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Calling Surveys and Visual Searches of Anuran Species: Reliability in the Palouse Region of Northern Idaho

Ryan J. Monello and R. Gerald Wright

Department of Wildlife Resources
University of Idaho
Moscow, Idaho 83843

The need to accurately monitor anuran species presence/absence and relative abundance is increasing in importance as a result of declines in species abundance and range sizes (Blaustein et al. 1994). The remaining habitat for anuran species often includes privately-owned lands. Because of concerns over access and impacts on these lands, the use of non-intrusive monitoring techniques can be especially important. For example, in Latah County, Idaho, where this study was conducted, stillwater breeding areas are almost solely the result of ponds created by private landowners. Calling surveys and eyeshine/visual searches are typically two of the most non-intrusive methods used for monitoring anuran species in stillwater habitats. However, these methods are often only carried out one or two times a season in the spring and/or summer (Moyle 1973, Freedman and Kerekes 1985, Ross and Esque 1995) possibly leading to estimates of uncertain accuracy. Multiple surveys may in fact be necessary throughout the spring and summer season at the same breeding sites in order to gain meaningful results, especially in the case of a declining or rare species.

This paper reviews the results of a study employing multiple calling surveys and eyeshine/visual searches made during a spring and summer anuran study in northern Idaho. Three stillwater native anuran species were surveyed: the Pacific chorus frog (Pseudacris regilla), the western toad (Bufo boreas) and the spotted frog (Rana pretiosa), with the latter two displaying recent declines in adjacent areas of the inland Pacific Northwest (Stebbins 1985, Hayes 1994a, Hayes 1994b, Ross and Esque 1995). Eyeshine searches were conducted for the spotted frog and western toad, and calling surveys were performed to determine the status of the Pacific chorus frog. The objective of this paper is to determine if the proportion of surveys which yielded accurate presence/absence and relative abundance results are sufficient enough to be able to document species ranges and changes in abundance.

Study Area
The study included 30 artificially-created ponds and their surrounding habitats located within a 40 km radius of Moscow, Idaho. Moscow is located on the central Washington/northern Idaho border in Latah County. Surface areas and depths of the ponds typically ranged from 200 m2 to 300 m2 and 5 m to 7 m respectively. Ponds were situated in forested (9), agricultural (4), grassland (11), and residential areas (6) with elevations ranging from 800 m to 1000 m. Higher elevational areas in Latah County support forested areas consisting of conifers, predominantly ponderosa pine (Pinus ponderosa), grand fir (Abies grandis), and Douglas fir (Pseudotsuga menziesii). Lower elevational areas and southwest exposures support a variety of shrubs, including snowberry (Symphoricarpos albus), ninebark (Physocarpos capitatus), red twinberry (Lonicera utahensis), ocean spray (Holodiscus discolor), serviceberry (Amelanchier alnifolia), common wild rose (Rosa nutkana), and dwarf rose (Rosa gymnocarpa). However, the largest percent of the county is dominated by grasses and agricultural development (Schaub and Larsen 1978).

The ponds were surveyed from March 1996 to August 1996. Sampling consisted of ten to fourteen nocturnal surveys and four to six afternoon surveys. Anuran occurrence was determined by the following methods: dipnetting, eyeshine/visual searches, call identification, and egg searches. Dipnetting was conducted in the summer after tadpoles had emerged, by sweeping a dip net through the water at various depths from the pond shore along 5 m intervals. The goal of dipnetting was to aid in accurate species determination and reproductive status, not to index the relative abundance of a species. Egg searches were conducted throughout the spring and consisted of searching water and submerged vegetation within 2 m of the pond shore. The eyeshine/visual method consisted of walking or wading along the entire shore at day or night using a flashlight to search for frogs and toads (Heyer et al. 1994, Stebbins 1985), and allowed for easy identification while anurans remained motionless. Finally, calling surveys followed North American Amphibian Monitoring Protocol methods and consisted of three-minute survey periods (Shirose et al. 1995) set up at four equally-spaced distances around each pond. Surveys began one minute after arrival (one minute being the minimum delay) at each calling station, and all were performed at least 2 m from the pond shore before and after other methods were undertaken.

