Oaks are generally regarded as having more associated species of wildlife than any other native trees in England. These organisms range from bacteria to fungi, lichens (fungi in symbiosis with algae), free algae, mosses, vascular plants, invertebrate animals, birds and mammals.

Ancient oaks are particularly important as habitats for many of these organisms. Not only do they provide 'services' for a very long time at the same place, but as they grow larger and age, they provide additional niches for yet other organisms to exploit. This leads to a situation where ancient oaks can be the only habitat for specialized organisms present in a wider landscape. When capacity to disperse is limited, as for many invertebrates, the close proximity of several ancient oaks is important, as over time the specific habitats in a single oak will change and eventually disappear.

Three groups of organisms may serve as examples: fungi, lichens and invertebrates. Not all are necessarily associated with ancient oaks, but the species mentioned here usually are restricted to these. Among the fungi two important categories are mycorrhizal fungi and saprotrophic fungi. The first are associated with the small roots of the tree and are symbiotic; the second kind are dead wood consumers. We find the 'fruiting bodies' (mushrooms and toadstools) of the first on the ground around the tree, while those of the wood decaying fungi appear on the tree itself. The three mycorrhizal mushroom genera Russula, Cortinarius and Inocybe have respectively 129, 105 and 72 oak-associated species in Britain. However, few if any of these species are limited to ancient oaks, which is to be expected: the living root tips of ancient oaks are not ancient. Among the saprotrophs the bracket fungi that cause brown rot in the heartwood are the most characteristic for ancient oaks, partly because these oaks provide lots of food. Beefsteak fungus (Fistulina hepatica), oak mazegill (Daedalea quercina) and oak polypore (Piptoporus quercinus) are among those associated with ancient and veteran oaks. Unlike fungi, lichens require sunlight and are therefore only found on the surface, from twigs to bark and dead wood.

Many of the lichens associated with ancient oaks are slow growing and dependent on stability and continuity of this habitat. There are different species present on the wetter and on the drier side of large oaks. More than any other organisms, lichens are sensitive to air pollution; as a result of this many species, such as the spectacular leafy lichen Lobaria pulmonaria or lungwort, are now confined to the west of the country. Brown-rot is probably the key invertebrate habitat provided by older oaks. The most characteristic insects that live in the decaying wood of ancient oaks are beetles. As the conditions change with progressing decay, so do the species of beetle that live in the wood as larvae. Some are very rare in the UK, examples are Gnorinus variabilis, Hypebaeus flavipes (only known from Moccas Park in Herefordshire), Lacon quercus and Trinodes hirtus. One of the most extraordinary finds to have been made in Windsor Great Park in recent years is the false scorpion Larca lata, known as the Oak-tree Chelifer.