CB9 · Ecosystems and material cyclesTopic 9 — Ecosystems and material cycles
Ecology, biodiversity and the recycling of materials.
Revise the key ideas
Organisms, populations, communities and ecosystems
An individual organism is one living thing. A population is all the organisms of one species in a particular area at a particular time.
A community consists of the populations of different species living in an area. An ecosystem includes that community and its non-living environment, including interactions between them.
A habitat is the place where an organism lives. Different species can share a habitat while using different resources.
Organisms need resources such as food, water, space and suitable conditions. Plants also compete for light and mineral ions; animals may compete for food, mates and territory.
Interdependence means organisms depend on others, for example for food, pollination or shelter. Changing one population can affect other species in the community.
An ecosystem includes both biotic and abiotic components. Do not use population, community and ecosystem as interchangeable terms.
Abiotic and biotic factors
Abiotic factors are non-living influences, including temperature, light intensity, water availability, soil conditions and pollutants.
Biotic factors involve living organisms and their interactions, including predation, competition, disease and the availability of food organisms.
Species are adapted to particular conditions. A substantial change in temperature, rainfall or other conditions can change survival, reproduction, abundance and distribution.
Competition occurs when organisms need the same limited resource. Competition can occur within a species or between different species.
In a simplified predator–prey cycle, prey numbers rise first, giving predators more food. Predator numbers may rise after a delay, increasing predation and reducing prey numbers; predator numbers may then fall.
Real populations are also affected by other foods, disease, migration and environmental changes. Predator–prey cycles and food-web responses are not guaranteed to follow one fixed pattern.
Feeding relationships, parasitism and mutualism
Food chains and webs show feeding relationships. An arrow goes from the organism eaten to the organism eating it, showing the direction of energy transfer in food.
Producers, such as green plants, make biomass using photosynthesis. Primary consumers eat producers; secondary consumers eat primary consumers, and tertiary consumers feed further along a chain.
A food web links several chains, so a consumer may have more than one food source. If one prey population falls, its predators may fall or switch to other prey, affecting those populations too.A simplified food web: a population change can affect several linked species.
A change can have indirect effects: fewer predators may allow a prey population to rise, increasing pressure on the prey's food. Explain the links and qualify predictions using other relevant factors.
A parasite benefits at its host's expense, often living on or inside it. For example, a flea feeds on a mammal's blood; harm to the host does not mean that it must die immediately.
Mutualism benefits both partners. A pollinating bee gains nectar or pollen while helping a flowering plant reproduce.
Legumes and bacteria in their root nodules can also be mutualistic: bacteria provide fixed nitrogen, while the plant supplies carbohydrates and a suitable habitat.
Core practical: random quadrat sampling
Abundance describes how common a species is, measured using counts, density, frequency or percentage cover as appropriate. It is not always simply a total population count.
A quadrat is a frame enclosing a known area. It is useful for sampling plants and animals that remain in place, rather than animals that quickly move into or out of the frame.
For a representative area estimate, mark a study area and use random coordinates to choose quadrat positions. Choosing only convenient or visibly crowded patches introduces bias.
Identify the species consistently, count individuals and use a consistent rule for plants touching the frame. Use percentage cover instead when distinct individuals cannot sensibly be counted.
Repeat with enough non-overlapping samples distributed across the study area. Record quadrat size, locations and counts so the method can be evaluated.
Density = total count in sampled quadrats ÷ total area sampled. Estimated population = density × total suitable habitat area.
For ten 0.25 m² quadrats containing 40 plants in total, sampled area = 2.5 m² and density = 16 plants per m². Across a representative 100 m² area, the estimated population is 1,600.Use the total sampled area, not just the area of one quadrat.
Alternatively, multiply the mean count per quadrat by the number of quadrat-sized areas in the habitat. Convert all areas to the same units before calculating.
An estimate assumes samples represent the area. Patchiness, identification errors, too few quadrats or sampling only part of the habitat can reduce reliability; the estimate is not an exact census.
Core practical: belt transects and environmental gradients
A belt transect samples a strip across an environmental gradient, such as from shaded woodland to open grassland, using quadrats along a measured line.
A continuous belt uses adjacent quadrats; an interrupted belt uses quadrats at specified intervals. Record distance along the transect and keep quadrat area and counting rules consistent.Record distribution across a gradient; this is different from random area sampling.
