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

Results and Discussion

Effect of site and method on number of butterflies and number of species. -- As expected, log number of butterflies (F = 28.26; df = 1,88; p < 0.0001) and log number of butterfly species (F = 18.33; df = 1,88; p < 0.0001) increased significantly with increasing log hours of search effort. Adjusting for increasing log hours of search effort, both site (F = 11.29; df = 6,6; P = 0.0047) and method (F = 8.50; df = 1,6; P = 0.0268) affected the number of butterflies counted. Significantly more butterflies were counted per hour effort using the checklist survey than the transect survey (back-transformed LSMEANS, adjusted for mean hours effort, are 41.1 and 25.8 for checklist and transect, respectively). Chase Lake National Wildlife Refuge had the most butterflies counted per hour effort and Burning Coal Vein Campground the least (Table 1). Because site and method did not indicate an interaction in the ANCOVA (F = 0.39; df = 6,88; P = 0.8851), the differences in number of butterflies counted between checklist and transect are fairly consistent across all sites. However, the number of butterfly species counted, adjusting for log hours of search effort, did not differ significantly among sites (F = 0.46; df = 6,6; P = 0.8139) or between methods (F = 2.11; df = 1,6; P = 0.1962), nor was there any indication of an interaction between sites and method (F = 1.00; df = 6,88; P = 0.4275) (Table 1).

These findings indicate that while the checklist count produced significantly higher rates of encounter with individual butterflies, it did not produce a significantly higher likelihood of encountering more species on a given date. With 146 species on record, North Dakota has one of the smallest butterfly faunas of any state or province in the Great Plains. The typical hypothetical list for a North Dakota county contains approximately 70 species, including invaders and seasonal immigrants. Species richness at any site on any particular date is bound to be low under such circumstances. Also, a number of localized or sedentary species were detected only during checklist counting (see Appendix 1), but the low frequency of such encounters rendered them statistically undetectable in this test.

Table 1. - Number of butterflies and number of butterfly species counted per hour of effort, 1995, by survey method, expressed as LSMEANS + standard error (SE)

No. butterflies counted/h No. species counted/h
LSMEANS (SE)A Count/hrB LSMEANS (SE)A Count/hr
Lostwood National Wildlife Refuge Checklist 4.98 (0.47) 62.8 2.37 (0.22) 4.2
Transect 4.44 (0.44) 36.4 1.84 (0.21) 2.3
J. Clark Salyer National Wildlife Refuge Checklist 4.52 (0.41) 39.5 2.40 (0.19) 4.4
Transect 3.74 (0.49) 18.3 2.22 (0.23) 3.6
Chase Lake National Wildlife Refuge Checklist 5.36 (0.50) 92.1 2.17 (0.23) 3.4
Transect 4.51 (0.48) 97.2 2.33 (0.22) 4.0
Sully's Hill National Game Preserve Checklist 4.85 (0.47) 55.1 2.22 (0.22) 3.6
Transect 4.20 (0.52) 28.6 2.25 (0.24) 3.7
Sheyenne National Grassland Checklist 4.13 (0.39) 26.6 2.32 (0.18) 4.0
Transect 3.79 (0.42) 18.8 1.94 (0.20) 2.6
Summit Campground Checklist 4.26 (0.37) 30.4 2.17 (0.17) 3.4
Transect 4.26 (0.41) 30.4 2.31 (0.19) 3.9
Burning Coal Vein Campground Checklist 3.80 (0.40) 19.0 2.24 (0.19) 3.6
Transect 2.88 (0.39) 7.3 1.83 (0.18) 2.3

A LSMEANS are in log-scale [ln(Y + 1) transformation], as used in ANCOVA.
B Backtransformed LSMEANS, adjusted to per hour of effort by dividing by backtransformed mean hours of effort = 2.30.

