The aliens are winning: how can we control invasive species and improve our quality of life?

In Ireland we have more alien plant species (987 species) recorded than native plants (952 species)

Grey squirrels, Sitka spruce, ash dieback disease and Japanese knotweed are all highly damaging invasive alien species in Ireland. Among other impacts, invasive species threaten nature, impact on food supply and reduce people’s quality of life through the spread of disease.

Grey squirrels outcompete the native red squirrel and harbour a disease that disproportionately affects the reds. Sitka spruce, while widely planted as a commercial forestry tree with benefits for some, regenerates easily from seed and takes over heath and boglands, profoundly changing their ability to sequester carbon and provide water services.

Ash dieback disease is rapidly killing one of the most abundant trees in the country, thinning out hedgerows and reducing native ash wood for hurleys. Japanese knotweed pushes its way through tarmac and concrete, reducing land values and slowing down development with big economic consequences.

In Ireland we have more alien plant species (987 species) recorded than native plants (952 species). This is a microcosm of the global situation, where there are more alien plant species than native species on a quarter of all islands on the planet.


Travel by people and trade of goods from one region to another are largely responsible for the introductions of alien species. Some of these introductions are purposeful, such as the introduction of Sitka spruce from Canada for use in forest plantations. Others, such as zebra mussels, are hitchhikers, most likely on boats imported from Britain in the early 1990s when economic conditions for second-hand boat sales were favourable.

Globally there are 37,000 alien species established, with 200 more aliens recorded each year and introductions of species outside of their native ranges accelerating. The recent report on invasive alien species by the Intergovernmental Science Policy Platform on Biodiversity and Ecosystem Services (IPBES) is the culmination of five years of work by hundreds of researchers who have painstakingly compiled, critiqued and evaluated the evidence for the impact and effective management of invasive plants, animals, fungi and microbes.

The IPBES report comprehensively demonstrates that invasive species are one of the five big direct drivers of change in nature, alongside land- and sea-use change, direct exploitation (over harvesting), climate change and pollution. These drivers can also work together.

The conversion of forest to agricultural land fundamentally changes the conditions that native species in that location are adapted to. The species that are most likely to get to these altered locations are those transported by humans and tolerant of our habits. It is not surprising, therefore, that invaders take over when we change ecosystems. Invaders can, however, thrive in even the most remote and seemingly pristine ecosystems, with several alien invertebrate species and plants found in Antarctica having hitchhiked with researchers, tourists, fishers and supply ships.

Invasive alien species are implicated, either solely or together with other drivers, in more than 60 per cent of global species extinctions

Islands, especially remote islands with specialised native species found nowhere else, are particularly vulnerable to species invasions. The brown tree snake was introduced accidentally to the island of Guam, an isolated US territory in the western Pacific, after the second World War and proceeded to cause the extinction of several native bird species. Invasive alien species are implicated, either solely or together with other drivers, in more than 60 per cent of global species extinctions.

About 10 per cent of alien species turn out to be damaging to nature or people. The other 90 per cent are currently relatively benign, but if conditions change they represent a pool of potential future invasive species, like sleeper cells waiting for a call-up. Species introduced to Ireland from warmer climates may barely manage to survive in our climate but as the climate changes, conditions for them improve and their populations can build up and become problematic.

In Ireland we are not adequately managing invasive alien species. The IPBES report clearly shows prevention of introductions through good biosecurity practices and co-operation with other jurisdictions, and management through eradication, containment and ecosystem restoration, can be effective if these measures are adequately resourced and co-ordinated. The best management outcomes are achieved with close collaboration across sectors and borders.

Management of invasive species will have multiple benefits for several sectors, from agriculture to tourism. It is our responsibility not just to manage the impact of existing invasions but to prevent the invasion of more species in the future with unpredictable effects on our ecosystems, economy, livelihoods and quality of life.

Yvonne Buckley is an ecologist and professor of zoology at Trinity College Dublin