Topic 19: Organisms and their environment

Every living thing depends on others and on its surroundings. In an ecosystem, energy from the Sun flows through living organisms while nutrients such as carbon and nitrogen are recycled again and again. This topic shows you how to follow that flow of energy along food chains, how to read ecological pyramids, and how populations rise and fall — the big picture of how nature keeps itself going.

Food chains, food webs and trophic levels

A food chain shows the transfer of energy from one organism to the next, beginning with a producer. The arrows point in the direction the energy travels (from the eaten to the eater). A food web is a network of interconnected food chains and gives a more realistic picture of feeding relationships in a habitat.

Each stage in a food chain is a trophic level:

• Producers make their own food by photosynthesis (usually green plants) — the first trophic level.
• Primary consumers are herbivores that eat producers.
• Secondary consumers eat primary consumers; tertiary consumers eat secondary consumers.
• Decomposers (bacteria and fungi) break down dead organisms and waste, releasing nutrients back into the environment.

A consumer gets energy by feeding on other organisms. A predator hunts and kills its prey; both terms describe roles within these chains.

Energy flow and ecological pyramids

Energy enters most ecosystems as light energy, which producers capture during photosynthesis. As energy passes along the chain, much is lost at each level through respiration (released as heat), movement, and undigested material in waste. Because so much energy is lost, food chains rarely have more than four or five links.

This energy loss explains the shapes of ecological pyramids:

• A pyramid of numbers shows how many organisms are at each level. It can be an odd shape — one large tree may support many insects, giving a narrow base.
• A pyramid of biomass shows the dry mass of organisms at each level and is almost always a true pyramid, narrowing towards the top.

The inefficiency of energy transfer is also why it is more efficient to feed people on crops directly than to feed crops to animals and then eat the animals — energy is lost at each extra step.

The carbon and nitrogen cycles

Nutrients must be recycled because the Earth’s supply is limited. In the carbon cycle, carbon dioxide is removed from the air by photosynthesis and returned by respiration, combustion of fossil fuels and the activity of decomposers. Carbon is locked away for long periods in fossil fuels and limestone.

In the nitrogen cycle, plants need nitrogen to make proteins but cannot use nitrogen gas directly:

• Nitrogen-fixing bacteria convert nitrogen gas into nitrogen compounds (some live in root nodules of legumes).
• Decomposers break down proteins in dead matter, releasing ammonium compounds.
• Nitrifying bacteria convert ammonium compounds into nitrates, which plants absorb.
• Denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the air.
• Lightning also fixes some atmospheric nitrogen.

Populations and the sigmoid curve

A population is a group of organisms of one species living in the same area at the same time. When a few organisms enter a new habitat with plenty of resources, their numbers follow a sigmoid (S-shaped) growth curve:

• Lag phase — few individuals, slow growth as they adjust.
• Exponential (log) phase — rapid growth with abundant food and space.
• Stationary phase — growth levels off as limiting factors take effect.

Factors that limit population size include food supply, predation, disease and competition for space, water and light. The maximum population an environment can support is called the carrying capacity.