Lichens consist of protoctists and fungi living in close association. Both types of organism benefit from this association. Lichens are found in a large range of habitats, including on the trunks of trees. Fig. 2.1 shows an example of a lichen, the salted shield lichen, *Parmelia saxatilis*, on the trunk of a tree. [Figure 2.1] The red deer, *Cervus elaphus*, is found in many parts of the world including north-west Europe. Red deer are found in a large range of habitats, including mixed woodlands. Mixed woodlands contain many different tree species. Fig. 2.2 shows a male red deer. [Figure 2.2] Red deer have increased in numbers in the mountainous regions of Scotland in north-west Europe since the 1960s. Scientists think that grazing of plants by red deer has affected the abundance of certain lichens found growing on the trunks of trees in mixed woodlands. To investigate this, the scientists excluded red deer from certain mixed woodland areas (exclosures) in 1995, allowing the exclosures to remain ungrazed. In 2013, the scientists compared the abundance of the salted shield lichen in the exclosures and in the grazed areas. The scientists used the grid shown in Fig. 2.3 to determine the abundance of the salted shield lichen on the tree trunks. [Figure 2.3] For each tree sampled: • The grid was placed against the tree trunk at three different heights: base, middle and upper. • The scientists counted the number of circles that contained the salted shield lichen. • The number of circles that contained the salted shield lichen was used to calculate the percentage cover of salted shield lichen on the tree. This study was carried out over the course of the three months (June, July and August) in the summer of 2013 in the exclosures and the grazed areas. [Table 2.1]