Surveys Of Calling Amphibians In North Dakota

Results and Discussion

Eight of the nine anurans known to occupy North Dakota were encountered in the survey: Plains spadefoot toad (Scaphiopus bombifrons), Canadian toad (Bufo hemiophrys), Woodhouse's toad (B. woodhousii), Great Plains toad (B. cognatus), gray treefrog (Hyla versicolor), western chorus frog (Pseudacris triseriata), northern leopard frog (Rana pipiens), and wood frog (R. sylvatica). We did not record the American toad (Bufo americanus). Locations where each species was detected relative to the range of each as described by Hoberg and Gause (1992) are given in Figs. 1-8. /1/2/3/4/5/6/7/8/

For three species, we had detections outside the Hoberg-Gause ranges. The Plains spadefoot toad was detected in Towner and Wells counties, both somewhat east of the limit suggested by Hoberg and Gause. We did not find it in the western half of the state, which is within the Hoberg-Gause range. Bowers et al. (1997) also found Plains spadefoot toads east of the indicated range. We found Woodhouse's toad in Bottineau, Eddy, and Cass counties; the species was present at two hexagons in the last-named county. These counties are north and east of the range indicated by Hoberg and Gause. We recorded the Great Plains toad in two of the northernmost counties, Bottineau and Towner, just north of the range indicated by Hoberg and Gause.

Conversely, we found Canadian toads only in the northeastern portion of the published range, and the gray treefrog only in the northernmost county of the North Dakota range. The northern leopard frog, although having a statewide distribution, was detected only in the southeastern quadrant of the state. Much of North Dakota had been under drought conditions for a number of years, ending in 1993. Conceivably, certain species had not rebounded from those adverse conditions by 1995, thus resulting in reduced abundance and range.

For analyses of features that might influence the detection of calling amphibians, we concentrate on the five species that were recorded at more than 10 stations (Table 2), but mention conditions under which the three uncommon species were recorded.

Date.--Plains spadefoot toads were documented from late May through mid June. We recorded the Canadian toad in both early and late June (Fig 9). Woodhouse's toads, Great Plains toads, and northern leopard frogs were recorded more frequently than expected in late May through early June. Bowers et al. (1997) showed calling activity of Woodhouse's toads varied irregularly from mid May until the beginning of July. They detected Great Plains toads most commonly throughout June. They found northern leopard frogs in the first three weeks of May and late June-early July. In Wisconsin, northern leopard frogs were recorded more frequently in late April surveys than in surveys in late May or later (Mossman and Hine 1984). Bishop et al. (1996) indicated that the peak period for northern leopard frogs in Ontario was from 1 April or earlier to 31 May. The chorus frog was more common than expected throughout May, and less common in June. Bowers et al. (1997) detected them throughout their surveys but peaking prior to mid June. Chorus frogs were more frequent in Wisconsin during the early survey period (15-30 April; Mossman and Hine 1984). The only occurrence of wood frogs in our survey was on 22 May; those of the gray treefrog were on 26 June. Bowers et al. (1997) found wood frogs only in early May, and gray treefrogs in mid June and early July. In Wisconsin, wood frogs were detected in April surveys (Mossman and Hine 1984), and treefrogs were noted from late May through June. Bishop et al. (1996) listed peak periods as 1 April - 17 May for the wood frog and 9 May - 8 July for the tetraploid gray treefrog.

Time of night.--We noted Plains spadefoot toads between 2330 and 0200. Canadian toads had a clear peak at about 2400 hours (Fig. 10). Woodhouse's toads in our surveys were heard more frequently from about 2300 to 0130 hours, after which calling diminished; results of Bowers et al. (1997) were similar. Great Plains toads followed a similar pattern, except beginning slightly earlier; Bowers et al. (1997) detected them regularly between 2130 and 0230 hours. The only gray treefrogs were heard at 2400 hours or just before. Conversely, Bowers et al. (1997) detected them mostly 0100 and 0200 hours. Chorus frogs called independently of time of night; observations closely matched time of survey. Bowers et al. (1997) recorded greater numbers between 2030 and 2200 hours, although they found them during all time periods. Northern leopard frogs were detected somewhat more frequently than expected from 0030 to 0130, after which time they were not detected. Bowers et al. (1997) noted them mostly during 2130-2200 hours and 0030-0100 hours. Wood frogs were noted only at 0130 hours.

