Human impact and biodiversity

Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem.

A great biodiversity ensures the stability of ecosystems by reducing the dependence of one species on another for food, shelter and the maintenance of the physical environment. The reasoning is worth following: where many species can fill a similar role, losing one does not bring down everything that depended on it. Where biodiversity is low, every species is load-bearing.

The future of the human species on Earth relies on us maintaining a good level of biodiversity. Many human activities are reducing biodiversity, and only recently have measures been taken to try to stop this reduction.

Rapid growth in the human population and an increase in the standard of living mean that increasingly more resources are used and more waste is produced. Both halves of that matter — more people, each using more.

Unless waste and chemical materials are properly handled, more pollution will be caused. Pollution can occur:

• in water, from sewage, fertiliser or toxic chemicals • in air, from smoke and acidic gases • on land, from landfill and from toxic chemicals.

Pollution kills plants and animals, which can reduce biodiversity.

Humans reduce the amount of land available for other animals and plants by building, quarrying, farming and dumping waste. Learn those four — a question asking how land use reduces biodiversity wants them named.

Peat bogs are the specification's worked example, and they carry two separate harms. The destruction of peat bogs, and other areas of peat, to produce garden compost reduces the area of this habitat and so the variety of plant, animal and microorganism species that live there. And the decay or burning of the peat releases carbon dioxide into the atmosphere — carbon that had been locked away for thousands of years.

There is a genuine conflict here, and the exam asks you to see both sides. Cheap compost helps growers produce more food at lower cost, which matters for food supply and for anyone on a low income. Conserving peatlands protects a rare habitat and keeps carbon out of the atmosphere. Peat-free composts exist but cost more and behave differently.

The honest answer is not that one side is simply right. It is that the cost of the cheap option is paid somewhere else — in habitat and in emissions — and the decision is about who bears it.

Large-scale deforestation in tropical areas has occurred to provide land for cattle and rice fields, and to grow crops for biofuels.

Evaluating the environmental implications means naming the consequences, not just disapproving:

• Less photosynthesis, so less carbon dioxide is removed from the atmosphere. • Carbon dioxide released when trees are burned, and by microorganisms decaying the felled wood. • Habitat destroyed, so biodiversity falls — tropical forest holds a very large share of the world's species. • Soil erosion, once the roots that held it are gone.

And the other side, which a full evaluation needs: the land produces food and fuel, and provides income in countries that need it. That is why deforestation continues despite the consequences being well known.

Levels of carbon dioxide and methane in the atmosphere are increasing, and contribute to global warming.

You should be able to describe some of the biological consequences:

• Loss of habitat, where low-lying areas are flooded by rising sea levels or where ice is lost. • Changes in the distribution of species, as the areas with suitable temperature and rainfall shift. • Changes in migration patterns, with some species migrating earlier or further, or not at all. • Reduced biodiversity, if species cannot migrate or adapt fast enough as conditions change.

Two points about the science itself are examinable. The scientific consensus about global warming and climate change is based on systematic reviews of thousands of peer reviewed publications — not on one study or one scientist, which is what makes it a consensus rather than an opinion.

At the same time, evidence in a complex system can be uncertain or incomplete: the climate has many interacting variables, the data goes back only so far, and predictions depend on models and on what humans choose to do next. Uncertainty about the details is not the same as doubt about the conclusion, and being able to say both of those things at once is what the question is testing.

Scientists and concerned citizens have put in place programmes to reduce the negative effects of humans on ecosystems and biodiversity. These include:

• breeding programmes for endangered species • protection and regeneration of rare habitats • reintroduction of field margins and hedgerows in agricultural areas where farmers grow only one type of crop • reduction of deforestation and carbon dioxide emissions by some governments • recycling resources rather than dumping waste in landfill.

The conflicting pressures are the real subject of this section. Every measure in the right-hand column costs money, or land, or yield that someone was relying on. Hedgerows take land out of production; protecting a habitat may block a development that would have provided housing or jobs; reducing emissions has an economic cost that falls unevenly between countries.

A strong answer says what is being traded against what, and then reaches a judgement — rather than listing the measures and assuming everyone agrees they should happen.

Food security is having enough food to feed a population.

The biological factors threatening it are:

• the increasing birth rate, which has threatened food security in some countries • changing diets in developed countries, meaning scarce food resources are transported around the world • new pests and pathogens that affect farming • environmental changes that affect food production, such as widespread famine when the rains fail • the cost of agricultural inputs • conflicts that affect the availability of water or food.

Sustainable methods must be found to feed all people on Earth.

Interpreting population and food production statistics is examined directly, and the trap is comparing the wrong things. Food production rising does not by itself mean food security is improving — if the population is rising faster, food per person is falling. Work out food per person where you can, and check whether the figures are for the same years and the same region before drawing a conclusion.

The efficiency of food production can be improved by restricting energy transfer from food animals to the environment. This can be done by limiting their movement and by controlling the temperature of their surroundings. Some animals are also fed high protein foods to increase growth.

The biology behind it follows straight from trophic levels: an animal that moves less respires less, and one kept warm does not need to release energy to maintain its body temperature. Both mean more of the biomass it eats is built into its body rather than transferred to the surroundings, so the transfer between trophic levels is more efficient.

Some people have ethical objections to modern intensive farming methods, and an evaluation question expects that to be part of the answer rather than an afterthought. The objection is not that the technique fails — it works, which is the point — but that the cost is borne by the animals, and that cheapness and welfare are being traded against each other. Reaching a judgement, and saying what you weighed, is what earns the final mark.

Fish stocks in the oceans are declining. It is important to maintain fish stocks at a level where breeding continues, or certain species may disappear altogether in some areas.

Control of net size and the introduction of fishing quotas play important roles in conserving fish stocks at a sustainable level.

How each one works is the examinable part. A larger mesh size lets young fish escape, so they survive to breed before they can be caught — the stock replaces itself. Quotas limit the mass of fish that may be landed, so fewer are removed than the population can replace. Both aim at the same thing: taking no more than the stock can regrow.