Species encountered by only one method or the other. -- When we segregated results by species found by both methods, by checklist method only, and by transect method only, there was clearly a dominance of sedentary, habitat-specialized taxa in the checklist-only category (see Table 2 and Appendix 1), particularly lycaenids [e.g., hoary elfin (Callophrys polia) at J. Clark Salyer National Wildlife Refuge] and hesperiids [e.g., Afranius dusky wing (Erynnis afranius) at Lostwood National Wildlife Refuge]. We interpret this to be at least in part related to habitat diversity at the site. We reasoned that the mobility of a given species is necessarily related to its chances of being detected by only one method (more sessile species detected primarily by checklist) or by both methods (more vagile species being detectable by both checklist and transect). However, confidence in such distinctions with respect to any single species is clearly related to the number of counts in which it was detected by only one method. In a majority of cases, detections occurred in only one or two counts out of an average of 27 transect and 26 checklist counts per site.

Odds ratios. -- When odds-ratio analysis was applied, on all but one site (Chase Lake National Wildlife Refuge Complex) the checklist method was again found more likely than the transect method to detect certain species. Not surprisingly, all species for which results were statistically significant (Table 3) showed greater odds of being detected by the checklist method than by the transect method; no species showed greater odds of being detected by the transect method than the checklist method. In most cases, detections appear to be related to habitats that were (1) not included in the transect or (2) specifically targeted by the observer during checklist counting. For example, at Burning Coal Vein Campground, the predominant habitat is dry, native, shortgrass prairie pasture, and the transect was broadly representative of that habitat. However, it also included substantial segments of brush and shaded woodland draws. The six species with significantly greater odds of detection by the checklist method at Burning Coal Vein Campground were all generalist denizens of open sunny environments, but most were also typical of disturbed habitats not represented in the transect -- roadside ditches [containing alfalfa (Medicago sativa) and sweet clovers (Melilotus spp.), which attracted the orange sulphur (Colias eurytheme), clouded sulphur (Colias philodice), and Melissa blue (Lycaeides melissa)] and patches of thistle (Cirsium spp.) (which attracted Baird's swallowtail (Papilio bairdii)). Variegated and callippe fritillaries (Euptoieta claudia and Speyeria callippe, respectively) are neither specialists nor otherwise localized in occurrence. Why these two species also showed greater odds of detection by the checklist method is not clear, although the fact that transect counts involved sizable nonprairie segments (woodland and brushy areas) may be a factor.

At Lostwood National Wildlife Refuge, two of the three species showing significantly higher odds of detection during checklist counting again were crop pests, the cabbage butterfly (Pieris rapae) and clouded sulphur, both of which are more commonly found along roadsides or in disturbed areas not included anywhere in the transect. The third, Afranius dusky wing, was specifically targeted during checklist surveys by searching areas with Thermopsis rhombifolia, its presumed larval host but a plant species absent from the transect area.

Although a large area of J. Clark Salyer National Wildlife Refuge includes widespread stands of exotic plant species in meadows and ditches, the transect path was established in the refuge's native and ecologically unique Sandhills area. The cabbage butterfly, characteristic of nonnative habitats, was the only species more likely to be detected by checklist than by transect counting at this site.

Table 2. - Summary of number of counts and number of butterfly species detected by checklist or transect method at seven federally managed sites in North Dakota, 1995

Checklist Transect No. species detected by both methods Total No. species detected
No. surveys No. species detected No. species unique to Checklist No. surveys No. species detected No. species unique to Transect
Lostwood National Wildlife Refuge 25 37 18 29 19 0 19 (51%) 37
J. Clark Salyer National Wildlife Refuge 24 45 24 22 22 1 21 (46%) 46
Chase Lake National Wildlife Refuge 25 30 7 24 25 2 23 (72%) 32
Sully's Hill National Game Preserve 16 36 12 15 29 5 24 (58%) 41
Sheyenne National Grassland 31 43 25 31 19 1 18 (41%) 44
Summit Campground 30 42 13 26 34 5 30 (67%) 47
Burning Coal Vein Campground 34 49 16 43 35 2 33 (67%) 51

