Ecologists and taxonomists: divergent traditions in twentieth-century plant geography
The distinction between taxonomic plant geography and ecological plant geography was never absolute: it would be historically inaccurate to portray them as totally divergent. Taxonomists occasionally borrowed ecological concepts, and ecologists never completely repudiated taxonomy. Indeed, some botanists pursued the two types of geographic study. The American taxonomist Henry Allan Gleason (1882–1975), for one, made noteworthy contributions to both. Most of Gleason's research appeared in short articles, however. He never published a major synthetic work comparable in scope or influence to the ecological texts of Clements, Schimper, and Warming. Despite exceptions such as Gleason, most plant geographers throughout the twentieth century have emphasized the distinction between ecological and taxonomic plant geographies. Why have these distinct traditions developed? In his book Geographical Ecology, Robert MacArthur has suggested a psychological explanation for the dichotomy: Unraveling the history of a phenemenon has always appealed to some people and describing the machinery of the phenomenon to others... The ecologist and physical scientist tend to be machinery oriented, whereas paleontologists and most biogeographers tend to be history oriented.46 Without necessarily rejecting MacArthur's explanation, my study suggests a more complex relationship between taxonomic and ecological plant geographies. At the turn of the century a group of botanists self-consciously defined a new area of botanical research. These ecologists defined their new discipline in opposition to what they believed was a moribund, nineteenth-century, natural-history tradition. They turned from historically oriented, descriptive, taxonomic plant geography to experimental physiology. The new ecological plant geography was to focus on communities rather than on species, on proximate environmental causes rather than on historical explanations, and on physiological experiments rather than on morphological descriptions. As we look back, much of the revolt from morphology was rhetorical. Ecologists never completely replaced species as units of distribution, nor did they set geography on an explicitly physiological basis. Indeed, much of early ecological research was, quite simply, descriptive. Plant communities were defined in terms of dominant species, representative life forms, or general physiognomy. The underlying physiological basis for community characteristics was more often assumed than demonstrated by experiments. Despite the fact that ecological plant geography was not a truly physiological specialty, it was significantly different from more traditional taxonomic plant geography. First, ecologists were less explicitly evolutionary in their approach than were taxonomists. Following Darwin, most taxonomic plant geographers viewed distribution in historical terms. In contrast, early ecologists tended to ignore the traditional geographic problems. Most ecologists were skeptical of historical explanations, emphasizing instead the proximate, environmental causes of distribution. While some nineteenth-century biogeographers had studied the correlation between climate and vegetation, twentieth-century ecologists focused much more sharply on the interactions between plant and environment. Plant ecologists did not place biogeography on a physiological basis, but by emphasizing physiology they laid the foundation for a more detailed understanding of adaption. This emphasis on physiology and environmental causation was a second distinguishing characteristic of ecological plant geography. Finally, the idea of the plant community, articulated by Eugenius Warming in 1895, provided ecologists with a unique perspective on the distribution of plants. For early ecologists, the community was more than an assemblage of species; it was an integrated unit. The distribution of these units became the major focus of ecological plant geography. Communities never completely replaced species as geographic units, and the distinction between flora and vegetation was often blurred. Nonetheless, ecologists were innovative in studying the distribution of structurally and functionally integrated groups of plants. In the twentieth century plant geography has occupied an anomalous position in biology. It has not developed into an autonomous discipline, nor has it been incorporated into the developing discipline of ecology. Ecologists and taxonomists have pursued fairly distinct styles of geographic research, with the result that two relatively independent approaches to the study of plant distribution have persisted.