IGCSE Biology 0610 · Chapter 14 notes · ~30 min read
Ecology & human influences
How energy flows through food chains and webs, how pyramids and the carbon and nitrogen cycles work, why populations level off, and how farming, deforestation and pollution threaten biodiversity and what conservation can do.
Food chains & websEnergy pyramidsCarbon & nitrogen cyclesPopulationsFood supplyPollutionConservation
ASby Aiza Ma'am
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No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end
In 30 seconds
- Energy comes from the Sun, is captured by producers and is passed along food chains. About 90% is lost at each step, so food chains are short. Carbon and nitrogen are recycled by photosynthesis, feeding, respiration and decay.
- A population grows in an S-shaped curve and levels off when food, competition, predation and disease limit it. Farmers raise yields with machinery, fertilisers, pesticides and selective breeding.
- Deforestation, pollution and habitat loss reduce biodiversity. Sustainable use and conservation protect forests, fish stocks and endangered species.
By the end you can
- Describe energy flow, food chains and food webs, and use the terms producer, consumer, herbivore, carnivore, decomposer and trophic level.
- Draw and interpret pyramids of numbers and biomass.
- Describe the carbon cycle.
- Define population, community and ecosystem, and describe the factors that limit population growth and the sigmoid growth curve.
- Describe how food production is increased, and the advantages and disadvantages of monocultures and intensive livestock production.
- Define biodiversity, and describe the causes of habitat destruction and the effects of deforestation.
- Describe the pollution caused by sewage, fertiliser, plastics, methane and carbon dioxide, and the greenhouse effect.
- Describe sustainable resources and the conservation of forests, fish stocks and endangered species.
- Interpret pyramids of energy and explain energy loss, why food chains are short, and why crops are more energy efficient than livestock. EXT
- Describe the nitrogen cycle and the roles of microorganisms in it. EXT
- Explain the phases of the sigmoid growth curve using limiting factors. EXT
- Describe the steps of eutrophication. EXT
- Explain the reasons for conservation, how AI and IVF help captive breeding, and the risks of a small population. EXT
1Energy flow in ecosystems
Food chains
Food chain — a diagram showing the transfer of energy from one organism to the next, beginning with a producer.
Producer — an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis.
Consumer — an organism that gets its energy by feeding on other organisms.
Herbivore — an animal that gets its energy by eating plants.
Carnivore — an animal that gets its energy by eating other animals.
Decomposer — an organism that gets its energy from dead or waste organic matter.
Trophic level — the position of an organism in a food chain, food web or pyramid.
- The Sun is the principal source of energy for almost all ecosystems. Producers capture it by photosynthesis (plant nutrition) and store it in chemical compounds.
- Energy is passed on when one organism eats another. At every step some energy is lost to the environment as heat (respiration).
- Trophic levels: producer (level 1), primary consumer (level 2, eats the producer), secondary consumer (level 3, eats the primary consumer), tertiary consumer (level 4), quaternary consumer (level 5).
Common mistake"The arrow points to the thing that gets eaten." No. The arrow shows the flow of energy, so it points towards the eater: grass → rabbit, never rabbit → grass.
Memory trickRead each arrow as "is eaten by": grass → rabbit means the grass is eaten by the rabbit.
Worked example
In the food chain grass → grasshopper → frog → snake → eagle, name the producer and each consumer.
- the chain starts with the producer: grass (trophic level 1)
- grasshopper eats grass = primary consumer (level 2, a herbivore)
- frog = secondary consumer (level 3)
- snake = tertiary consumer (level 4)
- eagle = quaternary consumer (level 5). The frog, snake and eagle are carnivores.
Food webs
Food web — a network of interconnected food chains showing the energy flow through part of an ecosystem.
- In the web, mouse → owl and mouse → snake → owl are two chains that share organisms. The owl is a secondary consumer in one chain and a tertiary consumer in the other.
- Webs show that changing one population affects many others, because the organisms are linked.
Worked example
Using the food web, predict two effects if a disease kills all the frogs.