Similarly, at Sullys Hill National Game Preserve, the transect was established almost entirely in woodland habitat. More than half the checklist counts (9 of 16) were conducted in prairie hay meadow. The single species showing higher likelihood of detection using the checklist method was the Aphrodite fritillary (Speyeria aphrodite), a wide-ranging species more characteristic of prairie hay meadows than of woodlands. In the same way, the transect at Summit Campground was established in a rugged landscape which included sizable segments of wooded coulee. The variegated fritillary, characteristic of more open, disturbed landscapes, was thus more likely to be detected during checklist surveys, which commonly included such areas.

In contrast to these examples, habitat throughout Chase Lake National Wildlife Refuge Complex is relatively homogeneous, open, rolling native prairie with very few other habitats. No difference in odds of detection was found for any species at this site.

Some highly localized and sedentary species were counted only during checklist surveys, but such events occurred infrequently enough that the test was unable to demonstrate significant results because of small sample sizes. In the odds-ratio test, odds are not related to actual abundance, but only to the fact of detection in a single transect or checklist walk. Consequently, a single detection may not have shown statistical significance even when many individuals were counted.

Major differences in odds of detection appear to be related only to very broad ecological parameters -- e.g., species of woodland vs. prairie or of native vs. cultivated habitats. A larger database would no doubt provide for further refinement of such distinctions.

Table 3.--Species having significantly higher probability of detection using checklist method vs. transect method, as determ ined by odds-ration test. N = Number of surveys conducted. D = Number of surveys where species was detected. Odds-ratio is the odds of detecting at least one butterfly of species I under Checklist vs. Transect method

Checklist Transect
Lower and Upper
95% Conf. Interval
Burning Coal Vein Campground Colias eurytheme
Colias philodice
Euptoieta claudia
Lycaeides melissa
Papilio bairdii
Speyeria callippe
Lostwood National Wildlife Refuge Pieris rapae
Colias philodice
Erynnis afranius
J. Clark Salyer National Wildlife Refuge Pieris rapae 24 11 22 3 4.746 1.190 18.925
Sheyenne National Grassland Limenitis archippus 31 17 31 6 4.735 1.565 14.328
Sully's Hill National Game Preserve Speyeria aphrodite 16 7 15 1 7.632 1.103 52.819
Summit Campground Euptoieta claudia 30 8 26 1 6.422 1.034 39.877
Chase Lake National Wildlife Refuge none              

There was clearly a dominance of "local" or "habitat specialist" taxa, especially lycaenids and hesperiids, in the checklist-only category at most study sites. We interpreted the proportion of species detected only by checklist to be an indirect indicator of a site's habitat diversity, for unless specialized habitats were included in the transect path, one would not normally expect to detect the obligate species they host. Conversely, generalist species which do tend to range widely also tended to appear in both kinds of counting method rather than exclusively in one or the other. Confidence in these conclusions with respect to any single species is no doubt related to the number of counts in which it was detected by only one of the methods; in a majority of such cases, only one or two counts were involved. However, when up to 25 different species appear in only one kind of count and not the other, one may confidently conclude that that method offers the more reliable means of assessing species richness. Table 2 and Appendix 1 show that within each site the checklist method identified many species that were not detected during transect surveys.

We believe these results indicate that the checklist survey method is the more efficient means for initial determination of a site-specific butterfly species list, especially in the case of large sites such as one encounters in the Great Plains. For continued, long-term monitoring, however, less time-consuming Pollard Walk transect counts offer a more focused, practical alternative. As noted in our introduction, the carefully constrained transect approach also supports more precise statistical analysis than is possible with "open-ended" checklist data. Pollard Walk transects are thus recommended once the resident butterfly fauna has been defined for a site by means of checklist counting. In such situations, permanent Pollard Walk routes may be specifically designed to take specialized habitats into account, favoring prolonged monitoring of those butterfly species that may be of particular conservation concern or interest at a site.

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