38
THE ESSEX NATURALIST
localities. The fact that a species was not recorded in an area
may either be because it is fairly rare and the sample was not big
enough to detect its presence, or because it is genuinely absent
from the area sampled. Table 5 shows the results of an attempt
to separate these two factors. The relative abundance of a species
is expressed as a percentage catch in those habitats where it de-
finitely occurred. A species that is rare where it occurs is likely
to be recorded in relatively few areas for this reason alone. If
a species is fairly abundant where it occurs then, if it is detected
in only a small proportion of localities, this is evidence of its real
absence from the other habitats. In fact all that can be safely
concluded from the table is that Clethrionomys, Apodemus
sylvaticus and Sorex araneus are both widespread and abundant
while the other species are rarer and somewhat more restricted.
The Maps
The maps which illustrate the species accounts are based
on one kilometre national grid squares. A single record from
a square is sufficient for that square to be marked with a symbol.
The type of record is indicated by the type of symbol (see the key
to the maps), and in the event of records of more than one type
being available for the same square, the following order of
priority is used:—
1. Live-trapping
2. Bottle-hunting
3. Casual records
4. Owl pellets. Owl pellet records are inherently less re-
liable because the owl may have carried its prey some distance
from the point of capture. However, owls are territorial in their
hunting and this source of error is not likely to be large.
Great care should be used in interpreting the maps. The
survey is not complete and a blank does not indicate an absence
of the species concerned. A thin scattering of records, through-
out the county, indicates that the species is widespread, but
rarer, than one with a dense scatter of records. Only when large
areas of the map are blank, while in related species the same
areas are not blank, should this be interpreted as good evidence
of absence.
Key to Maps
Note that, in addition to the records detailed in the tables
and appendix, the maps include additional records supplied to the
Essex Field Club mammal recorders. Old, pre-1960, records are
only shown for the rarer species: Harvest Mouse, Yellow-necked
Mouse, and Dormouse.
Symbols
pre 1960 post 1959
Longworth live-trapping
Bottle-hunt record
Owl pellets (includes 1959 records)
Casual records from all other sources