Organisms and the Environment

19.1 Energy flow

The Sun is the principal source of energy input to most biological systems. Plants (and other producers) capture light energy in photosynthesis and store it as chemical energy in glucose and other molecules. Most forms of life depend completely on photosynthesis, because it is the way that energy and food enter living things.

Terms

Food chains and food webs

A food chain shows the flow of energy from one organism to the next; the arrows mean "is eaten by" (the direction of energy flow).

$\text{grass} \to \text{rabbit} \to \text{fox}$

A food web is a network of interconnected food chains, which gives a more realistic picture because most animals eat more than one thing.

Energy transfer

Energy is lost at each trophic level (feeding level). Only about 10% passes on to the next level. Energy is lost:

• as heat from respiration (and used for movement and keeping warm);
• in waste (faeces and urine);
• in parts that are not eaten (such as bones, roots and fur), and in parts that are not digested.

Eating crops or livestock. It is more energy efficient for humans to eat crop plants than to eat livestock fed on crop plants, because energy is lost at each stage. Eating the plants directly means fewer energy-loss steps, so the same area of land feeds more people.

Pyramids

19.2 Nutrient cycles

The carbon cycle

The nitrogen cycle

Plants and animals need nitrogen to make amino acids and proteins, but they cannot use the nitrogen gas in the air (78%) directly.

1. Nitrogen fixation: nitrogen gas is converted to nitrates (or ammonium) by lightning, and by nitrogen-fixing bacteria (in the soil and in root nodules).
2. Absorption: plants take up nitrate ions through their roots, and use them to make amino acids and proteins.
3. Feeding and digestion: animals eat the plants, and digest the protein.
4. Decomposition: when plants and animals die (and in animal waste), decomposers break down the proteins to ammonium ions.
5. Nitrification: nitrifying bacteria convert ammonium ions to nitrates.
6. Denitrification: denitrifying bacteria in waterlogged soil convert nitrates back to nitrogen gas, which returns to the air.

Decomposers

Fungi and bacteria are decomposers. They secrete enzymes onto dead organisms and waste to digest them, and absorb the products. This recycles nutrients (carbon, nitrogen and minerals) back into the soil and air, so that producers can use them again.

19.3 Ecosystems and biodiversity

Population growth

The factors that affect the rate of growth of a population are:

• food supply (more food means faster growth);
• competition (for food, water, light, mates and space) slows growth;
• predation (being eaten) reduces numbers;
• disease reduces numbers.

The growth of the human population increases the demand for global resources (food, water, energy and land), and so the pressure on ecosystems.

19.4 Effects of humans on ecosystems

Deforestation

Causes: clearing land for farming (crops and cattle), for timber, for building and roads, and for mining.

Consequences:

• loss of biodiversity, and extinction of species that lose their habitat;
• loss of soil (without roots the soil is washed or blown away);
• flooding (fewer trees to absorb and slow the water);
• a higher concentration of carbon dioxide in the atmosphere (fewer trees for photosynthesis, and the burning or decaying of the trees releases carbon dioxide), which contributes to global warming.

Other impacts

• Over-harvesting of plant and animal species (for example fish and hunted animals) reduces their numbers and can lead to extinction.
• Introducing a non-native species can harm native species by competing with them, preying on them or bringing disease.

Pollution

Eutrophication.

1. Sewage and fertilisers add nitrate and other ions to the water.
2. Producers (algae and water plants) grow faster.
3. The producers die, and their decomposition increases.
4. The decomposers, which are bacteria, multiply and respire aerobically, using up dissolved oxygen.
5. There is a reduction in dissolved oxygen.
6. Organisms that need dissolved oxygen, such as fish, die.

19.5 Conservation

Reasons for conserving species and habitats:

• to maintain biodiversity (every species is part of food webs, and a source of foods, medicines and materials);
• to reduce extinction (extinct species cannot be replaced);
• to protect vulnerable environments (rainforests, coral reefs and wetlands).

Conserving forests: education (so people understand why forests are important), protected areas (national parks), quotas on how much timber can be cut, and replanting trees.

Conserving fish stocks: education, closed seasons (no fishing while fish breed), protected areas, controlled net types and mesh sizes (so that young fish escape), quotas (limits on the catch) and monitoring of numbers.

Energy is lost at each trophic level (by respiration, heat and waste). Eating grain directly has one fewer energy transfer, so more of the energy from the crop reaches people, and the same land can feed more.