264 
Psyche 
[December 
similarly showed that there was no significant difference in the order 
of dominance of the six plants mentioned. It does not appear that 
P. calif ornicus ameliorates the habitat as it does not clear vegetation 
as many harvester ant species do. I. humilis does not utilize the same 
nest sites as the displaced species and in fact, not a single I. humilis 
colony was found within two meters of an abandoned harvester ant 
colony. 
Pasfield (1968) found I. humilis displaced its neighbors at a max- 
imum rate of 274 meters (300 yards) per year in Australia. This 
value is higher than the 100 to 200 meters per year at Fort Shafter 
on the island of Oahu between 1940 and 1944 (Pemberton, 1944) 
or the average of 100 meters per year in the present study (Fig. 1). 
Fluker and Beardsley (1970) observed /. humilis displace P. megace- 
phala in Hawaii at about 66 to 100 meters per year. All these values 
seem low when compared to the displacement rate of 8 kilometers 
(5 miles) per year for native species by the fire ant S. saevissima in 
the Gulf states (Wilson and Brown, 1957). 
The effectiveness of competition in nature is best demonstrated by 
the impact of an invading species on the native fauna. It appears 
that here, there is a tremendous competition for nest space, which is 
the general case for highly aggressive territorial ant species such as 
Pheidole , Solenopsis, and Iridomyrmex (Wilson, 1971). Three as- 
pects of the populations biology of I. humilis gives this species a dis- 
tinct competitive advantage over the native harvester ants. The 
general aggressive nature of I. humilis as well as the large number 
of queens and method of proliferation allow these ants to move in 
and establish new colonies in a very short time. Raiding columns of 
workers clear the way and pioneer groups of workers and queens 
follow into freshly opened nest areas. 
Acknowledgements 
I would like to thank my uncle, Mr. Carl Erickson, for the gen- 
erous use of his property and to my cousins, Bob, Tom, David, and 
JoAnn for their assistance in various aspects of the study. Thanks 
are also due to Mrs. Mudie of the Department of Botany at San 
Diego State College for determination of the plant species and Dr. 
A. C. Cole, Jr. of the University of Tennessee for identification of 
the ant species. Mr. F. Slansky, Jr. and Drs. W. L. Brown, Jr., 
R. G. Helgesen, and P. P. Feeny read the manuscript and gave many 
helpful comments. A grant from the Grace Griswold Fund assisted 
with the publication costs. 
