Ecology & the Environment

What ecology is about

Ecology is the study of how living organisms interact with each other and with their non-living (physical) surroundings. To talk about it precisely, you need a small set of key terms that examiners use constantly.

Key terms A population is all the organisms of one species living in a habitat at one time.

A community is all the populations of different species living together in a habitat.

A habitat is the place where an organism lives (e.g. a rock pool, an oak woodland).

An ecosystem is a community of organisms together with the non-living parts of the environment, interacting as a unit.

A niche is the role of an organism in its ecosystem: how it feeds, where it lives, and how it relates to other organisms.

The non-living parts of an ecosystem are called abiotic factors (light, temperature, water, oxygen, soil pH). The living parts are biotic factors (predators, prey, competitors, disease).

Food chains and food webs

A food chain shows the flow of energy from one organism to the next, with arrows pointing in the direction the energy travels (from the eaten to the eater).

The organisms are grouped by how they get food:

Producers make their own food by photosynthesis — they are green plants and algae, and they start every food chain.
Consumers eat other organisms. A primary consumer (herbivore) eats producers; a secondary consumer eats primary consumers; a tertiary consumer eats secondary consumers.
Decomposers (bacteria and fungi) feed on dead organisms and waste, breaking them down and releasing nutrients back into the environment.

In reality organisms eat (and are eaten by) several others, so we draw a food web — many interlinked food chains. A food web is more realistic because it shows what happens when one population changes: removing a predator can let its prey increase, which then affects the prey's own food source.

Trophic levels and energy transfer

Each feeding stage in a food chain is a trophic level. The first trophic level is always the producers.

Energy enters the ecosystem when producers capture light energy from the Sun. As it passes up the chain, only about 10% of the energy at one level is passed on to the next. The rest is "lost" from the chain because organisms use it for living:

respiration releases energy, much of which becomes heat lost to the surroundings (especially in warm-blooded mammals and birds);
energy is used for movement, growth and other life processes;
not all of a meal is eaten or digested — energy stays in undigested material lost in faeces, and in uneaten parts like bones and roots.

Exam tip Energy is transferred, not destroyed or created. Say energy is "lost from the food chain" as heat, respiration, movement and waste — never that it "disappears".

Because so much energy is lost at each step, food chains are usually only 4 or 5 links long: there is not enough energy left to support another level. The same loss explains why feeding humans on plants (a short chain) is more energy-efficient than feeding them on meat.

Pyramids of number, biomass and energy

Pyramids are diagrams (drawn to scale) that compare trophic levels.

A pyramid of energy is always pyramid-shaped because energy is lost between every level, so each bar must be smaller than the one below it.

The carbon cycle

Carbon is constantly recycled between the air, living things and the ground. The store of carbon dioxide ($CO_2$) in the air is topped up and drawn down by four key processes.

Photosynthesis removes $CO_2$ from the air and locks carbon into plant sugars.
Respiration in plants, animals and microbes releases $CO_2$ back into the air.
Decomposition of dead organisms by microbes releases $CO_2$ as they respire.

Carbon that is not decomposed can, over millions of years, form fossil fuels (coal, oil, gas) — a store that combustion now returns to the air.

The nitrogen cycle

Plants need nitrogen to make proteins, but they cannot use the nitrogen gas ($N_2$) that makes up most of the air. The nitrogen cycle, run almost entirely by bacteria, converts it into usable nitrates.

Nitrogen fixation — nitrogen-fixing bacteria (in soil and in root nodules of legumes such as peas and beans) convert $N_2$ gas into nitrogen compounds. Lightning also fixes a little nitrogen.
Decomposition / ammonification — decomposers break down proteins in dead organisms and urea, releasing ammonium compounds.
Nitrification — nitrifying bacteria convert ammonium into nitrates, the form plants absorb through their roots. This needs oxygen (it works best in well-aerated soil).
Denitrification — denitrifying bacteria in waterlogged, low-oxygen soils convert nitrates back into $N_2$ gas, returning it to the air.

Watch out Don't confuse the bacteria. Nitrogen-fixing bacteria capture nitrogen from the air; nitrifying bacteria turn ammonium into nitrates; denitrifying bacteria turn nitrates back into nitrogen gas (the "bad" one for farmers).

The water cycle

Water also cycles: heat from the Sun causes evaporation from seas, rivers and (via plants) transpiration. The vapour rises, cools and condenses into clouds, then falls as precipitation (rain, snow). Water flows back to the sea as run-off and through the soil, and the cycle repeats.

Human impact on the environment

Air pollution. Burning fossil fuels adds extra $CO_2$ to the atmosphere. $CO_2$ is a greenhouse gas: it traps heat radiated from the Earth, causing the enhanced greenhouse effect and global warming, which drives climate change, melting ice and rising sea levels. Burning fuels containing sulfur releases sulfur dioxide ($SO_2$), which dissolves in rain water to form acid rain. Acid rain damages trees, makes lakes too acidic for fish, and erodes buildings.

Real world Power stations now fit "scrubbers" to remove $SO_2$ from waste gases, and many cars run on low-sulfur fuel — both reduce acid rain. Cutting fossil-fuel use (renewables, efficiency) is the main way to limit $CO_2$.

Water pollution and eutrophication. When fertilisers wash off fields, or untreated sewage enters rivers, they add lots of nitrates and phosphates. This sequence follows:

1. Extra nutrients cause algae to grow rapidly — an algal bloom covering the surface.
2. The bloom blocks light, so plants below die.
3. Decomposers multiply and respire as they break down the dead material.
4. The decomposers use up the dissolved oxygen, so fish and other animals suffocate and die.

This process is called eutrophication.

Deforestation. Cutting down large areas of forest:

reduces photosynthesis, so less $CO_2$ is removed and burning the trees adds more — worsening global warming;
destroys habitats, reducing biodiversity (species can be lost);
exposes soil to rain, causing soil erosion and flooding;
disturbs the water cycle as less water is returned by transpiration.

Conservation

Conservation means protecting ecosystems and the species in them while still using resources wisely. Practical ideas include:

protecting habitats in national parks and nature reserves;
replanting forests (reafforestation) and managing fishing with quotas;
recycling materials and reducing energy use to cut pollution;
captive breeding and seed banks to protect endangered species;
maintaining biodiversity, which keeps ecosystems stable and preserves species useful for food and medicine.

Exam tip Many ecology answers reward a chain of reasoning. For eutrophication or deforestation, write the steps in order (cause → effect → consequence) rather than a single fact — each correct linked step can earn a mark.