- frogs eat grasshoppers, so grasshoppers are no longer eaten by frogs
- so the grasshopper population increases, and they eat more grass, so the amount of grass falls
- frogs are also food for snakes: snakes lose a food source, so they eat more mice and the mouse population falls (or the snake population falls)
Exam tipFor "what happens if…" questions, follow the arrows one step at a time: fewer predators → more prey → more of the prey's food eaten → less of that food. Always say whether numbers increase or decrease.
Pyramids of numbers and biomass
- A pyramid of numbers shows the number of organisms at each trophic level. A pyramid of biomass shows the mass of living material (dry mass) at each level.
- To draw one: the producers go at the bottom; make each bar's length match the numbers or mass on a scale; centre the bars; label every level.
- A pyramid of numbers is not always a pyramid. One large oak tree feeds hundreds of insects, so the bottom bar is narrow. A pyramid of biomass is fairer because it allows for the size of the organisms, so it is almost always pyramid-shaped.
- A pyramid of energy EXT is always pyramid-shaped (see below).
Common mistakeDrawing a pyramid of numbers with every bar the same width, or putting the top predator at the bottom. Bars must be drawn to scale and the producers go at the base.
Worked example
An oak tree (1) supports 3000 aphids, which are eaten by 50 ladybirds, which are eaten by 1 bird. Describe the pyramid of numbers and explain its shape.
- bars from the bottom: 1 tree, 3000 aphids, 50 ladybirds, 1 bird
- the aphid bar is the widest and the tree bar is a very narrow bar
- so it is not a normal pyramid shape
- one very large producer supports many small consumers
- a pyramid of biomass would be a normal pyramid, because the tree has a huge mass
Energy loss between trophic levelsEXTENDED
- EXT Only about 10% of the energy in one trophic level passes to the next. The rest is lost, as heat from respiration, in movement, in waste (faeces and urine) and in parts that are not eaten (bones, roots) or not digested.
- EXT Why food chains are short: after four or five steps there is too little energy left to support another level, so food chains rarely have more than five trophic levels.
- EXT Why crops are more efficient than livestock: people who eat crops take energy from the second trophic level. If the crop is fed to animals first, about 90% is lost before people eat the meat. The same field feeds more people when it grows crops.
% efficiency = energy passed to the next levelenergy in the previous level × 100
both energies in the same unitsWorked example
A producer level holds 20 000 kJ. If 10% is passed on at every step, how much energy reaches the secondary consumers?
- primary consumers: 20 000 × 10% = 2 000 kJ
- secondary consumers: 2 000 × 10% = 200 kJ
- so 99% of the original energy (19 800 kJ) has been lost on the way
Worked example
A cow eats grass containing 50 000 kJ and stores 5 000 kJ in its body. Find the percentage efficiency of the transfer.
- efficiency = energy passed on ÷ energy available × 100
- = 5 000 ÷ 50 000 × 100
- = 10%
Worked example
A field of wheat holds 100 000 kJ of food energy. Compare the energy people get if they eat the wheat with the energy if the wheat is fed to cattle and people eat the beef (10% efficiency).
- eating the wheat: up to 100 000 kJ is available to people
- via cattle: 100 000 × 10% = 10 000 kJ in the beef
- so eating the crop gives about 10 times more energy from the same field
The carbon cycle
- Plants take carbon dioxide from the air for photosynthesis and build carbon compounds (sugars, starch, proteins, fats). Animals get carbon compounds by feeding.
- All living things release CO₂ by respiration (respiration): plants, animals and decomposers.
- When organisms die (and when animals produce waste), decomposers (bacteria and fungi) break the material down and release CO₂ by respiration.
- Over millions of years buried dead organisms can form fossil fuels (coal, oil and natural gas). Burning (combustion) fossil fuels and wood returns CO₂ to the air.
| Process | What happens to carbon | Effect on CO₂ in the air |
|---|---|---|
| photosynthesis | CO₂ is built into sugars and other compounds | removes CO₂ |
| feeding | carbon compounds pass from one organism to the next | no change |
| respiration | carbon compounds are broken down | adds CO₂ |
| decomposition | dead remains and waste are broken down by decomposers | adds CO₂ |
| fossilisation | carbon is locked away in coal, oil and gas | removes carbon from the cycle |
| combustion | fuels and wood are burnt | adds CO₂ |
Memory trickPhotosynthesis Pulls CO₂ in. Respiration, Rot (decay) and Roasting (burning) Return it to the air.
