What you'll learn

• How ecosystems are organised, from one organism to a whole ecosystem.
• How materials such as carbon, nitrogen and water cycle through living and non-living parts.
• Why microorganisms are vital decomposers.
• How abiotic factors, biotic factors, interdependence and competition affect communities.

Ecosystems: living and non-living parts working together

A species is a group of organisms that can reproduce together to produce fertile offspring. One member of a species is an individual organism.

A habitat is the place where an organism lives, such as a pond, woodland, field or rock pool.

The living parts are called biotic components. Examples include plants, animals, fungi, bacteria and protists. The non-living parts are called abiotic components. Examples include light intensity, temperature, water, soil pH and mineral ions.

Ecosystems are organised in levels:

• Individual organism: one living thing, such as one frog.
• Population: all the organisms of one species in an area, such as all the frogs in one pond.
• Community: all the different populations living together, such as frogs, fish, pondweed and microorganisms.
• Ecosystem: the community plus the abiotic conditions, such as water, light and temperature.

Materials cycle through ecosystems

Materials do not get “used up” forever. Atoms are recycled between the biotic living parts and the abiotic non-living parts of ecosystems.

Examples of cycled materials include:

• Carbon, found in carbon dioxide, carbohydrates, fats and proteins.
• Nitrogen, needed to make proteins and DNA.
• Water, needed for chemical reactions, transport and habitats.

Microorganisms and decomposition

A microorganism is a microscopic living organism, such as a bacterium, fungus or protist. Many microorganisms act as decomposers.

During decomposition, microorganisms release enzymes onto dead material and waste. These enzymes break large biological molecules into smaller soluble substances. The microorganisms absorb some of these substances for growth and respiration.

Decomposition is important because it:

• Releases mineral ions back into the soil for plants to absorb.
• Returns carbon to the atmosphere as carbon dioxide through respiration.
• Prevents dead material and waste from building up.

Decomposition is usually faster in warm, moist, oxygen-rich conditions because microorganisms respire and grow more quickly.

The carbon cycle and water cycle

The carbon cycle moves carbon between organisms and the environment.

Main stages of the carbon cycle:

• Plants take in carbon dioxide from the air for photosynthesis.
• Carbon becomes part of plant biomass, such as glucose, starch, cellulose and proteins.
• Animals obtain carbon by feeding on plants or other animals.
• Plants, animals and microorganisms release carbon dioxide by respiration.
• Dead organisms and waste are broken down by decomposers.
• Combustion of fuels releases carbon dioxide into the atmosphere.

The water cycle moves water between land, living organisms, oceans and the atmosphere.

Main stages of the water cycle:

• Evaporation: liquid water changes into water vapour.
• Transpiration: water vapour leaves plants, mainly through leaves.
• Condensation: water vapour cools and forms clouds.
• Precipitation: water falls as rain, snow, sleet or hail.
• Runoff: water flows over land into rivers, lakes and seas.
• Infiltration: water soaks into soil and groundwater.
• Uptake by roots: plants absorb water from the soil.

These cycles are vital because carbon is needed to build biomass, and water helps maintain habitats, transport substances and move nutrients through ecosystems.

Feeding relationships and biomass flow

A producer is an organism that makes its own food, usually by photosynthesis. Green plants and algae are producers.

A consumer is an organism that feeds on other organisms. Animals are consumers.

A food chain shows feeding relationships in a simple line. A food web shows many connected food chains in a community.

Biomass is the mass of living material. In food chains and food webs, arrows show the direction that biomass is transferred.

A stable community has population sizes that stay fairly constant over time. Food webs help stability because organisms may have alternative food sources if one population changes.

Calculating percentage mass

You may be asked to calculate the percentage of a total mass. Use:

$$percentage\; of\; total\; mass=\frac{mass\; of\; part}{total\; mass}\times 100$$

Abiotic and biotic factors

A factor is something that can affect organisms in an ecosystem.

Abiotic factors are non-living conditions, including:

• Temperature.
• Light intensity.
• Moisture level.
• Soil pH.
• Water availability.
• Mineral ion concentration.

Biotic factors are living factors, including:

• Predators.
• Food supply.
• Competition.
• Disease.
• Availability of mates.

These factors affect where organisms live, how well they grow and how successfully they reproduce. For example, low light intensity can reduce photosynthesis in plants, and too little moisture can cause plants to wilt.

Organisms often have adaptations, which are features that help them survive and reproduce in their environment. For example, plants in shady areas may have larger leaves to absorb more light.

Sampling communities

You cannot usually count every organism in a large habitat, so ecologists take samples.

A quadrat is a square frame used to sample organisms in a small area. A transect is a line placed across a habitat, often used to see how organisms change along an environmental gradient, such as from shade to sunlight.

To make results more reliable, samples should be repeated and placed randomly, unless you are deliberately investigating a gradient.

Interdependence and competition

Interdependence means organisms in a community depend on each other. This can involve food, shelter, pollination, seed dispersal, gases and recycling nutrients.

Important types of interaction include:

• Predation: one organism, the predator, kills and eats another organism, the prey.
• Mutualism: both organisms benefit.
• Parasitism: one organism benefits while the host is harmed.

Examples:

• Bees and flowering plants show mutualism: bees get nectar, and flowers are pollinated.
• Nitrogen-fixing bacteria in root nodules of leguminous plants, such as clover, can be mutualistic: bacteria gain sugars and shelter, while the plant gains useful nitrogen compounds.
• Leaf miner larvae show parasitism: they live inside leaves and feed on the plant tissue, harming the plant.

Competition happens when organisms need the same limited resource. Plants may compete for light, space, water and mineral ions. Animals may compete for food, territory and mates.