An Analysis of Post-Wisconsin Biogeography of the Prairie Peninsula Region Based on Distributional Phenomena Among Terrestrial Vertebrate Populations

Abstract

During recent years a mass of distributional data has been assembled as ranges of many animals in eastern North America have become known in greater detail. As our knowledge increased, evidence has accumulated to suggest that biogeographic events of the postglacial period played a significant role in determining present distributional patterns. Since this evidence permits generalizations remarkably parallel to inferences drawn from other biogeographic data, the time is at hand for a summary of the distributional information and an attempt to correlate data from the several sources. As a working hypothesis, a reconstruction is offered of the probable steps involved in producing the present distributional complexity of the midwestern and northeastern United States. In a broadly inclusive sense, this geographic area is the Prairie Peninsula region. I have profited by discussing this subject with a number of zoologists; the great number of these contributors prevents mentioning each one by name. I have drawn freely from published accounts without citing the sources of information in every case. The distributional maps, for the most part, have been constructed by consulting a number of published studies, and my interpretations are not necessarily those of the authors of these studies. I am indebted to my associates Drs. C. W. Collinson, J. C. Frye, H. B. Mills, H. H. Ross, K. P. Schmidt, R. B. Selander, H. M. Smith, L. J. Stannard, and H. B. Willman for reading all or parts of my manuscript and to Dr. S. A. Minton and Roger Conant -for calling my attention to certain illustrative distributional patterns. The highly complex patterns of present animal distribution reflect the availability of suitable habitat (sensu lato), the operation of such broad limiting factors as temperature and humidity, the faunal history of the region, the amount of human disturbance, and various combinations of these factors. Accordingly, attempts to reconstruct dispersals of animals are speculative, and a synthesis built on a limited amount of evidence is necessarily sketchy and diagrammatic. Considerable revision of certain assumptions and modification of several of the illustrative examples will doubtlessly be necessary. Nevertheless, a general picture emerges from the data available that is in essential agreement with present concepts of postglacial biogeography. This picture is greatly simplified in the present case by three conditions. First, the terrestrial vertebrates (amphibians, reptiles, and small mammals) are among the most reliable zoogeographic indicators because (a) their inability to fly and large size reduce the probability of transport by wind, (b) they are terrestrial and accordingly not likely to have their ranges altered by floods and the process of stream capture, (c) they are more or less chained to a specific type of habitat and not sufficiently motile to cross moderate-sized barriers of unsuitable habitat, (d) they tend to have relatively small home ranges, and (e) they have been studied in enough detail in eastern North America for their taxonomy and distribution to be comparatively well known. Second, the synthesis presented herein is based almost entirely on interpretations of relict populations and of trends in geographic variation, the two most indicative and lucid of distributional phenomena. When apparently relict populations exist some distance from the main segment of range of a species, it is assumed that either (a) the species is actively colonizing new areas at the present time or (b) the range of the species once encompassed the site of the relict but has since become disjunct. For reasons outlined in the preceding paragraph, saltatorial colonization of new areas by terrestrial vertebrates under natural conditions is deemed unlikely. However, animals are transported by man, and it is conceivable that a gravid female or a pair of waifs so transported could initiate a new colony in a suitable habitat well removed from the natural range of the species. Among the more obvious means by which this sort of colonization might occur at e the deliberate release of individuals, the escape of pets, and the inadvertent transport of animals in shipments of materials or in balled roots of trees and shrubs. Even if these considerations can be ruled out, some caution is still advisable in interpreting relicts because of the possibility that the specimens on which records are based may bear erroneous locality data or may be misidentifications. Even when a population is known to exist as a relict, there is a chance that it is isolated because human activity has been responsible for the destruction