Air pollution due to motor vehicles is an environmental issue that needs constant monitoring. Lichens consist of protoctists and fungi living in close association. The protoctists and fungi each benefit from this association. Lichens are found in a large range of habitats, including on the surface of tree trunks (tree bark), as shown in Fig. 2.1. [Figure 2.1] Lichens are sensitive to air pollution. The abundance of lichens decreases with increasing levels of air pollution and so lichens can be used to monitor levels of air pollution. Some scientists investigated the relationship between distance from a road and levels of air pollution. The scientists investigated this relationship by measuring the abundance of lichens on the tree trunks in a woodland beside a busy road. The scientists used the grid shown in Fig. 2.2 to determine the abundance of lichens on the tree trunks. [Figure 2.2] For each tree sampled, the scientists: • placed the grid against the tree trunk • counted the number of circles that contained lichen • used the number of circles that contained lichen to calculate the percentage cover of lichen on the tree • measured the distance from the tree to the road.