Indicator assessment
Two charts are presented in this indicator.
The first shows proportion of non-native species in survey samples
for birds, mammals, plants and marine organisms. This shows an
overall increase of 23% in the period 1990-2007, suggesting
non-native species are becoming more widespread in the countryside
in general. Given that the majority of these species are not
considered to have a high impact on native biodiversity, this chart
is shown to provide context and is not assessed.
The second chart shows the change in the
number of species identified as highly invasive (i.e. of greatest
threat to native biodiversity) established across more than 10% of
the land area of Great Britain. This has increased in the long term
in all ecosystems (freshwater, marine and terrestrial) although
there has been no change in the freshwater environment since
2000.
Description of trends
There is considerable variation in the trends
for different species groups (Figure 11 (iii)). The
proportion of non-native birds and mammals increased relatively
rapidly over the period compared with plants. There has been an
apparent fall in the proportion of non-native species in the marine
environment, although it is not clear why this should be,
especially given that the extent of invasive marine species is
increasing. It is possible that there has been a tendency to
under-record NNS in recent years.
Not only are the trends different for
different species groups, the absolute proportions also vary.
Generally, small organisms have a lower non-native proportion than
larger ones. There are relatively few non-native insects compared
to vertebrates. One well-recorded group, the butterflies, has been
omitted completely, because there are currently no established
populations of non-native butterflies in Britain. Lichens are
omitted for a similar reason. Non-native proportions for 1990
ranged from 0.54% for birds to 1.04% for bryophytes and 1.07% for
marine organisms. Mammal values are shown separately because
they were based on frequency (encounters or signs) in Breeding Bird
Survey squares, and could not be divided by the total number of
mammals in such squares (as this is not known).
Figure 11 (iii). Proportion of non-native species in separate
samples of birds, mammals, plants and marine organisms, 1990 to
2007
Relevance
The United Nations Convention on Biological
Diversity (CBD) identifies invasive species as second only to
habitat loss in terms of threats to biodiversity. Most
non-native species are not invasive but those that are can spread
disease (e.g. signal crayfish), modify ecosystems (e.g.
rhododendron), drastically reduce populations of native species
(e.g. American mink) or hybridize with native species (e.g. ruddy
duck).
Under the CBD, the United Kingdom has an
international obligation to address the impacts of invasive
non-native species. In 2008, the UK Government published the
Invasive Non-native Species Framework Strategy for Great
Britain.
Background
This indicator is based on species
distribution data assembled and held by the British Trust for
Ornithology (BTO), Centre for Ecology and Hydrology (CEH) and
Marine Biological Association (MBA) and supplemented by data from
the National Biodiversity Network Gateway.
Figure 11 (i) is based on the number of
non-native species records as a proportion of total records in each
of the samples set out in Table 11 (i). The combined proportion for
all taxa is the weighted geometric mean of the other values.
Weights for the mean were selected to give equal weighting (0.4 and
0.4) to terrestrial plants and animals and to give a moderate
weighting (0.2) to marine life.
Table 11 (i). Calculation of
index of non-native frequency
|
Species group
|
Weight
|
Observation
|
Index calculated
|
|
Birds (Breeding Bird Survey)
|
0.2
|
Counts of individuals per transect in 1-km
square
|
Non-native count as proportion of total
count
|
|
Mammals (Breeding Bird Survey)
|
0.2
|
Presence in Breeding Bird Survey (BBS) square;
data for 2001 interpolated (foot-and-mouth outbreak)
|
Proportion of BBS squares in which the
observer made records of six selected non-native mammals
|
|
Vascular plants, random plots (Countryside
Survey)
|
0.15
|
Presence in random 14 m square quadrat (‘X
plot’)
|
Frequency of non-native, non-crop species as a
proportion of all species recorded
|
|
Vascular plants, riverside plots (Countryside
Survey)
|
0.15
|
Presence in quadrat by watercourse (‘S’ and
‘W’ plots)
|
Frequency of non-native, non-crop species as a
proportion of all species recorded
|
|
Bryophytes (BRC dataset)
|
0.1
|
Presence in hectad (10 km square)
|
Frequency of non-native hectad records as a
proportion of all hectad records
|
|
Marine (Marine Life Information Network)
|
0.2
|
Record in database
|
Frequency of non-native records as a
proportion of the total
|
Trends in the extent of invasive species were
derived by a two-stage process. First, lists of non-native
species were screened by expert panels to categorise their degree
of threat to native biodiversity. This took account of the
evidence for significant harm to biodiversity and ability to
rapidly disperse (note that economic impacts were not considered,
although the method lends itself to this option). Forty-nine
species were placed in the highest threat category (Table 11
(ii)).
