Information and Advisory Note Number 112
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Nativeness or non-nativeness of species
1. Introduction
1.1 Much of the work of Scottish Natural Heritage's is implicitly, or at times
explicitly, concerned with discriminating between native and non-native species
of plants and animals. Virtually all of the work relating to the UK Biodiversity
Action Plan is focused on native species. The work on re-introducing red kites (Milvus
milvus), white-tailed eagles {Haliaeetus albicilla) and the possibility of
re-introducing beavers (Castor fiber) is predicated on the native status of
these species.
1.2 In contrast, site management, control measures are frequently targeted
against non-native species, such as rhododendron {Rhododendron ponticum),
Japanese knotweed (Fallopia japonica), grey squirrel (Sciurus carolinensis) and
Sika deer (Cervus nippon). Generally there is a common belief that these
non-native (alien) species occupy the ecological niches for which native species
are adapted, and therefore that those native species will suffer depleted
population sizes or even eventually become extinct.
1.3 This Information & Advisory (l&A) Note sets out criteria that have been used
to determine whether or not species should be considered as native. It also
establishes a new framework of six categories of species for considering their
nativeness, and their links with the natural heritage.
1.4 The l&A Note does not explore aspects of aesthetics, or the quality of the
countryside as experienced by visitors who, for example, might marvel at the
attractiveness of woodlands full of flowering rhododendron in the spring time!
It also recognizes that some species which are non-native in Scotland are
nevertheless components of 'cultural landscapes'; for example, beech (Fagus
sylvatica) trees might not be locally native, but can be visually important in
many landscapes.
2. Criteria
2.1 Webb (1985) made a systematic attempt to establish a series of criteria that
could be used in aiding a decision as to whether a plant species is native or
not. His eight criteria, with a short commentary on each, are as follows.
• Fossil evidence: a fossil record provides
evidence that the species is native if, and
only if, that fossil can be dated to between
the last glaciation (c. 10,000 years ago) and
the advent of neolithic agriculture
c. 7,000 years ago. However, fossil evidence is relatively scarce, especially
for animals. There are two other caveats to the use of fossil evidence. First,
it is generally accepted that the assessment of nativeness starts at the last
Ice Age, and hence fossils from previous inter-glacials are disregarded, whether
or not the species could exist in current climatic conditions. Second, the
presence of a post-glacial fossil does not mean that the species could exist at
that location today; many Arctic species moved north at the end of the last Ice
Age and are now extinct at lower latitudes because suitable climates no longer
exist.
• Historical evidence: historical data are more useful in establishing that a
species is non-native, since for some species dates of introduction or first
discovery are known. Historical evidence can also be essential in determining
that a species was present, as for example the presence of the beaver {Castor
fiber) in Scotland in the fourteenth century.
• Habitat: if a species lives in a more or less natural habitat then it is
likely to be native, whereas if it only lives in man-made
habitats it is more likely to be non-native. This criterion can at best only be
indicative, and at worst it can be misleading.
• Geographical distribution: although only indicative, a species with a disjunct
distribution is less likely to be native, even although many non-vascular plants
and fungi, and some invertebrates, have very disjunct distributions. For this
criterion, one needs to compare distributions across Europe (or more widely). If
there is a huge geographical separation then it is important to gain
corroborative evidence if a species is to be considered as native.
• Frequency of known naturalization: if a species is becoming naturalized in
similar habitats in other countries, it does not prove that it is not native in
this country, but it makes it more likely to be non-native.
• Genetic diversity: whilst such data are available for only very few species,
the greater the genetic diversity, the greater the chance that the species is
native (except, of course, for small relict populations of native species).
However, care in interpretation is required because some populations,
particularly near the edge of the geographical range, may have been formed from
a small number of 'founders'. This would appear to be the case in relation to
the natterjack toad (Bufo calamita) whose British populations apparently have
little genetic diversity.
• Reproductive pattern: probably only applicable to plants, but if a species
fails to reproduce sexually (instead reproducing entirely vegetatively) it is
less likely to be native. The converse is not, of course, true because many
non-native species reproduce sexually.
