Hunters in the Forest Canopy

Abstract

The Rich leaf-eating populations of the forest canopy support an even richer assemblage of active parasites and predators, whose influence on the level of their numbers has attracted much discussion and some serious field study, but whose exact role is still by no means understood. I shall not enter into all these questions, but only try to describe some of the situations that illustrate the ideas in this book: especially the oak galls and their small special communities, the insect and spider enemies of aphids, and the titmice and some other small insectivorous birds, about all of which there is valuable information from Wytham Woods. The leafy universe in which these animals spend much or all of their lives in summer is really a rather peculiar arrangement of cover. Although some of the insects (including leaf-miners and gall-formers) manage to dwell inside the plant tissue, most of them (such as larger caterpillars, Heteroptera, leaf-hoppers and aphids) are fully exposed on the leaves, though almost always on the undersides, a situation which presents little protection from hunting insects and spiders but perhaps a slight obstacle to small birds. The real protective system, apart from any camouflage that may deceive birds, seems simply to be the enormous number of leaves on a tree and the time and energy required to search among them separately. Professor J. D. Ovington has kindly provided me with an abstract of some estimates by Burger, of the number of leaves on representative oak⁵³ and beech^52,54 trees in Switzerland.