The terms population, community, habitat and ecosystem, producers, consumers and decomposers, food chains and food webs, the flow of energy through trophic levels and why it is lost, pyramids of numbers and biomass, and the carbon and nitrogen cycles.

What this dot point is asking

CCEA Double Award wants the key ecology terms, the roles of producers, consumers and decomposers, how to read food chains and webs, why energy is lost up a food chain, pyramids of numbers and biomass, and the carbon and nitrogen cycles. The "10 percent" energy rule and the recycling of carbon and nitrogen are the central ideas.

Key ecology terms

Organisms have roles in feeding: producers (green plants) make their own food by photosynthesis; consumers eat other organisms (herbivores, carnivores, omnivores); and decomposers (bacteria and fungi) break down dead organisms and waste.

Food chains and webs

A food chain shows the flow of energy from one organism to the next, with arrows pointing from the food to the feeder, for example: grass to rabbit to fox. The position an organism feeds at is its trophic level. A food web is several interconnected food chains, showing that most animals eat more than one type of food.

Energy flow and loss

Energy enters as light captured by producers. As it passes up the chain, most is lost at each level:

• Heat lost in respiration.
• Energy used in movement.
• Energy lost in waste (faeces and urine) and uneaten parts.

Only about 10 percent of the energy passes to the next trophic level. Because so much is lost, food chains rarely have more than four or five levels.

Pyramids of numbers and biomass

A pyramid of numbers shows how many organisms are at each level; it can be an odd shape if one large producer (like a tree) supports many consumers. A pyramid of biomass shows the total mass at each level and is almost always a true pyramid, because so much energy and mass is lost at each step.

The carbon and nitrogen cycles

In the carbon cycle, carbon dioxide is removed from the air by photosynthesis, returned by respiration, combustion of fuels, and decomposition of dead matter by decomposers.

In the nitrogen cycle, nitrogen is recycled by bacteria: nitrogen-fixing bacteria turn nitrogen gas into nitrogen compounds, decomposers release ammonia from dead matter, nitrifying bacteria turn ammonia into nitrates that plants absorb, and denitrifying bacteria turn nitrates back into nitrogen gas.

Examples in context

Example 1. Why we get more food eating plants. Growing crops for people to eat directly wastes less energy than feeding the crops to animals and eating the animals, because each trophic level loses about 90 percent of the energy. This explains why a vegetarian diet can feed more people per hectare.

Example 2. Burning fossil fuels and carbon. Combustion of coal, oil and gas releases carbon dioxide that was locked away for millions of years, adding to the atmosphere faster than photosynthesis can remove it. This links the carbon cycle to the human impact studied next.

Try this

Q1. Define the term ecosystem. [1 mark]

• Cue. A community of organisms together with the non-living parts of their environment.

Q2. Roughly what percentage of energy passes to the next trophic level? [1 mark]

• Cue. About 10 percent.