36
THE ESSEX NATURALIST
Results and discussion of methods
Table 4 summarises the relative abundance of the small
mammals in the main surveys described above. The fact that
one species (the Dormouse, Muscardinus avellanarius) was not
detected by any of these means is commented on further under
that species heading below.
Table 4
A comparison of the results from four small mammal sampling methods
a. The extensive live-trapping survey (see appendix 1)
b. The intensive live-trapping survey at Coptfold (see table 1)
c. The bottle hunts (see table 2)
d. Owl pellets (see table 3)
The great differences between the results from the three main
methods, and the fact that each method revealed a different
species as the 'most abundant' indicates the degree of caution
necessary when interpreting the results of such surveys. Clearly
each method gives a biased catch and some of the reasons for
this are as follows —
Live-trapping. To avoid theft of traps, woodlands or hedgerows
were usually selected as trap-sites. Bottles and Owls sample
different habitats. Also the overnight trapping and the absence
of pre-baiting is likely to increase the proportion of Apodemus
spp. Voles tend to be trap shy and both shrews and voles are
partially diurnal. The very lightest species are likely to be
underestimated by trapping as a certain pressure is necessary
to fire the mechanism. Species such as Harvest Mice (Micromys
minutus) which live above ground level for part of the year will
be underestimated for this reason.
Bottles. Most bottles are dumped in roadside hedgerows and
lay-bys. Very few were collected from mature woodland. Since
the bottles trap over a long period they may detect species
which migrate through the area—these could be missed by
short-term live-trapping. The bottles are not biased against light
species and the narrowest necked bottles would only allow the
smaller species and individuals to enter. Studies by Margaret
Wise at Royal Holloway College have shown that different