Aliens and Introductions

A discussion paper by David Pearman & Alex Lockton.

Amended Jan 2007

1. Invasiveness

A common theme in conservation circles in the UK recently has been the issue of invasive alien species: plants introduced from other countries that become established in the wild in Britain. A lot of money and effort now goes into eradicating aliens, and they are almost invariably seen as a bad thing. The truth, however, is not so simple.

One of the main reasons people worry about invasive alien species is that they are known to cause problems in other parts of the world – usually on remote islands where the native flora and fauna have evolved in isolation and are especially vulnerable to introductions. In such situations, an ecological imbalance can result in the extinction of entire species.

This extreme situation, however, rarely pertains to Britain. Almost the entire flora and fauna has arrived here in the last 10,000 years, since the retreat of the last Ice Age. These islands do not have distinct endemic ecosystems that need to be protected in this way. In fact, many of our rarest and most treasured plants (the archaeophytes) are recent arrivals, brought here by people in the last two thousand years or so, or as natural immigrants as the climate became increasingly temperate.

So, if the main reason for fearing the introduction of alien species is invalid, is there any other harm they cause which would justify eradicating them? Are there not some plants that do create problems?The answer, again, is not a simple one. Yes, there are some introduced species that can become rather abundant under certain circumstances. Rhododendron can be a pest on acid, peaty soils, and Canadian Pondweed can be very abundant in eutrophic water bodies (although recent studies have shown it to be declining). But there are just as many native species that cause problems: Bracken, Brambles, Common Nettle and Ivy are currently some of the most invasive plants in the British flora.

The danger with viewing alien species as a particular threat is that it distracts from the important issues facing nature conservation in Britain and diverts resources that might be better used elsewhere. When an individual species becomes overwhelmingly abundant in a particular habitat, it is usually because that habitat is being mismanaged or polluted – for example, by eutrophication. It does not matter whether the problem plant is native or exotic: the best solution is to restore a healthy vegetation community, not to attempt to eradicate one species whilst leaving the underlying problems unaddressed. Thus there seems little point in spraying a road-verge to eliminate Japanese Knotweed, just to leave the brambles, coarse grasses and even the frequently-invasive Cow Parsley to take its place.

The subjects of ecosystem stability, invasiveness of individual species, and habitat management are all promising ones for research, and might appeal to students or researchers at postgraduate level. Surprisingly, perhaps, although the UK government has a policy of eradicating New Zealand Pigmyweed, there is no published evidence whatsoever that it harms native species. Another common alien aquatic invader, the attractive Himalayan Balsam, is a frequent subject of vilification, with even less evidence of any threat to anything. The only way to formulate good policy is to base it on firm evidence, and that has clearly failed in these instances. There is now available good research information which suggests that the threat from Japanese Knotweed is minor, and it is even viewed by some as beneficial (Gilbert, 2001), yet hysteria on this species regularly surfaces in all sections of the media.

2. How big a threat are alien plants?

Perhaps surprisingly we doubt whether anybody knows. The information available to all government and nature conservation bodies is that contained in the New Atlas (Preston, Pearman & Dines, 2002) and we doubt whether anybody has anything more comprehensive.

To put things into some perspective there are around 1500 native species in Britain, with around about 1700 aliens, of which about 1200 have been recorded more than a very few times. Of that 1200 between 20 and 50 have been identified as anything more than a local threat to our native vegetation, and these have been introduced over hundreds of years. Small numbers indeed!

In the new Atlas the results were displayed at the scale of 10km squares, but underlying, these, particularly in the lowlands where one finds the vast majority of the alien plants, was a wealth of much finer detail. This gives the BSBI the ability to see fairly well what is actually going on in the countryside, a facility that we respectfully suggest is lacking from almost every other commentator.

Take a simple example of a species notorious in the press at least, Japanese Knotweed. This occurs in Dorset (where one of us lived for 20 years and collected his share of the data for the Atlas) in all the 10km squares, and in 236 2km squares out of a possible 739; so one might think a frequent pest, but it very, very rarely occurs in more than 3 sites in each 2km square. Take another cause celebré, Giant Hogweed (a superb garden plant!). Our Atlas shows it in 14 10km squares in Dorset, but in fact it is in only 18 2km squares and in 20 sites (and only in any numbers at two of those). In other words it is a really rare plant in the field. But Bramble, Ivy, Gorse and Nettle are in every square and are hundreds of times more frequent than any of the above.

