Geoscience Reference
In-Depth Information
recorded the relative area of the three predominant
vegetation formations. These vegetation formations
were classified into phytosociological associations
(Mueller-Dombois and Ellenberg, 1974; Goldsmith
et al.
, 1986). Conservation status assessment was
restricted to those habitats that were the most
predominant vegetation formation in any of the
polygons, while habitat area was calculated using
second and third dominant vegetation types as
well.
River Habitat Survey (RHS; Raven
et al
., 1997;
Environment Agency, 2003) was carried out on
300 sites to characterize the physical structure
of the river channel. Habitat Modification Scores
(HMS) were calculated for each 500 m RHS
survey length to determine the extent of artificial
modification to the channel. For some river
channels surrogate HMS scores were also estimated
from aerial photographs.
The first specific indicator (relative area covered
by each habitat) reflects the importance of
geographical extent for nature conservation
purposes (Meffe and Carroll, 1997; Primack, 2008).
To achieve favourable conservation status in our
system the observed habitat area should exceed
an expected value (Table 16.1). The 'expected'
area was calculated by dividing the river network
into 500 m lengths, with 25 m buffers on either
side of the channel. This produced 1451 units,
each approximately 2.5 ha in area (Figure 16.3).
Units that were not severely affected by human
activities were identified using three main criteria:
(i) absence or very minor hydrological alteration,
so units located downstream from large dams with
capacity to regulate high flows were excluded; (ii)
absence or very minor morphological alteration,
defined by an HMS score
200 calculated from
field survey and aerial images (Fern andez
et al.
,
2011); (iii) absence or very limited intensive
land management - those units where tilled
land, intensive pasture, forest plantation or urban
developments exceeded 10% of the total area were
excluded.
The relative habitat area was calculated for
each of 12 river types (IH Cantabria, 2010) by
incrementally combining data from relevant 500
m lengths (Figure 16.4). The 'expected' value
used to compare observed values was set as the
tenth percentile (P
10
) for each of these incremental
distributions and is therefore dependent on the
length of each assessment unit and river type
(Annex 1). Favourable conservation status was
only achieved if the observed area of each habitat
was at least the calculated P
10
value for the given
assessment unit area size and river type (Annex 2).
Habitat structure and composition were
calculated on the basis of a qualitative assessment
of different variables that characterize important
aspects of vegetation communities (Pettit and
Naiman, 2005). The six variables used depended
on the dominant vegetation type (e.g. forest,
shrubs, aquatic macrophytes) in each assessed
polygon. Table 16.1 shows how each variable was
used to determine whether overall conservation
status was favourable or not. Favourable status
was only achieved when all six variables for each
dominant habitat in each polygon were favourable.
In addition, the habitat could only reach favourable
status when the relative area of the habitat with
favourable structure and composition exceeded
60%; this rule was used to ensure that more
than half the existing habitat patches could be
expected
to
be
in
an
ecologically
functioning
condition.
Habitat vulnerability assesses the capacity to
tolerate the effects of human activities (De Lange
et al
., 2010). It was calculated by combining
two variables - habitat sensitivity and degree
of exposure to pressures. Habitat sensitivity was
determined by assigning a score (1
=
=
nil; 2
low; 3
high) derived for several
pressures using a Delphi method involving 22
experts. The average sensitivity score was then
calculated for each typical pressure. The degree of
exposure to pressures was assessed by estimating
the overlap between the dominant habitat and the
area affected by human pressures in each polygon,
the latter calculated using specifically designed
ARCGIS tools (IH Cantabria, 2010). A habitat was
considered vulnerable if more than 10% of it
coincided with a high sensitivity activity or more
than 20% of a medium sensitivity activity within
a polygon. Favourable conservation status of the
habitat within the assessment unit could only be
achieved if the non-vulnerable relative area of the
habitat exceeded 60%.
=
medium; 4
=
