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Results of the first anuran calling survey in Nebraska.

ABSTRACT

In 1997, the Nebraska Game and Parks Commission began an anuran calling survey using routes following those of the upland game bird surveys. At each stop the observer recorded start and stop times, air temperature, cloud cover, precipitation events, wind speed (Beaufort scale), species heard calling, and the frequency of calls heard. An index of calling frequency was standardized on a scale of I (no call overlap) to 3 (full chorus). The number of sites occurring within a species' range (n) was determined based upon the number of sites per survey route and species distribution in Nebraska. A mean call index was calculated based upon the value reported for each species. Thirty-eight routes were completed, of which 17 were surveyed during each of the three sample periods. Thirteen routes were sampled twice and eight were surveyed only once. Thirty-two, 21, 18, and 14 routes were surveyed during April, May, June, and July, respectively. All anuran species known to occur in Nebraska except Gastrophryne olivacea, the Great Plains Narrow Mouth Toad, were recorded during the survey period. Hyla chrysocelis, Acris crepitans, and Pseudacris triseriata were heard most frequently. Spea bombifrons, Bufo cognatus, and Rana caatesbeiana were heard least frequently (< 5% of total sites within known range). Bufo americanus was heard at four sites along two routes.

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The status of amphibian populations worldwide is under debate (Wyman 1990). Though it has been noted that amphibians are becoming more difficult to find, data from long-term, large-scale, multi-taxa census studies are largely unavailable, and reasons for declines are difficult to discern (Pechmann et al. 1991). Since the late 1980s, efforts have been made to determine the extent to which amphibian populations are declining and their causal mechanisms (Blaustein et al. 1994; Corn 1996; Pechmann et al. 1991; Pechmann and Wilbur 1994).

In 1994 the North American Amphibian Monitoring Program (NAAMP) was initiated to collect reliable data pertaining to the abundance, distribution, physiology, and reproductive status of amphibians. The NAAMP suggests several methods for standardized data collection, including anuran calling surveys, terrestrial salamander surveys, regional amphibian and reptile counts, and aquatic surveys of egg masses and tadpoles. This paper reports the results of the first anuran calling survey conducted in Nebraska and addresses concerns for future programs of this nature in Nebraska.

METHODS

In 1997, the Nebraska Game and Parks Commission (NGPC) initiated an anuran calling survey following NAAMP protocol in an effort to establish base-line census data for detecting long term spatial and temporal changes in anuran populations. Amphibian breeding occurs in Nebraska from March through August, with each species having specific periods during which it can be heard in breeding choruses (Billings 1973; Hudson 1953; Lynch 1985). NGPC biologists were trained to identify calls of individual species and mixed choruses of frogs. Anuran survey routes followed those of the upland game bird surveys (Taylor, J.S., Nebraska Game and Parks Comm., pers. comm.). Routes were 16-48 km, with travel direction assigned randomly. With one exception, each route consisted of 10 wetland areas, > 500 in apart to prevent site-to-site call overlap (one route in Gage County had four additional wetland areas). Data were collected for each route at least once during each of three periods (1-28 April, 7 May-4 June, and 13 June-10 July).

For safety and reliability, no survey was conducted during heavy rain or wind > 24 km/hr. Surveys were conducted < 30 minutes of sunset following periods of rain of high humidity. At each stop the observer recorded start and stop times, air temperature, cloud cover, precipitation events, wind speed (Beaufort scale), species heard calling, and the frequency of calls heard. Approximately five minutes were spent at each wetland area during the auditory survey. Two minutes were assigned to wait time, allowing calling to resume, and three minutes for recording observations. An index of calling frequency was standardized on a scale of 1 (no call overlap) to 3 (full chorus).

The number of sites occurring within a species' range (n) was determined based upon the number of sites per survey route and species distribution in Nebraska (Lynch 1985). If a route was surveyed more than once in a given month, the route was counted only once and each site at which anurans were heard was counted only one time. A mean call index was calculated based upon the value reported for each species. For the purpose of calculating the mean call index, sites visited more than once a month were treated independently so the mean call intensity could be calculated for each month.

