Geographically, islands are defined as areas of land surrounded by water. This results in a degree of biological isolation – it is difficult for terrestrial organisms to colonise small islands which lie far from a large land mass. Other habitats of similar types may also be isolated from each other. Mountain tops form ‘islands in the air’ surrounded by a ‘sea’ of lowland which may form, for some species, a barrier as difficult to cross as an expanse of sea is for many terrestrial organisms. Similarly, lakes are watery ‘islands’ surrounded by land, and movement of aquatic species from one lake to another is hindered by this land barrier. Organisms that inhabit patchy habitats, such as rotting logs or dung, also face the challenge of moving from one isolated habitat to another. Rotting logs and dung piles are special islands in that the resources they provide to the organisms that colonise them dwindle with time and, therefore, these are not only isolated habitats, but also temporary.