levelbiomass at next trophic level×100Choosing the denominator
The denominator is the previous trophic level — the level the energy or biomass came from. If your answer is over 100%, you have probably put the numbers the wrong way round.
Calculating transfer efficiency and biomass percentage
A grass population contains 20 000 kilojoules of energy. Rabbits feeding on the grass gain 1 500 kilojoules. The rabbit biomass is 300 kilograms, and the fox biomass is 24 kilograms.
- For energy transfer from grass to rabbits, use the energy gained by rabbits over the energy in grass: 1 50020 000×100=7.5%\frac{1\,500}{20\,000} \times 100 = 7.5\%200001500×100=7.5%.
- For biomass transfer from rabbits to foxes, use fox biomass over rabbit biomass: 24300×100=8%\frac{24}{300} \times 100 = 8\%30024×100=8%.
- Both percentages are low, showing that most energy and biomass from the lower trophic level is not transferred into the next trophic level’s biomass.
Human interactions within ecosystems
A sustainable action is one that can continue long term without using resources faster than they are replaced or causing permanent damage to ecosystems.
Humans can interact with ecosystems positively, negatively, or both depending on how carefully the activity is managed.
Fish farming and non-indigenous species
Fish farming, also called aquaculture, means rearing fish in controlled conditions for food. A non-indigenous species is a species introduced to an area where it does not naturally live. If it spreads and causes harm, it is called an invasive species.
| Interaction | Possible benefit | Possible biodiversity risk |
|---|---|---|
| Fish farming | Provides food and may reduce pressure on wild fish stocks. | Crowding can spread disease and parasites; waste, uneaten food and chemicals can pollute water; escaped fish may compete with wild fish. |
| Introducing a non-indigenous species | May provide food, control pests, or be useful in farming. | It may outcompete native species, eat native organisms, bring disease, or have no natural predators. |
Evaluating fish farming
A coastal fish farm produces salmon, but some fish escape and waste enters nearby water. Explain one benefit and two risks.
- The benefit is that farmed salmon can provide food without catching as many wild salmon, which may help protect wild populations.
- Escaped farmed salmon may compete with wild fish for food or breeding sites, reducing native biodiversity.
- Waste and uneaten food add nutrients to the water, which can encourage algal growth and damage the ecosystem.
Eutrophication
Eutrophication is nutrient enrichment of water, often caused by fertilisers or sewage entering rivers, lakes or ponds. Nitrates and phosphates are mineral ions that plants and algae need for growth, but too much can trigger an algal bloom, a rapid growth of algae on the water surface.
A decomposer is an organism, such as a bacterium or fungus, that breaks down dead material. During eutrophication, decomposer bacteria increase and use up dissolved oxygen in aerobic respiration. Low oxygen levels can kill fish and other aquatic animals.
Explaining a fish die-off
After heavy rain, fertiliser runs into a lake and many fish die. Explain the sequence.
- Fertiliser adds extra nitrates and phosphates, so algae grow rapidly and form an algal bloom.
- The algal bloom blocks light, so underwater plants cannot photosynthesise enough and start to die.
- Decomposer bacteria break down the dead plants and algae, increasing in number.
- The bacteria use dissolved oxygen for aerobic respiration, so oxygen levels fall and fish suffocate.
Do not skip the chain
In eutrophication questions, do not jump straight from “fertiliser enters water” to “fish die”. The marks usually come from explaining the linked sequence.
Maintaining biodiversity
Maintaining biodiversity is important locally and globally. A more biodiverse ecosystem is usually more stable because if one species decreases, other species may still provide food or carry out similar roles.
Biodiversity also benefits humans. It supports pollination, soil formation, clean water, medicines, crop varieties, tourism and healthy food webs.
Conservation means protecting and managing organisms and habitats so species do not become extinct. Conservation of animal species may involve protected areas, breeding programmes, legal protection, reducing poaching, and restoring habitats.
Reforestation means planting trees or allowing woodland to regrow where forest has been removed. It can increase habitat area, provide food and shelter, reduce soil erosion, absorb carbon dioxide during photosynthesis, and connect separated habitats.
Not all tree planting is equal
Reforestation improves biodiversity most when it restores mixed native woodland. A single-species plantation may store carbon but support fewer species.
Food security
Food security
Food security means people having reliable access to enough safe, nutritious food for a healthy life.
Food security is affected by biological and environmental factors:
- Increasing human population increases demand for food, water and farming land.
- Increasing animal farming and meat or fish consumption uses more land, feed and water. Energy is lost between trophic levels, so feeding crops to animals and then eating the animals is less efficient than eating crops directly.
- New pests and pathogens can reduce yields. A pest damages crops or livestock; a pathogen causes disease.
- Environmental change caused by human activity, including climate change, can alter rainfall, increase droughts or floods, damage soil, and reduce crop growth.
- Sustainability issues include using land for biofuels, which are fuels made from recently living material, instead of food crops. Food production also depends on agricultural inputs such as fertilisers, pesticides, fuel, machinery, water and animal feed; if these become too expensive, yields or access to food may fall.
Linking human choices to food security
A country uses more farmland to grow crops for cattle feed and biofuel. A new crop pathogen also spreads. Explain how this could reduce food security.
- Land used for cattle feed or biofuel is not being used directly to grow food for people, so less food may be available.
- Feeding crops to cattle loses energy between trophic levels, so less human food biomass is produced from the same land area.
- The new pathogen reduces crop yield, which lowers supply and can increase food prices.
In the exam
- For energy-transfer questions, state where energy goes: respiration, thermal energy, waste, uneaten parts, and undigested material.
- For percentage calculations, use the previous trophic level as the denominator and multiply by 100.
- For human-impact questions, link the action to survival, reproduction, population size and biodiversity.
- For eutrophication, write the sequence in order instead of jumping straight to “fish die”.
Check yourself
- Why does a pyramid of biomass usually get narrower at higher trophic levels?
- How would you calculate the percentage of biomass transferred from grass to rabbits?
- How can reforestation and a non-indigenous species have opposite effects on biodiversity?