Common mistake"Plants only take in CO₂, and animals only give it out." Plants also respire all the time, so they give out CO₂ too. In the light they take in more than they give out.
Exam tipCarbon moves between organisms in compounds (sugars, proteins, fats) but through the air as CO₂. Name the process on every arrow in a carbon-cycle answer.
The nitrogen cycleEXTENDED
- EXT About 78% of the air is nitrogen gas (N₂), but plants and animals cannot use it directly. Plants take up nitrates from the soil and use them to make amino acids and proteins. Animals get protein by eating plants (or other animals).
- EXT Nitrogen fixation: nitrogen-fixing bacteria turn N₂ into nitrogen compounds (ammonium or nitrates). Some live in root nodules of legumes (peas, beans, clover) and some in the soil. Lightning also fixes some nitrogen.
- EXT Decay: decomposers break down dead remains, urea and faeces, releasing ammonium ions.
- EXT Nitrification: nitrifying bacteria change ammonium ions into nitrates.
- EXT Denitrification: denitrifying bacteria change nitrates back into N₂. This happens in waterlogged soil with little oxygen, and it lowers soil fertility.
| Microorganism | Its role in the nitrogen cycle |
|---|---|
| decomposers (bacteria and fungi) | break down dead remains, urea and faeces into ammonium ions |
| nitrifying bacteria | change ammonium ions into nitrates |
| nitrogen-fixing bacteria | change nitrogen gas into nitrogen compounds |
| denitrifying bacteria | change nitrates into nitrogen gas |
Exam tipFarmers rotate crops and grow legumes such as clover or peas, because the bacteria in their root nodules add nitrogen compounds to the soil. This reduces the fertiliser needed for the next crop.
Memory trickFIX puts nitrogen IN (gas to compounds). NITRIFY makes nitrates. DENITRIFY takes nitrates out to the air.
2Populations and communities
Population, community and ecosystem
Population — a group of organisms of one species, living in the same area at the same time.
Community — all the populations of different species living in the same area (ecosystem).
Ecosystem — a unit containing the community of organisms and their environment, interacting together. Examples: a pond, a rainforest, a decomposing log.
Memory trickPopulation = ONE species. Community = MANY species. Ecosystem = community + the non-living environment.
Worked example
Which term describes (a) all the frogs in a pond, (b) all the living organisms in the pond, (c) the pond including its water, mud and sunlight?
- (a) frogs are one species in one place = a population
- (b) many species living together = a community
- (c) living organisms and their non-living environment = an ecosystem
What limits population growth
- A population grows when births are greater than deaths. It stops growing when limiting factors act.
| Limiting factor | How it limits the population |
|---|---|
| food | too little food: animals starve and fewer young are born |
| competition | organisms compete for limited food, space, light (plants) or mates, so some die or fail to breed |
| predation | predators kill and eat their prey, so prey numbers fall |
| disease | pathogens spread quickly in a crowded population and kill individuals |
The sigmoid growth curve
- Lag phase: slow growth, because there are few individuals to reproduce (and the cells are adjusting to the new conditions).
- Exponential (log) phase: rapid growth, as the number doubles at regular intervals.
- Stationary phase: the population stays level. Births equal deaths.
- Death phase: the population falls, because deaths are greater than births. This happens with microorganisms in a closed container, for example a fermenter.
- EXT Why? In the exponential phase there is plenty of food and space and few predators or diseases, so births far exceed deaths. In the stationary phase limiting factors (food, competition, predation and disease) balance births and deaths. In the death phase food runs out and waste products build up.
- EXT The sigmoid (S-shaped) curve is the lag, exponential and stationary parts together.
Worked example
A culture starts with 100 bacteria that divide every 20 minutes. How many bacteria are there after 2 hours of exponential growth?
- 2 hours = 120 minutes
- 120 ÷ 20 = 6 doublings
- 100 × 26 = 100 × 64 = 6 400 bacteria
- in a real culture growth would slow down before this, as food runs out
Exam tipDo not say "nothing happens" in the stationary phase. Births and deaths are still happening, but they are equal, so the total stays level.
