None of the University of Chicago Department of Botany's achievements loomed larger than the new field of plant ecology developed by Henry Chandler Cowles (1869-1939). Cowles was a graduate student of John M. Coulter, a member of the Botany faculty from 1897 onward, and chairman of the department from 1925 until his retirement in 1934.
When Cowles arrived at the University of Chicago for graduate studies in 1895, Eugenius Warming's Plantesamfund had just been published. John Coulter introduced the Danish scientist's theories to his students in classroom lectures, and Henry Cowles was so fascinated that he learned Danish so he could read Warming's entire text in its original language. While pursuing his botanical studies with Coulter, Cowles also worked with two noted University of Chicago physical scientists, geologists Thomas C. Chamberlin and Rollin S. Salisbury. From them he developed an understanding of physiography and an appreciation of the importance of landforms as factors in the shaping of plant life.
Cowles incorporated all these ideas into his own formulation of ecology, a theory of dynamic vegetational succession first expressed in his Ph.D. thesis, "The Ecological Relations of the Vegetation on the Sand Dunes of Lake Michigan" (1898). Cowles based his thesis on detailed field work he had undertaken in the Indiana Dunes, a wild, unsettled region of beaches, sand dunes, bogs, and woods along the southern shore of Lake Michigan about twenty-five miles from the University of Chicago.
Cowles argued that the natural succession of plant forms in time could be traced in physical space as one moved inland from the open lake beach across ancient shorelines through the shifting dunes to the interior forest. Along this route, scrubby beach grass would give way to flowers and more substantial woody plants, cottonwoods and pines would be seen yielding to oaks and hickories, and one would finally encounter the climax forest of beeches and maples. Cowles's thesis had an immediate and far-reaching impact. Published serially in 1899 in the Botanical Gazette, "Ecological Relations" became one of the most influential works in American plant science and quickly established Cowles's reputation as the first professional American ecologist.
Two years later, Cowles published his second important study, The Physiographic Ecology of Chicago and Vicinity; A Study of the Origin, Development, and Classification of Plant Societies (1901). Here Cowles applied the theory of ecology that he had developed in the Indiana Dunes to the entire range of plant communities found throughout the Chicago region. By showing that the natural processes of succession and climax were not confined to an isolated environment such as the Dunes, Cowles demonstrated that the composition of plant life in any setting must be understood as the result of constant flux and change in relations within plant communities and among communities and their environs.
Although Cowles did not publish a great deal more on the theory of ecology, he wielded great influence as an enthusiastic and inspirational teacher. Beginning with the earliest trips with his students to the Indiana Dunes in 1897, Cowles made group field study of plant communities an essential point of entry to ecological studies. Annual summer field ecology parties ranged across the continent, from Mt. Desert Island in Maine to the western prairies, from Alaska to New Mexico and California.
Cowles also played an influential role in the development of professional ecological studies. In December 1914, Cowles and his former students were leaders in assembling twenty-two ecologists to found the Ecological Society of America, an association that represented the merging interests of biologists and zoologists studying ecological communities. Cowles later served as president of the Ecological Society of America, and he also headed several other professional organizations, among them the Association of American Geographers, the Botanical Society of America, and the Phytogeography and Ecology section of the International Botanical Congress.
Henry C. Cowles and his theories and methods attracted many students to the Department of Botany, and these students in turn secured positions at other universities and research institutions. One study of scientific influences by Douglas Sprugel in 1980 concluded that of the seventy-seven recognized American scientists dominant in the field of ecology from 1900 to the early 1950s, no fewer than forty-six were students of Cowles or were directly influenced by professional mentors who had been students of Cowles.
The Chicago school of ecology is generally regarded as one of the most influential forces in the development of ecological studies. While Cowles's theories of ecological community and plant succession have been modified and extended by more recent generations of scientists, they continue to be considered among the most significant departures in the modern understanding of the natural environment.