Who's eating who?
Living in the wild is pretty dangerous!
You spend all the time trying to find your dinner and avoid ending up as someone else's. We can look at who eats whom in the environment by setting up food chains and food webs. They help us by breaking up the environment into understandable bits.
With a food chain you always start off with a plant as they produce all the energy in the environment from photosynthesis. A plant is therefore called a producer.
Herbivores are animals that eat plants and they are placed next in the food chain. They consume the plants and so are also the primary consumers.
To show that the herbivore eats the plant in our chain we use an arrow.
The direction of the arrow is very important. It shows who is eaten by who.
So in our example the pea plant is eaten by aphids, which are in turn eaten by the ladybirds, and so on.
If the aphids are the primary consumers here, or the herbivores, the ladybird is the secondary consumer. Then the robin is the next (tertiary) consumer, and so on.
So in our example the ladybird, robin and hawk are all examples of this.
The robin does not just eat other animals but will sometimes also eat fruit and nuts. Therefore you could actually call it an omnivore.
The hawk is the top carnivore as nothing else will eat it.
Our example was very simple. Usually in natural ecosystems they are more complex, this is because one predatory species will usually eat more than one prey species.
What would happen if you were an animal that only ate one animal or plant?
If that food source ran out, you would starve to death. Not a good idea.
Another good idea for eating up your greens! Animals tend to eat food from a variety of sources, different plants and animals. Therefore our food chain is not very accurate, it does not cover all the connections.
In order to obtain a more accurate idea of what is going on we need to construct a food "web":
The food web attempts to show which organisms eat any other animal. They can get pretty complicated but are still as easy to read as a food chain. Just remember the arrows.
Now think about what happens if one animal species gets wiped out.
What do the robins do if all the ladybirds disappear?
They just start eating more caterpillars and slugs.
The top carnivore, the hawk, might still be in trouble if all the robins got eaten by cats.
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.
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.
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!
A nice way to show the size of different populations of organisms is by building a pyramid.
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.
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.
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.
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.
Adapting to Survive
Any organism has to keep up with any changes in their environment. They have to adapt in order to survive.
Adaptation means making changes in what you do or how you do it. Without the necessary changes you will fail to compete and die.
A favourite exam question is to ask you to think why a particular characteristic helps an animal or plant survive. Each has to adapt to its own niche - the place where it lives and fits into the local environment.
For example: a rabbit has the following adaptations to its life as a prey animal:
AdaptationSurvival advantage Large ears To hear predators Thick brown fur Good camouflage and for warmth White tail To act as a warning flag to other rabbits when its running away! Fast and agile running To avoid predators Eyes on side of head Allows all-round vision to watch for predators
All those things are fairly obvious aren't they! Now try this list for the ultimate predator, the lion. They are all jumbled up so you need to match them up.
Cycling through Nature
Food webs and the size of populations has probably made you realise how interconnected things are in the environment.
This is not just true about the plants and animals but it is also the case for all the chemical molecules present on the Earth. The atoms that make them up cannot be destroyed, they just get moved around and around.
So if you look at a particular type of chemical you can follow it as it goes through a cycle in the environment. The best known are the carbon and nitrogen cycles.
First of all the chemicals need to get out of the animals or plants that they are in, this occurs through the process of decomposition.
All things die, sadly. But their molecules go marching on!
The cells and bodies of plants and animals decay by the action of soil bacteria and fungi. They are often called the decomposers. (There's a joke about Beethoven there somewhere).
The molecules released during decay are absorbed by the bacteria, fungi and also back into the soil from where plants can draw them up through their roots.
The decomposers work best at their optimum conditions since they use enzymes. The kind of conditions needed are shown nicely by the humble compost heap.
There are five necessary conditions or factors for good decomposition:
- Organic material: plant or animal material to decay
- Water: the right amount of moisture
- Oxygen: air must be able to get to the material
- Warmth: the temperature cannot be too hot or too cold
- Decomposers: bacteria or fungi are needed to do the job
You would not get decomposition if one of these was missing.
The molecules released by this decay process include the most important elements carbon and nitrogen which have their own environmental cycles.
Once carbon compounds from plants and animals are released by decomposition they are taken into the soil. Slowly they are broken down by bacteria and released back into the air in the atmosphere.
There is only one route in which carbon compounds return and that is by photosynthesis carried out by plants. That is the only way that animals can get glucose, without plants there would be no animals on the Earth! As well as carbohydrates, plants also form fats and proteins.
The material from plants can decay or be burned and release the carbon back into the atmosphere as carbon dioxide. Otherwise this material can be transferred to animals as the plants are eaten.
The animals, or those that eat them, also form the molecules into carbohydrates, fats or proteins within their own bodies.
Nitrogen is not a glamorous element. It is not very reactive, and does not look particularly attractive!
The air contains 78% nitrogen but it is not altered during our breathing. It cannot be used directly by plants or animals.
Nevertheless nitrogen is important in Biology. It occurs in DNA, proteins, nitrates (NO3) and ammonium compounds (NH4). Plants need nitrates to grow well.
Nitrogen ends up in these molecules through 3 different routes:
- Nitrogen fixing bacteria
- Artificially � through the synthetic, human reaction called the Haber process
The nitrogen cycle links these different compounds and routes in a continuous and inter-linked series of reactions.
The nitrogen-fixing bacteria live in a mutualistic relationship in the root nodules of Leguminous plants (Legumes), such as clover. The bacteria provide the plant with nitrates and receive glucose in return, so both benefit.
Other bacteria live in the soil itself, they make nitrates and energy for themselves.
Lightning is the other natural way in which nitrogen can return to the soil. The energy in a lightning bolt is enough to zap nitrogen and oxygen in the air together to make the oxides of nitrogen. These dissolve in water in rain to form nitrates.
Now see if you can sort the descriptions out to match the names of the different bacteria.
Just work through the cycle stuff until you have got it. Its not too hard, honest!