Surveys of calling amphibians in North Dakota

Amphibians have received increased attention in recent years from the scientific community and general public alike. Many populations throughout the world have declined, or have been extirpated, often without an apparent cause. Concern about the status of amphibians has translated into a growing interest in systematic and statistically sound monitoring programs. Several extensive efforts to monitor populations of calling amphibians are in place, and more are under development. Necessary for the design of appropriate surveys is an understanding of the behavior, especially vocalization, of the various species, and how it varies by geographic location and environmental conditions. In 1995 we conducted roadside surveys of calling amphibians along 44 routes in North Dakota. We describe results of that survey, with special attention given to variables that influence detectability of calling amphibians. Unlike similar studies, we accounted for the amount of time observers spent listening for amphibians under different conditions. We found that the optimal conditions for a single survey for North Dakota in that year would be in early June, between the hours of 2300 and 0130, with ambient temperatures above 13 deg. C, and with no rain and little or no wind or moonlight. Multiple surveys in a year would yield better results, of course, especially for the wood frog (Rana sylvatica), which is most active earlier in the season. Studies such as ours should be replicated in space and time to ensure a well-designed survey.