VIII. ECOLOGY - INTERPRETATION OF A SOURCES OF LIFE AFTER GLACIATION T the time of the glaciation of North America, the lower Lake Michigan region, with which we are here interested, was submerged in an ice sheet. (See map,1 page 319.) The lake formerly extended over a much wider area than it does at the present time. All the life now occupying this region has become established here since the decline of the glacial ice sheet. Whence comes the source of life found here at the present time is not fully determined. But much thought has been expended on the general subject of post-glacial dispersal of North American animals and plants by workers in this field. Especially notable are the researches carried on by the United States Biological Survey. Adams2 and others have given special attention to this most interesting subject, and the reader is referred to their published papers. In general, it may be said that the fauna and flora of the northern United States, east of the Great Plains, are geographically related to those of the southeast. In other words, from the southeastern part of the United States we have derived a great part of our fauna and flora, except in the case of some species which are supposed to have their origin in the north. Other species are also known to have come from the arid, southwestern part of the United States. The Mississippi Valley has been an important highway for the dispersal of forms from the southeast. The study of the local flora and fauna over great areas will ultimately throw much light on 1 See Leverett's published map in one of the Bulletins of the Geological and Natural History Survey of Illinois, published by the Chicago Academy of Science. 2 Biological Bulletin, Vol. IX, 1905, and others. the source and dispersal of species. There are many observed ways by which plants and animals are disseminated. For instance, in plants, the wind is perhaps the most important means of scattering seeds and fruits. Examples of this method are shown in the winged fruit of the maple, and the fluffy seeds of the milkweed and thistle. Water, also, has been another factor of great importance, as well as snow and the wind combined. Animals aid the dispersal of fruits and seeds by eating the fleshy, edible kinds, such as the cherry, bittersweet, raspberry, and the poison ivy; the undigested seeds being dropped in the excreta. There are also explosive fruits and seeds that possess a peculiar mechanism for shooting the seeds some distance away from the place of growth, while still others are provided with creeping mechanisms. Lastly, man has inadvertently scattered many weed species which were contained in adulterated clover and other crop seed. Free-moving animals, through migration, become widely dispersed, the wind here, as in the case of seeds, aiding them in these movements into new fields. Young mollusks which frequent ponds are often carried on the feet of water birds and thereby transported to new and similar habitats where they may start a new colony. HABITATS OF PLANTS AND ANIMALS Merriam and Allen, as well as others, believe that definable life zones are apparent over different regions of North America. The dependencies upon the habitat of free-moving animals, however, are not often clearly perceptible. These relations of the environment are generally not so evident, for instance, as the physical effect that water or light exerts upon fixed plants. But in some special cases they are quite marked. The physiographic features in the landscape doubtless have a great deal to do with determining the habitat of animals and plants. The real habitat of an animal is determined by the place in which it habitually breeds.1 Woodworth, in an article on "The Relation between Base 1 See classified habitats at the end of this book. 2 American Geologist, XIV, 1894. Map of the Great Lakes Region, showing the former glaciated White portion indicates ice sheet; the fine vertical area. lines, older drift; fine dots, driftless area. Chicago, Dune Park, and Lakeside, Michigan, noted. Levelling and Organic Evolution," has shown that base levelling processes influence the evolution of a species by erecting new and destroying old barriers. In this way there is caused isolation, or intervention of crossing between a separated section of a species or kind on the one hand, and intermingling of species on the other. Clements' remarks that the beginning of all the primary and many secondary successions of plant life is to be sought in physiographic processes which produce A view on the Des Plaines River. At the edge of the stream are shrubs and tree societies. new habitats or modify old ones. On the other hand, most of the reactions which continue successions exert a direct influence upon the form of land. Along this line, Cowles 2 has recently asserted that, according to well-defined laws governing topographic geography, namely, the action of water in producing denudation and deposition and ultimate base levelling, there occurs at the same time a succession of plant societies which, after a time, reach a climax stage. As years pass by, 1 "Research Methods in Ecology." 2 Botanical Gazette, 1901. |