Monday, 25 February 2013

Aliens vs Invasives

Over the last couple of days,a couple of people on Twitter have flagged up a Comment Is Free article on alien species (  It's not a bad article (although as ever, don't bother reading the comments), but it manages to confuse & combine alien with invasive and effectively ends up arguing strongly for the status quo.

Alien species are not necessarily invasive. The two terms are often heard - often used - together, but in fact, the two are very different.  The official definition of an alien species (from the IUCN via the Convention on Biological Diversity, CBD: is 'a species, subspecies, or lower taxon occurring outside of its natural range (past or present) and dispersal potential (i.e. outside the range it occupies naturally or could not occupy without direct or indirect introduction or care by humans) and includes any part, gametes or propagule of such species that might survive and subsequently reproduce' - basically, it's a species which people have moved to a new area.  

A recent review of the non-nativeness of the British countryside (overall, about 0.5%: data at  found that more than 2,700 alien species are at large in Britain, from Neolithic introductions such as the Large Tabby moth (Aglossa pinguinalis) to very recent arrivals like the Demon shrimp (Dikerogammarus haemobaphes).  A study on plants in Britain (Williamson & Fitter, 1996) found that, of every 100 alien species introduced to a new area, only 10 will establish wild populations, and only 1 will go on to become invasive.  In this context, 'invasive' has a strict definition, again from the CBD: a species which 'is an agent of change, and threatens native biological diversity'.  

This is the crux of the matter. An alien species may be invasive - for instance Himalayan Balsam (Impatiens glandulifera), Japanese Knotweed (Fallopia japonica), and the Harlequin ladybird (Harmonia axyridis), but so can native species. A decade ago now, I worked for the RSPB at their Aylesbeare Common reserve, a heathland site in East Devon. We spent a huge proportion of our time chopping down and otherwise killing plants which were invasive on the heathland - all of them natives, mainly Silver Birch (Betula pendula), Bracken (Pteridium aquilinum), and Bramble (Rubus fruticosus agg).  Aliens, like the pine trees scattered across the heathland, we generally left alone - they weren't spreading, so weren't invasive, so weren't a problem.

Some alien species can go beyond this, and become integral to an ecosystem. The rabbits (Oryctolagus cuniculus), mentioned in the CIF article as a long-established alien species, are crucial to the maintenance of chalk downland, and the 1950s/60s outbreak of myxomatosis in rabbit populations contributed significantly to the extinction of the Large Blue butterfly (Maculinea arion) and the near-extinction of at least three more butterfly species.

Of course, some aliens are disruptive, and there's no good way of predicting which species will establish, and which will displace or feed on natives, becoming problematic invasives. That's why border controls are strict: they're based on the precautionary principle.  We can't tell which species will cause havoc when they arrive, and it's virtually impossible to eradicate a species once it's established, so the best bet is to prevent arrival in the first place.

Some of the most notorious alien species are invasives.  The Harlequin ladybird has been linked to significant declines in native ladybirds (Roy et al., 2012), and the effects of Rhododendron & Japanese Knotweed are well known, but the vast majority of alien species fall between these two extremes.  These species lead lives of quiet obscurity with few, if any, effects on native ecosystems, like the Bryony ladybird (Henosepilachna argus) in Surrey or the Yellow-tailed Scorpions (Euscorpius flavicaudis) in Sheerness.  Consequently there's no control measures for them - they may be aliens, but they come in peace.

The flexible approach - taking each case on its merits - is presented as the big message at the end of the CIF article. But it's already common practice with alien species, in Britain and far beyond. The mere fact of a species' origin is, in practical terms, essentially immaterial: what's important is the effect it has on native ecosystems, adapted to the local area and each other.  


Roy, H.E., Adriaens, T., Isaac, N.J.B., Kenis, M., Onkelinx, T., San Martin, G., Brown, P.M.J., Hautier, L., Poland, R.L., Roy, D.B., Comont, R., Eschen, R., Frost, R., Zindel, R., Van Vlaenderen, J., Nedvěd, O., Ravn, H.P., Grégoire, J.-C., de Biseau, J.-C. & Maes, D. (2012b) Invasive alien predator causes rapid declines of native European ladybirds. Diversity and Distributions, 18, 717-725.
Williamson, M. & Fitter, A. (1996) The Varying Success of Invaders. Ecology, 77, 1661-1666.

1 comment:

  1. What a great blog post! Many people, the Guardian too, seem confused (sometimes deliberately, perhaps) about the difference between "alien" and "invasive". There is, as you say, a world of difference between "non-native" Little Owls, which surely have no objectors, and invasive/destructive Japanese Knotweed or American Mink.

    It's tiring to read articles which either, at one extreme, scream that all alien species are here to take over and kill everything or, at the other end of the scale, shout that being careful about bringing new species into the country is somehow intolerant and dicriminatory, despite the massive risks to biodiversity involved. Both extremes have their own agendas, neither of which has ecology at its heart.

    The precautionary approach prevents many diseases and invasives coming in and threatening other species. It's also good you point out the more complex cases such as Rabbits, which benefit eg chalkland wildlife but might not be so popular with farmers, or Ring-necked Parakeets, which have increased massively but the effects of this are not yet fully known.

    There's also the reintroduction issue - formerly native species returned to their historical ranges (Wild Boar are currently making it into a lot of newpapers!).

    The best approach is surely to adopt a New Zealand-style filter and limit any new aliens coming in to help avoid say another Dutch Elm disease, whilst living with and enjoying the numerous presumably benign aliens we already have, such as Little Owl, Chinese Water Deer and the Bryony Ladybird.