Ecosystems

The components of an ecosystem

An ecosystem is a community of living organisms together with the non-living parts of their environment, all working as one system. It can be as large as the Amazon rainforest or as small as a rock pool, a rotting log or the leaf litter under a single tree. What makes it an ecosystem is that energy flows through it and nutrients are recycled inside it, so the plants, animals, fungi and bacteria are linked to one another and to the water, soil and air around them.

The terms nest inside one another, and that is exactly what examiners test. A habitat is the place where an organism lives: the pond, the hedgerow, the dead wood. A species is a group of organisms that can breed together to produce fertile offspring. A population is all the members of one species in one area at one time, and a community is all the populations of different species living and interacting in the same area at the same time. A pond therefore holds one habitat, several populations such as frogs, water fleas and reeds, and one community.

The niche of an organism is its role in the ecosystem: what it eats, where and when it feeds, how it reproduces and what feeds on it. The habitat is the address; the niche is the job. Two species cannot survive for long with exactly the same niche in the same habitat, because one will out-compete the other for the same resources. A predator is an animal that hunts and kills others for food, its prey is the animal it eats, and an apex predator sits at the top of a food web with no natural predators of its own - a wolf, a lion or an orca.

Biotic and abiotic components

The biotic components of an ecosystem are the living parts, and each one is described by the way it feeds. A producer makes its own food by photosynthesis - plants on land, algae and phytoplankton in water. A primary consumer is a herbivore that eats producers, and secondary and tertiary consumers eat other animals; an animal that switches between plant and animal food can be a primary consumer in one food chain and a higher consumer in another. Decomposers, mainly bacteria and fungi, break down dead organisms, fallen leaves and animal waste, digesting the material outside their bodies and absorbing the nutrients, which are then released into the soil or water.

The abiotic components are the non-living physical and chemical conditions: temperature, water, oxygen, carbon dioxide, salinity, light and pH. Each of them limits where organisms can live. Light controls where photosynthesising plants can grow, so it sets the base of every food chain. In water, dissolved oxygen limits which fish and invertebrates survive, and warm water holds less oxygen than cold water, which is why a heatwave or a discharge of cooling water from a power station can kill fish. pH decides which plants can grow: acid moorland suits heather while chalk grassland suits lime-loving plants, and acid rain or acid mine drainage can shift a whole community.

Biotic and abiotic factors act together. Two plants may compete for light while both are limited by how much water the soil holds, and a change in one factor usually moves several populations at once rather than just one.

Photosynthesis and respiration: energy in, energy out

Almost all the energy in an ecosystem enters as sunlight captured by producers. In photosynthesis, plants, algae and some bacteria use energy from sunlight trapped by chlorophyll, the green pigment found in chloroplasts, to join carbon dioxide from the air to water taken up from the soil: carbon dioxide + water -> glucose + oxygen. The glucose is used for growth, stored as starch, converted into cellulose for cell walls or built into proteins with nitrates from the soil, and the oxygen is released as a by-product. Oxygen production by photosynthesis is what makes animal life on Earth possible.

Aerobic respiration is the reverse process: glucose + oxygen -> carbon dioxide + water. It happens in every living cell, in plants and in animals, day and night, and it releases the energy stored in glucose for movement, growth, active transport and warmth. Because much of that energy is released to the surroundings as heat, respiration is the main reason why only a small fraction of the energy in one trophic level ever reaches the next.

Keep the two equations apart and learn each one with its arrow. Chlorophyll is a pigment rather than an organ: it absorbs light energy, so a plant kept in the dark turns yellow and cannot photosynthesise even though it still respires. In an undisturbed ecosystem the two processes balance each other, but where fossil fuels are burned the carbon dioxide released is added to the air far faster than photosynthesis can remove it.

• Photosynthesis: carbon dioxide + water -> glucose + oxygen (needs light and chlorophyll).
• Aerobic respiration: glucose + oxygen -> carbon dioxide + water (needs no light and happens all the time).
• Both happen in plants; only respiration happens in animals.
• Chlorophyll captures the light energy that drives photosynthesis.

Food chains, food webs and trophic levels

A food chain shows the order in which energy passes from one organism to the next. The arrow means 'is eaten by', so the energy points towards the eater: marsh grass -> grasshopper -> frog -> heron. Each step is a trophic level: the producer is the first trophic level, then the primary consumer, the secondary consumer and the tertiary consumer.

A single chain is a simplification. A food web shows the many overlapping chains in a habitat joined together, and it models the ecosystem far better because most animals eat more than one kind of food and are eaten by more than one predator. In a food web the same animal can be a primary consumer in one chain and a secondary or tertiary consumer in another. That matters when something changes: a species with several food sources survives a shortage of one of them, and removing one species spreads its effects through the whole web instead of breaking a single line.

Energy does not travel along a food chain efficiently. Only about 10% of the energy at one trophic level is passed to the next, and the other 90% is lost during the organisms' life processes: movement, respiration, which releases energy to the surroundings as heat, digestion, because some food is not broken down and passes out as waste, and excretion of the waste products of metabolism. Energy is also lost in the parts of an organism that are never eaten at all, such as roots, bark, bones, hair and shells.

• Producer (first trophic level) -> primary consumer -> secondary consumer -> tertiary consumer.
• The arrow shows the direction in which energy is transferred, not just what eats what.
• Only about 10% of the energy at one level is passed to the next; about 90% is lost.
• Energy is lost in movement, respiration, digestion and excretion, and in uneaten parts of the body.

Ecological pyramids: numbers, biomass and energy

An ecological pyramid represents the trophic levels of a food chain as bars stacked one above the other, so you can see at a glance how much of something is present at each level. There are three kinds, and you must know what each one measures and what it cannot show.

