Alien plants: Do they come from space?

It seems there is some confusion on alien species and their invasion.

Species of either vegetables or animals discovered in a place they do not belong are defined as alien (or exotic).1 Not all of them have a negative impact on the hosting environment; however, the effects should never be underestimated.1

alienplants2A considerable number of species that we commonly see around us are not autochthonous (i.e. native); furthermore, each species we know arrived from somewhere, somehow and at a certain time. Therefore, how do we categorise them?

Botanically speaking, any plant species that appeared in a new habitat after 14922 is defined as neophyte (i.e. non-native). This year (when Christopher Columbus discovered America trying to reach India) is the symbolic start of global human movement and long distance trades. More recently, in invasive ecology, “any species transported intentionally or accidentally by a human-mediated vector into habitats outside its native range”3 are defined as introduced species. Moreover, any living or non-living carrier that transports living organisms intentionally or unintentionally4 is defined as a vector. For example, a reasonably common vector for an unintentional plant introduction could be a migratory bird as well as your muddy boot after a hiking weekend.

The introduction process is not dangerous itself.1 A good example of intentional introduction with no disastrous consequences could be the Cypresses’ arrival in Tuscany (Italy). It is hard to think about Tuscany without picturing in your mind bucolic landscapes with rows of Cypresses. However, the phylogenetic history of Cypressus spp. seems to start in the Middle East, where the Cypress was considered useful by Ancient Romans to surround graveyards, and consequently introduced in Italy. In fact, its roots system grows vertically rather than horizontally, causing no damage to the graves. However, taking into account the time of it’s relocation, the Cypress is now considered by definition a native Italian species.

On the other hand, the introduction in Europe of the Japanese knotweed (Fallopia japonica (Houtt.), also intentionally introduced, became one of the world’s worst invasive species.5 Philipp von Siebold brought this species from Japan to Holland in the first half of 1800. Then, in 1850, a specimen of this plant was requested by The Royal Botanic Gardens at Kew, because of it’s similarity to bamboo and quick growing time. Unfortunately, today thousands of hectares of British land are plagued by clones of the first von Siebold’s sample.6,7 In fact, Japanese knotweed is capable of extraordinarily fast-paced vegetative reproduction (there are no male plants in the UK), is known for its strong pioneer qualities, and has disruptive consequences on surrounding wildlife.8

Photo: West Norwood News

In any case, in order to consider the colonisation as successful after the introduction, plants need to go through the naturalisation process, which means overcoming reproductive barriers in the new habitat.1 Still, an introduced and naturalised alien plant is not synonymous with danger, rather it means a species that might survive and subsequently reproduce.3

Accordingly, this raises the question of when we can consider an alien plant dangerous.

Alien species become invasive when they are detrimental for autochthonous species. The Convention of Biological Diversity (2013) define invasive species as any alien species whose introduction and/or spread threatens biological diversity.9 They are generally euryoecious, which means they are able to live in variable conditions and possess strong adaptive properties, such as a fast reproduction process and quick propagation. Thanks to these features, invasive species are able to win in interspecific (i.e. between different species) competition for resources.10 Moreover, invasive species are able to change their adopted environment, altering the habitat and transforming it into an uncomfortable environment for native species. Consequently, the native species are generally more inclined to disappear, reducing their home territory and risking extinction.11 It is this difference that means that all the invasive plants are alien species, but not all alien species are invasive.

What is the solution? The removal of invasive species and the safeguard of sensitive native species could cost Europe around €12 billion per year. It is a complex issue; The European Parliament would like to promote more restrictive laws about introduction and cultivation of alien plants, in order to prevent the introduction of the dangerous species among them.12 Is it a good idea to fine people who grow alien plants? Could collecting tropical flowers in your own garden become illegal? Is it wise to negatively consider all the non-native plants that have enriched biodiversity across Britain in the last century, bringing natural, medical and culinary benefits?

So far, researching only plants that are a real invasive concern seems to be the best solution, preventing their introduction and spreading. In conclusion, prevention is better than a cure. Eradicating invasive plants can be very expensive, while being aware of the hazards caused by invasive alien plants is cheaper and more effective.


  1. Richardson, D. M., Pyšek, P., Rejmánek, M., Barbour, M. G., Panetta, F. D., & West, C. J. (2000). Naturalization and invasion of alien plants: concepts and definitions. Diversity and distributions, 6(2), 93-107.
  2. Allaby, M. (2012). A dictionary of plant sciences. Oxford University Press.
  3. ISSG – Invasive Species Specialist Group (2008). Guidelines for the Prevention of Biodiversity Loss Caused by Alien Invasive Species (online)., accessed 29th August 2015.
  4. ICES – International Council for the Exploration of the Sea (2015).  Code of Practice on the Introduction and Transfer of Marine Organisms 2005 (online). Danmark,, accessed 29th August 2015.
  5. Kurose, D., Furuya, N., Matsumoto, M., Djeddour, D. H., Evans, H. C., & Tsuchiya, K. (2009). Identification of an Aecidial Rust on Fallopia japonica. J. Fac. Agr., Kyushu Univ, 54(1), 53-57.
  6. Hollingsworth, M. L., & Bailey, J. P. (2000). Evidence for massive clonal growth in the invasive weed Fallopia japonica (Japanese Knotweed). Botanical Journal of the Linnean Society, 133(4), 463-472.
  7. Djeddour, D. H., & Shaw, R. H. (2010). The biological control of Fallopia japonica in Great Britain: review and current status. Outlooks on pest management, 21(1), 15-18 (Abstract).
  8. Townsend, A. (1997). Japanese knotweed: a reputation lost. Arnoldia, 57, 13-19.
  9. CBD – Convention Biological Diversity (2015). Glossary of Terms (online). Montreal, Canada,, accessed 20th August 2015.
  10. Maron, J. L., Vilà, M., Bommarco, R., Elmendorf, S., & Beardsley, P. (2004). Rapid evolution of an invasive plant. Ecological Monographs, 74(2), 261-280.
  11. Vila, M., & Weiner, J. (2004). Are invasive plant species better competitors than native plant species?–evidence from pair‐wise experiments. Oikos, 105(2), 229-238.
  12. Warwick, H., (2014). Don’t fear the alien invasion – Our landscape is defined by foreign plants. The Guardian, March 2014.


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