RESULTS

Thirty-eight routes were completed (Fig. 1), of which 17 were surveyed during each of the three sample periods. Thirteen routes were sampled twice and eight were surveyed only once. Thirty-two, 21, 18, and 14 routes were surveyed during April, May, June, and July, respectively. All anuran species known to occur in Nebraska (Lynch 1985) except Gastrophryne olivacea, the Great Plains Narrow Mouth Toad, were recorded during the survey period (Table 1). Hyla chrysocelis, Acris crepitans, and Pseudacris triseriata were heard most frequently. Spea bombifrons, Bufo cognatus, and Rana caatesbeiana were heard least frequently (< 5% of total sites within known range). Bufo americanus was heard at four sites along two routes.

[FIGURE 1 OMITTED]

DISCUSSION

Of the 11 anurans known to occur in Nebraska, Gastrophryne olivacea and Bufo americanus have extremely limited ranges, whereas most other species have nearly statewide distributions (Lynch 1985). Other taxa included in Nebraska's herpetofaunal composition include Acris crepitans, Bufo cognatus, Bufo woodhousii, Hyla chrysocelis, Pseudacris triseriata, Rana blairi, Rana catesbeiana, Rana pipiens and Spea bombifrons.

Gastrophryne olivacea is known from only two counties in south central Nebraska (Lynch, J.D., unpublished), only one of which (Gage County) was surveyed. Thus, due to its scarcity, it is not surprising that this species was not recorded. Pseudacris triseriata occurs commonly throughout the state and we expected it to be recorded at more sites. This species is likely the most commonly encountered frog in the state and is known to begin calling as early as March and continue through August (Lynch 1985). Spea bombifrons and Bufo cognatus have statewide distributions and appear at breeding sites after heavy rains during late spring and early summer. We believe these species may have been under-represented due to the limited time they occur at breeding sites, or because wetlands along survey routes were not the ephemeral pond types used by these organisms. Rana catesbeiana was also heard infrequently (< 5% of sites within the species' range). Rana catesbeiana occurs only in permanent bodies of water and therefore, depending on the types of habitat present along survey routes, may simply have not been encountered. Bufo americanus is known from only two localities in Nebraska (Burt and Sarpy counties) and appears to be restricted to hardwood forest habitats along the Missouri River (Lynch, J.D., pers. comm.). The presence of B. americanus was reported from one route in both Dodge and Saunders counties. It is known that B. americanus and B. woodhousii interbreed and their calls can be easily confused or misidentified. It is also possible that B. americanus has extended its range in Nebraska. Further work is required to verify these species.

Calling surveys, such as this one, are limited to those amphibian choruses that can be heard at the time the route is driven. It is possible that the frogs and toads in the area may not be calling for a variety of reasons, ranging from predators being in the vicinity, to poor environmental conditions, to the presence of the observer. Problems may also exist in interpreting the size of a chorus due to external noise pollution (e.g. irrigation motors, passing vehicles, and overhead powerline noise). Further, observer bias may contribute to misidentification of calls if, for example, certain species are expected at a site or within a particular geographic range. No verification procedure was in place to validate the calls reported and therefore the extent of misidentification is unkown. For example, Rana pipiens, occurring north of the Platte River, was recorded on a route well outside its range (in Pawnee County) and may have been mistaken for R. blairi, which occurs south of the Platte.

Calling surveys do not measure reproductive success. For example, Hyla chrysocelis was heard frequently during this survey (32.8--38.9% of total sites within its range), yet in a concurrent study no larval sample of H. chrysocelis was taken from sites within this species' range during the same time period (McLeod 1999). Therefore we recommend concurrent larval sampling at sites where calling surveys are conducted to determine whether breeding had in fact occurred. In addition, a verification procedure should be established to ensure the accuracy of field data collected if it is to be useful in the future for determining the status of Nebraska's amphibian populations. A possible solution is to have observers work in pairs, experienced with beginner. This would ensure better-trained observers for future studies. Last, we recommend surveys be conducted three times each month from April through July to establish a more complete record of anuran distribution.

Long-term monitoring programs are essential to accurately evaluate the status of amphibian populations. The 1997 anuran calling survey is a first step in establishing such a database in Nebraska.
Table 1. Anuran calling survey results by period for Nebraska, 1997.
Mean call indices were calculated based upon values from sites at
which the species was found. The total number of sites within the
species' (n) reflects distributions presented by Lynch (1985).
Species codes are as follows: Hyla chrysoscelis (196), Acris crepitans
(191), Pseudacris triseriata (214), Bufo americanus (172), Bufo
woodhousii (188), Bufo cognatus (175), Rana blairi 224), Rana
catesbeiana (228), Rana pipiens (239), Spea bombifrons (142),
Gastrophryne olivacea (218).