Measure the relevant abiotic factor at sampling positions, for example light intensity or soil moisture, as well as species abundance or cover.
Repeat transects where possible and compare patterns. Keep the measurement procedure consistent and consider weather, time of day and identification errors.
A transect uses regular sampling positions (systematic sampling) to investigate how species distribution changes along an environmental gradient. It is not random sampling. Only use it to estimate the whole habitat if the sampled area represents that habitat.
Plot abundance or cover against distance or a measured abiotic factor. A correlation can support an explanation but does not prove that one factor alone caused the pattern.
Frequency (%) = quadrats containing the species ÷ total quadrats × 100. A species in 6 of 10 quadrats has 60% frequency; that does not mean it covers 60% of the ground.
Work safely in the study area, avoid unnecessary disturbance and leave organisms and habitats intact. Record the method and environmental conditions so comparisons are meaningful.
Biodiversity and conservation
Biodiversity is the variety of living organisms in an area. More individuals of one species do not automatically mean greater biodiversity.
Maintaining biodiversity protects species and interdependent relationships. Diverse ecosystems may be more resilient to disturbance, although outcomes depend on the particular species and event.
Species can provide food, medicines and other useful materials, while ecosystems provide services such as pollination. Conservation also values species beyond their direct uses to people.
Conservation can protect and restore habitats, reduce pollution, regulate harvesting and protect threatened species locally and globally.
Reforestation restores tree cover and can provide habitat, store carbon and support biodiversity. Using suitable native species and varied habitats can be more beneficial than a single-species plantation.
Captive breeding can increase numbers of threatened species for possible reintroduction. It needs genetic diversity, suitable habitat and management of the original threats to support long-term success.
Controlling an invasive population, sometimes including culling, may protect native species. Evaluate effectiveness, animal welfare, unintended effects and alternatives rather than assuming that killing competitors is always beneficial.
Fish farming and non-indigenous species
Overfishing can reduce wild fish populations and disrupt food webs. Fish farming can provide food and may reduce some harvesting pressure on wild stocks, depending on the farming system and feed sources.
High stocking density can increase stress and disease transmission. Waste and uneaten feed can add nutrients to nearby water; these are possible impacts rather than proof that every farm is unhealthy.
Escaped farmed fish may compete or breed with wild populations, and parasites or diseases can spread. Better siting, waste management and biosecurity can reduce risks.
Medicines used in some systems can affect nearby organisms or contribute to antimicrobial-resistance concerns. Evaluate evidence and management practices rather than assuming every farm uses antibiotics routinely.
A non-indigenous species has been introduced outside its native range. Some become invasive and harm native populations through competition, predation or disease; not every introduced species becomes invasive.
An introduced predator or competitor can change several populations indirectly through a food web. Predict effects using the organism's interactions and the environmental context.
Eutrophication: nutrients and oxygen loss
Eutrophication can occur when excess nutrients, such as nitrates or phosphates from fertiliser runoff or sewage, enter water and stimulate rapid algal growth.
A dense algal bloom reduces light reaching submerged plants, reducing photosynthesis and potentially causing plant death. Algae also eventually die and provide material for decomposition.
Microorganisms decompose dead material and respire aerobically, using dissolved oxygen. Increased decomposition can greatly reduce oxygen concentration.
Fish and other organisms requiring oxygen may die or leave when oxygen is too low. The main immediate problem is oxygen depletion, rather than carbon dioxide alone.Microbial respiration links decomposition to dissolved-oxygen loss.
Some organisms can survive very low oxygen levels, so eutrophication changes the community rather than necessarily killing everything in the water.
Reducing nutrient inputs through appropriate fertiliser use and sewage treatment helps prevention. Explain the sequence from nutrient addition to oxygen loss, rather than saying fertiliser directly poisons all fish.
The carbon cycle
Carbon is present in carbohydrates, lipids, proteins and other components of biomass. Materials cycle between organisms and the non-living environment; energy flows through ecosystems and is not recycled in the same way.
Photosynthesis takes carbon dioxide from air or water. Producers use its carbon atoms to make organic molecules such as glucose.
Feeding transfers carbon-containing material from producers to consumers and between consumers. Carbon atoms are rearranged rather than created anew at each transfer.