A pyramid of numbers counts the individual organisms at each level. It is quick to collect, but it misleads easily, because one large producer can support a huge number of small consumers: a single oak tree supports thousands of insects, so the bottom bar is narrower than the bar above it and the pyramid is inverted. A pyramid of biomass measures the mass of living material at each level in a given area, usually as dry mass, so it takes account of size as well as number. It can also be inverted in a sea, because the phytoplankton are eaten as fast as they reproduce, so the mass of the consumers present at any one moment is greater than the mass of the microscopic producers.

A pyramid of energy shows the energy passing through each trophic level over a period of time, measured in kilojoules per square metre per year (kJ/m2/year). It is always a true pyramid, because energy is lost at every step and the amount reaching a level can never be greater than the level below it. If a question gives you bars that decrease steadily from bottom to top, the safest answer is the pyramid of energy.

Competition, predation, pollination and invasive species

Organisms in a community are linked by competition, predation and pollination. Competition happens whenever two organisms need the same resource and that resource is in short supply: light, water, space, food, a mate or a nesting site. Competition between members of the same species is intraspecific and competition between different species is interspecific. The individuals that are better adapted win the resource and reproduce, so competition is the pressure that drives natural selection.

In predation a predator hunts and kills its prey. The two populations are linked in a cycle that lags: when prey are plentiful the predators raise more young, predator numbers rise, heavy predation then reduces the prey population, and the fall in food is followed by a fall in predator numbers before the cycle begins again. Predators also keep the prey population healthy by removing young, weak and sick individuals, and an apex predator can control grazing all the way down the food web.

Native species evolved in an ecosystem and are held in check by their natural predators, competitors, parasites and diseases. An invasive (non-native) species has been carried to a new area by people and often spreads rapidly because the natural enemies that controlled it at home are absent, because it grows or breeds faster than the native species, or because it can exploit a habitat the native species cannot. It then out-competes native species for light, food or space, or simply eats them. Water hyacinth blocking the waterways of Lake Victoria, the grey squirrel replacing the red squirrel across much of the United Kingdom and the zebra mussel spreading through the Great Lakes are all well-known cases.

Pollination is the transfer of pollen from the anther of a flower to the stigma, and it is followed by fertilisation and then by seed and fruit formation. In insect pollination the pollen is carried by an insect pollinator such as a bee, a butterfly or a moth, and the flower is built to attract it with bright petals, scent, nectar and sticky or spiky pollen. In wind pollination the pollen is light and produced in enormous quantities, and the anthers and feathery stigmas hang outside the flower so that the wind can carry the pollen away and catch it; grasses, maize and oak are wind-pollinated and have no scent or nectar. Animal pollinators are a service that many crops depend on, so where pollinators are lost through pesticide use, disease or the loss of wildflower habitat, fruit and seed yields fall.

• Competition: two organisms need the same resource and there is not enough of it, so the better adapted individual survives and reproduces.
• Predation: predator and prey numbers rise and fall in a cycle, with the change in predator numbers lagging behind the change in prey numbers.
• Native species are controlled by natural enemies; invasive species usually spread because those enemies are missing in their new habitat.
• Insect-pollinated flowers: bright petals, scent, nectar and sticky pollen. Wind-pollinated flowers: dull, no scent or nectar, huge quantities of light pollen, and anthers and stigmas hanging outside the flower.

Nutrient cycles and what makes an ecosystem stable

Energy flows through an ecosystem and is eventually lost as heat, but nutrients are recycled. Decomposers - bacteria and fungi - break down dead organisms, fallen leaves and animal waste and release the carbon, nitrogen and minerals locked inside them, so the same atoms can be used again by plants. Without decomposers an ecosystem would run out of nutrients no matter how much sunlight it received, and dead material would pile up unbroken.

The carbon cycle must be known in full. Plants remove carbon dioxide from the air by photosynthesis and build it into glucose. Carbon then passes along food chains by feeding. Respiration by plants, animals and decomposers returns carbon dioxide to the air, and decomposition is simply respiration carried out by bacteria and fungi on dead material. Carbon leaves the cycle when dead organisms are buried in mud or sediment with little oxygen, where over millions of years the remains form fossil fuels; burning them - combustion - returns that carbon to the air in a moment instead of over millions of years, and this is what has raised the carbon dioxide concentration of the atmosphere. Forest fires also release carbon by combustion.

Nitrogen is recycled in the same way, although nitrogen gas in the air is unreactive and most organisms cannot use it directly. Nitrogen-fixing bacteria, many of them living in the root nodules of legumes such as peas, beans and clover, convert nitrogen gas into nitrogen compounds, and lightning fixes a small amount. Decomposers break down the proteins and urea in dead material and waste to ammonium compounds, which nitrifying bacteria convert into nitrates; plants absorb those nitrates to build their own proteins, and animals obtain nitrogen by eating plants. Denitrifying bacteria return nitrogen gas to the air and close the cycle. Because nitrogen is the nutrient that most often limits plant growth, it is the main ingredient of the fertilisers used in farming.

A stable ecosystem keeps its populations roughly steady and recovers after a disturbance such as a drought, a fire or a disease outbreak. Stability comes from high biodiversity, so that if one species is lost another can take over its role, from long and complex food webs that offer alternative food sources, from efficient decomposition and nutrient cycling, from a full set of trophic levels including predators, from enough incoming energy from the sun, and from abiotic conditions that stay inside the range the organisms can tolerate. Human activity breaks these links: pollution, habitat fragmentation, over-harvesting and introduced species remove the species and the recycling that hold the system together, which is why management concentrates on restoring them - removing an invasive species, reconnecting habitats with corridors, cutting pollution or reintroducing a missing predator. Every one of those strategies has limits, because removing a species is expensive and rarely complete, corridors take years to be used and predators may come into conflict with farmers.