Revision - GCSE Biology - Population Size

Population size

Temperature, the availability of water, the amount of light available (more so for plants) and the levels of carbon dioxide and oxygen are the four main factors which affect life. The size of a single population depends on

  • The total amount of food available
  • The competition for food
  • Amount of light (plants only)
  • Disease

Plants compete for space (and light, which comes from the space), water and nutrients, and animals compete for space, food and water. Fairly similar really. Things usually thrive if

  • They aren’t being eaten
  • They aren’t getting ill
  • They have enough food, water space etc
  • They are better than the competition at getting food, water etc

In predators, (as with any species) the numbers in the population is determined by the amount of food. However, as the number of predators increases, the number of prey decreases, meaning there is less food so the number of predator decreases and so on.

Being adapted to your lifestyle and environment is very important

The polar bear The camel The cactus
  • Large size compared to surface area and rounded shape minimizes heat loss.
  • Small ears reduces heat loss
  • A thicker layer of blubber is present for food and as insulation
  • Thick hairy coat is very insulating
  • White coat acts as camouflage
  • Big feet good for climbing across the snow
  • Greasy coat means water falls off and stops heat loss from evaporation
  • Strong swimmer and runner to catch prey
  • It can store lots of water, and drink up to 70 gallons at once
  • Little sweating and concentrated urine reduces water loss
  • It can tolerate big temperatures changes and not sweat
  • Large feet spread the weight on the sand
  • All the fat is stored in the hump so there is no body fat which would conserve heat
  • Large surface area increases heat loss
  • Sandy colour for camouflage
  • No leaves = no stomata for water loss
  • Its surface area to volume is 1000 times smaller than normal plants, which reduces water loss
  • Water is stored in its thick stem
  • Protected by spine to stop herbivores eating it
  • Roots are shallow but cover a huge area – if it rains the cactus will get lots of water.

Pyramids of number

In pyramids of numbers, as you move up one trophic level, the number decrease. Sometimes, there are exceptions, like when you have one tree at the bottom, or when 300 parasites of living on one goat. Biomass pyramids are the sensible way to do things (biomass, how much the organisms would weigh if you put them together)

Through the different trophic levels energy is lost. Plants convert a fraction of the suns energy into glucose. This glucose (chemical energy) moves through food chain, decreasing slowly as it is used for life processes such as reparation. A lot (especially in mammals) is used for keeping warm. It is also lost in droppings, which you can prove still have chemical energy because they burn.

Food chains are related to food webs. In the food chain below, the weeds are the producers, the tadpoles are the primary consumer, the water beetles are the secondary consumer and the pike is tertiary consumer or top predator. Usually food chains are only this long because the energy lost between trophic levels means

Weeds  tadpole’s  water beetles  pike


Food production can become more efficient if you use as few trophic levels as possible. Plants give us much more energy than meat, as only 10% of what beef cattle eat becomes useful meat for us to eat. However, just eating crops can be bad as you don’t get the correct proteins. Also, some land is bad for growing crops like fell sides and Mooreland.

When animals are grown, by artificially keeping them warm and stopping them moving you reduce the energy output they have, so they need to feed less, costing less money so you can have more cattle.

Intensive farming produces lots of good quality, varying and cheap food over little land all year around. However, removing hedges and spraying insecticides destroys the natural environment of many animals, and the removal of grasslands and meadows is priceless. If fertilisers are leached into rivers they cause eutrophication (later) and pesticides disturb the food chain.

Modern faming produces lots of good quality food. However, this is often at the expense of the environment, with new techniques often causing serious unforeseen problems. Now the Europe is overproducing food, there is no reason for farms not to switch to organic farming, where you can produce good quality food without harming the environment. While it can be more expensive and creates less food, its is a carefully managed ecosystem. The top things which can be done to promote this is

  • Use of organic fertiliser
  • Reforestation and the setting aside or land
  • Biological control – While this may not be as effective as pesticides, they do not damage the ecosystem.

Nowadays, new development has to be sustainable, which means that it can be maintained without damaging the possibility of future generation doing the same. Most development today has to be able to be kept up and running with minimal damage to the environment. Burning of fossil fuels and landfills are both not very maintainable, but because of the increasing population size there is not much else that can be done right now. However, because of this developers have to create the “cleanest” power stations and build buildings that won’t damage the environment in the future.



While pesticides kill insects which keep crops, they do not discriminate. This means that they kill all the insects, including things like bees and beetles. This is turn means that the numbers of insect eating birds fall (when prey falls, predator falls, except in this case, because it is not the predator which is responsible for the decreasing number in its prey, the neither the number of prey nor predator will bounce back). This is an example of a food web and the consequences of removing an element of the food web.

