The Oak trees of Ireland have more animal species associated with them than any other tree species, and truly must be regarded as the supermarkets of the forest. Over 200 species of insects inhabit the Oaks and nearly 20 species of bird. Native species of tree have more insect species associated with them than introduced trees because invertebrate life has evolved with the tree over a longer period of time. However, another possible reason for the richer insect fauna on Oak trees is that it reflects the warmer climates in which Oaks originated. Thus as Oak trees advanced into Ireland after the last Ice Age, they brought with them an already well established insect fauna.
The two basic requirements of animals which are provided by plants such as trees are food and shelter. Ultimately, all animals depend on plants for food since it is only green plants that can absorb the sun's energy and, by photosynthesis, convert carbon dioxide and water into carbohydrates, the building blocks of life.
As animal species have developed a relationship with plants which is more than just accidental, they have had to adapt to overcome four specific problems; feeding, attachment to the plant, desiccation, and dispersal.
To make use of a plant as a food, the mouthparts and digestive systems of animals have become adapted both to consume the plant (or part of it) and to overcome any chemical defences the plant has developed to prevent itself from being eaten. For example, the front teeth of deer (incisors) have become adapted as cutting blades to nip off the leaves and the back teeth (premolars and molars) have become adapted to grind up the plant material. Sap-sucking bugs have mouth parts adapted to pierce the veins in leaves and suck out the sap.
For smaller animals there is also the problem of attachment to the plant and thus to its food source. The feet of caterpillars function like suckers sticking the insect to the leaf. The Tree Creeper, a bird which feeds on the insect life present on trees uses its long claws and stiff tail feathers to hold on to the trunks of the trees.
There is a wide range of plant material available to woodland creatures not just from the Oak trees, but from the whole Oak forest ecosystem. Leaves, fruit bark rotting wood and plant juices such as sap and nectar are all available as the first link in a variety of food chains. However, the consumers may vary their diet and feed in more than one food chain. In this way they can exploit alternative food sources when their preferred food source is scarce or take advantage of a glut of another food source.
Thus the various food chains interconnect and there are so many possible connections that every organism is in reality part of a complex food web rather than a link in a simple food chain.