Populations - Higher Level

Ecology is the study of relationships between living and non-living factors in the environment. The place where an organism lives is called its habitat. Within any habitat there are many factors affecting the type and number of organisms that can live there. There are many different types of habitat which support different organisms. Each organism must find all it needs to survive and reproduce within its habitat. The interacting network of organisms and their physical environment is called an ecosystem.

The number of organisms of the same species living in a habitat is called a population. For example, in a garden pond there may be a population of goldfish and a population of frogs. The human population has grown very rapidly during the last two hundred years. Until about 1800 the world population was rising slowly. Then a population explosion started. There are many factors which have caused the human population to rise so dramatically. The information in this table on population growth is incomplete. Using the mouse, drag the letters beside the factors to the correct place in the table to complete it.

The continued rise in the human population will lead to serious problems. In wealthy countries there will be more old people than young. Fewer people will earn money and pay taxes, and more will want pensions and extra healthcare. This can cause economic and social problems. In poorer countries the demand for food, clean water, electricity and transport will be impossible to meet.

People who study the environment are called ecologists. They often need to know the size of a population so that they can monitor it over a period of time to see if it is changing. If it was, they would look for reasons to explain the change. How many daisies are there on your school field? You couldn’t go out and count them all because there are far too many. Instead you would have to estimate them. One way of doing this is by using a quadrat, which is a sampling square.

The quadrat is randomly placed at different positions on the field and all the daises in each one are counted. You can then calculate the average number of daisies per quadrat. If the total number of daisy plants is 37, then the average number per quadrat is 37 divided by 10, which is 3.7. The area of one quadrat is 1m², so on average there are 3.7 daisies per 1m² of field. If the field has an area of 100 x 100 which equals 10,000 m², you could fit 10,000 of these quadrats into it. So, the estimated number of daisies in the whole field would be 3.7 x 10,000, which equals 37,000. Remember, this wouldn’t be the actual number of daisies in the field. It would give a reasonable estimate of the population size. This technique is usually used for plant populations but it can also be used to estimate the number of some types of animals, for example barnacles or mussels on a rocky shore, because these animals only move a little.

If you wanted to find out find out how the types of organisms varied over a changing habitat, you could use a transect. For example, you might study a pond and the dry bank around it. A string, marked at regular intervals, is stretched across the habitat you wish to study. Starting at one end of the transect, you record all the plants that touch the string where you have a mark. You can use symbols to represent the different species when you record your results. Studying the distribution of animals is difficult because they tend to move around. Some must be caught before you can identify and count them. There are a variety of techniques used to collect animals. Click on each graphic below to find out more. Click on Next when ready to continue.

If you want to estimate the size of an animal population, the capture-mark-recapture method can be used. For example, you can set up a pitfall trap to catch beetles. Then, you count the number of beetles caught and mark each one with a small spot of paint. The beetles are then released in the area you are investigating. Some time later, the trap can be set up again. The number of marked and unmarked beetles caught on day two is counted. Then a formula is used to calculate the total number of beetles in the area being investigated. The total number of beetles equals the number caught on day one, multiplied by the number caught on day two, divided by the number of marked beetles caught on day two. For example, if you caught 27 beetles on day one and 30 beetles on day two, but only six of these were marked, the total number of beetles in the area would be 135. By the way, it’s not a good idea to mark beetles with fluorescent paint. They will become easy prey for birds.

If a healthy population of wild animals enters an unoccupied area where there is plenty of food, space, shelter and no predators, its numbers will increase rapidly. More animals will be born than are dying. But this cannot continue for long. The space will become overcrowded and the animals will compete with each other for food. Predators will also arrive. More animals will therefore die and fewer will be born because the population will not be as healthy if there isn’t enough food. The resulting change in population size can be plotted on a graph called a growth curve. Click on each column of the graph to find out more about the growth of populations. Click on Next when ready to continue.