• Possible means of introduction: if no mechanism for introduction can be
discovered or deduced, there may be a greater chance that the species is native.
2.2 These eight criteria are indicative rather than definitive. For most
species, classification as 'native' or 'non-native' depends upon the balance of
probabilities, weighing the evidence for several of these criteria. The criteria
were extended by Preston (1986), by the addition of a ninth criterion.
• Entomological evidence: most plant
species support a guild of insect
herbivores, and hence plants that are
notably free of insect herbivores are less
likely to be native. At times non-native
species may be introduced with one or two insect herbivores, or they may be
attacked by insects that feed on related native species, but the total number of
herbivorous species on non-native plants is generally limited.
2.3 In using these nine criteria there is an implicit time-scale. In Britain,
the last glaciation is taken as the 'starting time' because very few species
could have survived the cold and ice cover. Thus, assessments of a species'
status take no cognisance of its presence during previous inter-glacial periods.
Species such as Norway spruce (Picea abies) and the Balkan heath (Bruckenthalia
spiculifolia), both of which are known as macrofossils in inter-glacial peat
deposits but neither of which re-appeared after the last glaciation, are treated
as non-native. Assessments are therefore conventionally, and hence arbitrarily,
made on the basis of species' distributions since the last Ice Age, or over the
last approximately 10,000 years in Scotland.
3. Categories of species
3.1 A geographical scale needs to be defined. Because Great Britain is an
island, it is useful to carry out assessments for the whole island, including
Scotland, England and Wales. However, whilst using a definition in relation to
Great Britain, the focus of this l&A Note is largely upon Scotland.
3.2 It should be noted that this l&A Note deals only with species and
subspecies. It specifically excludes variation at the genetic or molecular level
(e.g. any possible genetic difference between Scottish primroses (Primula
scotica) from the mainland and Orkney populations). Many species are naturally
moving around, or have been moved around, the countryside or marine environment
since the end of the last Ice Age. These differences, whilst important for plant
and animal breeding, are not considered further: there is therefore no
consideration of ecotypes, provenances, cultivars, breeds, varieties, etc.
Table 1. The six classes, two subdivided, for classifying species according to
their nativeness. The class names can be applied to the whole of Britain (B) or
just for a local (L) part of Britain.
3.3 In much of the literature, only two categories of species are recognised,
namely native species and non-native species. These can be viewed as two
opposite ends of a spectrum.
3.4 A classification is set out in Table 1. The remainder of this section, from
paragraphs 3.5 to 3.12, expands upon this classification, and gives examples of
species in each group.
3.5 Native species are presumed to be those that are present in Great Britain by
natural means. In general they migrated (or were transported by other species)
into Great Britain after the last Ice Age, without the assistance of humans.
Examples of native species include the common daisy (Bellis perennis), the rare
and endemic Scottish primrose (Primula scotica), the red deer (Cervus elaphus)
(though genotypes may have been imported from elsewhere), the salmon {Salmo
salar) and the golden eagle (Aquila chrysaetos).
3.6 Non-native species have been introduced to Great Britain, either
deliberately or accidentally, by humans. Plant examples include Oxford ragwort
(Senecio squalidus), that escaped from the Oxford Botanical Garden, Himalayan
balsam (Impatiens glandulifera) that escaped from private gardens, and
rhododendron (Rhododendron ponticum) which Salisbury (1961) indicated was
introduced to the British Isles in 1753 (although it was present during an
inter-glacial period prior to the last Ice Age). Non-native
animals include the fur farm mammals such as mink (Mustela vison), coypu (Myocator
coypus) and musk rat (Ondatra zibethicus), introduced to various places in the
British Isles in the 1920s and rapidly escaping and establishing wild-living
populations (Usher, 1987); and the ruddy duck (Oxyura jamaicensis) which escaped
from the Wildfowl & Wetland Trust's reserve at Slimbridge and by the mid-1960s
had established a number of wild-breeding populations (Snow, 1971).