Spotted frog and western toad relative abundance was determined by the eyeshine method. Total number seen for the entire pond was then recorded and classified as: 1) no observations, 2) 1 to 10 observations, or 3) more than 10 observations mapped. Pacific chorus frog abundance was determined by calling surveys. Individual frogs were marked on a map at each calling station, and total number heard at each pond was then recorded and classified with the same relative abundance scale as the western toad and spotted frog.

For all three species, the proportion of observations that yielded an accurate measure of presence/absence (herein referred to as a correct observation) was calculated by dividing the number of visits during which the species was observed (a correct observation) by the number of total visits to the ponds during the species peak reproductive season. Pacific chorus frog and western toad peak reproductive season was taken to be all observations made between high points in relative abundance measurements (Fig. 1). For the spotted frog, all surveys were included due to their tendency to remain at the ponds year round. Percentages of correct presence/absence was done only for the ponds which contained anurans.

The proportions of relative abundance measurements considered to be correct were calculated in a similar way. Here, the proportion of correct relative abundance observations was calculated by dividing the number of visits which yielded an estimation of the relative abundance (that matched the highest relative abundance classification over the season) by the total number of surveys in the peak breeding season (Fig. 1). For both presence/absence and relative abundance measures, the total number of visits was specific to each pond. For example, if the Pacific chorus frog began calling in March and ended in April at a particular pond, only that time frame was considered. Proportions were determined as follows: if 10 visits were made to a pond during the peak abundance period, with 8 of those yielding a Pacific chorus frog observation, then 80% of the presence/absence surveys were considered correct. Likewise, if relative abundance was determined to be category three (>10 observed to breed in the pond), and 6 of the 10 surveys made yielded category three classifications, then 60% of the surveys were considered correct
(Fig. 1).

Results and Discussion
Overall, the Pacific chorus frog was found to be the most widely distributed anuran, present at the majority of the ponds, while the spotted frog and western toad were observed to a lesser extent (Table 1). These results are consistent with the observations of other studies in that low numbers of spotted frogs and western toads were generally observed (Hayes 1994a, Hayes 1994b, Ross and Esque 1995). However, whether or not these species are declining in northern Idaho is uncertain; prior to creation of the many ponds, virtually no stillwater habitat was found on the Palouse region of Latah County, and historical information is scarce. Moreover, the continuing dispersal to ponds is evident by the spotted frog, for in 1996 three previously uninhabited ponds (with pond ages of 2, 15, and 26 years old) were inhabited by the spotted frog; two by juveniles and one by an adult.

Pacific chorus frog presence/absence data was determined correctly in 84% of the pond surveys, and spotted frog surveys yielded correct values 57% of the time (Table 1). However, western toad presence/absence data was determined to be correct in only 19% of the pond surveys (Table 1). Results for the latter two species indicate additional surveys throughout their peak breeding season might be more appropriate as opposed to one or two surveys during the spring or summer. This is especially important for the western toad, in which all breeding may take place within one week (Samollow 1980). When searching for a particular species, one alternative would be that used by Gulve (1994), who re-sampled ponds only when no observations were made. However, Pacific chorus frog surveys were quite accurate and seem to be reliable if done when general calling in the area is observed. Part of this reliability rests with the tendency of the Pacific chorus frog to be active most of the year, typically at ponds in April and May (Brown 1975, Schaub and Larsen 1978).