Also, if insecticides seep into rivers (this is true for the land also) then they are absorbed by plants. Then, each micro organism eats a tiny amount of the plant, and therefore gets a small dose. Then a fish eats lots of the micro organisms and gets a bigger dose. Then an eel eats lots of these fish, so he gets even more, and then otters eat the eels, getting a huge dose. This is because the pesticides are not got rid of by extraction, so the chemicals build up. Just this happened in England in the 1960s, where the otter population was pushed to the brink by a pesticide called DDT.



Fertilisers are essential to modern farming, as they contain nitrates (and phosphates) which plants remove from the soil. With these added nitrates the crops have a bigger yield. However, if these fertilisers get into rivers or lakes then it can cause eutrophication.

  • The fertiliser is washed (or leached) into the rivers via runoff from rain.
  • When it reaches the river or lake it causes the plants to grow very quickly
  • The plants die because of natural causes (and because there are so many they are competing for light)
  • Because of all the dead plants there are loads of micro organism decomposers which break down the dead plants
  • They use up all the oxygen decomposing the plants
  • Now, because there is no more oxygen the fish and other organisms all die
  • Raw sewage also has a similar effect


Fish farms

Factors to consider when choosing a species for fish farming

  • Can the fish complete its lifecycle in captivity?
  • Are the fish very susceptible to disease?
  • The fish should have a high conversion ratio (i.e. be good at turning food into flesh)

Fish farming creates a controlled environment which means that it doesn’t affect the wild fish.

  • The fish are kept in nets. This keeps predators (such as seals, birds and even human poachers) out. It also keeps them all together, and reduces the amount of swimming they can, which means energy is not lost on that trophic level
  • They are fed on a special diet which means they maximise energy transfer, and also to create the minimum amount of faeces. If there are lots of faeces then it can cause a localised eutrophication near the bottom, which means fish near the bottom can die. It also increases the chance of disease. Because of this the net is not anchored to the bottom
  • The eggs are artificially fertilized, and the young are reared in special tanks to minimize the threat from predators and from bigger fish. The special tanks have: A controlled temperature and high oxygen levels. *Growth hormone may also be added. Young of the same size are kept together.
  • Because the fish are kept so close together the danger of disease and parasites is high. One parasite, fish lice, can be treated with the chemical Dichlorvos.
  • However, pesticides are bad, as we have already seen, so using biological pest control is also very useful. *A fish called wrasse eats the lice and keeps the salmon nice and shiny, so they are kept with the salmon (wrasse is common only to salmon).
  • Other chemicals include fungicides – kills fungus and prevents fungal infections
  • Antibiotics – controls disease
  • Possible problems include – disease, faeces, chemicals could disrupt food webs, food could be expensive


Greenhouse production

  • You control the temperature
  • You can control the carbon dioxide levels
  • You can control the light levels (artificial lights)
  • You can control water levels
  • It is easier to prevent food loss from birds and herbivores (rabbits)
  • Good hygiene prevents disease and insects

This means that

  • You can grow non native species
  • You can prolong the growing season


conservation is about protecting the environment and it's habitants.

    • adavntages
    • it increases biodiversity
    • it keeps the historic herachcy of a area, countries eg. bald eagle is found in the flag of america

Animals and plants are killed either

  • Directly – Us killing ‘em
  • Indirectly – Us destroying their habitat

The barn owl

We have destroyed loads of flat grassland where they hunt for vowels (voles), consonants and mice. Pesticides have killed loads of their food too, and disused barns (where they live) are being knocked down or renovated. Also, a lot of the best hunting ground is near roads, so owls are often killed by traffic. To help remedy this, people can, replant hedgerows, allow areas of natural grassland and make nesting boxes in trees.


The cod has been over fished cos it’s so nice and tasty. Mmmmm. However, the problem with this is that numbers are running low, and soon it could become extinct. Luckily, we have made four plans to stop this happening

  • Fishing allowances, so the numbers caught are controlled
  • No more catching juvenile cod
  • No fishing during the three month spawning period
  • Use special mesh nets which let juvenile cod escape

Increasing population

Since the introduction of modern faming methods and advanced and effective medicine, the death rate on earth has been falling. While this is a good thing, as fewer people die from disease and famine, it means that the world’s population is spiralling out of control.

With more babies being born and fewer people dying, finding things like food, education and housing for people (especially in developing countries) is a serious problem, and often one which governments cannot cope with.

The increase in living standards also puts more pressure on governments. This subsequently means that raw materials and non-renewable materials and energy resources are being used up, more waste is being produced and unless waste is handled successfully (which it usually isn’t) more pollution will be created.

People take land by

  • Building
  • Farming
  • Dumping waste
  • Quarrying

Because of all this humans are damaging water, the air and the land

  • Water – Fertilizers, sewage and chemical dumping
  • Air – Smoke and gasses
  • Land – Toxic chemicals

Also See

Here are the other comprehensive GCSE Biology notes by Prometheus: