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Psyche 
[December 
Figure 1. The forward advance of I. humilis as it displaces P. cali- 
fornicus in an old field from October 1963 to October 1968. Reference 
markers are located at intervals of 100 meters (o). 
made at six month intervals, whereas only minor observations were 
made of the other two species. Studies were carried out in an old 
field from May 1963 to October 1968 in San Luis Rey, California, 
two miles east of Oceanside in San Diego County. The study field 
consisted primarily of sandy soil with Bromus rubens , Salsola kali , 
Sonchus olcraceous , Pleliotropium curassauicwn, and Brassica nigra , 
the dominant plants. The study area was almost rectangular, being 
300 meters wide by 500 meters long on the south side and 450 meters 
long on the north side. The total area of the field was 14.25 hectares. 
The field was bordered by California Highway 78 on the west, dirt 
field roads on the east and south, and by a grass lawn on the north. 
Two additional fields of 5(2A) and 7(2B) hectares were located 
at the southern edge of the main study field. Here studies on colony 
size, foraging distance, and food preference were carried out (Erick- 
son, 1972; and in manuscript). 
All colonies of P. calif ornicus and /. humilis were individually 
marked with color-coded wooden stakes placed one meter from the 
colony entrance. At intervals of approximately six months the posi- 
tion of each colony was noted on a large map, measured to the 
nearest one meter from the colony entrance using the reference 