The size of a population is affected by many factors, such as the total amount of food or nutrients available. Often these resources are in short supply and organisms must compete with each other for them. This is called competition for resources. Only the most successful competitors will survive; less successful organisms will die. The red squirrel has lived in British forests for thousands of years. In the 1940s the grey squirrel was introduced from America. The squirrels had to compete for food, but because the grey squirrel eats a more varied diet it was more successful and now far outnumbers red squirrels. Plants must also compete for resources such as water, air, nutrients from the soil, and sunlight in order to survive. Have you any large conifers in your garden? If you look underneath them, you’ll notice that there aren’t many plants growing there. This is because the conifers take in so much water and nutrients from the soil that other plants can’t survive. In a wood, few plants can grow under the shade of the trees because there isn’t enough light for them. Bluebells do survive in woods because they grow during very early spring. They photosynthesise and store most of next year’s food before the buds on the trees above open.

Organisms also compete for space. Some fish, reptiles and birds take over and defend a breeding territory against intruders of the same species. Have you noticed that you usually only see one or two robins in your garden? They sing to let other robins know that the territory is occupied. During their breeding season they are very fierce and drive other robins away. If a habitat becomes overcrowded, disease can spread rapidly, causing a reduction in population size. The populations of some organisms vary at different times of the year. Many plant populations fall in winter because there is less light and it is cold. This means that there will be less food for animals. To survive the winter some animals, like the hedgehog, hibernate. During their long sleep they slowly use up their stored fat supplies. Birds, like the swallow, migrate to warmer countries. Before migrating, the bird stores fat so that it will have enough energy for the long flight. In the wild, one habitat will support many different populations. The group of organisms sharing a habitat is called its community. For example, in a rock pool, the community is made up of fish, seaweed, crabs, winkles and anemones.

The different species present in a community will depend on others for their survival. For example, bees feed on pollen and nectar from flowers. Flowers depend on bees for pollination. This is called the interdependence of living organisms. Some organisms will be predators that hunt other animals, their prey. Grazing animals affect the populations of different plants. Fences keep grazing animals out, and this has an affect on plant life. In a pond community, the plants are very important to the survival of the animals in the pond. They produce oxygen when they photosynthesise. They provide food and shelter for animals, and a place for them to lay their eggs. Sometimes two organisms of different species live in a close association with each other and both organisms benefit from the relationship. This is known as mutualism. Pilot fish often swim beside a shark. They are protected by the shark because they swim into the shark’s mouth when there is danger. The shark benefits because the pilot fish clean its mouth when they eat the scraps of food left there. Nitrogen-fixing bacteria live in the roots of pea plants. The plant benefits from the extra nitrates that the bacteria provide, while the bacteria are protected inside the plant’s cells, and can use the sugars made by the plant.

To be successful in a particular habitat, organisms must be adapted to live there. This means that they have features which help them to survive. Click on each of the graphics below to find out more about the habitat and adaptations of some organisms. Click on Next when ready to continue.

Many organisms are adapted in the way they behave in order to survive in their habitat. Birds build their nests high up in the trees so that they are safe from predators. Sheep huddle together to keep warm in winter and shed their fleece in summer when it’s warm. Some trees shed their leaves in autumn to reduce water loss during winter when the ground may freeze.

All living organisms need energy in order to survive. Animals obtain their energy by eating other plants and animals; plants obtain their energy from the Sun in the process of photosynthesis. The way that energy flows through a living system can be shown in a food chain. For example, in this food chain some of the energy stored in the cells of the seaweed is passed on to the periwinkles when they eat it. Some of the energy in the periwinkles is passed on to the herring gull when it eats them. Nutrients will also be passed along the chain. The arrows are very important. They show the direction of the energy flow through the organisms.

In this food chain, the oak leaf is eaten by the caterpillars which are eaten by the robins which in turn are eaten by the sparrowhawk." There are special terms which are used to describe the different organisms in a food chain. At the start of every food chain is a green plant. It is called the producer because it produces its own food during photosynthesis using light from the Sun as its energy source. The Sun is therefore the source of energy for all living organisms. The animals in food chains are called the consumers because they consume, that means eat, other organisms. The organism which eats the producer is the primary consumer, this is eaten by the secondary consumer, which is eaten by the tertiary consumer. The stages in these relationships are called trophic levels, meaning feeding levels. To find out more about food chains, click on each graphic below. Click on Next when ready to continue.

