Northern Prairie Wildlife Research Center
There is a clear need for a widespread, systematic, and statistically sound monitoring program (Wake 1991, Blaustein 1994, Heyer et al. 1994). Monitoring methods need to be standardized in order to ascertain population trends reliably. The few extensive efforts to monitor amphibian populations in place are at the scale of state or province, and use somewhat different methods. There is currently an effort underway, the North American Amphibian Monitoring Plan, to provide a statistically defensible program to monitor the distribution and abundance of amphibians in North America, with applicability at the state, provincial, ecoregional, and continental scales. Development of a reliable and extensive monitoring program will require, in addition to adherence to statistical principles of survey design, an understanding of the biology of the species. The behavior of the various species of calling amphibians, and how it varies by geographic location and environmental conditions, is necessary information.
The most extensive monitoring has been attempted in eastern North America. Amphibian populations in the northern Great Plains have received relatively little attention, population monitoring or otherwise. An early monograph (Wheeler and Wheeler 1966) provided keys and distributional information on North Dakota populations. Hoberg and Gause (1992) presented a popular guide, indicating counties in which each species had been collected or was considered probable.
In 1995 we conducted roadside surveys of calling amphibians (Zimmerman 1994). These are somewhat analogous to the Breeding Bird Survey (Robbins et al. 1986), which has proven very effective for monitoring populations of many avian species over large areas. Our objectives were 1) to learn more about the distribution of calling amphibians in North Dakota, 2) to understand better some of the variables that influence the detectability of calling amphibians, and 3) to test a roadside survey approach in the northern Great Plains.