Levels of organisation

Ecologists describe living things at increasing scales. Learn this order:

• Population — all the organisms of one species living in an area.
• Community — all the populations of different species living together.
• Ecosystem — the community plus the non-living (abiotic) environment it lives in.

Interdependence: species in a community depend on each other for food, shelter, pollination and seed dispersal. A change to one species can affect many others in the same ecosystem.

Community or ecosystem?

Abiotic & biotic factors

The size and distribution of a population is affected by two kinds of factor:

• Abiotic factors are the non-living conditions: light intensity, temperature, pH, salinity, water availability and mineral (nutrient) levels.
• Biotic factors are the living influences: predation, disease/pathogens, food availability and competition from other organisms.

Organisms in a community compete for resources. Plants compete for light, water, space and mineral ions; animals compete for food, mates and territory.

Watch out: temperature, pH, light and salinity are abiotic (non-living). Predators, disease and competition between organisms are biotic (caused by living things). Both can raise or lower a population.

What feeding role is it?

Tap an organism, then tap the trophic role it plays in a food chain.

Abiotic or biotic?

Producers, consumers & food chains

Producers are photosynthetic organisms — green plants and algae. They make food (glucose) and build biomass, so they are the start of nearly every food chain. Feeding levels are called trophic levels:

• Producers — plants/algae; capture light energy in photosynthesis and store it as biomass.
• Herbivores — first-stage consumers that eat producers.
• Carnivores — consumers that eat other animals. A food web is many food chains linked together.

Key idea: the arrows in a food chain point in the direction the biomass and energy are transferred — i.e. from the organism being eaten to the one eating it.

Where does it all begin?

Transfer & loss of biomass

Only a small fraction of the biomass at one trophic level ends up as biomass in the next. On average about 10% is passed on. So where does the rest go?

• Not all of an organism is eaten (roots, bones, etc.) or digested.
• Biomass is used for growth, repair and maintenance of the organism's own cells.
• Biomass is lost in waste materials (faeces and urine).
• Biomass is used up in respiration, which releases energy (as heat) to the surroundings.

Why it matters: because so much biomass is lost between levels, there is less energy available higher up the chain. This limits the number of organisms and the number of trophic levels an ecosystem can support.

Your turn — transfer efficiency

Where does the biomass go?

Pyramids of biomass

We can draw the amount of biomass at each trophic level as a pyramid of biomass. Because biomass is lost at each step, each bar is smaller than the one below it, giving a pyramid shape.

To compare biomass fairly, scientists often measure the dry mass of samples and calculate a mean. The mean = total of all the values ÷ the number of values.

Your turn — mean dry mass

The carbon cycle & decomposition

Materials are constantly recycled through ecosystems. In the carbon cycle, carbon moves between the air, living things and the ground:

• Photosynthesis removes CO₂ from the air and locks carbon into plant biomass.
• Respiration by plants, animals and microorganisms releases CO₂ back into the air.
• Decomposers (bacteria & fungi) break down dead organisms, respiring and releasing CO₂, and returning nitrates and phosphates to the soil for other organisms.
• Burning fossil fuels (combustion) releases stored carbon as CO₂.

Decomposition rate: decay is faster when it is warm, moist and oxygen-rich (aerobic). In waterlogged or airless conditions decay is slower and anaerobic.

Removing carbon from the air

Biodiversity & sampling methods

Biodiversity is the variety and number of different species in an area. Higher biodiversity usually makes an ecosystem more stable. To study it we sample:

• Quadrats — square frames placed at random to measure the abundance of a species (count, or estimate % cover). Take many samples and find a mean.
• Transects — a line across the habitat; you record species along it to show how distribution changes (e.g. from shade into light).
• Capture–recapture — used to estimate an animal population you cannot count directly.
• Indicator species (e.g. lichens, freshwater invertebrates) reveal how polluted an environment is.

Fair sampling: place quadrats randomly (e.g. using coordinates) and take a large number of samples, so your estimate for the whole area is reliable and unbiased.

Your turn — capture–recapture

Match each term to its meaning

Tap a description on the left, then its matching term on the right.

Food security

Food security means having enough safe, affordable food for everyone. It is threatened by several factors:

• A rising human population needing more food.
• Changing diets in wealthier countries (e.g. more meat, which is a less efficient use of biomass).
• New pests and pathogens attacking crops and livestock.
• Environmental change such as drought or flooding, and the rising cost of farming.

Solutions include improving farming efficiency, controlling pests, and using genetic modification (GM) to breed higher-yielding or pest-resistant crops — but this must be balanced against protecting wildlife and biodiversity.

Efficiency link: because biomass is lost between trophic levels, eating producers (crops) directly feeds more people than feeding those crops to animals first and eating the meat.

Feeding more people

The big ideas to know

You've covered all four parts of Eduqas Topic 6 — organisation & feeding, material cycling, biodiversity and food security. Press Finish to see your score.