Changes in Toad (Bufo americanus) Responses to Abiotic Factors at the Northern Limit of Their Distribution

Abstract

Toad activity data were collected at three sites. The most southerly site was in the St. Lawrence Valley near Montreal and the other two sites were 70 km and 870 km to the N. Activity was related to time of day, weather variables and weather factors, and the latter two were compared. Duration of activity was related to day length; however, activity was less peaked after sunset in the far north. Positive relations with temperature and dominant weather systems were consistent at all locations; the effect was lagged 24 hr in the N. The positive relation with rainfall decreased toward the N. In the St. Lawrence Valley, there was increased activity with rainfall; however, the effect was lagged 24 hr in the Laurentians and was inconsistent in the N. The relation between these findings and burrowing behavior is discussed in the light of its possible selective value. INTRODUCTION The of herpetofauna can be limited in two conceivable ways. A particular species could be excluded from an area by conditions which affect either the survival of adults or their reproductive success. Since herpetofauna are greatly affected by climatic conditions, it follows that in areas where the terrestrial habitat would be suitable, regional climatic conditions could be the factors excluding a particular species. Smith (1980) gives as a classic example of temperature limitation on animal distribution the findings of Moore (1949), wherein the of three species of ranid frogs was related to the temperature at which their eggs developed and the length of their larval stage. This assertion is not necessarily correct because it neither defines the physiological tolerance range of the organisms nor describes the lack of availability of breeding areas beyond the known range of the adults. Pyastolova and Shvarts (1975) found no differences in the development or activity of tadpoles of Rana arvalis taken from different latitudes when raised in either cold or warm water environments. Since it has not been conclusively demonstrated that anuran is related to factors acting on the egg and larval stage, we wished to determine whether abiotic factors may be limiting of adults. We reasoned that if the activity response to a set of abiotic factors which have been shown to affect anuran activity changed at the edge of the range, this would be a strong indication that the abiotic factors were acting on the adult stage to limit their distribution. The mechanism involved could be the limitation of their activity to a level at which successful reproduction becomes too infrequent to maintain a viable population. The activity data sets we used were derived from surveys and a terrestrial-animal activity-monitoring program. The species chosen was the American toad (Bufo americanus) because of the vast quantities of data available on the species and because of its (Cook, 1964; Cook and Folinsbee, 1975) in relation to the location of the data collection sites. Toads are common throughout the St. Lawrence Valley and southern Laurentians. In surveys carried out for the Societe d'Energie de la BaieJames (S.E.B.J.) and HydroQuebec (H.Q.) in the La Grande Riviere drainage system and the more northerly