The challenge of invasive alien species

Recent news headlines showcasing the interest on invasive alien species

What are invasive alien species?

Whether they are animals, plants or microorganisms, species are very mobile within the biosphere. They (or their propagules) move: independently using their own energy, are transported via other organisms (e.g. hosts) or environmental processes (e.g. flowing water). It is possible some transported species (i.e. ‘alien’ to their new environment) will survive and colonise their new destination, unless limited by environmental factors or antagonistic organisms. When such introduced ‘alien’ species start to infest and negatively impact their new environment’s integrity, economic activity, and or health of native species they are termed as invasive alien species. This is a relatively young but rapidly growing field of study.

Some examples of well known invasive alien species 

Source: Global Invasive Species Database (http://www.issg.org/database/welcome/)

What are the impacts of invasive alien species?

The negative impacts of invasive alien species could be direct or indirect. Invasive species could directly impact native species through herbivory or predation; acting as vectors of disease transmission; or even physically damaging local structures.  On the other hand, they could indirectly affect the integrity of the ecosystem by disrupting food chains or biogeochemical cycles. Some examples of such impacts and the mechanism involved are discussed as follows.

(1) Displacement of native species. Over the last century, the British Isles native red squirrel (Sciurus vulgaris) was displaced from its habitat by the invasive North American grey squirrel (Sciurus carolinensis). This was a result of the superior competitive ability of the grey squirrel as well as the new diseases it brought over from the North American continent. The native red squirrel had no immunity and its population succumbed, to be replaced by that of the grey squirrel.

(2) Disrupting ecosystem processes. Bush fire is a regular (every 10 years or so) natural process occurring in Mediterranean climate shrub lands. It helps remove old vegetation while encouraging germination of dormant seeds. However, in South African sites dominated with invasive species such as pines (e.g. Pinus pinaster) and eucalypts (e.g. Eucalyptus camaldulensis), fires burn more intensely due to the nature of such fuels. This has consequence in reducing resprouting of native species from the soil seed bank, especially those near the soil surface.

(3) Impact on health. Invasive species such as giant hogweed (Heracleum mantegazzianum) are increasingly appearing in public green spaces in Britain. The poisonous sap of the hogweed can cause severe skin burns to people.

(4) Impact on the physical structures. Zebra mussels (Dreissena polymorpha) are known to attach themselves to physical structures e.g. metal pipes of water works and power plants, thereby clogging them. Physical removal is costly and is a major nuisance.

Impact of invasive species on biodiversity (South Africa)

How do invasions happen?

Invasions occur as a result of species being introduced into areas which they have not previously existed in. As such, species need to be transported to the new sites either through natural processes (e.g. transport by wind, water or geological incidents) or by human beings. Introduction by human beings, whether deliberate (e.g. commercial gardening) or accidental (e.g. escaped pets), is often rapid and many species could be introduced at a time. Global trade has been one of the main pathways and often is compounded by lax biosecurity regulations and lack of people’s awareness to the consequences.

Introduction doesn’t necessarily mean though that all such introduced species become established and grow to be invasive; however it is likely some will become. The establishment of such invasion can be better understood through the Invasion Triangle Framework [Perkins et al. (2011) Invasion triangle: an organisational framework for species invasion. Ecology and Evolution 1(4): 610-625]. This shows invasion as being dependent upon three characteristics:

(1) attributes of the potential invader i.e. how competitive the invader is;

(2) biotic characteristics of the potentially invaded site; i.e. describing how vulnerable the site is for invasion and;

(3) environmental conditions of the potentially invaded site i.e. how the conditions of the site will influence its invasibility. External influences such as climate change, land use change and external inputs such as nitrogen deposition also can affect invasion further by influencing any of the above three categories listed.

What can be done to tackle invasive species?

Controlling invasive alien species involves: prevention of establishment as a first step; and if establishment has already occurred, follow-up eradication. Prevention at the early stages of the invasion is considered the most economical and ecologically sound method. Otherwise once established, eradication tends to be costly and success is not always guaranteed.

Prevention methods often involve biosecurity measures such as quarantine of suspect items, as well as isolation through legislative and regulatory enforcement. Such prevention can also be supported by education/awareness efforts for the general public. In this respect, the public and conservation organisations can play a major role in monitoring for invasive species.

Eradication methods could be of physical nature (e.g. direct removal of invasive species by hand or mechanically); chemical (e.g. using toxic chemicals to target and kill the offending invader) or biological (e.g. by introducing natural enemies of the invasive species). Eradication can be effective if invasion is tackled early.

For enduring successful management of invasive species, a wholistic approach involving all stakeholders, scientists and policy makers is a requirement.