3.7 Between these two extreme categories, there is a number of intermediate
scenarios that make hard and fast classifications impossible. These are grouped
in Table 1 into the following four classes.
3.8 There is a group of species that no longer occur, but are known (or
presumed) to have occurred naturally at some time in the past. The distribution
of species is not static; some species will inevitably become extinct whereas
other species will arrive unassisted by humans. Birks (1997) listed some Arctic
plants, such as the poppy (Papaver sect. Scapiflora) and cassiope (Cassiope
spp.), which have not been seen in Scotland in historical times, yet which have
been discovered as post-glacial macrofossil deposits. Other species have become
extinct in Scotland through human intervention, either directly or indirectly.
The beaver (Castor Fiber) and the white-tailed eagle (Haliaeetus albicilla) are
examples of species which may have been hunted or shot to extinction: the corn
cockle (Agrostemma githago) is an example of a plant which may now be extinct as
a result of changing agricultural practices. How long does it take for the food
chain associated with these species to change so that they can no longer be
presumed to be native? These species could be termed 'formerly native'. This
class can be subdivided into those that could persist if they were re-introduced
(such as the red kite (Milvus milvus)) and those that probably could no longer
sustain a population in Scotland (such as cassiope (Cassiope hypnoides)). This
subdivision is essentially between those that have been lost through human
action and those whose biotic or abiotic environment has so changed that they
could live here no longer.
3.9 Some species that are native in part of
Great Britain have been introduced beyond
their natural geographical range. An example
of this is the hedgehog (Erinaceus europaeus),
which is undoubtedly native throughout much
of mainland Britain, but which never naturally reached many of the islands around the
Scottish coast. Its recent introduction to the
Outer Hebrides is creating problems for
ground nesting birds. Another example is the
spring gentian (Gentiana verna), which is
native in limestone grassland in Teesdale
(northern Pennines, England) and the Burren
(western Ireland), but which has been
introduced to the limestone grassland on
Inchnadamph NNR. Whilst native in England
and Ireland, it is probably best to regard all
such species as 'locally non-native' in
Scotland.
3.10 Some species were probably introduced by humans a long time ago, but have
now become part of the 'natural' food chain. Examples include the sycamore (Acer
pseudoplatanus), which Salisbury (1961) presumed was introduced by the Romans,
and the Orkney vole (Microtus arvalis), which is presumed to have been
introduced by the early human settlers of the main islands of Orkney (Corbet &
Southern, 1977). It would also include the rabbit (Oryctolagus cuniculus), which
now forms a major prey species for many predators. After how long a period of
time do we accept that these species have become functionally native by becoming
a part of the food webs of native species? This group of species is sometimes
described as 'naturalised', but as 'long-established' is preferred.
3.11 There are some species of recent occurrence whose natural spread to Great
Britain might have been associated with human activity. One example is the
collared dove {Streptopelia decaocto), which has undergone a rapid range
expansion northwestwards across Europe since about 1930. It first bred in the
UK, in Norfolk, in 1955, and was breeding in the City of Edinburgh a decade
later. Although this expansion appears to have been natural, it might have been
assisted by changed agricultural practices in which more cereal grains are left
on the ground. Will species that migrate north into Scotland as a result of
pollution-induced climate warming, such as the wall brown butterfly (Lasiommata
megera), also come into this category? How much human mediation must there be
before seemingly 'natural' colonists are deemed to be non-native? These species
should be termed 'recently-arrived'.
Species that arrive totally naturally, i.e. orchid seed blowing over from the
European continent, might be considered as a subdivision, separate from those
suspected of arriving through human activity. In a sense this is a 'holding'
group, pending clarification as to whether the species should be considered to
be native or non-native.
