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

Field Methods


Study sites. -- Seven widely separated North Dakota sites were selected. They were chosen because they differed ecologically and because they represented substantial area under public control. One Pollard-style transect route was surveyed and mapped at each site. In accordance with the methods of Pollard (1977) and Pollard and Yates (1993), each route was designed to traverse a range of native habitats deemed most representative of the majority of terrain at the site. Routes were also designed to represent a variety of topographical and physical aspects. Following are brief descriptions of the seven sites and their transect configurations.

1. Sheyenne National Grassland, Ransom County (28,433 ha). -- An 800-m rectangular Pollard Walk route was laid out, beginning at the southwestern end of the unit's North Country Trail segment, lat 46 23' 50" N, long 97 27' 52" W. The entire transect included true native tall grass prairie, although it lacked the usual diversity of forbs expected of healthy prairies in this region and was intermittently invaded by Kentucky bluegrass (Poa pratensis). Patches of willow (Salix spp.) and drier, shorter prairie were interspersed along the route. This site was selected to include native prairie hesperiids and satyrids as well as the regal fritillary (Speyeria idalia), then a candidate for listing by the U.S. Fish and Wildlife Service under the Endangered Species Act.

2. Chase Lake National Wildlife Refuge Complex, Stutsman County (4,248 ha). -- An 800-m rectangular Pollard Walk route was laid out, beginning at lat 47 03' 16" N, long 99 25' 08" W. It traversed relatively uniform rolling landscape representative of generally homogeneous native mid-grass prairie, which predominates throughout the complex.

3. Lostwood National Wildlife Refuge, Burke County (10,824 ha). -- A 700-m Pollard Walk route began NW of Thompson Lake at lat 48 39' 10" N, long 102 24' 42" W, thence leading to the E, between a service road and the western lake shore. It followed a prairie trail to the top of a rise, traversed a brushy area along the high ground W of the lake, and dropped into a wet meadow, as defined by Stewart and Kantrud (1971). The transect was representative of the majority of mesic mid-grass prairie microhabitats within the refuge.

4. J. Clark Salyer National Wildlife Refuge, McHenry County (23,756 ha). -- An 800-m Pollard Walk route was located in rolling aspen parkland SW of the refuge's popular Sandhills Walk. It began at lat 48 32' 06" N, long 100 31' 00" W, leading thence westward between the refuge's "scenic drive" and picnic area, through a rolling combination of sandy prairie and woodland margins that included quaking aspen (Populus tremuloides), serviceberry (Amelanchier sp.), leafy spurge (Euphorbia esula), wild cherry (Prunus sp.) and bur oak (Quercus macrocarpa). It included habitat for both prairie and woodland species indigenous to N-central North Dakota.

5. Sullys Hill National Game Preserve, Benson County (678 ha). -- A 1500-m Pollard Walk route was marked out along the exact path of the preserve's nature trail, beginning S of Sweetwater Lake, at lat 47 58' 30" N, long 98 58' 30" W, and ending on the NW shore of the lake near the preserve's amphitheater. The existing trail was judged to be the most logical and ecologically varied line of survey travel through woodland habitats, which cover nearly all of the northern unit of the preserve. Hay meadows SW of the main preserve were not sampled by transect counting, but were subjected to occasional checklist surveys. The transect included dense bur oak and basswood (Tilia americana) woodland with occasional shaded openings, an area of woodland stream margin, several exposures of N-slope grassland clearing, and a section of oak parkland with dogbane (Apocynum androsaemifolium), serviceberry and beaked hazel (Corylus cornuta) in the understory. The wide variety of habitats along this route produced an array of species generally within the Upper Transition Zone woodland fauna.

