An Introduction to Ecology

For most A-level Biology students, ecology causes problems. Before you start your revision, it might be worth looking at this topic from the point of view of the examiner. Why is it that so many students fail to gain marks on questions concerned with ecology? In this first section we will look at three main areas of concern and suggest some steps you might take to avoid making the same errors. At the very least, being aware of the problem should help.

Background knowledge
Examiners have to stick to the syllabus. They can’t ask you for facts about subjects which are not spelt out in the syllabus but they may expect you to use your knowledge to interpret information which is unfamiliar. For example, competition is probably part of your syllabus although potatoes are probably not. It would not be fair to set a question which asks for lots of detail about growing potatoes but it would be perfectly reasonable to ask you to interpret some information about competition between potato plants and weeds. By the time you have opened this book and decided to do some serious revision, it is probably getting a bit late to start thinking about your background knowledge. But there are one or to things that you might do:

Recognising that different species of organisms differ in the food they eat and the way they live
This point is probably best explained with an actual example. Suppose a question asks about food chains and you decide to illustrate what you have written with this example:

 

This seems fine, but let us have a critical look at it. Take the first link. There are more species of insect than of any other group of living organisms. Some, like the caterpillars of certain moths and many grasshoppers will eat grass, but a lot of them won’t. The term ‘insect’ then is much too vague. How about the next link. Sparrows are seed eaters, they do not feed on insects – another mistake.

Throughout this section on ecology we will use examples to illustrate general ideas. The organisms we will use are shown in Figure 3.1. Many of these should be familiar to you already, and they can all be found living in a clump of nettle plants.

Some of the common organisms which can be found on a clump of nettle plants
Some of the common organisms which can be found on a clump of nettle plants

Use of appropriate A-level language
It is not only what you say but how you write it that is important. Animals and plants compete for resources, they don’t ‘fight’ for them. Organisms have specific ecological niches in particular habitats, they don’t live in ‘nests’ or have ‘homes’. It is much easier to do something about this. Here is a list of eight basic ecological terms. Get to know them, get to know what each one means and, above all, get to use them. 

Environment
The environment of an organism is the set of conditions which surround it. The environment consists of a non-living or abiotic component and a living or biotic component.

Abiotic
Abiotic factors are those which relate to the non-living part of the environment. Nettle plants, for example, grow particularly well where there are high concentrations of phosphate ions in the soil. On the other hand, the number of aphids found on a nettle plant will be influenced by temperature and rainfall. Mineral ion concentration, temperature and rainfall are all abiotic factors.

Biotic
Biotic factors are those which relate to the living part of the environment. Competition between seven-spot ladybirds and two-spot ladybirds for aphids and the effect of predation on the numbers of aphids are examples of biotic factors.

Population
A population is a group of individuals belonging to a particular species. The members of a particular population will be found in a particular place at the same time. The two-spot ladybirds living in early spring on a patch of nettles growing in a corner of a field will form a population. Those that live on another patch of nettles on the other side of the field will form another population. Members of the same population are generally able to breed with one another; members of different populations seldom do.

Community
A community is the term used to describe all the populations of different organisms living in a particular place at a particular time. If we go back to our example of a patch of nettles, then the community is made up of all the living organisms present: the nettles, the aphids, the ladybirds and the various other organisms that we have not mentioned, such as the fungi and bacteria which live in the soil.

Ecosystem
An ecosystem forms the basic unit of ecology. It consists of the community of living organisms and the abiotic factors which influence them. Although ecosystems are generally considered as being separate from one another, they do influence each other. The use of pesticides on farmland in Europe and North America, for example, has resulted in an increased pesticide concentration in the tissues of the penguins that live in Antarctica.

Habitat
The habitat is the place where a particular population or community lives.

Niche
The meaning of this term is rather complicated. In very simple terms, the niche of an organism is the place where it is found and what it does there. It is a description of how an organism fits into its environment.

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