Different living organisms form communities. Understanding the relationships within and between these communities is important. Relationships are affected by external factors.
By the end of this section you should understand:
- why particular organisms are distributed within a particular habitat
- the methods used to collect environmental data
- how valid methods are used to collect data as proof of environmental changes
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Distribution of organisms
There are a number of physical factors that can affect the distribution of organisms:
- temperature
- availability of nutrients
- light intensity
- water availability
- availability of oxygen and/or carbon dioxide
For example, in order for most green plants to flourish they need areas where there’s sunlight and water available. However, there are certain plants, like ferns, which thrive in shaded regions.
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Collecting quantitative data
In order to find out what kinds of plants and animals are in a habitat and how many there are of each species sampling is carried. As it’s extremely hard to do a headcount of an entire organism population it makes more sense to take a sample from a particular area. The more results recorded the more valid and reproducible the end results are. This can be achieved in two ways.
Quadrats
A quadrat is a square shaped instrument which is usually made from wire. Due to the fact that it’s a particular size (usually 1m2) a number of comparative samples can be taken from a habitat. The number of species and organisms can then be identified and counted.
Transects
Instead of a solid square frame a line is made, in the form of maybe a piece of rope, and placed on the ground. Organisms are then recorded along the transect. This is carried out a set number of times at regular intervals. This method is useful when you want to observe a particular linear pattern along which organisms change.
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Data analysis
Once the results from sampling have been collected they need to be analysed. Popular ways in which to analyse data are to find the mean, median and mode. The following example looks at the number of daises in a field using a quadrat.
| Sample | Number of daises |
| 1
2 3 4 5 6 |
10
11 15 13 16 10 |
Mean
The mean refers to the average number of daises counted in each quadrat. It’s calculated by adding up all the results and then diving by the number of samples taken.
Mean = (10 + 11 + 15 + 13 + 16 + 10) 6 = 12.5
Median
The median is the middle value. In order to calculate it you need to put all your values in numerical order (so, smallest to largest). If you have an odd number of values then the answer is the middle number. However, if you have an even number of values then then the answer will be the mean between the two figures.
If you take the results above:
10 10 11 13 15 16
The median is the mean between 11 and 13 which is 12.
Mode
The mode is the number which appears most often.
In the results above:
10 10 11 13 15 16
The mode is 10 because it appears twice.