3.12 The six classes, and their subdivisions outlined in Table 1, are not
mutually exclusive. The category 'native' in a British context includes species
that are (i) 'formerly native', (ii) 'locally non-native', and (iii) 'recently
arrived' but for which a human causative agent is not suspected and/or a natural
means of arrival has yet to be determined. The 'non-native' category would
include (i) 'long established' species and (ii) 'recently arrived' species for
which a human causative agent is suspected but not proven. The six classes
provide a pragmatic framework within which to consider approaches to species
conservation.
4. Relevance to SNH's work
4.1 The IUCN has recognized the importance of biological invasions. Apart from
habitat loss and fragmentation, the impacts of non-native species are seen to be
the greatest threat to the planet's biodiversity. Taking such a global
perspective, there are innumerable problems caused by invasive, non-native
(usually referred to as 'alien') species. Taking a world view, some plants are
more serious than the rhododendron (Rhododendron ponticum) colonisation of
western Scottish woodlands; some animals more serious than the grey squirrel (Sciurus
carolinensis) or the New Zealand flatworm (Artioposthia triangulata) in the
Central Belt of Scotland; and some microorganisms, many of them bringing disease
to both wildlife and human populations, worse than the outbreaks of Dutch elm
disease caused by Ophiostoma novo-ulmi. It is only necessary to remind ourselves
of the damage caused by period plagues, and particularly the introduction in the
1840s of potato blight from South America.
Table 2. A summary of Williamson's (1996) conceptual framework for considering
invasions by non-native species.
4.2 A recent treatment of the subject by Mark Williamson (1996) showed that the
science has come a long way since the pioneering work of Charles Elton (1958).
Williamson's conceptual framework for non-native species is outlined in Table 2.
It can be criticised for attempting to make too many general rules; for example,
the 10% rules (points A1 and C6) are often queried. As an example, 21 of the 65
species of British mammals (32%) are non-native (Harris et a/., 1995), and 10 of
these might have caused (or are causing) problems (i.e.48% of the introduced
species, or 15% of all mammals, but what is unknown is how many species of
mammals have been introduced at one time or another and have failed to establish
a wild-living population). However, Williamson (1998) challenged the view that
there was consensus about which mammals were 'pests'; his survey identified
eight potential candidates for 'pest status', six
of which were non-native. Examples of the 10:10 (10% of 10%) rule not applying
to one small group do not negate its general applicability when applied to large
faunas and, especially, floras.
4.3 In Williamson's analysis, points A1 and C6 imply that only of the order of
1% of non-native species that reach Great Britain are likely to pose problems.
As yet there is no estimate of the number of all terrestrial animal or plant
species introduced, or of microorganisms (including diseases of both plants and
animals), or of non-native marine organisms that enter Scottish coastal waters,
particularly as ballast water discharge. Using the 10:10 rule, and guessing the
number of terrestrial and marine species introduced as possibly at least 2,000,
this implies that we are likely to have at least 20 problematic, non-native
species in Scotland either now or in the years to come. On a world scale this
might
seem minor, but on a Scottish scale it could pose considerable demands for
resources if these problematic species are to be managed and possibly
eliminated. The greatest difficulty is that we cannot predict which the problem
species will be (Usher, 1988).
4.4 Only limited resources have been expended in controlling non-native species.
Practical work has been undertaken on rhododendron clearance and in eliminating
Canadian pondweed (Elodea canadensis) in lochs in South Uist, and research is
on-going in developing an immuno-contraception technique to control the grey
squirrel (to favour the native red squirrel (Sciurus vulgaris)). The problems of
red deer hybridization with Sika deer (Cervus nippon) have led to the concept of
island refugia for red deer (Anon., 1997). In the freshwater environment the
signal crayfish (Pacifastacus leniusculus) has been found in two burns from
south-west Scotland, and the ruffe (Gymnocephalus cernua) has been introduced to
Loch Lomond by fishermen. This is not an exhaustive list, but it does indicate
the breadth of potential problems that might be faced in the terrestrial and
freshwater environments and the need for resources to be devoted to the
management of non-native species.