3Human influences on ecosystems
Increasing food production
| Method | How it raises yield | A possible problem |
|---|---|---|
| machinery | ploughing, sowing and harvesting are faster over larger areas | uses fuel (CO₂), costs money, removing hedges destroys habitats |
| fertilisers | add nitrates and other minerals, so plants grow faster | run-off pollutes rivers (see eutrophication) |
| insecticides | kill insect pests that eat the crop | also kill useful insects such as pollinators and can build up in food chains |
| herbicides | kill weeds that compete for light, water and minerals | fewer wild plants, so less food for insects and birds |
| selective breeding | crops and animals with higher yield and disease resistance | less variation, so one disease may harm the whole crop |
- Selective breeding and genetic modification are two ways to improve the crop plants and animals themselves.
Monocultures
- Monoculture: growing one crop (for example wheat) over a very large area, year after year.
| Advantages | Disadvantages |
|---|---|
| higher yield from the best variety | pests and diseases spread quickly because every plant is the same |
| cheaper: machines can plant and harvest the whole field at once | the crop uses the same minerals every year, so more fertiliser is needed |
| all the crop ripens together | fewer habitats and food sources, so lower biodiversity |
Intensive livestock production
- Intensive livestock production: keeping large numbers of animals (hens, pigs, cattle) indoors in a small space, with controlled feeding.
| Advantages | Disadvantages |
|---|---|
| more meat, eggs and milk from a small area, so food is cheaper | animal welfare problems: crowded, stressed and unable to move naturally |
| animals lose less energy as heat and movement, so more food energy goes into growth | disease spreads quickly, so antibiotics are used and can lead to resistance |
| easier to control feeding and health | waste pollutes water and animals release methane; large amounts of feed are needed |
Biodiversity and habitat destruction
Biodiversity — the number of different species living in an area.
- Habitat destruction happens because of land use (houses, roads, farmland), extraction of resources (logging, mining, quarrying, drilling) and pollution.
- Destroying a habitat removes the species that live there, breaks food webs and lowers biodiversity. A species that loses its food or shelter can become endangered or extinct.
Deforestation
- Why forests are cleared: timber, farmland (crops and cattle), roads and buildings and mining.
- Loss of biodiversity and extinction: the animals and plants that lived in the forest lose their habitat and food.
- Soil erosion and flooding: roots no longer hold the soil. Rain washes it away, taking nutrients, so the land becomes less fertile and rivers silt up and flood.
- More carbon dioxide: there are fewer trees to remove CO₂ by photosynthesis, and burning or rotting trees releases CO₂ (plant nutrition).
- Less rainfall (an extra effect): fewer leaves means less transpiration, so less water vapour enters the air.
Pollution
| Pollutant | Source | Effect |
|---|---|---|
| untreated sewage | released into rivers and seas | feeds bacteria that use up oxygen, so fish die. It also spreads disease-causing microorganisms |
| excess fertiliser | run-off from fields | nitrates and phosphates cause eutrophication |
| plastics | litter and rubbish dumped in the sea | animals swallow them or get tangled; they do not rot and break into microplastics that enter food chains |
| methane | cattle, rice fields, rubbish tips | a greenhouse gas |
| carbon dioxide | burning fossil fuels and forests | a greenhouse gas |
The greenhouse effect and global warming
- The ground absorbs sunlight and gives out heat. Greenhouse gases (CO₂, methane, water vapour) absorb some of this heat and send it back, which warms the Earth. This natural greenhouse effect makes life possible.
- Human activity adds extra CO₂ and methane, so more heat is trapped: the enhanced greenhouse effect, which causes global warming and climate change.
- Possible effects: melting ice and rising sea levels (coastal flooding), changing rainfall, more extreme weather, and habitats lost, so some species become extinct.
Common mistake"Global warming is caused by the hole in the ozone layer." No. These are different problems. Global warming is caused by extra greenhouse gases.
EutrophicationEXTENDED
Eutrophication — EXT the enrichment of water by nutrients (nitrates and phosphates) from fertiliser or sewage, leading to loss of oxygen and death of animals.
