Home Archived April 13, 2016

Survey Methods


There are multiple ways to sample amphibians. Techniques may be species, habitat, or season specific. Every technique is accompanied by underlying biases, limitations and assumptions that need to be considered carefully, in the context of the goals of each study, before the technique is implemented. Multiple techniques are often used in a single area, because one technique is generally not adequate to document all species of interest. Heyer et al. (1994) provides an excellent description of many techniques and their assumptions (Heyer, W.R., M.A. Donnelly, R.W. McDiarmid, L.C. Hayek, and M.S. Foster. 1994. Measuring and monitoring biological diversity standard methods for amphibians. Smithsonian Institution Press, Washington, D.C.). The SE ARMI Web site also provides good information on sampling design and technique limitations.

We have emphasized a metapopulation approach in our sampling design because amphibian populations are often linked among breeding sites by migration. The use of a metapopulation framework (in a large landscape, the random sampling of groups of sites rather than individual sites) will improve the efficiency of field work and may allow us to examine the role of isolation in local population dynamics.

We built a set of hierarchically nested drainage catchments from the USGS Elevation Derivatives for National Applications (EDNA) Project to determine our survey units. Drainage boundaries are overlaid onto National Wetlands Inventory (NWI) maps of the study area. This method allows us to aggregate “drainages” to a hierarchic level as necessary. The criterion for describing a drainage is the inclusion of 10–50 identifiable NWI water bodies. This exercise results in a variable number of potential survey units depending on the size and topography of the study area.

We conduct surveys of all potential amphibian breeding sites within the chosen drainage during times when amphibian eggs, larvae, or metamorphs are likely to be present if the species occurs in the area.

Due to small size or lack of topographic relief, some survey areas do not lend themselves to using drainages as unit boundaries. In these cases, such as our surveys on Bureau of Land Management lands in Wyoming and in Great Sand Dunes National Park and Preserve in Colorado, we use grid cells rather than drainages to define our study units.

Map of Glacier National Park, Montana
Watershed delineation for Glacier National Park 115 watersheds<br />
              (green) were sampled in 2001, not including sentinel sites.
Drainage delineation for Glacier National Park, Montana. Fifteen watersheds (green) were sampled in 2001, not including apex sites (white).
Map of Rocky Mountain National Park, Colorado
Drainage deliniation for Rocky Mountain National Park, Colorado. 8 drainages were sampled in 2002, not including sentinel sites.
Drainage deliniation for Rocky Mountain National Park, Colorado. Eight drainages were sampled in 2003, not including apex sites.

We use Visual Encounter Surveys to document the presence of amphibians Visual Encounter Surveys are effective in most habitats and for most species that breed in lentic (non-flowing) water. Surveys are conducted by a pair of workers walking slowly around the perimeter and shallows of a water body while searching for amphibians. Multiple transects are used for large wetland complexes such as marshes that cannot be surveyed completely. Most amphibians are identified by sight, but surveyors use dip nets in areas with vegetation or where water clarity is poor. We record all life history stages (eggs, larvae, juvenile, or adult) of each species Observed; however, detection of evidence of breeding activity (presence of eggs or larvae) is emphasized so that future surveys can use changes in the number and distribution of breeding sites for each species to assess status and trends. Data collected yields information on the presence of a species but does not establish absence, nor does it give reliable estimates of abundance.

Our goal is to survey a sub-sample of sites twice within a short time period (e.g., 2 weeks) so that we can estimate species- and time-specific detection probabilities. Collection of data in this manner will allow us to control for the effects of variation in timing of breeding by different species, and subsequently to estimate the proportion of the sampling area occupied by breeding populations of each species (MacKenzie, D.I., J.D. Nichols, G.B. Lachman, S. Droege, J.A. Royle and C.A. Langtimm. Estimating site occupancy rates when detection probabilities are less than one. 2002. Ecology 83:2248–2255).

Occupancy, or proportion of area occupied, by breeding populations of each species is the primary response variable used for detecting trends of amphibians in the Rocky Mountain region. Occupancy is based on a closed-population model that incorporates detection probabilities along with the number of water bodies where each species is detected to estimate the area occupied by each species. For common species, Occupancy is based on the proportion of sites where breeding populations are detected. For rare species such as the boreal toad, occupancy may be scaled up to the proportion of watersheds (or other large units of land) occupied rather than sites.

 Survey methods, National Elk Refuge, Grand Teton National Park. Photos: Debra Patla

Survey methods, National Elk Refuge, Grand Teton National
Park. Photographs courtesy of D. Patla

Data Collection*

Collective images of team member using a PDA.In general, our field data are housed in either an Excel® spreadsheet or an Access® database. We began our pilot year (2002) using handheld computers for all data collection in the field on all of our projects. Field technicians input data directly into a handheld using forms (the equivalent of datasheets) specifically designed for the task at hand. We are using Access® and Pendragon® to create dataforms that are loaded onto the handheld in the lab, and accessed and completed at the study site. Data is downloaded directly to the office PC through a "hot sync" from each handheld. We use waterproof cases (Aquapac®) to protect the handheld and a backup memory chip to prevent loss of data due to power or battery failure. Pendragon®, the application that enables data collection on the handhelds, was designed in Access. Therefore, all data that is collected with Pendragon® creates an Access® file.

These files can be linked together to match databases that have already been created locally in Access® or exported to Excel®. These files can be appended to existing Access® data files or copied and pasted onto an existing Excel® file. Because data can be downloaded directly from the handheld to a spreadsheet or a relational database, we find that using handhelds decreases transcription error, the number of illegible entries, and typographical errors. Using pre-designed forms on the handheld we can ensure completeness of recorded field information (it is possible to disallow the movement to the next box on the form if the current box is not filled in). There is also a substantial reduction in the time from data collection to data analysis.

We estimate that we save a minimum of 60 hours per summer inputting the data and in the quality assurance phase of the project. This represents a substantial savings based on 1.5 weeks of technician time.

Our data becomes part of the ARMI national database served from the USGS-Western Ecological Research Center, San Diego Field Station).

* Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.