Population Dynamics of the Manitoba Toad, Bufo Hemiophrys, In Northwestern Minnesota

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Spring emergence patterns, age and sex ratios, and changes in abundance over a three—year period of a population of the Manitoba toad (Bufo hemiophrys) were studied on a virgin prairie in northwestern Minnesota. Data were obtained from more than 7,500 individuals trapped as they emerged from their wintering sites. Analysis of the size distribution of 5,117 animals captured and measured upon emergence from two Mima—type mounds in the spring of 1961 shows that this population can be divided into only two ages classes, immatures and adults. Changes in age ratios of emerging toads suggest that some environmental force or forces probably affected the 1959—year class adversely and not the 1960 and 1961 classes. Analysis of weather data indicates that frost depth may be the factor responsible for the low survival of immatures in certain years. Sex ratios were determined for entire local populations as they emerged from their wintering sites. The effects of behavioral differences and mortality rates on sex ratios are considered, and the conclusion is reached that the adult sex ratio is equal. The time of first emergence and the peak of emergence appear to be dependent upon rising temperatures and precipitation, with the peaks occuring at about the same time in each of the three years studied. Our findings indicate that adults emerged earlier than immatures but that there is no apparent emergence difference between the sexes in the adult age class. Data on the distribution of toads emerging from the Mima—type mounds on the prairie indicate that nearly the entire population winters in these sites. The density of wintering toads is high in some mounds and low in others. Toads using a particular mound appear to constitute a distinct population. The factors which make some mounds more attractive than others have not been determined. It is possible that the differences may be related to the profile of the mound and or to the vegetation on the mound. Origin of these Mima—type mounds is not known. The volume of soil displacement by burrowing toads may be a significant factor in their formation in northwestern Minnesota.