Invasive species and the changing flora of the UK

Conservationists believe that invasive

alien plant species are second only to
habitat destruction as a cause of species
extinction and environmental decline!
They are creating an increasingly
significant impact on bio-diversity around
the world.The flora of the UK faces a
challenge from these non-native plants
on every side.

What are invasive alien

plants?
These are plants that have been
introduced relatively recently into an
area where their natural predators,
competitors and diseases may be absent
and there is little to control their growth. Kew information sheet T4
Their arrival can cause economic and/
or environmental damage and they are Above: Rhododendron ponticum is an
sometimes harmful to human health.
How do they get here? example of an ornamental plant
They can compete with native plants for Invasive alien species can arrive in a new that has invaded the countryside
nutrients, light and space and therefore region or country in many ways. Natural (Christina Harrison)
alter the natural vegetation of a habitat. boundaries, such as oceans, mountain
Not all newly arrived species become ranges and deserts, provide no barrier Invasive alien plants and
invasive, in fact it is only a small at all in this age of world trade and
percentage, but these few can cause global travel. the changing flora
severe problems. Many of the species in this country have
Some arrive hidden on ships, in bales of
The economic consequences of invasive hay, in packing materials or in crop seed. been here since the retreat of the ice
alien plants can be phenomenal. For Others are deliberately brought into the following the last Ice Age (13 to 11.5
example, they can affect water supplies country either as ornamental plants for thousand years ago). Others have
and spoil crops amounting to millions of gardens, or as medicinal or food plants. colonised the land since that time.
pounds worth of damage. In the USA the Given the right conditions they can However, since mankind has lived
cost per year for control of invasive alien spread along the same avenues that we here we have had an impact on the
plants is over $120 billion dollars (US use for transportation – roads, railways composition of the flora through the
Department of the Interior 1999). and canals. Species that become invasive introduction of plants that we
Predicting which species will become often have high growth rates and considered useful in some way.
invasive is a priority in the battle to produce many seeds, which, if adapted Plants are not as mobile as mammals,
conserve our environment. for wind dispersal, can quickly spread birds or insects, but they can still move
over a wide area.They are often also around according to where the best
Some native species, for example Bracken tolerant of certain stressful conditions conditions are for them to live. As the
(Pteridium aquilinum), can also be invasive such as drought, grazing, fire or wind. UK becomes warmer and with milder
(Emma Holmwood) All of these attributes are an advantage winters, many Mediterranean species are
in colonising disturbed soil such as on managing to survive in southern England
agricultural or waste land. Colonisation and Wales. At the same time, species
of natural habitats may follow colon- which thrive in a colder climate are
isation of these disturbed areas and the retreating northwards.
invasive alien species then compete with
native and naturalised plants. The Botanical Society of the British Isles
recently surveyed and counted 2,947
The landscapes most altered by invasive vascular plant species living in this
alien species are tropical oceanic islands country (not counting gardens), of which
such as the Hawaiian Islands, St Helena, 1396 were native, 149 species were
and Mauritius. However continental archaeophytes or ancient introductions,
areas also suffer, for example in Australia, and 1402 (almost half of the total) were
America and South Africa. neophytes or recent introductions
See links: www.plant-talk.org
www.invasivespecies.gov
www.defra.gov.uk
www.plantlife.org.uk

Invasive species and the changing flora of the UK

 Kew Information sheet T4 Invasive species and the changing flora of the UK

(i.e. naturalised after 1500 AD). Natives

include trees such as birches (Betula Most Wanted! • It grows in dense stands choking
pendula and B. pubescens) and oaks waterways. So far all measures to
(Quercus robur and Q. petraea) and the Some recently introduced species are control this plant have failed and it
Scots Pine (Pinus sylvestris). increasing rapidly in number and pose continues to threaten rare plants such
a threat to native and archaeophyte flora. as Starfruit (Damasonium alisma).
Ancient introductions include species Here are some selected examples:
such as the Common Poppy (Papaver 4. Parrot’s Feather
rhoeas) and the Sweet Chestnut 1. Japanese Knotweed (Myriophyllum aquaticum)
(Castanea sativa) whilst recent (Fallopia japonica) • A native of Central America, this
introductions include Japanese Knotweed • A tall herbaceous plant in the species has now colonised over
(Fallopia japonica) and Australian Swamp family Polygonaceae. 100 sites in southern England but it
Stonecrop (Crassula helmsii). • Green bamboo like stems. remains much less widespread than
• It grows up to 2 to 3 metres high. Crassula helmsii.
In general, over the past 40 years,
neophyte species including some 2. Giant Hogweed • It is mostly found in shallow ponds and
invasives have increased in frequency, (Heracleum mantegazzianum) currently threatens the native plant
whilst ancient introductions (or • From the same family as parsley Brown Gallingale (Cyperus fuscus).
‘archaeophytes’) have generally declined. and carrots – the Umbelliferae.
As a whole, native species have showed • It reaches up to 4 metres tall. Other problem alien invasive species
little change (New Atlas of the British • Imported into this country as a in the UK include Floating Pennywort
and Irish Flora; Preston et al 2002). garden plant in the late 1800s. (Hydrocotyle ranunculoides), Himalayan
However, these trends conceal the fact Balsam (Impatiens glandulifera),Water
that some natives, such as the Ghost 3. Australian Swamp Stonecrop Fern (Azolla filiculoides) and Campylopus
Orchid (Epipogium aphyllum), have or New Zealand Pigmyweed introflexus. C. introflexus is a moss from
become extinct in many areas. (Crassula helmsii) the Southern Hemisphere which arrived
• It takes a staggering £3 million a year in 1941 and has spread throughout the
Tropical islands suffer from invasive to control this plant in Britain alone! country since then vigorously invading
species too such as this Strawberry (Leach and Dawson 1999). heathlands. Invasive species also include
Guava (Psidium cattleianum) on Kauai • First grown in English gardens in 1927 much more recognisable species such as
(Christina Harrison) it has spread across the country at an Rhododendron ponticum and the Butterfly
alarming rate since the late 1970s. Bush (Buddleja davidii).

What can we do? Conservation strategy.

If you recognise an area of invasive Some of our rarest plants are under
plants such as those listed you should: direct threat from alien invasive plants,
including the Three Lobed Water
• Inform your local council as they may Crowfoot (Ranunculus tripartus) and
have an eradication program in place Starfruit (Damasonium alisma).The
for dealing with the plant in question Wildlife and Countryside Act 1981
if it is a notifiable weed. restricts the release of non-native plants
such as Japanese Knotweed and Giant
• Contact a national conservation Hogweed, as well as two aquatic species
agency that could deal with the and some algae, making it an offence to
problem. At present there is no plant them or cause them to grow in
Himalayan Balsam (Impatiens glandulifera) national co-ordination of eradication the wild.
measures.
The Global Invasive Species Programme
• Do not try to tackle the problem (GISP) provides a valuable service for
yourself without first seeking specialist conservation managers with its newly
advice. In the case of Giant Hogweed developed Global Invasive Species
this could lead to health problems if Database, helping people around the
you are not properly equipped! world identify the usual suspects!

• Do not put known invasive alien plant

species on your compost heaps as
they may still send out seeds, roots
and runners from the remains.

The best policy is always to do something

now and save time and money later!

© Copyright 2003, Board of Trustees Royal Botanic Gardens, Kew

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