Indicator description
Two charts are presented in this indicator. The first 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 per cent 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.
The second chart shows proportion of non-native species in
survey samples for birds, mammals, plants and marine organisms.
This shows an overall increase of 23 per cent 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.
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
per cent for birds to 1.04 per cent for bryophytes and 1.07 per
cent 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
hybridise 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
|
|
Codium fragile ssp. tomentosoides (green sea
fingers)
|
Corbicula fluminea (Asian clam)
|
|
Sargassum muticum (wire weed)
|
Dreissena polymorpha (zebra mussel)
|
|
Undaria pinnatifida (Japanese kelp, wakame)
|
Lithobates catesbeianus (American bullfrog)
|
|
|
Pacifastacus leniusculus (signal crayfish)
|
|
(b) Marine animals
|
Procambarus clarkii (red swamp crayfish)
|
|
Anguillicola crassus (swim-bladder nematode)
|
Pseudorasbora parva (topmouth gudgeon)
|
|
Botrylloides violaceus (a tunicate)
|
Sander lucioperca (pikeperch, zander)
|
|
Corella eumyota (a tunicate)
|
Trachemys scripta (common slider turtle)
|
|
Corophium sextonae (an amphipod)
|
|
|
Crassostrea gigas (Pacific oyster)
|
(e) Terrestrial plants
|
|
Crepidula fornicata (slipper limpet)
|
Carpobrotus edulis (Hottentot fig)
|
|
Didemnum vexillum (a tunicate)
|
Disphyma crassifolium (purple dewplant)
|
|
Elminius modestus (an acorn barnacle)
|
Fallopia japonica (Japanese knotweed)
|
|
Eriocheir sinensis (Chinese mitten crab)
|
Heracleum mantegazzianum (giant hogweed)
|
|
Gammarus tigrinus (an amphipod)
|
Impatiens glandulifera (Himalayan balsam)
|
|
Rapana venosa (rapa whelk)
|
Quercus ilex (evergreen oak)
|
|
Rhithropanopeus harrisii (dwarf crab)
|
Rhododendron ponticum (rhododendron)
|
|
Solidobalanus fallax (a barnacle)
|
Rosa rugosa (Japanese rose)
|
|
Styela clava (leathery sea squirt)
|
|
|
Tricellaria inopinata (a bryozoan)
|
(f) Terrestrial animals
|
|
Watersipora subtorquata (a bryozoan)
|
Arthurdendyus triangulata (New Zealand flatworm)
|
|
|
Branta canadensis (Canada goose)
|
|
(c) Freshwater plants
|
Cervus nippon (sika deer)
|
|
Crassula helmsii (New Zealand pigmyweed)
|
Harmonia axyridis (harlequin ladybird)
|
|
Hydrocotyle ranunculoides (floating pennywort)
|
Oxyura jamaicensis (ruddy duck)
|
|
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 Figure 11 (iv)).
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 per cent.
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
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.