Geoscience Reference
In-Depth Information
covered by peat, sand, boulder clay, alluvium and warp overlying distinct geologic-
al formations. Altogether, he made 34 transects and recorded 1165 roadside fields of
which 652 were pasture. The histogram in
Figure 40
shows that fen peat soils topped
the frequency score with 85 per cent mole occurrence, but sandy soils in the Vale of
Pickering, and alluvial soils in the Vale of York, were scarcely less good. All of these
were clearly much preferred to shallow soils overlying chalk, pastures subject to reg-
ular flooding, and recently reclaimed podzols on the edge of the South Yorks Moors.
Moles live to be about five years old. Males and females occur in roughly equal
numbers and have one litter a year containing about four young. For a population in
an area to remain stable, two- thirds must either die or emigrate to pastures new be-
fore the next breeding season in May. Usually the young are forced out of existing
runs, and it is then that one may notice new runs - for example in one's lawn. There
are always some empty niches and unexploited areas for a few moles to find but the
carrying capacity within a parish, say, normally remains fairly constant. Population
density can only increase when food resources increase significantly. This could hap-
pen if arable land, which supports rather low populations, is converted to grassland or
woodland under the agricultural 'Set-Aside' scheme in the future.
When the Dutch polders were created in the IJsselmeer, there was a dramatic
population increase, and a unique opportunity to study the spread of moles into virgin
land. This spread has been described in detail by J.Haeck, and some of the salient
features are given by Mellanby. The surrounding banks, or dikes, were constructed
and sown with grass and clover a few years before the rest of the polder was laid out.
These, therefore, provided the initial bridgehead for colonization from the mainland.
Because of their linear nature, they allowed measurements to be made of the rate of
spread. In the Nord-ost polder, this appeared to be two to three kilometres a year on
average - 20 to 30 week's digging at the rate Haeck actually measured. From the
dikes, moles spread gradually into the polders along the sides of canals, ditches and
roads. Pastures were often colonized first along fence lines where the soil was less
compacted.
It is interesting to compare this colonization by moles with the spread of earth-
worms and ground beetles which other researchers studied in the polders. Worms, or
worm egg-capsules, were almost certainly introduced on the roots of trees and shrubs
widely planted along roadsides and around farmsteads. Their subsequent natural dis-
persal in raw soil seemed to be very slow. Sampling in areas around colonies inocu-
lated into sown grassland suggested a spread of 4-9 metres a year depending on the
species. This slow spread was probably due to the low organic matter and unworked
structure in these raw soils. Subsequently, worm-worked soils could be seen in infra-