Temperature.--No relation to temperature was noted for the Plains spadefoot toad. The only departure from expected for the Canadian toad was that it seemed most active at temperatures near 70°F (Fig. 11). Woodhouse's toads also seemed more active at temperatures of about 65-70°F. Bowers et al. (1997) found no favored temperature range for Woodhouse's toads. Great Plains toads called more than expected when temperatures were 60°F or above, and less than expected at lower temperatures. In contrast, Bowers et al. (1997) indicated that they called actively across the full range of temperatures; their Fig. 5 shows a peak between 50 and 77°F, however. The gray treefrog was recorded when temperatures were near 65°F. Bowers et al. (1997) indicated that treefrogs called more frequently at temperatures between 59 and 77°F. Shirose et al. (in press) noted that a hot, dry period in June interrupted the breeding activity of gray treefrogs. We recorded the chorus frog slightly more frequently when temperatures were below 55°F, but it was heard at all temperatures encountered; Bowers et al. (1997), in contrast, suggested that activity declined uniformly with increasing temperatures. Northern leopard frogs were more commonly detected at temperatures of about 60 to 70°F. Bowers et al. (1997) found them more frequently when temperatures were between 41 and 59°F. The wood frog was heard when the temperature was only 40°F; Bowers et al. (1997) also noted more calling at lower temperatures (41-59°F).

Humidity.--No effect of humidity was apparent for the Plains spadefoot toad, Woodhouse's toad, Great Plains toad, chorus frog, or northern leopard frog (Fig. 12). The Canadian toad was more frequently heard than expected when the relative humidity was about 90 percent, and the gray treefrog and wood frog were encountered only when the humidity was 90 percent. Bowers et al. (1997) indicated that Woodhouse's toads and northern leopard frogs called more frequently when humidity was below 65 percent, whereas the other four species' calling activities peaked between 55 and 90 percent humidity.

Precipitation.--Precipitation seemed not to influence markedly detections of any species (Fig. 13), but calling was slightly more frequent than expected for all species when rain was absent except the rarely heard Plains spadefoot toad, for which the reverse was true. Bishop et al. (1996) had some limited evidence that gray treefrogs were detected more often during rain.

Wind speed.--The Plains spadefoot toad was recorded mostly under calm conditions, but was detected once when the Beaufort value was 4. Calling of the Canadian toad and Woodhouse's toad were not clearly correlated with winds (Fig. 14). The Great Plains toad was detected more frequently when winds were still, and less frequently when the Beaufort scale reading exceeded 1. The gray treefrog was detected only at a Beaufort reading of 1. The chorus frog was generally not influenced by wind speed, except for a minor dropoff at Beaufort 4; Bishop et al. (1996) detected a similar pattern for spring peepers (Pseudacris crucifer). The northern leopard frog and wood frog were recorded only under still conditions.

Cloud cover.--The Plains spadefoot toad and northern leopard frog were recorded somewhat more often under moderately cloudy conditions. Canadian toad calling was more frequent under heavy cloud cover (Fig. 15). Woodhouse's toads and Great Plains toads showed a reverse pattern, being detected somewhat more often when cloud cover was less than 50 percent. The two detections of gray treefrogs occurred under cloudy conditions. The chorus frog was unaffected by cloud cover. The only detection of wood frog was under conditions of medium cloud cover.

Moonlight.--Canadian toads, Great Plains toads, northern leopard frogs, and possibly Plains spadefoot toads were noted more often than expected under darker conditions (Fig. 16). The gray treefrog and wood frog were detected only under the darkest conditions. Chorus frogs and Woodhouse's toads seemed unaffected by moonlight.

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