6. Summit Campground, Little Missouri National Grassland (McKenzie Unit), McKenzie County (203,625 ha). -- An 800-m Pollard Walk route began at lat 47 32' 36" N, long 103 14' 36" W. It included the northern margin of a W-sloping wooded break, the southern margin of a 0.4-ha water impoundment, crossing the woodland proper to the southern margin of the break and passing thereafter along a ridge separating the woodland on the N and badlands on the S. The route included rocky mountain juniper (Juniperus scopulorum) and green ash (Fraxinus pennsylvanica) woodland, mixed grassland, wetland margin and badlands habitats.

7. Burning Coal Vein (BCV) Campground, Little Missouri National Grassland (Medora Unit), Billings County (212,430 ha). -- A 700-m Pollard Walk route began at lat 46 35' 30" N, long 103 26' 40" W, on the eastern end of the parking lot at the Columnar Junipers Overlook. The route included sage steppe, badlands terrain, and a shady wooded ravine. It included several hundred meters within habitat for Strecker's giant skipper (Megathymus streckeri), a species occurring in North Dakota only at this site, as well as habitat for a majority of upland species indigenous to prairie and sagebrush habitats of the Little Missouri River drainage.

Counting methods. -- We intended that each site would be visited at least twice during each of the state's three episodes of butterfly emergence and activity, as outlined by McCabe and Post (1976) and Royer (1988). These periods are 15 May - 5 June (spring emergents), 5 June - 5 July (early summer and first-brood bivoltine emergents), and 25 July - 15 August (late summer univoltine and second-brood bivoltine emergents). However, a delayed and somewhat rainy season required an adjustment of that schedule approximately two weeks toward autumn.

During each visit to a site, the observer tallied number of individual butterflies and number of species using both checklist and transect survey methods. Protocols for counting and recording generally followed Pollard (1977) and Pollard and Yates (1993). These were essentially identical for both methods, with the exception that in the transect method the recorder walked only along the precisely marked route, whereas in the checklist method the recorder was free to wander at will in active search of productive habitats, nectar sites, etc. These two methods were alternated continuously on a 2-h schedule throughout the day.

Weather permitting, each sampling day extended from approximately 0900 CDT to generally not later than 1800 CDT. At the beginning of each day, a coin was flipped to determine which sampling method was to be employed first. During both methods, the recorder walked at a steady pace of approximately 35 m/min. While specific habitats were intentionally sought out during checklisting, only butterflies actually seen within an estimated 2.5 m on either side of the recorder, within 5 m above the ground, and within 5 m to the front were actually counted. Butterflies behind the recorder or otherwise outside these limits were never counted. No active pursuit was made during either method, nor was undue effort made to count butterflies that hid themselves from view or that perched out of sight of the recorder during counting. When individuals flew ahead of the recorder, if the recorder could be certain that they had already been counted, they were ignored; if the recorder could not be certain, they were counted. Counting was curtailed whenever cloud cover was estimated to be greater than 50% or when wind speed exceeded 25 km/hr.

A net was carried, and when problem individuals could be netted without undue difficulty or active pursuit, they were netted and released as soon as identification had been made. In such cases, counting was stopped until the walk was resumed. Where distinction between two alternative species was not possible, the commoner of the two options was recorded. For the rare cases involving more than two alternatives, or when individuals eluded both identification and capture, identification was made to the lowest confident taxonomic level.

Between counting periods, voucher specimens were collected for any species not already on record for the county or the site. All such specimens bear the label designation "NBS Survey 1995" and are permanently on file in the North Dakota state voucher collection of the first author. To minimize the possible statistical effect of removal, no voucher was taken at any point closer than 100 m from the nearest point on a transect.

The generic taxonomy of Miller and Brown (1981), as amended by Ferris (1989), has long been employed in the technical literature and until recently in the annual Season Summary of the Lepidopterists' Society. However, a majority of amateur butterfly counters have begun to employ a different nomenclature, as recently published by the North American Butterfly Association (1995). For the sake of continuity with other counting efforts, the latter is essentially the nomenclature employed in this report.

