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Comparative Catch Efficiency of Sampling Techniques Used in Natural and Modified Terrestrial Habitats in Atlantic Canada

J. D. Adams, D. Clay, and B. Freedman

Department of Biology
Dalhousie University
Halifax, Nova Scotia
B3H 4J1
Ph. (902) 494-3737

[ Abstract ]

We have been examining amphibian populations and habitat in Fundy National Park, NB, Canada, since June 1995. Information from this research will be used to design a longer-term amphibian monitoring program.

Amphibians and their Habitats:
Anthropogenically-disturbed habitats often become unsuitable for certain species of amphibians. For example, logging causes: drying of the surface soil and leaf litter; an increase in surface soil temperature; a loss of woody debris; an increase in soil erosion and compaction; and other detrimental changes in amphibian habitat (Buhlmann, et al., 1988; Jones, 1988; Waldick, 1994). These changes are particularly unfavourable for terrestrial amphibians, which require moist, cool conditions for survival and egg and larval development (Jones, 1988).

Studies have shown that some amphibians prefer natural, mature forests over habitats altered by humans. Diller and Wallace (1994) examined Plethodon elongatus in California, and found that this terrestrial salamander prefers mature forest, but not necessarily old-growth forest. Bury (1983) noted that species richness and abundance were altered for forest-dependent amphibians after logging of redwood forests in California. Waldick (1994) also noted a decline in species diversity and abundance of amphibians in plantations when compared to reference forests in New Brunswick.

Some forestry practices, such as clearcutting and plantation establishment, change habitat conditions needed by some species of amphibians. Waldick (1994) found that conversions of mixed-species forests to conifer stands changed relative humidity and the composition and abundance of leaf litter in areas surrounding Fundy National Park. It can therefore be hypothesised that amphibians relying on these microhabitat features may be in danger of declining as a consequence of extensive forestry-related disturbances. Petranka (1994) believes that after clearcutting most resident salamanders die. Because Fundy National Park is bordered by areas managed for forestry, investigation of relative abundance and species richness of amphibians is important in determining whether amphibians in this area are being unduly affected.

The present study is examining amphibian abundance in both anthropogenically-disturbed and natural forest habitats. In the past, amphibian habitats have largely been categorised as either feeding or breeding habitat (Beiswenger, 1988). This project will confirm these general habitat preferences for terrestrial species occurring in the vicinity of Fundy National Park, as well as quantify specific microhabitat preferences by correlating habitat variables with amphibian abundance and diversity.

Studies that attempt to correlate habitat variables with species presence and abundance must be interpreted with caution, because cause-and-effect relationships are not established by correlations (Diller and Wallace, 1994). Further experiments are needed to explore whether relationships are dependent. However, correlative research is still useful for identifying potential habitat relationships and for suggesting habitats for protection.

General observations have been made regarding amphibian habitat in eastern North America. Specific habitat tolerances have not, however, been investigated much in eastern Canada, particularly in habitats that have been modified by humans, for example, through agriculture or forestry. Also, because habitats of salamanders differ among species and sometimes among geographic areas within species (Tilley, 1973), baseline data on species occurring in Fundy National Park must be obtained before a specific management plan can be developed for this protected area. These baseline data are also important for assessing the effects of various anthropogenic activities on amphibians, such as forestry and road building (Beiswenger, 1988).

In view of these circumstances, we are examining populations of amphibians within various terrestrial habitats in Fundy National Park. This research will provide information regarding habitat preferences by amphibians, which have varying preferences for terrestrial micro- and macro-habitats. This project will provide much needed baseline data that can serve as a reference for establishment of a longer-term monitoring program in the park.

Site Description:
Fundy National Park was established in 1948 and occupies an area of 207 km2 on the south-east coast of New Brunswick (Cooper and Clay, 1994). Because the park is relatively young, there are legacies of pre-establishment, anthropogenic activities, such as logging, agriculture, introductions of non-indigenous plant species, and extirpations of indigenous species and communities. The present mosaic of habitats in the park have, therefore, resulted from a combination of natural and anthropogenic stressors. At present, plantations and clearcuts extensively border the park boundaries. The park is increasingly becoming insularized and cannot be managed in isolation from its surrounding landscape (Freedman et al., 1996). In view of the above circumstances, the ecological integrity of Fundy National Park may be at risk.

The present study is examining amphibians in ten terrestrial habitats within and adjacent to Fundy National Park. The anthropogenically-disturbed habitats include a partial-canopy black spruce (Picea mariana) plantation, a closed-canopy jack pine (Pinus banksiana) plantation, a closed-canopy red spruce (Picea rubens) plantation, an abandoned gravel pit, and an abandoned pasture. The natural areas consist of a mature maple-dominated forest, a budworm-damaged, conifer-dominated forest, and three stands of mature mixed-species forest. These sites were chosen to provide a diverse representation of terrestrial habitats within and surrounding the park.