 Species code 196 191 214 172
 n 90 244 384 0

1-28 Mean Call Index 0.00 2.00 2.33 0.00
April No. Routes 0 2 25 0
 No. Sites 0 3 228 0
 % of Total 0.00 1.23 59.38 0.00

7 May- Mean Call Index 1.86 1.88 2.56 0.00
4-Jun No. Routes 6 12 28 0
 No. Sites 35 40 162 0
 % of Total 38.89 9.02 42.19 0.00

13 Jun- Mean Call Index 2.14 2.30 2.09 2.00
10-Jul No. Routes 6 12 19 2
 No. Sites 29 37 85 4
 % of Total 32.22 15.16 22.14 4.00

 Species code 188 175 224 228
 n 384 384 384 384

1-28 Mean Call Index 0.00 1.00 1.38 1.00
April No. Routes 0 1 11 1
 No. Sites 0 3 21 1
 % of Total 0.00 0.78 10.55 0.26

7 May- Mean Call Index 1.70 1.70 1.27 1.70
4-Jun No. Routes 18 6 8 5
 No. Sites 71 10 11 10
 % of Total 18.49 2.60 2.86 2.60

13 Jun- Mean Call Index 1.68 2.50 2.05 1.18
10-Jul No. Routes 13 3 9 7
 No. Sites 37 4 19 17
 % of Total 9.64 1.04 4.95 4.43

 Species code 239 142 218
 n 254 321 13

1-28 Mean Call Index 1.73 0.00 0.00
April No. Routes 6 0 0
 No. Sites 22 0 0
 % of Total 8.66 0.00 0.00

7 May- Mean Call Index 1.26 2.00 0.00
4-Jun No. Routes 7 4 0
 No. Sites 19 12 0
 % of Total 5.92 3.74 0.00

13 Jun- Mean Call Index 0.00 1.56 0.00
10-Jul No. Routes 0 4 0
 No. Sites 0 18 0
 % of Total 0.00 5.61 0.00


ACKNOWLEDGMENTS

We thank M. Fritz for graciously providing materials and background information, and J. Lackey and C. Colt were involved in overseeing the 1997 survey and data entry. This project would not have been possible without the time and efforts of the observers.

LITERATURE CITED

Billings, J.T. 1973. A partial definition of the fundamental niche of larval Rana pipiens (Ranidae, Amphibia). M.S. thesis, University of Nebraska-Lincoln.

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. Conservation Biology 8: 60-71.

Corn, P. S. 1996. Effects of ultraviolet radiation on Boreal Toads in Colorado. Ecological Applications 8: 18-26.

Hudson, G. E. 1953. The amphibians and reptiles of Nebraska. Nebraska Conservation Bulletin 24: 1-146.

Lynch, J. D. 1985. Annotated checklist of the amphibians and reptiles of Nebraska. Transactions of the Nebraska Academy of Sciences 13: 33-57.

McLeod, D. S. 1999. A re-survey of amphibian populations in Nebraska after twenty years: A test for declines. M.S. thesis, University of Nebraska-Lincoln.

Pechmann, J. H. K., D. E. Scott, R. D. Semlitsch, J. P. Caldwell, L. J. Vitt, and J. W. Gibbons. Declining amphibian populations: The problem of separating human impacts from natural fluctuations. Science 253: 892-895.

--, and H. M. Wilbur. 1994. Putting declining amphibian populations in perspective: Natural fluctuations and human impacts. Herpetologica 50: 65-84.

Wyman, R.L. 1990. What's happening to the amphibians? Conservation Biology 4: 350-352.

David S. McLeod * and John T. Vaughan

School of Biological Sciences

University of Nebraska-Lincoln

Lincoln, Nebraska 68588-0118

david@mail.nehs.hc.edu.tw

Kevin E. Church

Advanced Telemetry Systems, Inc.

470 First Avenue North, Box 398

Isanti, Minnesota 55040

Daylan Figgs

Wildlife Research Section

Nebraska Game and Parks Commission

Lincoln, Nebraska 68503-0307

* Author for correspondence.
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Author:Mcleod, David S.; Vaughan, John T.; Church, Kevin E.; Figgs, Daylan
Publication:Transactions of the Nebraska Academy of Sciences
Date:Jun 1, 2001
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