The feeding relationships between organisms in an ecosystem can be seen in food chains. Sampling allows us to measure the abundance and distribution of these species.

There are three different types of average which are all useful in science. They are called the mean, median and mode.

The mean is the most common type of average we use. To calculate the mean you add all the values together and divide by the total number of values.

Two students completed an ecological investigation into the dandelions on the school field. They randomly placed ten quadrats in shaded and sunny areas, in order to count the dandelions in each. Their results are below.

Quadrat number | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
---|---|---|---|---|---|---|---|---|---|---|

Number of dandelions per quadrat in shade | 4 | 4 | 6 | 0 | 1 | 4 | 2 | 3 | 6 | 5 |

Number of dandelions per quadrat in sun | 6 | 5 | 7 | 8 | 4 | 5 | 8 | 5 | 5 | 3 |

The students wanted to compare their results, thus they calculated the **mean** for each. They added up all the dandelions in the shade, which came to 36 and all those in the sun which came to 57. They divided each of these numbers by ten to calculate the two means, as there were ten numbers from the ten quadrats.

Quadrat number | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | Total | Average |
---|---|---|---|---|---|---|---|---|---|---|---|---|

Number of dandelions per quadrat in shade | 4 | 4 | 6 | 0 | 1 | 4 | 2 | 3 | 6 | 5 | 36 | 3.6 |

Number of dandelions per quadrat in sun | 6 | 5 | 7 | 8 | 4 | 5 | 8 | 5 | 5 | 3 | 57 | 5.7 |

The number of dandelions per quadrat is given to one significant figure. Usually the same number of significant figures would be used in the results of a calculation, but in this case when finding the mean of a series of integers, giving the answer to two significant figures is acceptable. Using three significant figures (eg 3.60) would be unacceptable as an inappropriate level of precision is implied.

The mean can be used to estimate the total number of dandelions.

If the area of the field in the shade is 3 m^{2} and the quadrat area is 0.25 m^{2} then the estimate is calculated by:

(3.5 × 4) = 14. This is the estimate for the number of dandelions in 1 m^{2}.

14 × 3 = 42 dandelions on the whole field.

To calculate the median, a set of numbers are placed in increasing order of size. The median is the middle number in the list. The two students took an even number of readings, and they calculated the median as the mean of the two middle numbers.

The median for shade is four because both middle numbers are four and the median for sun is 5.5 as it is halfway between five and six.

Number of dandelions per quadrat in shade | 0 | 1 | 2 | 3 | 4 | 4 | 4 | 5 | 6 | 6 |
---|---|---|---|---|---|---|---|---|---|---|

Number of dandelions per quadrat in sun | 3 | 4 | 5 | 5 | 5 | 6 | 6 | 7 | 8 | 8 |

The mode is the value that appears the most often. In the shade, the mode is four because there are three values of four. In the sun, it is five because there are three values of five.