Náttúrufræðingurinn

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Náttúrufræðingurinn - 1995, Qupperneq 103

Náttúrufræðingurinn - 1995, Qupperneq 103
within ptarmigan breeding habitat and that at least 1 km separated different sites. A map and a compass were used to locate the sites out in the field. Point counts for ptarmigan cocks were done in early and late May. The counts were only conducted in the early morning hours (06:00- 10:00) and in the late afternoon (17:00-22:00). During the middle of the day the territorial cocks rest and are difficult to spot. A count was conducted for exactly 20 minutes at each site. All cocks seen or heard within that period were counted and the distance to them meas- ured with a range finder. The hikes between the count point sites were used as transects. Two parameters were measured for each cock sighted during transects: I. sighting distance (r.) measured with a range finder; 2. sighting aiigle (q). Using these parameters perpendicu- lar distance of the bird from the transect line was found using the formula: y.= r. x sin q. The length of the transect was measured from a map. Only cocks within 500 m of the transect line (perpendicular distance) were used in the analyses. The program DISTANCE was used to estimate density and confidence interval (Laake et al. 1991). The results from the transects were used as an unbiased estimator of how well the point counts reflected density. The vegetation studies were done in June- August. Six randomly placed transects were laid out wilhin a 500 m radius of each count point site. Each transect was 25 m long. Meas- urements included cover of 11 plant species (Table 1) and unvegetated areas under the measuring tape, and frequency of 16 species or plant groups (Table 1) measured in three 50x50 cm frames on each transect; the frame was divided into 25 small 10x10 cm squares. To have a 100% frequency, the plants had to be regislered in all 75 small squares on the transect. Also measured was the height of the vegetation at 5 cm intervals along the 25 m transect. The plants measured are either im- portant food for the ptarmigan or dominant species in the communities and some are botli. Average figures for each plant species for the 6 transects on each point count site were used to describe the vegetation of the site. Not in- cluded in the analyses were some few rare di- cotyledons, all monocotyledons, mosses and lichens. The data matrix of 27 plant variables x 36 sample points was analysed by the ordination technique of detrended correspondence analy- ses (DECORANA) with no downweighting (Hill 1979a) and by TWINSPAN classification (Hill 1979b). Percentage cover and frequency val- ues were arcsine transformed. A total of 94 ptarmigan cocks were regis- tered during the point counts (average 2.2 cocks/site, range 0-9, .v 2.52). The cocks ranged up to 750 m from the sample points but most were within 100-400 m (Fig. 3). As rela- tively few birds were observed during the point counts, no attempt was made to convert these numbers to density figures. These raw figures were used when comparing ptarmigan abundance and vegetation, under the asump- tion that they reflected density. The transect censuses were done to see if this presumption was valid. Total length of transects was 162.6 km and 234 cocks were observed. Densities varied according to areas, being highest on the coastal heathlands and declining inland (range 0.8^t.8 cocks/km2). Linear regression of the number of cocks observed in point counts on density derived from transects gave a signifi- cant relationship (Fig. 4), and the slope of the regression line was not significantly different from 1 (1.38±0.74). According to this the point counts should be a usable index on den- sity. Table 1 shows average values and range for the plants recorded at the sample count sites. Because of lack of time vegetation analyses were not done at six of the count point sites and they were therefore not.used in the vegeta- tion analyses. The most common shrub and heather species were Empetrum nigrum and Betula nana, the most common herbs were Bistorta vivipara and Thalictrum alpinum. The results ol the site ordination is shown in Fig. 5. The closer the points are in the graph, the more similar are the sites with respect to vegetation composition. The points arrange themselves fairly wcll in accordance with geo- graphic location, with coastal sites and the sites furthest inland at the opposite ends of the spectrum. In Fig. 6 some selected variables are superimposed on the ordination scattergram. The environmental factors responsible for the variation according to axis 1 of the ordination seem to relate to climate and grazing pressure. It is windy and wet on the coast and precipita- lion declines as we go inland. Grazing by sheep is heavier on the lower lying heathlands than inland. These factors are reflected among other things in the cover of unvegetated sur- faces and the height of the vegetation (Fig. 6). It is not perfectly clear what environmental factors account for the variation shown by axis 101
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