Some plants only grow in certain favourable conditions that suit them and so are generally confined to a particular habitat type. They form a very useful guide as to the type of habitat you are looking at. I stress that they can be indicators of a particular habitat type, they are not confirmation of a particular habitat type. There is always room for the 'exception to the rule' but the more indicator species you find the more likely it is that the habitat they indicate is actually the habitat type present. Most habitat types have indicator species attached to them and here are some very obvious examples to reinforce how the principle works.Primary Habitat:Imagine you have stopped in a remote car park, get out of your car and you are surrounded by trees. It does not need much detective work to know you have arrived at woodland! If you got out of your car and saw heather and gorse then that would certainly be heathland. If there was lots of open space with grass and occasional shrubs then you could be pretty sure you were looking at grassland.Those are very obvious examples and at primary habitat level you really do not need much knowledge, it is almost instinctive.Sub-habitat:Going back to our woodland example, if you take a look at the trees around you, you can start to be a little more specific about the type of woodland you are in. If there are lots of conifers in roughly parallel rows then it is pretty certain you are in a forestry plantation. If the trees are a mix of alder and willows then that indicates that the ground is usually wet and that you could be in wet woodland. The trees may be a mix of birch and oak, that would indicate acid woodland whereas a mix of ash and oak would indicate true broadleaf woodland. The types of tree are indicators to the type of sub-habitat.The same principle applies with other habitats. On heathland, if the heathers are mainly ling and bell heather then that would indicate a very sandy soil and dry heath. If the mix is bell heather and cross-leaved heath then the likelihood is that it is wet heath. You can find all three on the same heath with ling where it is driest and cross-leaved heath where it is dampest and bell heather in between. In very wet areas the heathers will disappear altogether and you will find reeds and sedges becoming more dominant.Micro-habitat:At the microhabitat level it becomes very much more species specific. Sea spleenwort is a kind of fern and is the only fern that can survive on rocks within the splash zone of the shoreline. Some lichen only grow on certain substrates and so in a churchyard one might find different lichen on granite headstones than on the flint walls of the nearby church. At the micro-level you are looking at species that have carved themselves a very particular niche where they can survive with little competition. Indicator species can tell you a lot about the habitat you are in and that, in turn, can alert you to what to expect as you wend your way along. Vegetation is used in formal classifications of habitat type but similar principles can be applied to animals too, especially insects like butterflies and dragonflies.