# Populations

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## Populations

#### Population sizes

Now you have started thinking about the numbers of animals, the population sizes.

A population is a group of animals of the same species living in an area.

These are never exactly the same but over time they fluctuate about a constant level. There are six factors that can affect the population size, these are shown in the following diagram:

If the birth rate increases the population size will increase.

If the death rate increases due to an increase in predators the population size will decrease.

If competition increases the population will decrease.

Competition occurs when another species that eats the same food (or occupies the same space) comes into an area.

The other two factors are less common. They arise if animals move from one area to another, usually in migration.

You need to be able to answer questions that suggest a factor and ask what effect it would have on population size. Other factors might be the effects of a disease, or of changes in available light, or space, among other factors.

Now lets look at how two populations interact.

How do the numbers of a predator effect the numbers of its prey?

You can see that as the number of herbivores increases it is followed slightly later by a rise in the population size of the carnivores. There is a time lag between the two. This is due to the time it takes for carnivores to adjust to the presence of more food i.e. to produce baby carnivores.

If too many herbivores are eaten and their numbers drop, what happens?

There is a time lag and then the numbers of carnivores also falls. This is because there are too few herbivores for all the carnivores to eat and so some starve.

It's a hard life!

#### Pyramids

A nice way to show the size of different populations of organisms is by building a pyramid.

If you count up how many of each type of organism you have you can set up a pyramid of numbers like this:

The width of each trophic level tells you how many there are of each type. As you go up the pyramid there are fewer individuals on each trophic level.

It can be done for an individual food chain such as the one we had originally. Although it looks a little odd since there are only a few pea plants.

This is because when food chains or webs include plants such as trees there are only a few but they are very large.

A different way to show these food webs is to do a pyramid of biomass.

To do this you take the total masses of the individuals at each trophic level. So you need to go round and weigh each plant or animal.

Have you tried to weigh a tiger?

The final pyramid of biomass looks similar to the pyramid of numbers.

Tricky!

The main difference between them is that pyramids of numbers can look 'wrong' as there might only be one tree on the bottom trophic level. Pyramids of biomass always look right, since a tree will weigh much more than anything else.

So in an exam always be careful that you know which pyramid is which!

#### Energy flows

All this talk of food!

But why do we need food?

We only eat because we need fuel for respiration.

Respiration gives us energy which we use for growth, activity and so on.

The Sun is the ultimate source of all the energy in the environment. However the producers can only capture a small fraction of it using photosynthesis.

When plants get eaten by an animal what happens to any of that energy stored in them?

It is passed on to the animal - the consumer. The same happens if that animal is then eaten by another.

However not all the energy is available, some is lost at each stage.

The reasons for this include the energy for staying alive and growing. Any extra energy is stored, usually in the form of fat.

The amount of energy and biomass lost at each stage is about 90%.

In a fully grown animal 90% of the biomass it eats is lost in the faeces - that's why you can burn them for fuel. Try it sometime!

So the amount of energy that gets to the top carnivore is a fraction of that given out initially by the sun. By the time the hawk gets its energy there is only about 0.01% of the energy present in the pea plants. Not much!

This explains why pyramids of biomass have the expected shape. The amount of biomass (or energy) gets less each time you move up to the next trophic level.

#### Efficient food

So little energy is left by the end of a food chain that they cannot get too long. In fact the shorter they are the better. That's why on those survival programmes they spend so much time eating worms!

A better idea is to eat plants yourself, like vegetables, fruit and grains. They will provide you with more energy than you would get from raising a herd of cows on the same piece of land.

Losing one trophic level means that you get more energy at the end. It is more efficient.

Before you become a total vegetarian you do need to remember about vitamins and minerals. Deciding on such a diet needs careful thought to avoid malnutrition.

Intensively reared livestock are kept in restricted conditions so that they do not 'waste' energy moving around. However, apart from the ethical questions, they still end up wasting energy from their food.

Land can feed more people if used for crops. However some land such as hillsides and marshy areas are not suitable for crops and so rearing animals on it is the most efficient use.