- EXT The five steps: (1) fertiliser or sewage washes into the water, (2) algae grow rapidly and cover the surface (an algal bloom), (3) the algae block the light, so plants below die, (4) bacteria decompose the dead plants and algae and use up the oxygen in respiration, (5) fish and other animals die from lack of oxygen.
- EXT Reduce it: use less fertiliser, do not spread it before heavy rain, and treat sewage before it is released.
Common mistake"The fertiliser poisons the fish." No. The fish die from lack of oxygen. It is the bacteria (not the algae) that use up the oxygen.
Exam tipWrite eutrophication answers as a chain with "so" and "because": nutrients → algae grow → light blocked → plants die → bacteria respire → oxygen falls → fish die.
4Conservation
Sustainable resources
Sustainable resource — a resource that is produced as rapidly as it is removed from the environment, so that it is not likely to run out.
- Examples: timber from a forest where trees are replanted as fast as they are cut, and fish caught at a rate that lets the stocks recover.
- Fossil fuels are not sustainable: they take millions of years to form.
Conserving forests and fish stocks
| Resource | Methods of conservation |
|---|---|
| forests | replant trees; cut only some trees (selective logging); protect areas as national parks; educate people; make and enforce laws |
| fish stocks | fishing quotas (limits on the catch); larger net mesh so young fish escape; closed seasons while fish breed; no-fishing zones; monitor the stocks |
Endangered species
- Causes of endangerment and extinction: climate change, habitat loss, hunting, pollution and introduced species (which compete with, eat or infect the native species).
- Methods of conservation: monitor and protect species and their habitats (for example in nature reserves), educate people, captive breeding programmes (in zoos), and seed banks that store seeds of rare plants.
- EXT Why conserve? To protect vulnerable ecosystems and food webs (losing one species affects others), to keep biodiversity and its genetic resources (future medicines and crops), for tourism and income, and for ethical reasons: we should not cause extinction.
- EXT Artificial insemination (AI) places sperm from a male into a female, so animals in different zoos can breed without being moved. IVF fertilises eggs outside the body, and the embryos are placed in a female. Both help to increase numbers and add genetic variety.
- EXT Risks of a small population: less genetic variation, so it adapts poorly to change; inbreeding (two harmful recessive alleles are more likely to meet), which lowers fertility and survival; one disease or disaster could wipe out the whole population.
Exam tipConservation answers score best when they give a reason as well as the method: "Seed banks store seeds so that if the plant becomes extinct in the wild, it can be grown again."
5Test yourself
Test yourself — cover the answers first
Quick check
- Define a food chain.
- What is the main source of energy for most ecosystems?
- Define a producer.
- What is a decomposer?
- In which direction does an arrow in a food chain point?
- What is a trophic level?
- Which trophic level do herbivores occupy?
- Why can a pyramid of numbers be inverted?
- Name two processes that add CO₂ to the air.
- Which process removes CO₂ from the air?
- Define population.
- Define ecosystem.
- Name two factors that limit population growth.
- What happens in the stationary phase of a growth curve?
- Give one advantage and one disadvantage of a monoculture.
- Define biodiversity.
- Name two effects of deforestation.
- Name two greenhouse gases.
- What is a sustainable resource?
- Name two methods of conserving fish stocks.
- What is a seed bank for?
- EXT Roughly what percentage of energy passes to the next trophic level?
- EXT Why are crops more energy efficient than livestock?
- EXT What do denitrifying bacteria do?
- EXT Which bacteria change ammonium ions into nitrates?
- EXT Why do farmers grow clover or peas in a crop rotation?
- EXT What causes the death phase in a closed culture?
- EXT Which organisms use up the oxygen during eutrophication?
- EXT What is the difference between AI and IVF?
