Habitat effects on small mammal biomass were also evident from the data 

 (figure 52 )• Biomass per grid was greater on the sagebrush than the grassland 

 grids during most months; this difference increased near the end of the trapping 

 period significantly and both biomass per one hundred trap nights and captures 

 per one hundred trap nights were positively correlated with the percentage of 

 absolute litter cover in each habitat category. However, no significant results 

 are evident when biomass and captures regressed on mean total live plant cover or 

 the percentage of absolute bare soil and rock cover. 



Biomass increase was calculated as the change in biomass between the two 150 

 trap night sample periods at each site (table 33). The statistic was standardized 

 by dividing by the time elapsed between sample periods as this was not constant for 

 all sites. Of twenty-one sample sites for which biomass increase was measurable, 

 only five showed positive increase. Negative increase, or a loss of biomass be- 

 tween sample periods may be due to the relationship of the timing of sampling to 

 habitat destruction due to localized livestock grazing, vegetative dessication 

 due to prolonged drought during mid-summer 1977, intensive habitat destruction by 

 grasshopper infestation during the same period, the effect of harvesting with snap- 

 traps, or late-season population declines. 



Strongly positive biomass increases were recorded at transects #23 and #33. 

 Both were sagebrush-dominated sites with relatively high percent absolute litter 

 cover. Both were sampled first during early breeding season and last during mid 

 to late breeding season. Consequently they both reveal the trend displayed in 

 figure 4 for sagebrush grids where captures increased sharply over the same period. 



It is evident that the highest biomass and species number of small mammals 

 was found in shrub habitats: tall coulee, western snowberry-prairie rose and 

 silver sagebrush. A trap line in tall coulee shrub showed by far the highest 

 biomass of all habitats sampled. Trap lines in western wheatgrass and saltgrass 

 habitats showed the lowest biomass of all sampled. 



Highest species number and biomass values were recorded for habitats with 

 high live plant cover; these were characteristically diverse, complex vegetative 

 communities, and most were of limited coverage in the study area (riparian bulrush, 

 silver sagebrush/western wheatgrass-blue grama, western snowberry-prairie rose, 

 and silver buffaloberry/western siiowberry-prairie rose). Excepting the sagebrush 

 habitat, these types were confined to locations of topographic irregularity such 

 as coulees, stream banks, and ephemeral stream bottoms. Such vegetative-top- 

 ographic complexes may be particularly sensitive to disturbance and difficult to 

 re-establish, indicating that an important portion of the small mammal diversity 

 on the study site may resist restoration following mining. 



Habitat types of broadest distribution over the study area support small 

 mammal communities dominated almost entirely by one species, the deer mouse. 

 This reveals a restricted prey base with limited flexibility for species replace- 

 ment if reclaimed habitats are unsuited to habitation by deer mice. 



179 



