Naturenet: Invasive Plant Species in the UK
Invasive Plant Species in the UK
by Alina Congreve
Human beings have benefited from the introduction of plant
species into the UK for over 2000 years. These have added to the variety and
nutrition to our food supply as well as adding colour and diversity to our
parks and gardens. However, a small number of introduced plant species are
now acknowledged to represent a risk to natural ecosystems. Invasive plants
can also represent a risk to economic activities including the ability to develop
land effected by invasive plants as well as causing erosion. Concern about
invasive species began in the 1950s and a large body of scientific research
followed, often looking at particular species that had already become a problem.
Now research is much more proactive, focusing on identifying species before
they represent a serious risk.
Invasive species represent a
problem because they are able to out-compete native plants.Most plants present no risk of becoming invasive in Britain.
To become invasive three events must happen: the plant must arrive; the plant
must survive; and the plant must thrive. This means that the new habitat must
be a close enough match for the plant’s native habitat that it can survive
without human cultivation. The term invasive is used in a number of different
ways:
- Any introduced non-native species.
- Introduced species that spread rapidly in the new region.
- Introduced species that have harmful impacts, particularly on native species.
The second definition is most commonly used by ecologists, while the third
definition is more common in legislation and policy papers.
For the purposes of this briefing, the term invasive plant species is defined
using the International Union for the Conservation of Nature (IUCN) definition:
“an alien species which becomes established in
natural or semi-natural ecosystems or habitat, is an agent of change, and
threatens native biological diversity”.
Invasive plant species represent a global problem and
impact particularly severely on small island countries and other regions,
such as mountainous regions that have remained geographically isolated. Physical
barriers such as oceans and mountains that would have acted as a barrier
to plants natural dispersal are now no longer effective. Some invasive species
arrive accidentally e.g. in packing material on ships. Others are deliberately
introduced as ornamental plants in gardens or as crops.
Before human settlement
it is estimated that about one species of vascular plants and protozoan arrived
in the Hawaiian Islands every 50,000 years. After the arrival of the Polynesians
this increased to 3–4 species per century. The current rate is about
20 new species annually (Planttalk, 2001). In the UK, our history as a trading
centre and our enthusiasm for plant collecting have been important historical
features affecting the introduction of invasive plants. While the UK has
not suffered as severely as some other regions, invasive plants have nevertheless
had a negative impact on our native biodiversity.
Native habitats
in Britain face a number of pressures including direct habitat removal, invasive
species, pollution and inappropriate management practices. It is widely acknowledge
that the second most important threat, after direct habitat removal is from
invasive species. Invasive species represent a problem because they are able
to out-compete native plants. By taking the light, nutrients and space normally
occupied by native plants, invasive species alter the environment. In their
new environment invasive species often lack natural predators, competitors
and diseases, leaving little to control their growth.
In a study by Pearman
and associates (2002) 2,947 vascular plants were found to be living in this
country (excluding gardens). Of these 149 were historic introduction (before
1500) and 1402 were recent introductions. This represents about half of the
total. Of these 1402 only a small number are recognised as representing a
major problem. Flowering plants that are identified as a problem include:
- Rhododendron (Rhododendron ponticum)
Rhododendron is one of the best know examples of a plant originally introduced
as an ornamental garden plant which then escaped and thrived outside a
garden setting. It outshades native plants and is causing problems in many
important oak and hazel woodlands, for example, on the West coast of Scotland.
- Giant Hogweed (Heracleum mantegazzianum)
Giant Hogweed also stops the growth of other plants by forming dense growth
that suppresses native plants and grasses. When the Hogweed dies back in
the winter it leaves riverbanks bare of vegetation, increasing erosion.
- Himalayan balsam (Impatiens glandulifera)
Himalayan balsam causes the same problems as Giant Hogweed, shading out native
plants and when it dies back in winter, leaving riverbanks exposed to erosion.
In addition to these problems it attracts pollinating insects away from
native plants by producing more nectar than any native European species.
In rivers and ponds invasive species such as the Australian
swamp stonecrop present a direct and immediate threat to one of our most
threatened native plants – the starfruit. The floating pennywort is another aquatic plant
that is causing serious problems by removing oxygen from the water which kills
fish, blocking drainage ditches, and causing local flooding. In 1999 English
Nature
put the cost of removal of this plant at £250,00-£300,000
per year.
Looking at the issue globally, invasive plants share certain characteristics.
They tend to reproduce vegetatively. When they do reproduce by seeds they
tend to have small seeds, short juvenile periods and low variability in seed
crops (Kolar and Lodge 2001). Work by Thompson and associates looked specifically
at native and non-native plants that had recently expanded their range in
Britain. Invasive plants were found to be more likely to be clonal than native
plants - this means one plant gives rise to another, and gives rise to another
with no sexual reproduction. Crawley and associates, who also worked specifically
on the UK, identified tall stature and long seed dormancy as important characteristics
of invasive plants. Work in Australian and North American has progressed
further with the development of statistical models to predict the likely
invasive qualities of plant species. Such models do not currently exist in
the UK and the most likely predictor of whether a plant represents a risk
of being invasive is whether the plant itself or a close relative has been
invasive in other countries.
