26 q 
Psyche 
[December 
markers placed at ICO meter intervals throughout the field (see 
Fig. i). The lines on Fig. i deliniate the limit of eastward ex- 
pansion of 7 . humilis colonies at the time the survey was taken. The 
new area added was then calculated by subtracting the total occupied 
area at the previous sampling date from the new total area occupied 
by 7 . humilis. This area was then divided by the number of colonies 
in the new added territory to get the mean area per colony values. 
At the same time, quadrat sampling with 30 randomly placed 2 
meter by 2 meter quadrats were carried out to determine the vegeta- 
tion characteristics. Weather data were taken from a station 3 miles 
northeast of the field site and averaged to get monthly mean tem- 
peratures and precipitation. 
Results 
The displacement of the three other ant species by 7 . humilis 
started slowly in October, 1963, but increased to an almost constant 
rate from 3 May 1964 to 4 October 1968 (Table 1, Fig. 1). The 
new area added by 7 . humilis during each displacement interval of 
six months was approximately 14000 m 2 ranging from 8318 m 2 to 
19988 m 2 in the study field. The mean area per colony of 7 . humilis 
for the whole occupied portion of the field increased during each 
sampling interval, whereas the mean area per colony in the newly 
displaced land was almost a constant 1400 m 2 per colony. The num- 
ber of colonies of 7 . humilis increased during each displacement in- 
terval as the displacement proceeded whereas the number of colonies 
of P. calif ornicus decreased except for a minor fluctuation due to 
flooding between 3 October 1965 and 1 May 1966 (Fig. 2). By 
October 1968, not a single colony of P. grallipes was observed in 
the study field, and by 5 March 1969 all colonies of P. calif ornicus 
and V. pergandei were located outside the boundaries of the study 
field. In their place remained 57 colonies of 7 . humilis. 
Discussion 
To explain this phenomenon of displacement, some comparison is 
necessary of the basic biology of the ant species involved. The nests 
of 7 . humilis are situated wherever there is sufficient moisture and 
where light is excluded, as under rocks and logs (Wood word, 1905, 
1910; Eckert and Mallis, 1937, Smith, 1947) or in shallow nests in 
the soil (Cook, 1953). These ants occur in a wide variety of habi- 
tats — swamps, beaches, lawns and gardens, roadsides, houses, and 
various woodlands (Crowell, 1968). 7 . humilis are exceptionally rest- 
less ants and normally emigrate one or more times a season in search 