Table 11 (ii). The forty-nine
most invasive non-native species identified for the
indicator
|
(a) Marine plants
|
(d) Freshwater animals
|
|
Sargassum muticum (Wire weed)
|
Pacifastacus leniusculus (Signal
crayfish)
|
|
Undaria pinnatifida (Japanese kelp,
wakame)
|
Procambarus clarkii (Red swamp
crayfish)
|
|
Codium fragile ssp. tomentosoides
(Green sea fingers)
|
Corbicula fluminea (Asian clam)
|
|
|
Dreissena polymorpha (Zebra
mussel)
|
|
(b) Marine animals
|
Pseudorasbora parva (Topmouth
gudgeon)
|
|
Tricellaria inopinata (a
bryozoan)
|
Sander lucioperca (Pikeperch,
zander)
|
|
Watersipora subtorquata (a
bryozoan)
|
Lithobates catesbeianus (American
bullfrog)
|
|
Corophium sextonae (an amphipod)
|
Trachemys scripta (Common slider
turtle)
|
|
Gammarus tigrinus (an amphipod)
|
|
|
Elminius modestus (an acorn
barnacle)
|
(e) Terrestrial plants
|
|
Solidobalanus fallax (a barnacle)
|
Carpobrotus edulis (Hottentot
fig)
|
|
Eriocheir sinensis (Chinese mitten
crab)
|
Disphyma crassifolium (Purple
dewplant)
|
|
Rhithropanopeus harrisii (Dwarf
crab)
|
Fallopia japonica (Japanese
knotweed)
|
|
Crassostrea gigas (Pacific
oyster)
|
Heracleum mantegazzianum (Giant
hogweed)
|
|
Crepidula fornicata (Slipper
limpet)
|
Impatiens glandulifera (Himalayan
balsam)
|
|
Rapana venosa (Rapa whelk)
|
Quercus ilex (Evergreen oak)
|
|
Anguillicola crassus (Swim-bladder
nematode)
|
Rhododendron ponticum
(Rhododendron)
|
|
Botrylloides violaceus (a
tunicate)
|
Rosa rugosa (Japanese rose)
|
|
Corella eumyota (a tunicate)
|
|
|
Didemnum vexillum (a tunicate)
|
(f) Terrestrial animals
|
|
Styela clava (Leathery sea
squirt)
|
Arthurdendyus triangulata (New
Zealand flatworm)
|
|
|
Harmonia axyridis (Harlequin
ladybird)
|
|
(c) Freshwater plants
|
Branta canadensis (Canada goose)
|
|
Crassula helmsii (New Zealand
pigmyweed)
|
Oxyura jamaicensis (Ruddy duck)
|
|
Hydrocotyle ranunculoides (Floating
pennywort)
|
Cervus nippon (Sika deer)
|
|
Ludwigia grandiflora (Uruguayan
Hampshire-purslane)
|
Muntiacus reevesi (Reeves’
muntjac)
|
|
Myriophyllum aquaticum
(Parrot's-feather)
|
Mustela vison (American mink)
|
|
|
Myocastor coypus (Coypu)
|
|
|
Rattus norvegicus (Brown rat)
|
|
|
Sciurus carolinensis (Grey
squirrel)
|
The second step was to categorise the extent
of each of the most invasive species based on the proportion of the
land area or length of coastline in which NNS were found in 1960,
1970, 1980, 1990, 2000 and 2008 (see Table 11 (iii)).
Figure 11 (iv) shows the number of species in
each category in each of the assessment years. The indicator
(Figure 11 (ii)) is compiled only from those invasive species in
extent greater than or equal to 10%.
There are of course limitations to this
approach – the most invasive species are based on expert judgment
(albeit using standardised assessment criteria) and the extent
value is based on relatively broad categories. The scoring system
is adapted from one used by the Belgian Forum on Invasive Species
(see web-links). Further details are available in a separate
research report (see web links). The indicator is a proxy for
impacts - it demonstrates a likely growing impact due to invasive
species establishment and spread.
Figure 11 (iv). Changes in the extent of invasive non-native
species in marine, freshwater and terrestrial environments, 1960 to
2008
Table 11 (iii). Invasion extent
of non-native species
|
Definition
|
Interpretation
|
Extent
|
|
Not present in territory
|
Absent
|
0
|
|
Present in territory and either not
established or with established populations that have not spread
more than 10 km from their source
|
Not or scarcely established
|
1
|
|
Established populations present less than 10%
of territory, with some having arrived from further than 10 km from
their source; or if more widespread then populations scattered and
sparse
|
Established but still generally absent or at
most occasional
|
2
|
|
Established populations present in 10-50% of
the territory
|
Established and frequent in part of the
territory
|
3
|
|
Established in more than 50% of the
territory
|
Widespread
|
4
|
Further development planned
Vascular plant data are based on Countryside
Survey. This is unlikely to be repeated before 2015. Although
the data for other species groups are generally available on an
annual basis, the impact indicator is more likely to be updated on
a 5-10 year cycle Defra is setting up a Central Repository for data
on non-native species in Great Britain. This will bring together
data on several additional species groups such as fish and will
provide a mechanism for reporting future changes.