- Biotic factors
- Climatic factors
- Edaphic Factors
- Producers & Photosynthesis
- Carnivores, Herbivores, Omnivores & Scavengers
- Food Chains
- Food Webs
- Carbon & Nitrogen Cycles
- Succession & Climax
Ecology is the study of the “homes” of animals and plants. Ecologists are interested in where animals and plants live and how they interact with each other. They answer question such as “What would happened to all the oak trees in a forest if the climate becomes drier?” and “Will there be more greenfly on a tree if the ladybirds are all destroyed by a disease?”. Today many people are worried about “Global Warming”, they try to predict what will happen to the world, and its animals and plants, if the average temperature of the world goes up.
Much of what you need to know is really commonsense, but you have to use the right language in your GCSE exams. So here are some key words.
The biosphere is the living world. It extends high into the atmosphere, to the bottom of the ocean, and deep down into caves. Since the biosphere is so complicated, many ecologists like to divide it up into smaller parts which can be understood more easily; these smaller parts are called ecosystems.
An ecosystem is a more or less independent part of the biosphere. e.g. a forest, lake, river, grassland, ocean. Although some animals might move between ecosystems, most of them remain in their own preferred environment. Ecosystems are conveniently divided into two part which are the place (habitat) and the living things (community). It is difficult to think of one without the other: e.g. an oak forest is an ecosystem, but if you take away the community of animals and plants, there would be no oak trees, so the habitat would not be the same.
A habitat is a place in which you find animals and plants. The kind of animals and plants which can live in a habitat obviously depend upon what the habitat is like. Is it very hot or cold? Is it very wet or dry? Is the soil very acid or alkaline? These are the climatic and edaphic factors. It also depends upon what other animals and plants live there. Large things like oak trees may provide shelter for animals against extremes of climate, but they could also prevent some plants from getting enough light for photosynthesis. These are the biotic factors.
The community consists of all the animals and plants living in one habitat. Different animals and plants will effect each other by competition, predation, grazing, sheltering and so on. So that we can understand these interactions, we need to look at populations of each species to find out if they are increasing or decreasing.
The community of animals and plants in an ecosystem is divided up into one population of each species. So in a forest there may be a population of oak trees, a population of squirrels, a population of greenfly and so on. The size of any one population may be influenced by the climatic, edaphic and biotic factors.
A food chain starts with a green plant. This is able to use simple inorganic chemicals to produce new organic substances: carbohydrates, fats, proteins and vitamins. The second link in a food chain is a herbivore, i.e. an animal that eats plants. The next link in the chain is a carnivore, i.e. an animal that eats another animal.
The shortest food chain has just two links: green plant and herbivore, e.g.
cabbage → human
Most food chains are much longer, but there are rarely more than four links, e.g.
oak tree → greenfly → small bird → hawk
phytoplankton → zooplankton → small fish → larger fish → sharks
Of course we don’t live on cabbages alone; it would be pretty boring and not very healthy. Food chains don’t show everything that an animal eats. A Food Web does show everything that each animal eats, but it might be a bit complicated too understand at first.
A food web is an enormous chart to try and show all the feeding relationships in a single ecosystem/community. (Don’t panic, the examiners at GCSE will give you very simple food webs.) A food web diagram lets you predict what will happen to one animal population if another population gets bigger or smaller. Ladybirds eat greenfly; so if the ladybird population gets smaller, they will eat fewer greenfly and therefore the greenfly population will get bigger.
A predator is any animal which eats another animal; e.g. spiders eating flies, lions eating zebras. Providing the prey species reproduces as fast as it is predated, its population will stay at a constant size. If the rabbits in a grassland ecosystem reproduce faster than the foxes predate them, the rabbit population will increase. Predators are consumers.
Grazers are also consumers. These are primary consumers because they are eating plants. Cows, rabbit, deer, goats and zebra are all grazers. If there are too many grazers, the plants will become over-grazed and this can be a problem because the plants may be protecting the soil from erosion. If the plants are over-grazed, then the soil may be eroded (washed away) when there is heavy rainfall.
Carbon & Nitrogen Cycles
The atmosphere contains a pool of Carbon Dioxide which is used by plants in photosynthesis to make carbohydrates e.g. starch. When animals eat plants, these carbohydrates are turned into animal carbohydrates e.g. glycogen. Eventually all animals and plants die and when bacteria or fungi decompose then, the Carbon is returned to the atmosphere as Carbon Dioxide. In the carboniferous period carbon was removed from the system because in the waterlogged forests or at the bottom of the sea, it was not possible for decomposers to do their job. All this carbon was locked up as coal, gas, and oil. The amount of Carbon Dioxide in the atmosphere decreased. Now that we are burning all this fossil fuel, we are increasing the amount of Carbon Dioxide in the atmosphere and some say we are contributing to “Global Warming”.
Nitrogen is also recycled by nature. Plants use nitrates from the soil to make proteins. Animals get their proteins by eating plants. Dead animals and plants release Ammonia when they are decomposed. Nitrifying bacteria in the soil convert Ammonium ions into Nitrate ions so completing the cycle. When farmers harvest their crops they effectively remove Nitrogen from the environment, so they must put it back or their crops will not grow in future years. Farmers might fertilize their fields with manure, artificial fertilizers or by rotating their crops. When farmers rotate their crops, each field has a crop of clover or other leguminous plant every three or four years. Clover plants have “nodules” on their roots which contain Nitrogen Fixing Bacteria. These bacteria can use Nitrogen gas from the air to make Nitrates.
Plants and animals compete with each other. This is a kind of race, if you win you survive, if you lose you die. Plants compete for physical space, for nutrients and water from the soil and for sunlight. Animals compete for territory, for food, and for mates.
Succession & Climax
If we stopped using the school playing fields and fenced them off so that nobody could walk on them, there would be a succession. At the moment you will find grass, plantains, dandelions and clover growing on the fields. We have an effect on the fields because we trample all over them and get someone to mow them twice a week. If we stopped doing this, other plants like brambles would seed themselves and start to grow. Then there would be hawthorn bushes and oak saplings. The kinds of plants that live on the fields would change. These changes are called a succession. Because the brambles and oak saplings are taller than the grass and plantains they would take over: all the grass, plantains, dandelions and clover would die out. After about 50 years the oak saplings would start to win the competition for light and the hawthorns and brambles would lose out. Eventually the playing fields would turn into oak forest. This is called a climax community because the oak trees would not be replaced by anything else.
My garden is almost under control. If I don’t like a plant I remove it. If there are bare patches on the lawn I sow new grass seed. My neighbour does not bother with his garden and it is on its way to becoming an oak forest. The acorns from my oaks trees can’t grow on my lawn because I mow them down, but in my neighbour’s garden they are free to grow up through the long grass and brambles. His garden is in the process of a succession.
It would all be much easier if we used a bulldozer to flatten all the hills and removed all the forests. We could turn the world into one enormous road and airport; never mind the animals and plants. Why should we try to conserve our ecosystems? Good question! Wait for this page to be updated, or think about it.