Loving The Alien: Why Not All Introduced Species Pose A Problem

Introduced, or alien, species have often (and occasionally deservedly so) had a bad rap. Theoretically, species that enter environments in which they are not native with the assistance of humans are only called invasive when they have harmful effects on the host ecosystem, but in reality we often have a biological bias when it comes to evaluating the impact of a species. Purple loosestrife (Lythrum salicaria) has been referred to as an ‘ecological disaster’ in North America, but scientific studies have failed to back this up (1). Nevertheless, the introduction of species into a novel environment can undeniably have disastrous consequences; see, for example, the introduction of brown tree snakes (Boiga irregularis) in Guam, a Pacific Island which has since lost at least 22 native bird species to predation (2). Globally, millions, maybe even billions of dollars are spent on the control and prevention of invasive species (though that doesn’t account for the billions that they provide). The US military alone spends up to $2 million searching cargo leaving from its Guam base to ensure they do not accidentally transport brown tree snakes into Hawaii or anywhere else on the continental mainland. This has led to a general mistrust of introduced species and a resistance to the idea of human-assisted introduction (excluding all the food crops, pets, and livestock that we routinely transport because they benefit us).

Brown Tree Snake (Public Domain)

Brown Tree Snake (Public Domain)

However, do attitudes need to change? Not all introduced species actually go on to become pests, or even to establish a flourishing population. For terrestrial plants for example, there’s a rough rule of thumb called the tens rule which holds that only 10% of introduced species (grown in gardens etc.) go on to form self-sustaining populations in the wild, and only 10% of those can be considered invasive. Some alien species may even have beneficial effects on environment, increasing biodiversity and ecosystem functions such as nutrient turnover (3). It may be time for scientists, policy-makers, and the general public to start seriously considering when purposeful introductions may be a good thing.

Rewilding, the reintroduction of species to their natural habitat, receives a lot of vocal support from many (e.g. George Monbiot). It could be thought of an introduction scheme, depending on your definition of native vs. non-native species. In Scotland, for example, a non-native species is any that only reaches the country with the help of humans, even if once native. If lynx or wolves were to be reintroduced as a way of managing large herbivores such as deer, they would be considered an alien species.

Some scientists have already started to embrace the idea of assisted migration. Human-driven climate change is affecting the home ranges of species all over the world. A common reaction to climate change is for populations to shift their home range in accordance with the change, generally towards the poles. This is not always possible however, for example when a sea or a city blocks their way. It has been suggested that we may need to translocate threatened populations around obstacles in anticipation of climate change affecting their range. Researchers in Finland are already investigating whether the Siberian primrose (Primula nutans var. jokelae), which grows on the northernmost shores of the Bothnian Bay will be affected by a warmer climate, in anticipation of the possibility of moving the variety to 500km north to shores on the Arctic sea (4).

If They Can't Stand The Heat (Nature, Vol 427, P 145)

If They Can’t Stand The Heat (Nature, Vol 427, P 145)

At first glance, intentionally introducing a non-native species to try and control an invasive species that has got out of hand may seem a bit backwards, but it is potentially a very clever idea. Alien species may become pests when in a new environment where they can run rampant without any native predators that are adapted to preying on them. Introducing a specific co-evolved predator as a form of biological control could achieve what millions of pounds and man hours cannot. The UK Environment Agency spends £2 million a year killing and uprooting the highly invasive Japanese knotweed (Fallopia japonica). In 2010, for the first time in the EU an insect was officially released to act as a biological control agent for a weed (5). The psyllid Aphalara itadori, a type of plant lice only found on Japanese knotweed, can do lot of damage. After ten years of extensive research it was released in 3 locations in the EU. It will take some time for psyllid populations to build up to the level where they will be able to have a significant impact, but no harm has yet been found to native species.

Aphalara itadori on Japanese Knotweed (CABI Communications)

Aphalara itadori on Japanese Knotweed (CABI Communications)

Finally, it’s possible that conservationists just need to become more pragmatic. With limited time and resources, it is highly unlikely that we can save everything. In many conservation circles there has been a gradual shift in focus from species to ecosystem functioning, the biological processes between a community of living organisms acting as a system. When keystone native species (e.g. pollinators) are endangered, then perhaps instead of wasting resources (or in addition to!) we could try introducing new species from neighbouring environments that could do the same job. Interestingly, the honeybee Apis mellifera, reportedly worth over $12 billion to the US economy and official insect of 12 states, is actually a European alien.

Intentionally moving species into new ecosystems is not without risk (look no further than cane toads and rabbits running rampant in Australia), but also has great potential. We are headed down a path there we may not have any choice but to start taking risks, albeit with the catchphrase of scientists everywhere: “further research is needed!”


Lavoie, C. (2010) Should we care about purple loosestrife? The history of an invasive plant in North America. Biological Invasions. 12(7): 1967-1999

Rodda, G.H. & Savidge, J.A. (20070 Biology and impacts of Pacific Island invasive species. 2. Boiga irregularis, the brown tree snake (Reptilia: Colubridae). Pac Sci. 61(3): 307-324.

Mascaro, J.R., Hughes, F. & Schnitzer, S.A. (2012) Novel forests maintain ecosystem processes after the decline of native tree species. Ecological Monographs. 82:221–228.

Hällfors, M., Vaara, E. & Lehvävirta, S. (2012) The assisted migration debate – Botanic gardens to the rescue? BGjournal. 9(1): 21-24.

Shaw, R.H., Bryner, S. & Tanner, R. (2009) The life history and host range of the Japanese Knotweed Psyllid, Aphalara itadori Shinji: Potentially the first classical biological weed control agent for the European Union. Biological Control. 49: 105-113.


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