4.5 There can also be problems in the marine environment. There are concerns
about the discharge of ballast water in Scottish coastal locations (especially
the Minch because of its potential to introduce species from other parts of the
world); Eno et al. (1997) reviewed the occurrence of non-native marine species
throughout UK waters. Recent studies undertaken jointly by SNH, the Scottish
Office and others have shown that there were motile phytoplankton in 133 of the
134 ballast water samples examined, as well as cysts of potentially toxic
species of diatoms (Pseudonitzchia spp.) and dinoflagellates (Dinophysis spp.
and Alexandrium spp.). It is suspected that the polychaete worm Marenzelleria
viridis, thought to be a North American species, reached the Firth of Forth in
1982 and the Firth of Tay in 1984 via ballast water discharges.
4.6 However, a few non-native species appear to have been accepted as additions
to our flora and fauna without causing any apparent deleterious impacts on our
native species. Examples would be the New Zealand willowherb (Epilobium
brunnescens) and the marine diatom, Biddulphia sinensis.
4.7 In conclusion, there seems to be some scientific basis for adopting a
precautionary approach to both the arrival and establishment of non-native
species (and, by inference, also to non-native genotypes). There is a practical
basis as well, deriving from experience in dealing with problem non-native
species such as grey squirrel, muntjac {Muntiacus reevesi) and Sika deer, mink,
rhododendron, giant hogweed (Heracleum mantegazzianum), and many more. If
non-native species become established, and start to impact on native species and
habitats, it can be extremely expensive to control them, even if control is
possible.
5. References
Anonymous (1997). A Policy for Sika Deer in Scotland. Deer Commission for
Scotland, Inverness.
Birks, H.J.B. (1997). Scottish biodiversity in a historical context. In
Biodiversity in Scotland: Status, Trends and Initiatives, ed. by L.V. Fleming,
A.C. Newton, J.A. Vickery & M.B. Usher. The Stationery Office, Edinburgh. pp.
21-35.
Corbet, G.B. & Southern, H.N. (ed.) (1977). The Handbook of British Mammals (2nd
edition). Blackwell, Oxford.
Elton, C.S. (1958). The Ecology of Invasions by Animals and Plants. Methuen,
London.
Eno, N.C., Clark, R.A. & Sanderson, W.G. (1997). Non-native Marine Species in
British Waters: a Review and Directory. Joint Nature Conservation Committee,
Peterborough.
Harris, S., Morris, P., Wray, S. & Yalden, D. (1995). A Review of British
Mammals: Population Estimates and Conservation Status of British Mammals other
than Cetaceans. Joint Nature Conservation Committee, Peterborough.
Preston, CD. (1986). An additional criterion for assessing native status.
Watsonia, 16, 83.
Salisbury, E. (1961). Weeds and Aliens. Collins, London.
Snow, D.W. (ed.) (1971). The Status of Birds in Britain and Ireland. Blackwell,
Oxford. Usher, M.B. (1987). Invasibility and wildlife conservation: invasive
species on nature reserves. Philosophical Transactions of the Royal Society of
London, B314, 695-710.
Usher, M.B. (1988). Biological invasions of nature reserves: a search for
generalisations. Biological Conservation, 44, 119-135.
Webb, D.A. (1985). What are the criteria for presuming native status? Watsonia,
15, 231-236.
Williamson, M. (1996). Biological Invasions. Chapman & Hall, London.
Williamson, M. (1998). Which mammals are pests? Imprint: the Yorkshire Mammal
Group Newsletter, 25, 26-30.
6. Acknowledgements
1 should like to thank the following people for
commenting on a draft of this l&A Note: Phil
Boon, Andy Douse, Sandy Downie, Noranne
Ellis, Fred Last, Greg Mudge, Sue and Michael
Scott, Chris Sydes and Des Thompson.
7. Contact
Professor Michael B Usher
Chief Scientist
Scottish Natural Heritage
2 Anderson Place
EDINBURGH
EH6 5NP
Tel: 0131-446 2401
Fax:0131-446 2406
E-mail: MBU@snh.gov.uk
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