|
GEOLOGY AND OTTERS
IOSF has undertaken eight years' research on the otter
populations of the Isle of Skye and below is a summary of the findings.
For the purposes of the study the island was divided into seven
geological coastal zones based on maps from the British Geological Survey.
In an initial survey of otter sightings in 1990
it was found that certain coastlines favoured
denser otter populations than others;
it seemed that the geology of the coastline had an effect on this
distribution with the Torridonian coastline having a much denser number than any
other rock type. The island was
divided into 500m coastal sections and the following variables were studied:
coastal type, inland vegetation, slope of shoreline, width of the
intertidal zone, otter activity per hour, number of holts, sprainting points,
number of spraints and freshwater pools.
60% of the coastline was covered and data sheets
completed for each section. Upon
analysis of the data, certain predictable patterns emerged:
the Torridonian zone had a higher proportion of all the 'otter variables'
than any other zone [mean numbers per 500m section (otters/hour = 2.4, holts =
0.4, sprainting points = 2.5, spraint numbers = 6.3 and freshwater pools = 1.4)]
compared with the lowest numbers in the Tertiary Intrusive zone [mean
numbers per 500m section
(otters/hour = 0, holts = 0.04, sprainting points = 1.5, spraint numbers = 3,
freshwater pools = 0.6)]. The
increased activity on the Torridonian coastal zone is attributed to this zone
having the greatest number of freshwater pools, a gently sloping shoreline with
a boulder intertidal zone and native woodland adjacent to the High Water Mark. A map of otters per km is shown here together with the
geology.
Until now, little was known about the relationship
between otter utilisation of the coastal zone and geology.
On Shetland, Milner (1978) categorised major coastal types and identified
eight major coastal types using 81 physical and 18 geological attributes.
Conroy and French (1985) classified the Shetland coast using over 200
attributes which were grouped into seven headings.
Still on Shetland, Kruuk (1989) grouped the coast into six types based on
evidence from maps, reports and prior knowledge of the area.
He found a strong relationship between numbers of holts and peaty coasts
with little agriculture and no high cliffs and a negative association between
holts and tall cliffs.
On Orkney there are few otters, in sharp contrast to
Shetland, (Kruuk, 1995). This
difference could possibly be attributed to distribution of prey species but it
seems likely that Orkney shores are just as productive as those of Shetland.
Green and Green (1997) stated, "that despite the productive nature
of Orkney's fresh and coastal waters, the otter population was less dense than
those of other island regions or parts of mainland Scotland".
A more likely reason for the difference is the geology of the Orkney
coast, which makes it a less suitable habitat for otters.
The Torridonian sandstone on Skye is impervious and has very low porosity allowing freshwater pools to build up easily on the coastal fringe. By contrast, Devonian sandstone has a high degree of porosity and permeability (many oil reservoirs are present in Devonian sandstone), and so it would be difficult for freshwater pools to build up on these rock types in the same numbers as on the Torridonian sandstone. The Orkney landscape is composed of Devonian sandstone and is well drained with rich farmland. The low density of otters from Dunnett Head to the Moray Firth could equally be a consequence of the geology of the coastal fringes, which also consist of Devonian sandstone.
References
Conroy, J.W.H.and French, D.D. (1985). Monitoring
otters in Shetland. Shell Oil Terminal
Report.
Green, J. and Green, R. (1997). Otter Survey of
Scotland 1991‑1994. Vincent
Wildlife Trust. London.
Kruuk, H., Moorhouse, A., Conroy, J.W.H. and Durbin, L.
(1989). An estimate of numbers and habitat preferences of otters (Lutra lutra)
in Shetland. Biological
Conservation, 49, 241‑ 254.
Kruuk, H. (1995). Wild Otters, Predation and
Populations. Oxford University
Press.
Milner, C. (1978).
Shetland Ecology Survey. Geographical
Magazine, 50, 730‑53
|
|