Relative abundance estimates of the Pacific chorus frog yielded correct results 71% of the time when their presence was recorded (Table 1). Thus, when presence/absence surveys were correct, relative abundance surveys were fairly reliable as well. However, it should be noted that incorrect relative abundance surveys were found throughout the Pacific chorus frog's breeding season, and did not increase at the beginning or end. This was due to the fact that only surveys between the highest relative abundance estimates were evaluated. Fluctuations within the peak reproductive season are therefore possible and should be considered when relative abundance estimates are undertaken. Western toad results were similar, with correct relative abundance values found in 19% of all surveys through the breeding season and 100% of the time when the species was found. However, spotted frog surveys yielded correct relative abundance only 22% of the time (Table 1). This again suggests the need for multiple surveys to be carried out in the case of spotted frog relative abundance, but this may depend somewhat on the surrounding vegetation present at the pond. Ponds surveyed in Latah County generally were surrounded by cattails (Typha latifolia) and relatively large amounts of overhanging vegetation making presence/absence data and relative abundance surveys more difficult, whereas a montane pond dominated by rocks might be more easily searched.

Egg searches for ponds with anuran species showed that only 5 out of 21 ponds (24%) contained Pacific chorus frog eggs, 8 of 13 ponds (62%) had spotted frog eggs, and no ponds contained western toad eggs. However, adults were present at all these ponds, suggesting either a few ponds supported reproduction among all species or that many ponds are unsuitable for breeding. Further yearly studies are needed though to determine this factor. Spotted frog eggs occurred in a large number of ponds, but they were typically only present in ponds with close proximity to other ponds (200 m) and an undisturbed environment (i.e., no surrounding agricultural or residential activities). Western toads may not be breeding in ponds at all, for juveniles were observed but eggs were not.

It appears the most appropriate number of samples to be made in amphibian monitoring needs to be somewhat specific to the species and study area. However, regardless of the species and area, this study suggests that multiple surveys are necessary to increase accuracy of amphibian monitoring in the future, especially in those cases where the species may be absent some of the time.

Literature Cited

Blaustein, A. R., D. B. Wake, and W. P. Sousa. 1994. Amphibian declines: judging stability, persistence, and susceptibility of populations to local and global extinctions. Cons. Bio. 8:60-71.

Brown, H. A. 1975. Embryonic temperature adaptations of the pacific treefrog, Hyla regilla. Comp. Biochem. Physiol. 51:863-873.

Freedman, J. M., and J. Kerekes. 1985. Acidity and associated water chemistry of amphibian habitats in Nova Scotia. Can. J. Zool. 63:97-105.

Gulve, P. S. 1994. Distribution and extinction patterns within a northern metapopulation of the pool frog, Rana lessonae. Ecology 75:1357-1367.

Hayes, M. 1994a. The spotted frog (Rana pretiosa) in western Oregon. Report for Oregon Dept. of Fish and Wildlife.

Hayes, M. 1994b. Current status of the spotted frog (Rana pretiosa) in western Oregon. Report to Oregon Dept. of Fish and Wildlife.

Heyer, W. R., M. A. Donnelly, M. S. Foster, L. C. Hayek, and R. W. McDiarmid. 1994. Measuring and monitoring biological diversity. Standard methods for amphibians. Smithsonian Institution Press. Washington D. C. 364pp.

Moyle, P. B. 1973. Effects of introduced bullfrogs, Rana Catesbeiana, on the native frogs of the San Joaquin Valley, California. Copeia 1973:18-22.

Ross, D. A., and T. C. Esque. 1995. Historical distribution, current status, and a range extension of Bufo boreas in Utah. Herpetological Review 26:187-188.

Samollow, P. B. 1980. Selective mortality and reproduction in a natural population of Bufo boreas. Evolution 34:18-39.

Schaub, D. L., and J. H. Larsen, Jr. 1978. The reproductive ecology of the Pacific treefrog (Hyla regilla). Herpetologica 34:409-416.

Shirose, L. J., C. A. Bishop, D. M. Green, C. J. MacDonald, R. J. Brooks, and N. J. Helferty. 1995. Validation study of a calling amphibian survey in Ontario. Abstract of paper presented at the Second NAAMP conference, Toronto.

Stebbins, R. C. 1985. A field guide to western reptiles and amphibians. Houghton Mifflin Co., Boston, Massachusetts. 336pp.


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Last Modified: June 2002