Organisms in their Environment

A population is all the individuals of a particular species that live in one area. For instance, you can have the population of fleas on a dog, or the population of fleas in El Salvador. If the conditions are perfect a population will grow. Eventually the population will get so big that there is not enough food, or not enough space, or too much disease, or too many predators, or some other factor that limits the population size, so the population stops growing.

We can draw graphs of population size against time which often have characteristic shapes. When nothing in the environment limits the growth of the population we see an exponential curve. This is where the graph gets steeper and steeper and steeper. This is what has happened to human populations throughout history. We have not yet reached our limit (though we will soon), but usually the graph stops getting steeper and flattens out, giving a characteristic sigmoid shape (sigmoid is like a flattened S shape).

In any habitat there will be many different populations that all have an effect on each other. This is called the community. The habitat will also affect the community because of such things as temperature, amount of water etc. The community and habitat together are known as the ecosystem.

An ecosystem gets all of its energy from sunlight. Plants absorb sunlight during photosynthesis and use it to produce complex chemicals such as glucose and proteins. An animal cannot make these chemicals, so to get them it needs to consume plants, or other animals that have consumed plants. When these animals and plants die the energy they contain is not wasted. Other organisms such as bacteria and worms feed on the decomposing remains of dead organisms. In this way, energy passes along a food chain from sunlight to plants (which we call producers), to animals (which we call consumers) to decomposers.

At the end of the food chain very little of the original energy from the sunlight remains for the decomposers. This is because much of the energy has been used up by the plants and animals further down the chain (of course remembering that we cannot create or destroy energy, only transfer it from one form into another ? in this case the organisms have transferred it mostly into heat energy). This is why the animals at the top of the food chain (the tertiary consumers), such as eagles, jaguars, wolves etc are generally quite rare, whereas plants and primary consumers (the animals that eat plants) are much more common.

We can represent the number or organisms in a food chain by using a pyramid of numbers. We call it a pyramid because usually there are less individuals the further up the food chain that you go. However this is not always the case. For example you can have thousands of insects living on one tree because the tree is so big. In this case it is better to use a pyramid of biomass (the dry mass of the organisms at each step in the food chain). This is because even though there are more insects than trees, there is a far greater mass of tree than there is of insects.

The pyramid of numbers can also tell us something about how humans can get more food from the land. If energy is lost on its way up the food chain then humans will get more energy out of their land if they grow crops than if they raise animals. An area of land may grow enough lettuces to feed one hundred people, but if you feed the lettuces to rabbits and then eat the rabbits you may only feed about 10 people, because typically only about 10% of the energy is passed from one level in the food chain to the next. Occasionally, it will still be important to eat some animals because they provide an important source of protein which only certain plants can provide.