The case of the Olympic park site illustrates the costs
and technical difficulties of control once an invasive species has become
established. Control would be more feasible if ‘problem’ species could be identified
earlier. Identifying species that could become invasive is not straightforward
because plants can react in unpredictable ways in their new environment. Because
of this plant conservation charities and scientists (Manchester et al 2000)
argue that proposed new species introductions should be subject to thorough
ecological characterisation and risk assessment before introduction.
Once
plant species have been identified as potential risk species quick and clear
mechanisms are then needed to update Schedule
9.
Plant conservation charities argue that there is already a case to add aquatic
plants to Schedule 9, which have proved invasive in mainland Europe including
the water hyacinth and water chestnut. Other important measures that need
to be considered in the future are the ways in which plants arrive in the
UK. There is a greater role for ecologists to have an input into boarder
controls – to reduce
the risk of accidental plant introductions. There is also an important role
for garden centres and gardening organisations to avoid the sale and promotion
of invasive plants. There is also a strong role for the general public to
avoid the accidental release of invasive species into native habitats.
A survey carried out in 2004 identified Japanese knotweed
in patches across the Olympic Park. It is concentrated along river and canal
banks, by the Velopark and beside the Aquatics Centre site. A number
of methods will be used to treat land affected. These include:
- Spraying with appropriate herbicides has been taking place
over the past year.
- The surface knotweed will be cut, placed in a skip and incinerated using
an expert, safe and clean system to prevent spreading.
- Where possible and particularly near water, as Japanese knotweed spreads
via watercourses, the soil will be sprayed with a suitable herbicide.
- Away from water knotweed may be buried within a special protective barrier
to prevent regrowth.
The costs of this treatment programme have been estimated
at £70 million.
The UK is affected by a number of European and international treaties which
refer to invasive plant species:
- Berne Convention Article 11 (2) b
- Species and Habitats Directive Article 22 (b)
- Birds Directive Article 11
- Convention on Biological Diversity (CBD) Article 8(h)
The Wildlife
and Countryside Act 1981, provides the main
controls on the release of non-native species into the environment. The Act
contains Schedule 9 Part II. This is a list of plants, which it is against
the law to “plant or
otherwise cause to grow in the wild”. Despite this listing, Japanese
knotweed has spread to nearly double its distribution over the past 20 years.
The responsibility for enforcing the legislation relating to endangered species
currently lies with the police. The Countryside
and Rights of Way Act of 2000 allowed for stricter enforcement and increased penalties for releasing invasive
plant species. However due to difficulties in tracing sources material there
have been no prosecutions under the Act. The new technology of DNA profiling
could improve this situation – allowing plants in the wild to be matched
with garden centre sources.
- Crawley, M.J. Harvey, P.H. and Purvis, A. (1995) Comparative
ecology of the native and alien floras of the British Isles. Phil. Trans. R.
Soc. London. Vol. 351 pp 1251-1259.
- English
Nature (1999) Overgrown and over here! TV gardener support Agencies
call for ban on plant menace. EN press release. 15 September 1999.
- Environment
Agency (2003) Guidance for the control of invasive weeds in
or near fresh water.
- Elton, C.S (1958) The ecology of invasions by animals
and plants, Chapman & Hall.
- IUCN (1999) Guidelines for the prevention of biodiversity loss due to biological
invasion.
- Japanese
Knotweed Alliance (2007) The plant and the problem.
- Kolar, C.S, Lodge, D.M. (2001) Progress in invasion biology: predicting
invaders. Trends in ecology and evolution Vol 16 No 4 pp199-204.
- Manchester, S.J.; Bullock, J.M (2000) The impact of non-native species
on UK biodiversity and the effectiveness of control. Journal of Applied Ecology
Vol. 37 No. 5 pp845-864.
- Plantlife (2000) At war with Aliens: changes needed to protect native plants
from invasive species.
- Plant Talk (editorial) No 24 May 2001. The cost of invasive plants.
- Pearman, D.A., Preston, C.D. & Dines, T.D. (2002)
The New Atlas of British and Irish Flora. Botanical Society of the British
Isles.
- Royal Botanical Gardens Kew (2003) Invasive species and the changing flora
of the UK.
- Royal Horticultural
Society (2005) Invasive non-native species.
- Sunday
Times (2007) Knotweed sets Olympics £70
million challenge. 25th March.
- Thompson, K. ; Hodgson J.G. ; Rich T.C.G (1995) Native
and alien invasive plants: more of the same? Ecography Vol. 81 No. 4 pp390-402.
Alina Congreve 5 July 2007
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