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Surveys of Calling Amphibians in North Dakota


Concern about the status of amphibian populations has heightened in the past several years, due in part to the realization that many populations throughout the world have declined, or even been extirpated, often without a proximate cause apparent (Barinaga 1990, Blaustein and Wake 1990, Phillips 1990, Wyman 1990, Wake 1991). The widespread nature of reported declines, and their rapidity, suggested a far-reaching environmental cause (Stebbins and Cohen 1995: 210). Not all authorities, however, accepted the claim that amphibians are suffering recent worldwide declines due to some unknown but widespread cause (e.g., Pechmann et al. 1991, Pechmann and Wilbur 1994). The high natural fluctuations in numbers of many amphibians complicates the detection of long-term trends and identification of causes of population change (Pechmann et al. 1991). That amphibians may display metapopulation dynamics, with decreases in some local population units coinciding with increases in others, complicates the detection of true long-term population declines (Gill 1978, Hayes and Jennings 1986, Sjögren 1991, Pechmann and Wilbur 1994, Johnson and Larson 1994). The tendency to revisit sites where earlier amphibian studies had been conducted may bias conclusions, because those sites may have been chosen at the high part of a normally fluctuating cycle (Johnson and Larson 1994). It is widely acknowledged, however, that numerous amphibian populations have suffered from habitat loss, habitat isolation, introductions of bullfrogs or fish, and other environmental insults (Stebbins and Cohen 1995). In light of population declines in many species of amphibians, it is important to gain a better understanding of their status.

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.

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