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[December 
ber of queens in each colony, the method of colony proliferation, and 
the omnivorous habits of these ants. These factors may also account 
for the great and rapid spread of I. humilis throughout the temperate 
regions of the world. 
Basic differences in food resources limit to some extent the amount 
of competition between 7 . humilis and the three harvester ant species 
(Table 2). 7 . humilis is highly omnivorous whereas the three har- 
vester ant species are only slightly omnivorous, being basically seed 
gatherers. I. humilis not only monopolizes the proven food sources 
but attempts to control the remaining foraging areas (Wilson, 1971). 
In the main study field, food, especially seeds, were very abundant. 
The food chambers of P. calif ornicus and I. humilis were always 
full when the colonies were excavated. In fields 2 A and 2B, the area 
was supplemented with approximately five pounds of mixed grass 
seed per month to determine the foraging characteristics and distances 
for P. calif ornicus. The seeds, colored with common food coloring, 
were fully acceptable to the ants, making up 43 to 59% of the 
P. calif ornicus food stores and 9 to 17% of the I. humilis food stores. 
The variously colored seeds were spread in concentric circles from 
a nest of P. calif ornicus every 5 meters to a distance of 30 meters. 
The maximum foraging distance for P. calif ornicus was about 10 
meters except in areas where there was an I. humilis colony in which 
case the harvester ants foraged no farther than 5 meters even though 
the I. humilis colony was 20 meters away. Even though both fields 
were supplemented with a little over 50 pounds of mixed grass seed 
per year, in one year P. calif ornicus was displaced 76 meters (2A) 
and 109 meters (2B) by I. humilis. It does not appear that this 
displacement is due to any overlap of a fundamental food dimension. 
At each sampling interval the mean area per colony of 7 . humilis 
in the newly displaced territory was approximately 1400 m 2 whereas 
the mean area per colony for the entire field increased from 1400 m 2 
to approximately 2600 m 2 during the five year period (Table 1). 
There thus appears to be a minimal area for a colony of I. humilis 
in the newly acquired areas and as these colonies become established 
and increase in population density, the colony requires a larger area. 
Michener (1942) working with P. calif ornicus encountered a 
similar displacement by 7 . humilis. He described in detail how in- 
dividual harvester ants would be set upon and killed by groups of 
7 . humilis. When temperatures are cool, Pogonomyrmex species tend 
to be sluggish and it is at this time that the Argentine ants torment 
the harvester ants as they forage around the nest (Michener, 1942). 