Because amphibian species vary in trapability, a variety of sampling techniques should be used (Bury and Corn, 1988). Pitfall traps, for example, may not adequately sample rare or fossorial species that move infrequently. To sample the species richness and abundance of amphibians in Fundy National Park, various methodologies were used.

Pitfall traps (transect). This method has been used since the summer of 1995. Twenty pitfall traps were established in each site every 10 m along a 200 m-long transect. Line transects were used instead of a grid because the lines encompassed a greater variety of microhabitats. Microhabitats are defined here as smaller habitats, such as leaf litter or coarse woody debris, within a larger habitat, such as a maple-dominated stand of forest.

The pitfall traps are constructed of 4 L or 6 L plastic buckets (diameter 20 cm) buried in the ground with the opening flush with the surface. A 40 cm x 40 cm untreated piece of plywood was placed over the top of each trap, raised approximately 5 cm, to provide cover. The cover helps prevent excess rain from entering the trap and provides shading that slows evaporation of moisture. The cover may also aid in attracting amphibians to the trap, as a form of refuge. Each trap contained damp moss and/or leaf litter to help prevent discomfort of trapped animals.

The traps were opened at the beginning of the field season (June or July 1995, depending on the site) and closed at the end of October 1995. The traps were opened the following year as soon as snow had melted (early May). Traps are checked every 3 to 14 days, depending on the season. Amphibians caught in the traps were measured for snout-vent and/or snout-tail length and then released at least 40 m perpendicular to the transect.

Pitfall traps (array). Each array consists of three 30 cm-high and 5 m-long drift fences made from 6 mil plastic cut into 1 m x 5 m-long strips. The strips were arranged in a Y-formation, with 3 pitfall traps in the centre of the array and one at each end (modified from Corn, 1994). The plastic was fitted between the three centre pitfall traps so that the direction the amphibians were moving could be determined. Two arrays were set up at each site. All arrays were operational by June 1, 1996 and were checked in the same manner as the pitfall transects.

Time-constrained surveys. These searches were conducted in each site during rainy weather which is assumed to be the best time to locate wandering amphibians. The surveyors actively searched for amphibians in moss, leaf litter, coarse woody debris, and under rocks. The searches are not random, and the surveyors are free to look in microhabitats where they think amphibians may occur, allowing for detection of rare or fossorial species that might otherwise be overlooked using other survey methods. The searches were conducted by two people for one hour each. Two surveys were done at each site in 1996.

Quadrat searches. These searches are a modified version of quadrat sampling (as described in Jaeger and Inger, 1994). Using a random numbers table, the location of a 5 m x 5 m quadrat was chosen in relation to sites along the pitfall transects. Various environmental variables were measured, and then the grid was thoroughly searched, by hand, for amphibians. A total of five quadrat searches were completed for each of the ten sites.

Microhabitat variables. Percent cover for the quadrats was estimated within the following categories: conifer > 2 m, conifer < 2 m, angiosperm > 2 m, angiosperm < 2 m, moss, ferns, leaf litter, bare ground, coarse woody debris, and rock. Percent cover was estimated for the entire 5 m x 5 m grid. Cover and other environmental variables were also measured in 1 m x 1 m plots including: canopy cover; soil pH, temperature, and moisture; substrate resistance; and air temperature.

The environmental variables will be correlated with amphibian species and abundance. Stronger correlations will suggest habitat choice by amphibians.

Results and Discussion:
Because this project is ongoing, the data have not yet been statistically analyzed. However, there are already useful observations of the different methodologies used and their efficiencies.

Pitfall transects and particularly pitfall arrays sampled the most number of species (Table 1). Of these two methods, the pitfall arrays are much more effective at catching amphibians (Table 2). The main difference between these two methods is the use of a drift fence connecting the traps. The drift fence increases the area the trap intersects and guides wandering amphibians into a pitfall trap. The material used for the drift fences was vapor-barrier plastic and this worked well. A preferred material is aluminum flashing (Corn, 1994), but this is expensive.

The quadrat searches only discovered three amphibian species (Table 1). However, Plethodon cinereus was caught frequently in these searches. This species was not effectively retained in the pitfall traps, unless the traps contained water, in which case the animals drowned.

The time-constrained searches did not uncover any species that were not found using another method. They were also more destructive than quadrat searches because the area searched encompassed a greater range. Time-constrained searches can be quite important though, if a particular species is located only in specific microhabitats.

Each of the methods used had disadvantages:
(1) The pitfall transects and pitfall arrays trap more than just amphibians. Shrews and, less commonly, mice and moles are sometimes found dead in these traps. When this happens, the traps are considered "disturbed" and taken out of the analysis since shrews can eat amphibians. Because shrew catching is common and frequent (occurring at least once each time a site is checked), with as many as five shrews in one trap, it is also a potential problem for the shrew population. Pitfall arrays catch more small mammals than pitfall transects for the same reasons that they catch more amphibians. Ant nests being built in the traps is another but less severe problem (occurring over a period of 2-3 weeks in one site). This problem can be solved by digging out the nest.

