The purpose of this exercise is to introduce you to the mapping of plant species, and for you to investigate the distribution of some examples. There are several sections, and you should answer the questions in all of them.
Distribution maps in Britain are usually based on the 10 km grid square and the tetrad. Look at the example here.
Over the map of Shropshire (plant recording was originally done by county societies), a grid of 10km squares is drawn. The squares, of course, are taken from the Ordnance Survey grid. Dots are placed in a square if a species occurs there. Now, the map uses 10 km squares, but the actual plant recording is done in 2 square km subunits called tetrads. The tetrads are not shown on the map, because they are too small, but the dots are placed on the map as if the tetrads were there. If you look at the example, you can see that the dots are not placed at random within a square but at regular intervals - because of the tetrad system.
There are two adjacent squares towards the centre of the map. One has two species marked, the other four. Draw larger versions of these squares to show the tetrads and where the plants occur.
Distribution maps have many uses. They can, for example, reveal whether species are becoming more or less common over time. They can also be used to try and understand the factors which affect the distribution of a particular species. This is done by mapping out where particular factors, like high ground, or water, occur, and then looking to see whether the distribution of a species matches the distribution of the factor.
Print out this map of the distribution of Yellow-wort (Blackstonia perfoliata). Now examine the examples of factor distribution: altitude, limestone, meres and canals, igneous and ancient rocks, railways. Which factor best explains the distribution of Yellow-wort ?
Often there is no good fit for a particular species. Give as many reasons as you can why this might happen.
The effect of an environmental factor can sometimes be seen by plotting what is known as a coincidence diagram.You take the distributions of several species that are often found in association with one another, and compare them all with the occurrence of a particular factor. Take the five species shown here, and draw one distribution map for them all. On this map, instead of putting a dot in a tetrad, put a number showing how many of the five species occur there. So if four of the species all occur in a particular tetrad, put 4 there, and so on.
Which of the factors from Question 2 best fits the new distribution diagram ? Compare how clear the association is using the coincidence diagram and using the single distribution from question 2.
Plot a coincidence diagram for the four species shown here. What factor (note that this is not amongst those from question 2) is likely to explain their distribution as a group ? To answer this, you will need to study maps of Shropshire.