Plants, animals and microorganisms respire, returning carbon dioxide to the environment. Plants respire as well as photosynthesise.
Death, fallen leaves and waste supply organic material to decomposers, including bacteria and fungi. Decomposition makes materials available again, while decomposer respiration releases carbon dioxide.
Some organic carbon can be stored for long periods, for example in fossil fuels formed under particular conditions over geological time. This does not happen to all dead material.Main transfer pathways; dead material does not all become fossil fuel.
Combustion of fossil fuels or biomass releases carbon dioxide. Deforestation can reduce carbon uptake and storage; reforestation can increase them.
The water cycle
Water is needed for cell reactions and is a major component of cytoplasm and blood plasma. It moves between oceans, freshwater, the atmosphere, land and living organisms.
Energy from the Sun drives evaporation from water surfaces. Transpiration from plants also transfers water vapour into the atmosphere.
Water vapour cools and condenses into droplets or ice crystals that form clouds. Condensation changes gas to liquid; it is not the same process as rainfall.
Precipitation returns water to the surface as rain, snow or other forms. Water may collect in lakes and rivers or return to the ocean.
Some water soaks into soil and rock (infiltration) and moves underground as groundwater. Some flows over the land as surface runoff. Rivers and groundwater can return water to the sea.Water moves between stores through several processes; cloud formation and rainfall are different steps.
Organisms take up water, and return it through processes including transpiration, respiration and excretion. Water is continuously redistributed rather than permanently used up by one organism.
Nitrogen and bacterial processes
Nitrogen is needed for amino acids, proteins and DNA. Although nitrogen gas is abundant in air, most plants cannot use it directly and take up nitrogen compounds such as nitrate ions through their roots.
Nitrogen-fixing bacteria convert nitrogen gas into ammonia or ammonium compounds. Fixation does not directly turn nitrogen gas into nitrate in a single step.
Some nitrogen-fixing bacteria live freely in soil; others live in legume root nodules. The bacteria and the legume can both benefit from their association.
Decomposers break down dead organisms and waste, releasing ammonium compounds into soil. This returns nitrogen from organic material to a form that can undergo further processing.
Nitrifying bacteria convert ammonium compounds to nitrites and then nitrates under oxygenated conditions. Nitrates can be absorbed by plant roots and used to build organic molecules.
Animals obtain nitrogen by eating plants or other animals, not by absorbing atmospheric nitrogen directly. Feeding transfers nitrogen-containing compounds through the community.
Denitrifying bacteria can convert nitrates to nitrogen gas, returning nitrogen to the atmosphere, especially in oxygen-poor soil. Distinguish this from fixation and nitrification.Fixation, nitrification and denitrification are distinct processes; decomposition returns organic nitrogen to ammonium compounds.
Fertilisers and crop rotation
Fertilisers supply mineral nutrients, including nitrogen compounds that can increase plant growth when these nutrients are limiting. They do not supply the carbon in glucose produced by photosynthesis.
Crop rotation changes the crop grown on a field in successive seasons. Including legumes can introduce biologically fixed nitrogen into the system.
Nitrogen in legume residues can become available to later crops after decomposition and bacterial processing. Nitrogen is not instantly transferred from atmospheric gas to every neighbouring root.
Returning suitable organic matter to soil can provide nutrients as decomposers act. Excess nutrient applications can still cause runoff and eutrophication.
Evaluate farming practices using crop yield, soil conditions, fertiliser demand and environmental impacts. More fertiliser does not always produce more growth if another factor is limiting.
Potable water and desalination
Potable water is safe to drink and need not be chemically pure water. It must contain acceptably low levels of harmful dissolved substances and microorganisms.
Freshwater treatment commonly removes solids by settling and filtration, then uses disinfection, such as chlorine, to reduce harmful microorganisms. Ordinary filtration does not remove all dissolved salts.
Where freshwater is scarce, desalination removes dissolved salts from seawater. Distillation evaporates water and condenses the vapour, leaving most dissolved salts behind.
Reverse osmosis is another desalination method: pressure pushes water through a suitable membrane while rejecting much of the dissolved salt.
Desalination provides an additional water supply but requires energy and suitable management of concentrated brine. Compare these costs and impacts with local water availability.
Different treatment steps have different purposes: filtration removes particles, disinfection reduces microorganisms and desalination removes salts. One step does not automatically replace all the others.
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