1. The Social Use of Space 69 



of home range, did not take place until day 21, which was just after the 

 final maximum expansion of home range for the other three species. 



By the following year (Table lb, NACSIM Release No. 6) this uncertain 

 social state had completely clarified. Three-day totals for 34 NACSM 

 lines were Clethrionomys (753), Peromyscus (38), Sorex (17), and Blarina 

 (1). Other details of the resultant social organization have*already been 

 treated in Section VIII, A. 



The studies (e.g., Figs. 10 and 13B) conducted during the past few 

 years in Montgomery County, Maryland, by Dr. Barbehenn and me, 

 further substantiate the indeterminancy of the small-mammal community 

 as a dynamic system. We have mostly sampled woodlands of 50 to 1000 

 acres. Peromyscus, Blarina, Sorex, and Pitymys are the more abundant 

 species. In every case Peromyscus is the dominant species. For it, the largest 

 catches per day occur during the first few days, and usually from day 1 

 there is a continuously declining catch per day with the exception of the 

 slight secondary increase resulting from expansion of home ranges by 

 gamma individuals (Fig. 10). Blarina is also nearly universally present. 

 The date by which 509f of the total is trapped consistently arrives several 

 days later for Blarina than for Peromyscus. In actuality, peak numbers 

 taken per day usually occur several days after initiation of trapping. Thus, 

 in these communities the home ranges of most Blarina are socially con- 

 tracted. When either Sorex or Pitymys is present, they are definitely sub- 

 ordinate to both Peromyscus and Blarina in terms of the degree to which 

 their home ranges are contracted. Their peak captures per day never occur 

 until after most of the two dominants have been removed. Relative num- 

 bers are quite another matter. It seems to be purely a matter of chance 

 whether either Pitymys or Sorex is present in any particular woodlot. 

 Either, both, or neither may be present. Their absence appears not to be 

 due to absence of requirements for food and shelter, but merely due to 

 failure to reproduce under circumstances of spatial isolation, when the 

 processes of social adjustment within a particular woodlot happens to 

 markedly reduce the numbers of some one species. These woodlots in subur- 

 ban to semirural Montgomery County, adjacent to the district of Columbia, 

 exist as ecological islands which must be characterized by a rather low 

 probability of receiving colonizers of these subordinate species. Further- 

 more, any one of the three subordinate species can become the most 

 abundant species in the community. In each case the most abundant species 

 has small home ranges whose centers lie within the interstices of the larger 

 home ranges of the dominant Peromyscus (as shown in Fig. IG). Blarina 

 was such a species in the study shown in Fig. 10. Had trapping been con- 

 tinued longer in the study shown in Fig. 13B, Pitymys would undoubtedly 

 have had a nearly 2:1 ratio of abundance to the dominant Peromyscus. 



