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Durham University News

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Global hotspots of established alien plants and animals revealed

(12 June 2017)

Rose-ringed parakeet (Psittacula krameri manillensis), originally from Asia and now found in London. Credit: Tim Blackburn

Islands and mainland coastal regions are the world’s hotspots for alien plant and animal species, according to new research.

An international team of researchers, led by Dr Wayne Dawson in Durham University’s Department of Biosciences, has provided the first global analysis of established alien species.

Their study is published in the journal Nature Ecology & Evolution.

The researchers say their findings show that more effective measures need to be put in place to prevent further introductions of invasive alien plants and animals into vulnerable ecosystems.

Hotspots of established alien species

The scientists studied existing data looking at eight groups including amphibians, ants, birds, freshwater fish, mammals, reptiles, spiders and vascular plants across 186 island and 423 mainland regions.

Hotspots of established alien species were found mainly in island and coastal mainland regions, the research concluded.

The top three hotspots with the highest numbers of established alien species were the Hawaiian Islands, New Zealand’s North Island and the Lesser Sunda Islands (Indonesia).

The Hawaiian Islands have high numbers of alien species in every one of the eight groups studied. These include guppies among fish (now globally widespread) and feral pigs among mammals.

Predatory mammals

New Zealand follows not far behind Hawaii with around half of its plant life consisting of established alien plants. Predatory mammals such as rats, cats and possums, have been a real problem for naïve native bird species which evolved without being used to land-based predators.

While not in the top three regions, the UK, and England in particular, still has more established alien plants and animals than many other global regions, according to the study.

These include parrots (the Indian rose-ringed parakeet, found across London), many plant species like Himalayan balsam (common along rivers and streams), mammals such as the grey squirrel (which has outcompeted the native red squirrel), and the noble false widow spider (introduced accidentally in cargoes of fruit).

Florida, in the USA, is the top hotspot among coastal mainland regions, with the Burmese python a well-known example of an invasive species among reptiles, while the state is also home to a large number of less well-known non-native ant species.

Islands and mainland coastal regions

A region’s wealth, density of human population and, to a lesser extent, climate affected the number of established alien species, though these factors varied between species and from region to region, the study reported.

Dr Wayne Dawson, in Durham University’s Department of Biosciences, said:

“Our research shows that, islands and mainland coastal regions contain higher numbers of established alien plants and animals, and this may be because these areas have major points of entry like ports.

“In general, regions that are wealthier, and where human populations are denser also have more alien species, but these effects are stronger for islands.

“More work is needed to understand whether these effects arise because more species are introduced to hotspot regions, or because human disturbance in these regions makes it easier for the newcomers to find vacant spaces and opportunities to thrive.”

Biosecurity measures

Global efforts aimed at preventing the further introduction of alien plants and animals could be focussed on islands and coastal mainland regions, the researchers said.

Dr Dawson added: “While species have been moved around the globe throughout history, more and more species will be introduced as the world becomes ever more connected, and the human population continues to grow.

“Many of these incoming species are useful, and won’t establish and spread in their new homes, but some will, with varying levels of impact on resident species and ecosystems.

“The challenge for us is to understand what the consequences are of mixing up the world’s species in this way, to decide how to deal with this change, and what measures we can put in place to try and prevent further introductions to the most vulnerable regions.

“This includes bolstering biosecurity measures at major points of entry to detect stowaway species, as is already done to an impressive degree in New Zealand.

“It should also include careful vetting of species proposed for introduction to an area, as done by organisations such as the European and Mediterranean Plant Protection Organization (EPPO), to filter out those species that are risky in terms of their potential to establish and have negative impacts.”

Examples of invasive alien species:

  • Rose-ringed parakeet (Psittacula krameri): Native to Africa and Asia, can compete with native birds in Europe for food and nest sites. Can be seen in many of London’s parks.
  • Grey squirrel (Sciurus carolinensis): Originally native to North America, has led to a decline in the UK’s Red squirrel population through competition and spread of squirrel pox. Also damages trees by stripping bark.
  • Wolf snake (Lycodon capucinus): Native to South-East Asia, introduced to Christmas Island (Indian Ocean). A predator of lizards, it poses a risk to threatened and endemic lizard species.
  • Scotch broom (Cytisus scoparius): Native of Europe, now invasive in New Zealand and North America. Outcompetes native vegetation and fertilizes soils with nitrogen from symbiotic bacteria that fix nitrogen from the air, and live in the roots.
  • Asian needle ant (Brachyponera chinensis): Native to Asia, introduced to the South East USA from Japan in the 1930s. Causes declines in native ant abundance, and its sting leads to anaphylaxis in many people every year.
  • Guppy (Poecilia reticulata): A popular aquarium fish from the Caribbean and South America, was also widely introduced to control malaria-carrying mosquitos. Now globally widespread, it is linked to declines in native fish populations in the USA. A predator of native fish eggs and aquatic insects in Hawaii, also a carrier of fish diseases.
  • Pig (Sus scrofa): Widely introduced as domestic animals, but escaped and established in the wild in many countries. Causes major disturbances to vegetation, promoting weeds, and as an omnivore, preys on native animals.
  • Himalayan balsam (Impatiens glandulifera): Native to the Himalayan mountains, introduced to UK and other parts of Europe as an ornamental. Now widespread, common on river and stream banks, where it increases soil erosion. Flowers are very frequently visited by bees.
  • Noble false widow spider (Steatoda nobilis): Native to Madeira and the Canary Islands, now spreading northwards in mainland Europe, and the UK. One of the few species of spider present in the UK that can deliver a painful bite to people.
  • Burmese python (Python bivittatus): Native to SE Asia, now invasive in Florida (including the Everglades), where it preys on a wide range of native animals.

* Image: Rose-ringed parakeet (Psittacula krameri manillensis), originally from Asia and now found in London. Credit: Tim Blackburn.

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