The Ecology of a Sheltered Clay Bank 169 
in point of numbers from year to year show very fine 
adjustment. As one species increased progressively from 
year to year, the other decreased. The details of this 
phenomenon are given on later pages. 
While 4. abrupta appeared about June 25 in 1917, a 
visit on May 28, 1918, showed this species already out 
and active. Early emergence this year affected most 
Species inhabiting the bank. Later I shall discuss the 
relation of this emergence to meteorological conditions. 
But even at this early date, the A. abrupta bees must 
have emerged five or six days previously because the 
work on their chimneys was in many cases far advanced 
or completed. Having been born a month earlier in the 
year than their parents of the preceding year had been, 
the population had the same number of days to their 
adult life and lived, builded, and many had died before 
June 28, Then, within three days after their disappear- 
ance came the white-banded bees again, following with 
all the precision of clock-work! The interesting item 
was that Anthophora abrupta had increased in numbers 
from 22 nest-building mothers in 1917 to 92 in 1918.* 
Not all of the 92 mothers had nests with turrets; only 
52 had full-sized chimneys, 15 with half-sized ones and 
25 nested in burrows devoid of chimneys. This increase 
was, as we shall later show, very decidedly at the ex- 
pense of the white-banded bees whose numbers this year 
rapidly decreased. What should we expect in the fol- 
lowing year, and what was the cause of the change in 
dominance? 
*It , that any accretions in numbers from 
ene tu this cis We A. abrupta were progeny from the 
t 
Since the latter species diminished in the bank from year to year, 
is evident that no new stock took up their abode there. 
