80 John B. Calhoun 



onomys and the more numerous Peromyscus. Regardless of which species 

 dominates in the actual encounter, the interspecific common characteristic 

 of the signal may be expected to assume only a negative quality by Clethri- 

 onomys. Some interactions among Peromyscus must be of a positive nature. 

 Thus, even though both species may detect the interspecies common com- 

 ponent of their signals with equal frequency, Peromyscus might be expected 

 to exhibit less contraction of its home range because this signal is less 

 aversive than it is for Clethrionomys. 



This line of reasoning applies also at times to relationships among three 

 species. In the unusual situation (Fig. 12) in which Blarina, Peromyscus, 

 and Clethrionomys were all c^uite abundant, the seciuential trapping data 

 clearly indicate that inhibition of home range nearly equally characterizes 

 all three species. 



In such a system, the extent and center of an animal's home range de- 

 pends not so much on characteristics of the habitat as on the current 

 density and origins of signals, and the temporal and spatial history of 

 interaction among members of the community during the most recent 

 generations. The absence of an animal in a locality cannot be construed 

 to mean unsuitability of the habitat. It is conceivably possible to delineate 

 microhabitat characteristics contributing to the animal's niche even under 

 such circumstances. However, it requires that we know an individual's 

 home range center and that we mark out stations along a circumference of 

 a circle having a radius which will result in a high probability of the animal 

 crossing that circumference. A 1-sigma radius should prove effective. 

 Examination of stations from such a series, which have a greater than 

 chance frequency of visitation, might lead us to isolate those conditions 

 which do contribute to an animal's niche. 



XL An Induced Invasion 



The major portion of this section presents the author's interpretation of 

 a study conducted by Webb and Rosasco (1953). It describes the response 

 of the red-backed mice, Clethrionomys, surrounding a 30-acre tract within 

 which continuous removal trapping was conducted for 80 consecutive 

 days. In this account it will become apparent that the concepts elaborated 

 remain inconclusive. Even so, their implications, when taken in conjunc- 

 tion with the other sections of this paper, warrant their presentation. 



A brief history of events leading up to Dr. Webb's study provides a 

 background for appreciating the objective. During the summer of 1950, 

 while I was in residence at the Roscoe B. Jackson Memorial Laboratory 

 as a National Institute of Mental Health Special Fellow, Dr. A. Dexter 