Checklist methodology. -- Preliminary checklists were based on hypothetically complete lists of regularly breeding resident species for each site. They were developed as a composite of formal records for the county of the site and all adjacent counties. Occasional or infrequent immigrant species were excluded, but species were included when it seemed likely that a lack of records derived from incompleteness of information rather than actual absence. A separate reporting form was used for each episode of checklist surveying.

With these lists, each site was subjected to comprehensive checklist search aimed at (1) identification and confirmation of all species occurring on the site and (2) determining the number of individuals of each species encountered per hour of search effort. In rare instances where identification was impossible without handling, an example was collected. Within the constraints of the above counting method, each butterfly encounter was tallied in a blank space provided next to the species name on the checklist.

Transect methodology. -- On the ground, each transect was composed of flagged 10-m segments and, where possible, of permanently placed, fireproof 100-m reference posts. This scheme was in turn represented on the reporting form by a series of 100-m linear scales, each divided into 10 boxes corresponding to the flagged 10-m segments. Butterflies encountered along the transect were identified and tallied by location within each 10-m segment. This made it possible later to define a precise location for each butterfly counted, and to tally (1) number of species and (2) number of individuals of each species encountered per hour of search effort. A separate reporting form was used each time the transect was surveyed.

Data analysis. -- To assess differences between the two survey methods in the number of individual butterflies and the number of butterfly species observed, we used analysis of covariance (ANCOVA) techniques. ANCOVA techniques were used because the number of individual butterflies observed and the number of species observed are expected to increase with increasing search time, in a way similar to that seen in species-area curves (Rosenzweig, 1995:8-22). Therefore, we used hours of searching effort as a covariate to adjust the number of individual butterflies and number of butterfly species before comparing the two survey methods. We first summarized the data by summing the number of butterflies, the number of species, and the total amount of survey effort (hours) across surveys within a survey date, for each method at each site. For the ANCOVAs, sites were considered to be random blocks in a completely randomized block design, with survey date the sampling unit (Milliken and Johnson, 1984:52). The number of butterflies and number of species were ln(Y+1) transformed to linearize their relationship with hours of effort, which was also ln(Y+1) transformed (Rosenzweig, 1995:8-22). ANCOVAs were done using the general linear models procedure (PROC GLM) of SAS (SAS Institute Inc., 1989), with 0.05 the significance level. We report means as least squares means (LSMEANS) (SAS Institute, Inc., 1989) where appropriate.

We also computed odds-ratios (Agresti, 1990:15) to address the question of whether a particular butterfly species was more likely to be observed using one survey method as opposed to the other. An odds-ratio, which is not a probability, is a ratio of the ratio of two odds. Computationally, the odds ratio of checklist (c) to transect (t) is:

GIF -- equation 1

where

GIF -- c = odds in favor of detecting versus not detecting a particular butterfly species using the checklist,

GIF -- t = odds in favor of detecting versus not detecting a particular butterfly species using the transect,

GIF -- c = probability of detecting a particular butterfly species using the checklist, and

GIF -- t = probability of detecting a particular butterfly species using the transect.

Therefore, an odds-ratio of 1.0 implies an equal chance of observing a particular butterfly species using the checklist approach as compared to the transect approach. We estimated the odds-ratio of the checklist survey method vs. the transect survey method for each butterfly species observed at each site using each survey count within each survey date as an independent observation. For species with zero counts for one of the methods, we added 0.5 to all counts in estimating the above probabilities (Agresti, 1990:54). To test whether we were more likely to observe a particular butterfly species using the checklist method or the transect method, we computed and compared asymptotic 95% confidence intervals of each odds-ratio to the value 1.0. Here, a confidence interval with its lower limit greater than 1.0 implies that the probability is greater of observing a particular butterfly species using the checklist approach as opposed to the transect approach. A confidence interval with its upper limit less than 1.0 implies that the odds are greater of observing a particular butterfly species using the transect method as opposed to the checklist method.


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