Burger 



Chapter 16 



Inland Habitat Associations in British Columbia 



The percentage of the sampled sites in which the 

 mean frequency of detections exceeded a given threshold 

 was then plotted (fig. 6). This should facilitate ranking a 

 particular site, relative to other sites, or guide decisions on 

 how important surveyed sites might be on a provincial or 

 regional basis. The trends in the Queen Charlotte Islands 

 and on Vancouver Island were surprisingly similar. These 

 indicate, for example, that about 18 percent of all sites in 

 these areas had mean densities exceeding 40 detections 

 per survey. If a manager decided to preserve all sites 

 above this threshold, then one would expect about 18 



percent of the potential sites to be included. These trends 

 should obviously only be used as guides, since some low- 

 density sites might be important in places where there are 

 few high quality sites. 



These data were derived from relatively few surveys 

 (means for Queen Charlotte Islands and Vancouver Island 

 were 3.2 and 1.6 surveys per site, respectively), made in a 

 single year (1990 and 1991, respectively). By contrast, the 

 surveys made in Carmanah-Walbran-Nitinat used fewer sites, 

 but were much more intensive (mean 3 1 .4 surveys per site) 

 and covered four years. Not surprisingly, the threshold pattern 



Queen Charlotte Is. 

 Vancouver Island 



Carmanah-Walbran 



10 20 30 40 50 60 70 80 



Threshold (mean no. detections per survey) 



Queen Charlotte Is. 

 Vancouver Island 



Carmanah-Walbran 



1990-1991 



Carmanah-Walbran 



1992-1993 



10 20 30 40 50 60 



Threshold (mean no. detections per survey) 



70 



80 



Figure 6 A: plot of the percentage of sites in which the mean frequency of Marbled Murrelet detections 

 exceeded the thresholds on the x-axis. Data from the period 1 May through 31 July in the Queen Charlotte 

 Islands (1 58 surveys at 50 sites in 1 990; Rodway and others 1 991 ) , Vancouver Island (209 surveys at 1 51 

 sites in 1 991 ; Savard and Lemon in press), and Carmanah-Walbran-Nitinat (471 surveys at 1 5 sites in 1 990- 

 1 993; Burger 1 994). B: the same plot as A, but with the Carmanah-Walbran-Nitinat data separated into two 

 periods: 1990-1991 (176 surveys at 12 sites) and 1992-1993 (297 surveys at 14 sites). 



160 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