(2) Pitfall traps can fill with water or become completely dry. Both of these problems can cause amphibian mortality. Species of anurans can survive in water-filled traps whereas P. cinereus and Notophthalmus viridescens drown. However, if traps dry out, some amphibians can become dehydrated and die. Moist moss was placed in the traps to prevent mortality, however, without frequent trap checking, some mortality is inevitable.

(3) Some amphibian species can escape, such as Pseudacris crucifer and P. cinereus. This problem can be overcome by modifying the cover of buckets to create a funnel at the opening of the bucket, preventing animals from escaping (Corn, 1994). However, due to logistical constraints, we did not use this method.

(4) Animal disturbance of traps can sometimes be a problem, but this was not observed often. In fact, the drift fences held up well, and only some minor repairs using a staple gun and duct tape were necessary.

In conclusion, the most efficient and effective method for locating amphibians in terrestrial habitats is pitfall arrays. Although installation is labor-intensive, once installed these provide the best means for trapping many species of amphibians. The actual traps (buckets) can be left in the ground for years if a long-term monitoring program is required, with the fence being replaced yearly. Materials can be inexpensive and still reliable. It is recommended that some method of searching be conducted with the trapping to increase the number of species recorded. Quadrat searching is best, because it disrupts less habitat and is better standardized than time-constrained searching.

References Cited:
Beiswenger, R. E. 1988. Integrating anuran amphibian species into environmental assessment programs. In R. C. Szaro, K. E. Severson, D. R. Patton (eds.), Management of Amphibians, Reptiles, and Small Mammals in North America, pp. 109-128. USDA. Forest Service, Gen. Tech. Rep. RM-166.

Buhlmann, K. A., Pague, C. A. Mitchell, J. C., and Glasgow, R. B. 1988. Forestry operations and terrestrial salamanders: techniques in a study of the cow knob salamander, Plethodon punctatus. In R. C. Szaro, K. E. Severson, D. R. Patton (eds.), Management of Amphibians, Reptiles, and Small Mammals in North America, pp. 38-44. USDA Forest Service, Gen. Tech. Rep. RM-166.

Bury, R. B. 1983. Differences in amphibian populations in logged and old growth redwood forest. Northwest Sci. 57(3): 167-178.

Cooper, L. and Clay, D. 1994. An historical review of logging and river driving in Fundy National Park. Unpublished manuscript of Parks Canada, Alma, N.B., Res. Notes of Fundy Nat. Park No. 94-05. pp. 73.

Corn, P. S. 1994. Standard techniques for inventory and monitoring (Straight-line drift fences and pitfall traps). In W. R. Heyer, M. A. Donnelly, R. W. McDiarmid, L. C. Hayek, and M. S. Foster (eds.), Measuring and Monitoring Biological Diversity. Standard Methods for Amphibians, pp. 109-118. Smithsonian Institution Press, Washington and London.

Diller, L.V. and Wallace, R. L. 1994. Distribution and habitat of Plethodon elongatus on managed, young growth forests in north coastal California. J. Herpetol. 28 (3): 310-318.

Freedman, B., Woodley, S. and Forbes, G. 1996. The Greater Fundy Ecosystem; planning for an ecologically sustainable landscape. In Proceedings: IUCN World Congress, Montreal, Quebec, in press.

Jaeger, R. G. and Inger, R. F. 1994. Standard techniques for inventory and monitoring (Quadrat sampling). In W. R. Heyer, M. A. Donnelly, R. W. McDiarmid, L. C. Hayek, and M. S. Foster (eds.), Measuring and Monitoring Biological Diversity. Standard Methods for Amphibians, pp. 97-102. Smithsonian Institution Press, Washington and London.

Jones, K. B. 1988. Comparison of herpetofaunas of a natural and altered riparian ecosystem. In R. C. Szaro, K. E. Severson, D. R. Patton (eds.), Management of Amphibians, Reptiles, and Small Mammals in North America, pp. 222-227. USDA Forest Service Gen. Tech. Rep. RM-166.

Petranka, J. W. 1994. Response to impact of timber harvesting on salamanders. Conserv. Biol. 8:302-304.

Tilley, S. G. 1973. Life histories and natural selection in populations of the salamander Desmognathus ochrophaeus. Ecology. 54: 3-17.

Waldick, R. C. 1994. Implications for forestry-related habitat conversion on amphibians in the vicinity of Fundy National Park, New Brunswick. M. Sc. thesis, Dalhousie University, Halifax, N.S.

U.S. Department of the Interior
U.S. Geological Survey
Patuxent Wildlife Research Center
Laurel, MD, USA 20708-4038
Contact: Sam Droege, email:
Last Modified: June 2002