Geoscience Reference
In-Depth Information
mounds as latrines, and one exceptionally acidic and nitrate-rich ant-hill in Colorado
was deemed to have been used as a coyote 'lamppost'!
These various changes wrought by ants are clearly reflected in the vegetation.
Indeed, there is probably no other soil animal which modifies the vegetation so con-
spicuously in this way in temperate regions - though subterranean grazers such as
root aphids and wireworms probably affect the competition between some plant spe-
cies. Certain small annual plants, such as thyme-leaved sandwort and wall speedwell,
and a few deep-rooted perennials, such as thyme and rock-rose, are particularly as-
sociated with ant-hills. The annuals need bare soil for germination, and they flower
and set seed early in the year before drought becomes severe. The perennials must
withstand periodic droughts and having soil continually heaped on them.
Table 5
lists
some of the plants found to be more abundant or noticeably scarce on ant-hills, com-
pared to surrounding grassland, on chalky and acidic soils. On heathlands, the mounds
may be distinctive in supporting heather. Mole-hills also provide germination niches
for plants, as described later.
One of the most interesting recent discoveries about ant-hills is the relationship
between their size and the age of the grassland where they occur. A multidisciplinary
team of a botanist, an entomologist, a soil scientist (D. F. B.) and an historical geo-
grapher were studying the ecology of Porton Down in Wiltshire when they noticed
that the average size and density of of ant-hills differed markedly in adjacent areas.
Old maps and records showed that these areas had been cultivated in the past and
more recently had fewer and smaller ant-hills than older areas. It was known that ants
continue to build their nests so long as they remain in residence, for some nests had
been under continuous observation for 22 years. However, if mounds become shaded
by vegetation, or are taken over by another ant species, they stop growing and even
decline again. Faced with a vista of ant-hills, the problem was to decide what meas-
urement of size or density would be the best guide to age. Was there any consistency
from site to site, or from one soil type to another?
King devised an empirical formula based on the mean volume of the five largest
ant-hills out of a thousand counted out in a standard area:
Age of field since last ploughing (years) = Mean volume V (litres)/1.04+5.0
where the volume of a mound was calculated from the mean height (h) and mean ra-
dius (r) by V = πh (3r
2
+ h
2
)/6
This formula seemed to be reliable for chalk grasslands between 15 and 165
years old in Wiltshire; a former golf course at Porton was predicted to be 56 years
'old', and subsequently found to have been abandoned 56 years before. It also gave
surprisingly good results for other datable grasslands on alluvial soil in Surrey, clay
