AMPHIBIANS AND REPTILES

Patrick T. Gregory and Linda A. Gregory

INFLUENCES ON THE DISTRIBUTION OF AMPHIBIANS AND REPTILES IN THE MONTANE CORDILLERA

In Canada, the vast majority of species of amphibians and reptiles occur near the U.S. border. Farther north, the number of species decreases rapidly; only one (the Wood Frog) makes it north of the Arctic Circle, although the Northern Chorus Frog comes fairly close. There are no exclusively Canadian species of amphibians or reptiles, all ranging into the U.S.A., although a very few (e.g. Wood Frog, Fig. 4) are predominantly Canadian in distribution. Why some species extend their ranges fairly far north, whereas others are confined to the south, is an interesting question. We so far have only a limited understanding of what limiting factors and organismal attributes are involved. Possibilities include: thermal physiology; dietary requirements; reproductive strategies (size, frequency and numbers of offspring); availability and proximity of summer and winter habitats; and reproductive mode (e.g. oviparity vs. viviparity). Although only a few species occur in the extreme north (Larsen and Gregory, 1988), amphibians and reptiles are, collectively, nearly ubiquitous elements of the fauna of the Montane Cordillera, being completely absent only from the highest elevations.

Decreasing species diversity with latitude is a general pattern in amphibians and reptiles and most other groups of organisms, but the particular pattern of distribution within a region is also a function of the geological history and present physiographic features of that region. In the Montane Cordillera, distributions of organisms are influenced by the historical fact of relatively recent glaciation, which necessitated recolonization from unglaciated areas, mainly to the south, and by the significant topographic variation of the region, which influences both the physical ability of organisms to disperse to new areas and the climate of local areas. Approximately 10000 years ago glaciers extended across the Montane Cordillera to south of the U.S. border. Recolonization routes for amphibians and reptiles into the Montane Cordillera are restricted primarily to the plateaus (e.g. Thompson-Okanagan Plateau) and valleys (e.g. Rocky Mountain Trench), which extend north from the southern refugia. The north/south alignment of the mountains within the Montane Cordillera limits extensive east/west movements, leading either to restricted distributions of species or isolation of populations in different parts of a species’ range. Local climate also will influence significantly the kinds of organisms that can live in an area. For example, the hot, dry southern interior of British Columbia, especially the Okanagan Valley, is essentially a northern extension of the Great Basin of the U.S.A.. Many of the species that occur there, including amphibians and reptiles, are desert species found nowhere else in the Montane Cordillera Ecozone, even in its southern parts.