A further subject for research on which some progress has been made in Europe, but as yet little here, is the time-lag between the arrival of an alien plant in cultivation, and its appearance, if it makes that transition, in the wild. Recent work in Germany (Kowarik, 1995) (only one of a number of recent papers) gives a surprisingly long time-lag for woody plants, so we might only now being seeing the rapid spread of plants introduced 100 to 200 years ago.     

3. What is an alien plant?

Work in the last five years, initially as a basis for a fresh look at status for the New Atlas, has brought together a wealth of information which has given all of us who wish to use them new tools to make better judgments.

The advances in detailed distribution mapping (by grid squares) both in the British Isles and in much of N. Europe, the new research available in the fields of archaeobotany and in the interpretation of the nomenclature of early gardening works, both in medieval times and up to 1800, now allow a broader view to be taken. This can often be coupled with recent European work on alien and native status and on archaeophytes. Thus two of the Atlas authors have participated in papers on Archaeophytes and Neophytes, and have advocated using a range of criteria to assess status. In a country such as ours, almost completely modified by man, it would be surprising if there were not more plants than already identified that had actually been introduced in the past.

The question of protection and value of these non-natives needs a fresh look by all of us.

Of course resources themselves are scarce, but this, if I may describe it, obsession with native status, which extends in the opposite direction to blaming aliens for many of the ills facing our native flora, is surely misplaced and simplistic. We would like to extend the parameters of species worthy of protection to those species which are an integral part of the mosaic which is being valued and conserved - thus, amongst others, one might preserve Fritillary, Fly Honeysuckle, Meadow Clary and perhaps Wall Germander (in its Sussex coastal site) as being part of a valued semi-natural habitat, Rough Marsh Mallow, Compact Brome, Wall Bedstraw, Jagged Chickweed, Cut Leaved Germander and possibly Proliferous Pink as species very persistent in open habitats, Field Cow Wheat and Greater Yellow Rattle as part of the cornfield weed mosaic we now value (and possibly potential archaeophytes too). We appreciate this is more subjective than the natives ‘good’, aliens ‘bad’ approach, but we do feel that species that have that cultural resonance, act in harmony with other species that we value in their habitats and have a long association in those sites, could well be included in conservation efforts. This would be all the more possible since almost all conservation efforts are perforce concentrated on the management of the habitat rather than the individual species therein. Furthermore the list would not be long.

Summary

There is more data available, at a finer resolution, than most people are using. There is new thinking (certainly in the last ten years) on status and time scale for spreading into the wild that has not yet entered the mainstream of conservation. There is an urgent need for focused distribution work at varying scales and habitats to show just how widespread are alien plants and in what places they are really found. There is new work on species such as Himalayan Balsam, which seems to show that despite all the hype, there is little discernable detrimental effect on native vegetation that is not already dominated by grossly nithrophilous plants. This all needs pulling together, coupled with the new survey work.

The basis of this note is from an article in BSBI News (Pearman 2004) which has been discussed by the society’s Science & Research Committee, together with subsequent research. The BSBI would welcome research into this and other related themes, and can offer to help with supplying data. Please contact the Science & Research Committee in the first instance.

Further reading

Defra. 2003. Review of non-native species policy, report of the working group. Department for Environment, Food & Rural Affairs, London. www.defra.gov.uk.

Gilbert O. 2001. Figs, Japanese knotweed and Himalayan balsam enhance the urban ecology of Sheffield. In Alien Species: Friends or Foes? Pp. 52-56. Glasgow Naturalist, vol. 23, supplement.

Koowarik, I. 1995. Time lags in biological invasions with regard to the success and failure of alien species. In Pyšek P, Prach K., Rejmánek M & Wade M, eds., Plant invasions - general aspects and special problems. Amsterdam: SPB Academic Publishing. Pp15-38.

Pearman, D.A. 2004. Invading Aliens - on Invading Natives? BSBI News 96, 41-42.

Pearman, D.A. 2007. ‘Far from any house’ - assessing the status of doubtfully native species in the flora of the British Isles. Watsonia 26:271-289.

Pearman, D.A. 2007. Invasive Aliens – and Natives. Plantsman in press.

Peretti, J.H. (1998) Nativism and nature: rethinking biological invasion. Environmental Values, 7, 183-192.

Walker, K. & Pearman, D. 2004. Flowering Plants - England. British Wildlife 16, 438-440.