Non-native Invasive Species

Non-native (alien) invasive species are the second biggest threat to biodiversity, after habitat destruction.  They are defined as non-indigenous species that adversely affect habitats they invade, ecologically, environmentally or economically.  There are numerous examples, with many introduced into Britain during Victorian times.  Some well-known species that adversely affect biodiversity within Britain's woodlands are listed below:

- Pontic rhododendron (Rhododendron ponticum)
- Japanese knotweed (Fallopia japonica)
- Spanish bluebell (Hyacinthoides hispanica)
- Sycamore (Acer pseudoplatanus)
- Grey squirrel (Sciurus carolinensis)
- Reeves's muntjac deer (Muntiacus reevesi)

Some of these species are listed under Schedule 9 of the Wildlife and Countryside Act 1981 and it is an offence to plant or deliberately release them in the wild.  Guidance on the control of alien invasive species and other aspects of this subject can be found on a dedicated website hosted by the Central Science Laboratory.

These invasive non-native species are a threat to our native flora and fauna in a number of ways:

• They often grow and reproduce more quickly than native species and therefore outcompete the latter for resources of light, water, nutrients, food and space.
• They may inhibit the growth of neighbouring plants by the release of biomolecules (allelopathy).  These toxic compounds can remain in leaf litter for several years after the allelopathic plant has died or been removed.
• They can adversely affect ecosystem functions such as altering fire regimes, nutrient cycling and hydrology.
• They can, if closely related genetically, hybridise with native species, leading to genetic pollution.
  
  At Native Forestry, Rhododendron ponticum is our most hated plant!  Widely planted as an ornamental in gardens, owing to its impressive purple flowers, it has spread throughout woodlands and other habitats on acidic soils, such as heathlands, acidic grassland and peatland.  In ideal conditions, it quickly reproduces by seed or vegetatively from stem buds and root suckers.

The damage to native flora and fauna can be dramatic.  Understorey vegetation can be completely eliminated since the tall shrub forms dense stands that cast deep shade and create a layer of toxic leaf litter.  Certain animals that require understorey vegetation, such as the dormouse, are similarly displaced by rhododendron.

Control of R. ponticum is often expensive since it needs to be carried out over large areas and for several years to be sure it has been eradicated.  Even after it has been cut and herbicide treated or ripped out, regrowth can occur from the seed bank, which persists for many years, and from root suckers.  The toxic humus layer left behind can retard new growth of native plants for up to seven years, making the post-control habitat barren and vulnerable to further invasion.

Our much-loved common bluebell Hyacinthoides non-scripta is also under threat from the introduced Spanish bluebell Hyacinthoides hispanica; the two species freely hybridise and both the resulting hybrid Hyacinthoides x massartiana and the Spanish bluebell produce highly fertile seed that enables both to invade areas of our native bluebell woodlands.

Internationally, our native species can become invasive introductions in other countries.  In parts of Australia, for example, holly and ivy have become invasive and are considered a serious threat to biodiversity within their natural ecosystems.

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