Define organism population community and ecosystem and give an example of each

Here are a few of the fundamental words in ecology, which are simple, but may be easy to mix up because they are so similar. It is, however, quite important to be clear of what they mean. I will here try to explain how they differ by defining them and giving a few examples to illustrate how they could be applied.

A habitat is basically the site where an organism or a group lives. It may be anything from a stone in a lake, on which algae grows, to a forest containing all sorts of creatures. Note that groups within a habitat do not need to be of the same species. However, one usually speaks of habitats of individuals, species, or larger groups. For instance, the habitat of the algae would be the stone in the lake, and the forest could be the habitat of a single bear – regardless of what other organisms live there and how they are geographically distributed; here we are interested in the bear, so we define the habitat as its home range, and all that falls within it will arbitrarily be a apart of its habitat.

An ecosystem is similar to a habitat, but with one crucial difference: ecosystems are self-sufficient, cyclical systems; nutrients are transported around within the ecosystem and ideally never leave it. For instance, organic carbon goes from organism to organism through digestion, and is taken from the atmosphere by photosynthesis while cell respiration returns it. In other words, the various nutrientcycles should apply to every ecosystem. One could say that an ecosystem is a special, more complex type of habitat. Moreover, just like habitats, ecosystems can vary in size: one could speak of the entire biosphere (the area on Earth containing life) as an ecosystem, but a single lake, or perhaps even a single tree, could comprise a minor ecosystem. The criterion is only that it must be nutritionally self-sufficient.

A population is a group of individuals of the same species. Strictly speaking, the individuals do not have to be closely related, or live in a group, although in many cases it is meaningful that they are or do. I could for example define a population as all Helix pomatia (a snail species) living in my garden – they may or may not be close relatives, and they may or may not live in tight groups, and in depends on the case whether this actually matters or not. I restrict the population to those living in my garden, because this is the area that I am interested in, say, because I wish to exterminate them, and I will not be bothered about the slugs in my neighbour’s garden. In short, you may define the size and geographic range of your population as you wish, but it should be in relation to what you want to use it for.

A community is all organisms or populations in a habitat. Compare this to the above example that a single bear’s habitat may contain various other organisms: the forest is the bear’s habitat, and all organisms living within it is the community of the forest. In other words, the community is not defined around a single individual or population, but around the habitat. Community and habitat go hand in hand: a habitat is where something lives, and a community is what lives there (but bear in mind that the habitat and community may focus on different organisms, as in the example above). A community differs from a population in that it rarely consists of a single species, but most often includes all various sorts of organisms, simply because it contains all living creatures in the area, including microbes, fungi, animals and plants.

To give an example that illustrates these terms, let us look at an ordinary freshwater lake. The lake could be said to be an ecosystem, if we presume that the nutrients are recycled within it (there is exchange of gases back and forth with the air at the lake’s surface, but, over time, the net exchange would be zero). The lake could also be a habitat, in which case all organisms living in it are its community: all plankton, algae, sediment bacteria, insect larvae, crustaceans, molluscs, fish, and so on. There could also be smaller habitats within the lake, for instance a rock at the bottom, whose community then could be algae, molluscs, etc. (but a crustacean that occasionally walks on the rock when searching the sediments for food would not be part of its community; the organisms must live there). The lake contains various populations, including a shoal of Rutilus rutilus (roach), colonies of Volvox plankton, and several generations of Chironomus plumosus (a mosquito species, whose larvae hatch and live under water).