Exam-style
- Describe the roles of the Sun and of producers in a food chain. [3]
- Draw a food chain with four organisms and label a producer and a secondary consumer. [3]
- Describe how carbon in the air becomes part of an animal and returns to the air. [4]
- Describe the sigmoid growth curve of a population. [4]
- Describe the advantages and disadvantages of growing crops as a monoculture. [4]
- Describe the effects of deforestation. [4]
- Explain how increasing the amount of methane and carbon dioxide leads to global warming. [3]
- Describe how conservation of a fish stock can be achieved. [3]
- EXT Explain why food chains rarely have more than five trophic levels. [3]
- EXT Explain why more people can be fed from a field growing wheat than from the same field used to feed cattle. [3]
- EXT Describe the nitrogen cycle, including the roles of bacteria. [5]
- EXT Explain why a population stops growing and levels off. [4]
- EXT Explain how the use of fertiliser on a field can lead to the death of fish in a nearby river. [5]
- EXT Explain why it is important to conserve endangered species, and why small populations are at risk. [5]
Show answers
- a diagram showing the transfer of energy from one organism to the next, beginning with a producer
- the Sun
- an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis
- an organism that gets its energy from dead or waste organic matter
- towards the organism that eats (the direction of energy flow)
- the position of an organism in a food chain, food web or pyramid
- the second (primary consumers)
- one large producer (such as a tree) supports many small consumers
- respiration, decomposition, combustion
- photosynthesis
- a group of organisms of one species living in the same area at the same time
- a unit containing the community of organisms and their environment interacting together
- food, competition, predation, disease
- the population stays level because births equal deaths
- easy to plant and harvest or higher yield; pests and diseases spread quickly or lower biodiversity
- the number of different species living in an area
- loss of biodiversity, soil erosion, flooding, more carbon dioxide
- carbon dioxide and methane
- one produced as rapidly as it is removed, so it is not likely to run out
- quotas, larger net mesh, closed seasons, no-fishing zones
- storing seeds of rare plants so they can be grown again
- about 10%
- people eating crops take energy from the second trophic level, so less energy is lost than when the crop is fed to animals first
- change nitrates into nitrogen gas
- nitrifying bacteria
- bacteria in their root nodules fix nitrogen, which adds nitrogen compounds to the soil
- food runs out and waste builds up, so deaths exceed births
- bacteria decomposing the dead plants and algae
- AI places sperm into a female; IVF fertilises eggs outside the body
- the Sun is the source of energy
producers capture the energy by photosynthesis
the energy is stored in chemical compounds and passed to consumers when they are eaten - starts with a producer, for example grass → grasshopper → frog → snake
arrows point towards the organism that eats
grass labelled producer and frog labelled secondary consumer - plants take in CO₂ by photosynthesis
carbon is built into carbohydrates and other compounds
the animal eats the plant
CO₂ is returned by respiration (or decomposition after death) - slow growth at first (lag)
then rapid exponential growth
then levels off in the stationary phase
may fall in the death phase - advantages: easy to plant and harvest with machines, cheaper
high yield from the best variety
disadvantages: pests and diseases spread quickly
lowers biodiversity or uses up the same minerals, needing more fertiliser - loss of habitats, so extinction and lower biodiversity
soil erosion because roots no longer hold the soil
flooding
more CO₂ in the air because fewer trees photosynthesise - they are greenhouse gases
they absorb heat given out by the Earth and send it back (trap heat)
so more heat stays in the atmosphere and the Earth warms up - set quotas to limit the catch
use nets with larger mesh so young fish escape
close fishing in breeding seasons or create no-fishing zones
monitor the fish population - only about 10% of the energy passes to the next level
the rest is lost as heat, in waste and in parts not eaten
so there is too little energy left to support another level - people eating wheat take energy from the second trophic level
if cattle eat the wheat, about 90% of the energy is lost in the cattle
so less food energy reaches people as beef - nitrogen-fixing bacteria change nitrogen gas into nitrogen compounds
decomposers change dead remains and urea into ammonium ions
nitrifying bacteria change ammonium into nitrates
plants take up nitrates to make protein which animals eat
denitrifying bacteria change nitrates back into nitrogen gas - limiting factors act
food runs short or there is competition for food and space
predation and disease increase
so deaths equal births and the population stays level - fertiliser is washed into the river
nitrates cause algae to grow rapidly (algal bloom)
algae block the light so plants die
bacteria decompose the dead material and use up oxygen
fish die from lack of oxygen - keeps biodiversity and protects food webs
genetic resources for medicines and crops (or tourism, ethics)
small populations have less genetic variation
inbreeding increases harmful recessive alleles
a disease could wipe out the whole population
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