Geoscience Reference
In-Depth Information
In Europe and North America several species of ants build ant-hills, but in this
country they are almost always the product of the little yellow ant
Lasius flavus.
Mound building may be partly a response to climate, as in southern Europe and in
high mountains the yellow ant lives under stones. It is very common in southern Bri-
tain where ant hills can reach extraordinary densities if undisturbed.
T.J.King counted ant-hills in a large number of grasslands on both chalky and
acidic sandy soils using 1000 square metres as his standard sampling unit. Four counts
from Roche Court Down on the Porton Ranges in Wiltshire ranged from 110 to 400
in Oxfordshire, he surveyed 8450 square metres and counted 1947 ant-hills, while
several estimates from smaller areas in Wiltshire and Sussex fell between 1200 and
3500 per hectare. The highest density was on acidic grassland in the Gower Peninsula
in south Wales where he encountered 497 ant-hills in 1000 square metres, almost one
in every two square metres.
Ant-hills have a definite local micro-climate caused both by their capacity to
absorb warmth from the sun on their south-facing slopes and by their free-draining
nature. Much of the rain runs off the surface of a mound, and that which penetrates
can quickly drain away through the galleries which permeate it. Dew, frost and snow
lie for longer on the north side of mounds than on the south, where, on the other hand,
temperatures can often reach 40°C on bare soil on summer days. The mounds are
built little by little by ants bringing up soil particles in their mandibles from a metre
or more deep. These particles are glued to the existing surface of the mound and to
plants, some of which continue to grow and thus provide a framework. The coarser
sand grains (more than 0.5mm), are less easily transported, and so the mound comes
to have a disproportionately large amount of fine soil particles compared with the sur-
rounding soil - 84% as against 16% in the case of one study.
Not only are the physical properties distinctive but there appear to be chemical
differences as well. These have been noticed in Britain, Europe and the United States.
In Britain, higher quantities of extractable phosphorus have been found in some in-
stances, while the organic matter content of the mounds is generally lower than in
the soil around. Ant-hills always have a higher potassium concentration than the sur-
rounding soil. They are often more alkaline as well, probably due to nutrient-rich
clay particles brought up from lower down the profile. In Polish meadows, a simil-
ar increase in pH, phosphorus and potassium was accompanied by marked changes
in the bacterial and fungal microflora. The researchers attributed the increased mi-
crobial activity to the decomposition of organic material imported into the nests. Not
all chemical differences may be directly caused by ants; rabbits are known to use
