TRIFFIDS THEY ARE NOT

Some of you will be old enough to remember John Wyndham’s 1951 post-apocalypse horror story and later film The Day of the Triffids, when an off-world plant invaded earth and started killing people. It was of course pure fantasy, but I do wonder whether some of the more alarmist stories about real-life earthly Invasive Alien Species (IAS) have their roots in that undeniably frightening story. Perhaps that is why many have stopped calling colonising species that are not autochthonous IAS and renamed them linguistically more clumsily INNS (Invasive Non-Native Species). There is nothing wrong, though, in calling species which are not native “aliens”, because that is a perfectly accurate term in the context.

Botanists have now gathered many data on plants which are colonising the UK from elsewhere. Unsurprisingly, since we have been an inveterate nation of gardeners, plant importers and botanical explorers for centuries, plus the more recent effects of species migrating northwards as the planet warms, it is a very long list. Some of these neophytes persist and become naturalised here: others die out.

As ecological scientists, Betts know well that species and ecosystems are highly dynamic. Colonisation occurs, species disperse and there is a constant ebb and flow as communities adjust to competition, predator–prey relationships, and environmental change. Professor John Harper's Thespian analogy of climate and substratum providing scenery and stage for a large cast of plant and animal players, which come and go, is an apposite concept: the players act out a tragedy dominated by hazard, struggle, and death, in which there are few survivors (Harper 1977[1]).  One might add that the play is delivered in a foreign tongue, without a plot and at a scale of space and time outside normal human experience to which it is challenging to relate.  Nonetheless, it is by probing the intricacies of the system, testing hypotheses, and recording observations over time, that understanding may be gained. Whilst some IAS undeniably affect the stability, diversity and richness of native communities and ecosystems adversely, we should not, I feel, leap too quickly to anthropocentric conclusions about them. Similarly, slavishly demanding that “native” species are always “better” is simplistic and indicates a failure to appreciate ecological dynamics and the status and behaviour of species in ecosystems.

The best-known botanical IAS in the UK are probably some of those on the Wildlife and Countryside Act 1981, Schedule 9. Under this Act it is illegal to plant or otherwise cause to grow in the wild any plant taxon so listed. There are thirty-four land plant taxa and seventeen aquatic taxa on the UK list, of which twenty-one terrestrial and fourteen aquatic taxa are relevant to England and Wales. Full details can be found at https://bit.ly/IASUKS9. The aquatic IAS are mainly listed because their excessive growth interferes with drainage, water flow controls and wetland ecology. Terrestrial species may out-shade or out-compete natives or have other adverse environmental impacts.

Let’s briefly look at four of the well-known botanical IAS that we at Betts often come across: Japanese knotweed Reynoutria japonica, Indian balsam Impatiens glandulifera, giant hogweed Heracleum mantegazzianum and New Zealand pigmyweed Crassula helmsii.

[1] Harper, J. L. (1977). Population biology of plants. Academic Press, London, UK.

Japanese knotweed is infamous because it is so invasive and can grow through concrete and brick structures, causing costly damage. Its infamy may be deserved but I have a sneaking admiration for this plant (and other IAS I must confess). It is not unattractive, as you can see. It is now widespread and frequent through almost all the British Isles and Ireland on waste ground and along railway lines, rivers, canals, roads and sea-loch shores. It was first found in the wild in Britain in 1886 and hails from Japan, China and Korea. Its great ecological triumph is to be able to grow   on volcanic lava fields and scree, an attribute that makes it problematic in towns and other built environments. It is prone to forming dense clumps which out-compete other vegetation. All plants are male-sterile clones and it spreads through fragments of its rhizomes, very small pieces of which are viable and readily transported with soil, vegetation waste, or by floods. It is not at all easy to eradicate from a site it has colonised.

Indian (sometimes called Himalayan) balsam is one of the fastest spreading IAS in Britain and has now colonised water courses, damp woods and other moist ground throughout swathes of the lowlands. It is an attractive garden escape – there are still garden varieties about, like the red-flowered one in my photo, but they are usually pink, as the inset – and was first found in the wild in Middlesex in 1855. It is a tall (2m) annual growing quickly and strongly from the abundant seed it produces every year (which children love to “pop” from the ripe seed pods, but really shouldn’t) and can even shade out dense stands of nettles! When it dies back after frosts, it leaves a soggy mush covering the ground. As well as its ability to out-compete native flora, there is some evidence that the copious nectar it produces means that pollinating insects are preferentially encouraged to it rather than pollinating our other wild flowers. Cutting or cattle-grazing it to stop it flowering, and thus eliminating the seed bank over time, can control it, but seed washed from upstream re-invades riparian areas in times of flood. Defra have introduced a rust fungus as a potential means of biological control, but it does not seem to have spread much in wild populations so far: we have not seen the fungus on any of our sites and we have Indian balsam along most of our streams and rivers now.

Perhaps my favourite amongst the IAS, because of its sheer majesty, is giant hogweed. This is an enormous version of the common hogweed of our waysides, grasslands and woodlands, growing to a whopping 5.5m tall and, like its small relative, is a perennial herbaceous plant and member of what we used to call the umbellifer family that are now the Apiaceae. It is native to the western Caucasus and was introduced to British gardens and deliberately planted along watercourses in the 19^th century. First recorded in the wild in Cambridgeshire in 1828, it has spread throughout the British Isles where it colonises waste ground, roadsides and rubbish tips as well as the banks of rivers and streams. Unfortunately, it is phototoxic – all its tissues contain furancoumarin which causes a nasty blister-forming dermatitis in humans in the presence of sunlight. Giant hogweed should never be meddled with without suitable personal protective equipment. This has led to a pogrom against it which, though understandable, is a little regrettable. It can spread quickly through its copious seed, so is likely to stay with us despite its persecution.

 

New Zealand pigmyweed is a very different plant: it is an emergent aquatic perennial with long, densely packed trailing stems, growing in shallow or fairly deep water (to about 3m) or on wet mud or similar substrata where it forms dense mats, floating or attached to the substratum, that block sunlight and aggressively out-compete other flora. An antipodean native, it arrived in Britain as an aquarium plant but soon escaped, probably from emptied aquaria, and was first recorded in the wild in Essex in 1956.  It has rapidly colonised lowland wetlands and is tolerant of waters that vary widely in chemistry. It has tiny (1–2mm diameter) white flowers in the leaf axils (inset bottom left) and long, fine roots (right). It is difficult to control, not least because of collateral damage risk to coexisting aquatic and mesic fauna and flora. Herbicides, hot foam, hot water and dyes have been tried without great success.

Betts Ecology keep a careful watch for IAS on our sites and do apply the latest control methods where necessary and possible and where they do not cause adverse environmental impact such that the “cure may be worse than the disease”.

 © Betts Ecology