Organisms in the Environment

An ecosystem is made up of several different species which depend on each other for food and shelter. The number of different species in an ecosystem is referred to as biodiversity. Humans have had a negative impact on biodiversity over the past several decades.

Definitions

You need to be able to understand and define the following keywords which can be used to describe different groups within the environment:

• Population: all the members of a single species which live in the same area

• Habitat: the area in which an organism lives

• Community: all the populations of different species which interact together in the same habitat

• Ecosystem: a community of organisms and the habitat in which it lives (i.e. both the biotic and abiotic factors)

For example, if you live in England then you are a member of the UK population (members of the Homo sapien species living in the same area) and your habitat is the specific area in which you live (such as the city of London). The community would be the interaction of humans with their pets and the animals and plants which we buy from the supermarket to consume. The ecosystem in which we live is all of those things, together with non-living factors such as the weather.

Investigating population size

To get an idea of the size of a certain population, we use sampling techniques such as quadrats to count the organisms in a small representative area of the habitat and scale up our data to estimate the total number of organisms in that population.

Method:

• Place the quadrat randomly within the habitat - for example, using a random number generator to produce coordinates.

• Count the number of your chosen organism within the quadrat. For organisms which are difficult to count individually, you can record the percentage cover instead.

• Repeat this process so that a large number of quadrats are used - this will increase the reliability of our results.

The limitations of using quadrats to estimate population size are that sometimes it can be tricky to decide whether certain organisms which are half in-half out of the square should be counted. To prevent this, scientists use standardised techniques, such as always counting plants which overlap the quadrat on the left and bottom and ignoring organisms on the top, right sides. In addition, certain organisms may be mis-identified as the wrong species so its a good idea to use a key.

Biodiversity

Biodiversity refers to the number of different species within a community. Areas such as tropical rainforests host a high number of different organisms so are described as having high levels of biodiversity. The Amazon rainforest is believed to be home to approximately 2.5 million different insect species, with only a fraction of these having been officially identified and recorded by scientists. More extreme environments, such as the arctic have much lower levels of biodiversity. Human activities such as deforestation, pollution, overfishing and hunting have resulted in a significant decline in biodiversity.

Abiotic and biotic factors

Both non-living (abiotic) and living (biotic) factors affect the population size of a particular species. Abiotic factors are things which we do not consider to be alive, such as the amount of rainfall, the pH of the soil or the temperature. Biotic factors are anything to do with living things, such as predation, the number of prey and disease (remember that disease can be caused by living organisms, such as bacteria, and is then passed between living organisms).

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