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Checklist and "Pollard Walk" Butterfly Survey Methods on Public Lands

Introduction


Guidelines for management of native prairie butterfly populations have recently appeared in the literature (Moffat and McPhillips, 1993; Swengel, 1996). However, more precise information on which species are present at a particular site and how they might respond to management practices such as prescribed burning, weed control or grazing, is typically not available to personnel responsible for such operations. Gaining detailed local knowledge about butterfly populations on public lands is therefore crucial to the development of long-term conservation strategies, particularly for areas in which species may be declining or imperiled.

This situation is exacerbated by the fact that beyond our very basic recognition of their proclivity for natural conditions, we lack a clear understanding of many of the factors that influence the distribution and abundance of native prairie butterflies, particularly in the extensive public lands of the northern Great Plains. Until very recently, we have also lacked any systematic approach to monitoring their distributions and numbers. Recently, Stanford and Opler (1993) and Opler (1995) have provided a comprehensive picture of species distributions in the U.S., including the Great Plains states, through county-by-county (dot map) records of formally reported occurrences. The Fourth of July Butterfly Counts of the Xerces Society/North American Butterfly Association (Swengel, 1990) offer a basis for long-term monitoring of selected sites by volunteer groups. A standardized methodology for undertaking and sustaining local butterfly monitoring programs at public sites is now needed. Our objective in this study was to define more clearly the relative roles of unrestricted comprehensive search (hereafter "checklist") and restricted "Pollard Walk" (hereafter "transect") methods of butterfly counting at seven extensive, federally managed sites across North Dakota (see fig.1).

GIF -- figure 1
Fig. 1.--Distribution and location of study sites within North Dakota: 1. Sheyenne National Grassland (28,433 ha); 2. Chase Lake National Wildlife Refuge Complex (4248 ha); 3. Lostwood National Wildlife Refuge (10,824 ha); 4. J. Clark Salyer National Wildlife Refuge (23,756 ha); 5. Sullys Hill National Game Preserve (678 ha); 6. Summit Campground, McKenzie Unit, Little Missouri National Grassland (203,625 ha); 7. Burning Coal Vein Campground, Medora Unit, Little Missouri National Grassland (212,430 ha).

Checklist Surveys. -- Checklist surveys are employed primarily to confirm the presence of species and sometimes the number of individuals of each species for the survey site. Besides meteorological data and raw numbers, few other variables are taken formally into account. One important advantage of checklist counting is that an observer is free to search out places where butterflies typically would breed or congregate. Another is that checklist counting is procedurally simple; the recorder need merely identify and count without regard to other factors. As S. Droege (pers. comm.) has noted, such anecdotal data are far better than none at all and can easily be produced by informed volunteer amateurs and hobbyists.

Because it imposes few procedural constraints, checklist counting is also more flexible than transect sampling. For example, it allows immediate response to periodic changes in site condition (e.g., daily changes in wind, or weekly, monthly or annual redistribution of larval host plants, nectar sources, etc.). Species presence may thus be confirmed without sophisticated research design or secondary data analysis and with a minimum of effort. This combination of procedural freedom and economy of effort is arguably the most important feature of checklisting.

As Hellawell (1991) has noted, however, such "open-ended" survey approaches frequently are "inadequate to meet the rigors of statistics." Patches covered and time spent in a given patch may differ by observer or from count to count, for example. Relative abundance is difficult to estimate accurately across a series of checklist data sets unless the total number of sets is very large (S. Droege, pers. comm.). Swengel (1990) has reviewed a variety of such problems with Christmas Bird Count data, including those stemming from year-to-year inconsistency in counting routes, differences in method and observer, differences in observer numbers, concern over accuracy in identification, locational biases related to where observers live, and problems with sample size in relation to statistical requirements. Since the Fourth of July Butterfly Counts are modeled after the Christmas Bird Count system, we reasoned that these problems are of similar relevance when counting butterflies.

Hellawell (1991) has also noted that "(difficulty with) open-ended monitoring strategy is avoidable, provided that clear objectives are set and a true monitoring yardstick is defined at the outset." If the purpose of checklist counting is merely to confirm the presence of certain species or to define the scope of a faunal list, strict control of variables may be of secondary concern. [Samways (1994) has emphasized the importance of "before" and "after" checklist documentation of species richness in rapidly declining environments, for example.] However, if continuous monitoring or indexing of actual or relative abundance is a concern, as for example in working with rare or endangered species, then a more carefully designed sampling model is essential. For this purpose, the transect method developed by Pollard et al. (1975), and later adapted by Pollard (1977, 1982), Pollard and Yates (1993), and others, has frequently been employed by butterfly counters, particularly in the United Kingdom.

Pollard Walk surveys. -- "Pollard Walk" surveys employ fixed travel routes during counting. More rigorous statistical analysis of Pollard Walk transect data is possible because counts are conducted in a much more uniform manner with respect to area covered and time spent. Moreover, fixity of extent and location of transects allows subsequent or concurrent study of multiple factors (e.g., floral and faunal studies on the same transect). Definite extent and permanent location also make frequent replication possible. This uniform delimitation of parameters, which allows confident longitudinal monitoring, is one of the most important features of transect sampling.

Unfortunately, the finite extent and precise location of a fixed transect also introduces the likelihood that some localized, sedentary species may never appear in a count. Given commitment to long-term sampling of a fixed transect, it may not be possible to account for periodic changes in larval habitat or adult nectar sources that occur off the transect. Since confirmation of the presence of a particular species is limited to the transect, the surveyor is less able to generalize the status of a given species across an entire site. These factors underscore the importance of original transect location and layout, especially in terms of habitats or microhabitats included and potential influences of day-to-day changes in wind, sunlight and other environmental factors.

Because both checklist and transect approaches have been in common use elsewhere, we wanted to define more clearly the difference between them in the numbers of species and individuals a surveyor would detect per equivalent unit of search time. Specifically, our null hypothesis was that there would be no significant difference between the two methods either in terms of (1) the number of individual butterflies detected per hour of effort, or (2) the number of butterfly species detected per hour of effort.


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