Niche Breadth and Niche Overlap 



Standardized niche breadth (table 38) reflects the portion of sampled hab- 

 itats occupied by a given species and the relative proportion of its total cap- 

 tures contained in each habitat. Its usefulness lies in predicting which species 

 will be most affected by development covering many habitat types. Those species 

 with most narrow niches are most likely to suffer proportionally greater impact 

 as a consequence of their limited distribution. Species with broad niches are 

 likely to suffer proportionally less impact. 



The deer mouse, which was the dominant species, showed the broadest niche. 

 The western harvest mouse and Wyoming pocket mouse were very similar in niche 

 breadth. The two voles and two sciurids had the most narrow niches. Under- 

 representation of the thirteen-! ined ground squirrel in snap-trap data discussed 

 earlier may have negatively biased its niche breadth estimate. The species with 

 most narrow niches are confined to habitats of limited distribution-, therefore, they 

 stand to suffer proportionally greater impact than the deer mouse. 



As noted by Krebs and Wingate (1973), species may "occupy different habitats 

 or may overlap each other" in the same habitat. From table 38 the occurrence of 

 niche overlap is obvious. The quantify the degree of overlap, Pianka's (1973) 

 formula for niche overlap was used. Results are shown in table 39. All species 

 combinations showed low overlap, the most significant of which were: 1) the wes- 

 tern harvest mouse with the meadow vole, and; 2) the deer mouse with the Wyoming 

 pocket mouse. Twelve of the species combinations showed no overlap. 



Home Range Size 



Home range dimensions were calculated to describe movements of individuals. 

 They assist in estimating rodent density for live-trapped habitats. They are 

 useful in impact prediction in that they provide a scale against which that of the 

 proposed development can be judged. For example, if the movements of a species 

 are confined to a measured area within a given habitat, the impact of destroying 

 any amount of that habitat can be predicted for that species. Subsequently, 

 interspecific comparisons and predictions can be made. 



As described under Methods, restrictive conditions were applied to data used 

 in calculating home range dimensions. Consequently, sample sizes are frequently 

 too small to permit generalizations on data collected from grids as small as 1.82 

 ha (4.6 acres). From a group of captures of one individual all of which fall on 

 a line, home range length may be calculated whereas area may not. Table 40 shows 

 home range area and length for cricetid rodents live-trapped at Circle West. Dur- 

 ing April and June capture data were inadequate for home range calculations. July 

 through October data showed a generally decreasing trend over time in both home 

 range and length. This relationship is expected in light of increasing density 

 over the same period as seen in figures 51 and 52. Exceptions were the western 

 harvest mouse on grid #2 (area and length) and the deer mouse on grid #2 (length) 

 where increased home range dimensions were recorded from September to October. 

 Data are inadequate for interspecific comparison and no general trends are apparent 

 other than gross similarity between deer mouse and western harvest mouse home ranges. 

 Averaged home range dimensions indicate that mean home range area decreases tc 300 m^ 

 (3240 square feet) in September and is constant at least through October. I'lean 



181 



