157 



influence in determining the kind of trees that compose what the 

 plant ecologists (Cowles and others) consider the climax forest of 

 eastern North America — the maple-beech forest. It has long been 

 known (Packard, '90, p. 515) that the beech has remarkably few in- 

 sect enemies, perhaps about fifty species being recorded. Its associate, 

 the hard maple (Acer sacchariini) , has many more, and the oaks and 

 hickories, which are largely absent from the climax forest and char- 

 acterize the changing stages preceding the climax, are preyed upon 

 by more insects than any other of our trees, their number possibly 

 equaling the sum total of all the other forest-tree insects. 



A good example of the combined influence of physical and organic 

 factors is seen in the huge rafts of driftwood which have accumulated 

 in the Red River of Louisiana and Arkansas (Veatch, '06) — (Pis. 

 XXX and XXXIV) — on such an extensive scale that hundreds of 

 acres of the bottoms were flooded and the forests killed, producing 

 vast quantities of dead and decaying wood. With the opening of 

 the drainage canal, connecting Lake Michigan with the Illinois River, 

 the bottoms were so flooded that willows, maples, cottonwoods, etc.. 

 on the lowest ground were killed along the river for many miles, and 

 presented a view similar to that shown on Plate XXXV. In this 

 manner vast quantities of dead and decaying wood have been made 

 available as food and habitat for wood-inhabiting invertebrates. 



7. Interrelations within the Forest Association 



The dependence of the animal upon the physical and organic en- 

 vironment is primarily a phase of the problem of maintenance. In 

 the forest these relations are so intricate, and involve the lives of so 

 many kinds of animals, that a forest, like the prairie, must be looked 

 upon as a mosaic composed of a vast number of smaller animal, or 

 biotic communities, each one not only interrelated at many angles 

 within itself, but similarly connected with the other communities of 

 the forest. Walsh ('64, pp. 549-550) has given us a graphic ac- 

 count, not of the forest as a whole but of one of its smallest units — 

 those which he found clustered about the galls of willow trees, the 

 willow leaf-gall community. He says : 



"Nothing gives us a better idea of the prodigious exuberance of 

 Insect Life, and of the manner in which one insect is often dependent 

 upon another for its very existence, than to count up the species which 

 haunt, either habitually or occasionally, one of these Willow-galls, 

 and live either upon the substance of the gall itself or upon the bodies 

 of other insects that live upon the substance of the gall. In the single 

 gall S [alieis]. brassicoides n. sp. there dwell the Cecidomyia which 