What happens to all these organisms when they die? Dead plants and animals are broken down by organisms called decomposers. These are usually bacteria and fungi. The chemicals in the dead organisms are released back into the environment to be recycled and used by other organisms. This information can be added to food chains.

In real life an animal usually feeds on a variety of other organisms and may be eaten by several predators. The complex feeding relationships within an ecosystem can be shown as a food web. A food web is made up of several food chains linked together. If the population of one species changes it can affect the numbers of other organisms in the web. If a disease killed the rabbits, grass, grasshoppers, and badgers would increase in numbers. There would be more grass because there would be less rabbits to eat it. If there were more grass there would be more food for the grasshoppers, so their population would increase. If there were more grasshoppers the number of badgers would also increase because they too would have more food. If there were fewer rabbits there would be less food for the foxes and weasels, so their populations would decrease.

Pyramids of numbers are diagrams which show the number of organisms at each trophic level in a food chain. The number of organisms at each level is represented by a bar which is proportional to the size of the population. Pyramids of numbers are useful because they tell us how much food is available in a food chain. This food chain: rose leaves ® greenfly ® ladybirds ® blue tits ® cat can be displayed as this pyramid of numbers: The bar for the producers is always at the bottom. Most food chains produce a pyramid of this shape, showing that the number of organisms decreases along the chain. One cat would need to eat several blue tits, and these would need to eat many ladybirds, to supply their energy requirements. The ladybirds would eat lots of greenfly and all these would consume many rose leaves. Some food chains produce unusually shaped pyramids. If we redraw this pyramid, but count the rose bush as one plant, rather than lots of leaves, the pyramid would look quite different.

Pyramids of biomass are another way of showing the information in a food chain. But in this case, the total mass of organisms at each trophic level is displayed rather than the number of organisms. The food chain: rose leaves ® greenfly ® ladybirds ® blue tits ® cat gives this pyramid of biomass. Pyramids of biomass almost always give a typical pyramid shape. This is because energy is lost at each trophic level.

Not all the energy absorbed by a producer from the Sun will reach the top carnivore. This is because energy is lost at each trophic level. Some energy is wasted in the uneaten parts of the plants and animals. Each organism will transfer some of the energy as it respires, grows, makes complex chemicals, and releases faeces and urine. Birds and mammals use a lot of energy to keep warm. On average, only about 10% of the available energy is passed on from one trophic level to the next, so there is progressively less energy available for the consumers further along the chain. This pattern of transfer and loss also happens to chemicals in food. Animals at the end of a food chain have fewer chemicals to use, so need more food. This explains why pyramids of biomass have a typical pyramid shape.

Food chains involving many links are rare. This is because less energy is available at each trophic level. There is rarely enough energy to support more than four levels. Which of these two food chains would be the most efficient? Click on the correct answer. B is the most efficient food chain because it is a shorter chain, so less energy would be wasted. It has been calculated that one hectare of land used to grow corn could feed 9.48 people from the food chain corn ® cow ® human, whereas the same land could provide food for 94.8 people from the food chain corn ® human. This has obvious implications on food production for the human population. If humans ate less meat, the land could be used more efficiently to produce energy for more people, and a larger world population could survive. This is one argument in favour of vegetarianism. Farmers manage their farms to increase the efficiency of food production. This often involves the use of chemicals, and these chemicals can cause environmental problems. Click on the each graphic below to find out more about chemicals used in farming. Click on Next when ready to continue.

To improve the efficiency of meat production, many animals, for example chickens and pigs, are factory farmed. The animals are kept in specially constructed units where their movement is restricted and the temperature is controlled. As a result, they use less energy and gain weight more quickly. Their food intake is controlled so that they take in the optimum amount for growth. The animals are also protected from predators, so more of them survive to be eaten by humans. This allows farmers to produce meat more cheaply. The animals are kept in cramped conditions and cannot behave naturally. Many people also say that meat and animal products produced in this way don’t taste as good as those produced by free range methods. What do you think? You could compare the flavour of a free range egg to a battery produced egg. What are your views on the welfare of farm animals? Would you pay more for organic produce?