Sexual segregation in black-tailed deer: Effects of scale

We studied sexual segregation in a migratory population of Columbian black-tailed deer (Odocoileus hemionus columbianus) on Big Flat in the Trinity Alps of northern California. During 20 June-20 August 1992, we examined the effects of spatial and temporal scales on measurements of sexual segregation by dividing our study area into 56 16-ha quadrats, the smallest area in which we could map locations of deer accurately. We observed and recorded locations of deer by driving a 9-km fixed transect each morning and evening. We analyzed observations made near parturition (29 Jun-31 Jul) to determine spatial separation of the sexes, and to avoid a bias from deer migrating to and from Big Flat. We combined adjacent quadrats to obtain sampling units of 32, 48, 64, 80, 96, 112, 128, 160, 176, and 192 ha, We arbitrarily required a quadrat to contain observations with greater than or equal to 90% of a single sex to be considered segregated. We hypothesized that the scale of our analyses would influence measures of sexual segregation. We analyzed the degree of sexual segregation on a spatial (all observations of a quadrat combined for the entire 5 weeks) and a temporal-spatial scale (samples of a quadrat analyzed by week) to test the null hypothesis that quadrat size was unrelated to the degree of sexual segregation. For all quadrat sizes, the degree of segregation was greater (10-25%) for temporal-spatial than spatial analysis, but the pattern in declining degree of segregation with increasing quadrat size was similar for both methods. We assessed whether male:female ratios departed significantly from the 0.27:1.00 ratio for our 5-week sample of the study area to determine if the ratio of deer available to sample biased our measure of sexual segregation. Male:female ratios differed significantly from 0.27:1.00 for all quadrats, but the degree of difference declined with increasing quadrat size. We used a one-tailed runs test to determine whether the sexes were distributed contagiously (i.e., segregated) across our study for all sizes of quadrats. This analysis suggested that quadrats of 80 ha should be used to sample the degree of sexual segregation in our study area. The size of the sampling unit had considerable effect on degree of sexual segregation we measured; failure to consider scale in such samples holds the potential to misinterpret data. We suggest that natural variation in patches of vegetation may provide more reliable sampling units for measuring sexual segregation than arbitrarily chosen sizes of quadrats, or that the effects of scale need to be evaluated.