NNI =Xnn /Nnran = 1.3107/1.162
= 1.128 (Yardley Hill (south-facing))
NNI =Xnn /Nnran = 2.7/4.3644
- 0.62 (Daisey Plain (north-facing))
Based upon the above index scale, it would appear that Yardley Hill tends towards a random
distribution pattern whilst Daisey Plain shows some degree of clustering.
Discussion
Initial counts of individual anthills confirmed that there were 74 nests on Yardley Hill (south-
facing) and 21 nests on Daisey Plain (north-facing), a ratio of approximately 3.5 ; 1. Some evidence
exists to support the theory that colonies prefer a relatively high humidity in their nests as this aids
the construction and cohesion of internal building infrastructure, and certainly Daisey Plain has
been shown to have higher soil moisture levels.
So why the bias in numbers of anthills towards the drier slopes of Yardley Hill? It would appear
that the south-facing slope warms up earlier in the day and would receive a greater input over the
year of incident solar radiation. This helps the ant population to check their circadian rhythm
against sunrise or sunset from time to time and as this biological clock is also temperature-dependent,
it can enable inner nest workers to work continually without visiting the surface. The same could
not be said for the shadier slopes of Daisey Plain. Although this north-facing area has increased
levels of soil moisture this actually dampers any amelioration of temperatures because of the resultant
increase in soil conductivity and the reduced quantity of sunlight.
The difference in mean nearest neighbour distances of 1.3m (Yardley Hill) and 2.7m (Daisey
Plain) appears to correlate well with the higher densities found on Yardley Hill, which also helps to
explain nest distribution patterns for these aggressive territorial insects. A larger area to colonise
will mean that there is less of a tendency for territorial competition, and so the development of a
system of ramifying galleries and chambers in support of its colony results. Such a system seems to
start as a random pattern but with increasing colonies moves towards regularity as space becomes
a limited resource. Unfortunately the statistics don't always reflect what is seen in the field.
Caution is always needed when using statistical analysis, particularly when calculations are dependent
upon accurate field measurements and the appropriate selection of the sample quadrat area.
Further observation has shown the height of the nests on Daisey Plain to be higher than those on
Yardley Hill. Could this be a function of the need to obtain more heat by increasing surface area
for the cooler, shadier north-facing slope? More 'A' level investigations awaited!
Background to work and acknowledgements
This article arises from work carried out at Hawkwood Lodge, Epping Forest, with sixth form
students following a course in biology 'A' level (September 1998), from St. Edwards C. of E.
School, Romford, Essex and supplemented by work carried out by the author (July 2001).
References
BRIAN, M.V. (1977) Ants. New Naturalist series, Collins, London.
BRIAN, M.V. (1979) The Wortdof the Ant Hill. Faber.
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