Alien plants cover approximately ten per cent of South Africa and are spreading constantly.

Where alien plants take root

An international team of biologists have compiled the first global overview of naturalisation and spread of alien plant species. The survey was based on the statistics for “naturalised” plants, which that include all species that grow and reproduce in the wild outside their original geographic range.

For the first time, a scientific survey provides a global overview of the naturalisation and distribution of plants on continents outside their native ranges. Under the leadership of Professor Mark van Kleunen at the University of Konstanz,Germany, biologists from the University of Vienna, Austria, The Czech Academy of Sciences, the German Centre for Integrative Biodiversity Research and the University of Göttingen, Germany, analysed data for 481 mainland areas and 362 islands in collaboration with a further 33 research institutions all over the world. The areas surveyed represent around 83 percent of the Earth’s land surface.

According to the researchers’ survey, humankind has been responsible for the establishment of at least 13,168 plant species – approximately 3.9 per cent of the flora world-wide – outside their natural ranges.

The results of this research were published in the scientific journal Nature in August 2015.

According to the survey, North America has become home to the largest number of alien plant species, as almost 6,000 have naturalised in this continent, followed by Europe with more than 4,000. Compared to their area, the Pacific Islands had the most naturalised plant species, indicating that islands are easier to invade than mainland areas. Regions in the Northern Hemisphere are the largest “donors” of naturalised species to other parts of the world, with Europe and non-tropical Asia in the lead.

In the course of the four-year research project, the biologists collected regional lists of naturalised plants from all parts of the world, and compiled them in the global databank called GloNAF - Global Naturalized Alien Flora. “It was a great challenge to standardise the names of plant species. There are large regional discrepancies in the names used for the same plant species in different countries,” lead author van Kleunen reports.

The survey was based on the statistics for “naturalised” plants. This category of plants includes all species that grow and reproduce in the wild outside their original geographic range. These figures are not exactly the same as the statistics for so-called invasive plant species, those that rapidly spread in their non-native ranges. Many of the latter noticeably impact their new environment as well as other species.

“Our survey is purely descriptive to start with,” van Kleunen explains. “It details where alien plant species have spread world-wide and where they come from. With the GloNAF database that we built, we can now begin to ask questions about the biological mechanisms driving these processes”. What contributes to the spread of alien plant species? Which plant characteristics foster their spread into different environments? How important are evolutionary relationships between the naturalised and native plants? “Our data can now also be used to make predictions about which species could become dominant in which regions,” van Kleunen concludes.

Original publication:
Mark van Kleunen et al. Global exchange and accumulation of non-native plants. Nature 2015. Doi: 10.1038/nature14910, 


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