Red Listing Freshwater Issues Protocol
(notes for freshwater biodiversity assessments)
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Extent of Occurrence
Area of Occupancy
Location
Severe Fragmentation
Extent of Occurrence:
Definition
“Extent of occurrence is defined as the area contained within the shortest continuous
imaginary boundary which can be drawn to encompass all the known, inferred or
projected sites of present occurrence of a taxon, excluding cases of vagrancy. This
measure may exclude discontinuities or disjunctions within the overall distributions of
taxa (e.g., large areas of obviously unsuitable habitat) [but see 'area of occupancy',
below]. Extent of occurrence can often be measured by a minimum convex polygon
(the smallest polygon in which no internal angle exceeds 180 degrees and which
contains all the sites of occurrence).” (IUCN 2001)
Riverine Species
Catchments are defined using Level 6 of the Hydro 1K watersheds GIS layer. All
catchments with confirmed records of a species are included. Adjacent (i.e.
connected) catchments are added in if it is reasonable to infer the presence of the
species there, according to expert opinion. Areas where the species is not likely to be
found are deleted from the range, based on the known ecology of the species (e.g. if a
species is known to be limited to headwater streams). This gives the species range
map, whose total area is the Extent of Occurrence of the species. If there are
discontinuities within the range (e.g. intervening river basins with no species records),
these are not included in the EOO. So in the example below, the area of catchments a
& d is calculated to give the EOO of the species, while catchments b & c are not
included.
Lacustrine Species
The EOO for lacustrine species is the surface area of the lake (e.g., Lake a). If the
depth range of the species is known, the unoccupied depths can be eliminated from
the calculation (e.g. Lake b).
Marsh/swamp species
The EOO for marsh/swamp species is the area of the marsh or swamp.
Area of Occupancy:
Definition
“Area of occupancy is defined as the area within its 'extent of occurrence' (see above),
which is occupied by a taxon, excluding cases of vagrancy. The measure reflects the
fact that a taxon will not usually occur throughout the area of its extent of occurrence,
which may contain unsuitable or unoccupied habitats. In some cases, (e.g.,
irreplaceable colonial nesting sites, crucial feeding sites for migratory taxa) the area
of occupancy is the smallest area essential at any stage to the survival of existing
populations of a taxon.” (IUCN 2001)
Riverine Species
Use a 2x2 km (i.e., 4 km²) grid - see user guidelines.
The grid method will in some cases over
estimate the AOO for the species. Therefore,
it may be useful to also calculate the AOO by
multiplying the length by the width of the
stretches of river the species occupies (e.g.,
river sections a, b & c) and adding these
areas together. This will allow us to identify
cases where the different AOO estimation
methods result in different categories being
allocated.
Lacustrine species
If the species is pelagic and occurs lake-wide, the AOO and EOO will be the same. If
known, use the area of the specific habitats that the species occupies (e.g., rocky shore
areas or river mouths).
Marsh/swamp species
The AOO and EOO will often be the same. If known, use the area of the specific
habitats that are occupied by the species.
Location:
Definition
“The term ‘location’ defines a geographically or ecologically distinct area in which a
single threatening event can rapidly affect all individuals of the taxon present. The
size of the location depends on the area covered by the threatening event and may
include part of one or many subpopulations. Where a taxon is affected by more than
one threatening event, location should be defined by considering the most serious
plausible threat.” (IUCN 2001)
Location is intrinsically linked to the main plausible threat to that species. It is the
area likely to be affected by that threat (single event). Some examples:
Introduced species
Introduced species often affect the whole river catchment or lake area, as they can
easily spread throughout. In such cases, the lake or a river catchment is one location.
Pollution
In a river system, pollution is likely to affect the area from the source of the pollution
downstream, and in some cases all the way to the river mouth (depending upon the
nature of the pollutant). In such cases, the river catchment can be two or more
locations. Occasionally pollution can make the species distribution in a river system
one location (e.g., if the point source is at the headwaters of the stream or affects the
entire population (which may only be found in one tributary or only in lower reaches
of the river)).
Pollution in a lake, especially a large one, may not affect the whole lake (depending
upon the nature of the pollutant). In such cases, the lake would be two or more
locations.
Dams
The actual construction of a dam can destroy habitats. However, a dam constructed in
a river that stops a migratory species reaching its spawning grounds would make the
species distribution in that river catchment one location.
Water Extraction
Water extraction from a lake may affect the entire population in that lake (e.g., the
species may be restricted to habitat at the edges of the lake and this may be affected
by reduced water levels; prolonged water extraction may even affect the lake ecology
by increasing salinity, etc.). In such cases, the lake is one location.
Severe Fragmentation:
Definition
“The phrase ‘severely fragmented’ refers to the situation in which increased
extinction risks to the taxon results from the fact that most of its individuals are found
in small and relatively isolated subpopulations (in certain circumstances this may be
inferred from habitat information). These small subpopulations may go extinct, with a
reduced probability of recolonization.” (IUCN 2001)
A taxon can be considered to be severely fragmented if most (>50%) of its total area
of occupancy is in habitat patches that are (1) smaller than would be required to
support a viable population, and (2) separated from other habitat patches by a large
distance.
i.e., if >50% of the total AOO is broken into areas that support small, non-viable
populations AND there is no (or very little) chance of natural recolonisation of these
areas (because the nearest subpopulation is too far away), the species distribution is
severely fragmented.
Recolonisation
Recolonisation could be in the form of flood events linking two or more lakes or
rivers.