Saturday, May 20, 2017

Skunk Cabbage and Monty Don

Almost four years ago I wrote a little piece about Himalayan Balsam. It's here:
It's Himalayan Balsam flowering season again.

Last night Monty Don, on BBC2's Gardeners' World, told us all about the terrible scourge overwhelming us that is Skunk Cabbage, Lysichiton americanus. I wondered just what harm, exactly, was this rather spectacular plant doing. So I did, as one does, a bit of Googling.

Photo: Kfediuk

First up was this from an RSPB site (what more responsible body than the RSPB, I thought).
Skunk Cabbage - Lysichiton americanus
Stephen Corcoran, biodiversity officer at the Cairngorms National Park Authority, said: “The Cairngorms National Park is one of the most important areas for biodiversity in the UK with a fascinating range of native plants and animals. We want to keep this a special place and it is vital that we all help to prevent non-native species from becoming established. This is because some non-native plants and animals can threaten the National Park’s biodiversity by out-breeding or out-growing our native plants and animals. These non-native species can spread diseases and result in significant economic impacts on our agriculture, forestry and fisheries. I recommend that you choose native plants or plants suggested in Plantlife’s “A guide to plants you can use in place of invasive non-natives” leaflet available on their website (www.plantlife.org.uk) in your garden. By making this choice you will help to prevent the spread of invasive plants like skunk cabbage.”

https://www.rspb.org.uk/our-work/rspb-news/news/325662-curse-of-the-skunk-cabbage

Stephen Corcoran doesn't really tell us exactly what harm it does vaguely alludes to threats to biodiversity, spread of disease and economic impacts. He gives no references to scientific research.
So I checked the Non Native Species Secretariat(NNSS). This is the responsible government-sponsored organisation and ought to know best. Here's what they say about disease:
In GB gardens, skunk cabbage is generally free of diseases

Or did Corcoran mean diseases affecting people?  The NNSS again:
Health and Social Impact: None known.

Ah well, what about the "significant economic impacts on our agriculture, forestry and fisheries" of which Corcoran write.

Economic Impact: None known.
Now far be it from me to suggest that the RSPB is just making stuff up, but if what they say is not backed up by the responsible government body then they ought, at least, to provide references to the evidence that backs up their assertions.

Of course for the NNSS to have L. americanus on their list there must, surely, be some problem. But what is it? They say it's been grown in gardens since 1901 and escaped by 1947. They give the 'species status' as "Widespread (c.400 hectads) but not generally common. It is apparently increasing in lowland and semi-upland Britain except for the English Midlands and drier parts of Eastern England." I'm not sure that 'apparently is a useful scientific term, but this statement does not suggest imminent skunk cabbage armageddon.
So let's see how the NNSS describes what environmental impact can be attributed to this invasive:
Sanderson (2013) reported a significant decrease in numbers of associates within two riverine woodlands invaded by Lysichiton americanus in the New Forest, Hampshire, UK. Reports of it having similar adverse impacts on swamp communities in Germany require confirmation for naturally-spreading populations, as at the most affected site it had been deliberately planted in many different locations by a gardener.
That Sanderson (2013) looks like a reference to science though the NNSS does not actually provide a proper publication source. Google Scholar, however, finds it in milliseconds. And it turns out that it is not a reference to a paper published in a recognised peer-reviewed scientific journal but is, rather, a report of a study conducted by N A Sanderson BSc MSc for Hampshire and Isle of Wight Wildlife Trust. Here it is.
I'm tempted to point out the various flaws which, had this report been submitted for peer-review in the hope of publication, an independent reviewer might have sent it back for revision, but let's not go there. It is worth noting that the report was commissioned by the Wildlife Trust for The New Forest Non-Native Plants Project. Now this is a project which jumps in to its task on the back of some major assumptions. This from the NNSS again:
The New Forest Non-Native Plants Project was established in May 2009 to stop the spread of invasive non-native plants in the New Forest area, particularly in wetland habitats and along rivers and streams.
The Project aims to:
identify where these plants are a problem;
arrange for control work to be carried out by volunteers and contractors;
commission research into control methods;
raise awareness of the need to control these plants and to prevent them spreading into our countryside.
So there we have it, the project has decided before it starts that there is a problem. The first aim is not to established scientifically rigorous evidence for whether or not there is a problem. The existence of a problem is already settled and the project just has to locate it and deal with it, whatever it is.
And just to get the emotional framing established they present a picture with a scary sign, not exactly the radiation symbol or other standard hazard warning sign, but the message is clear.


But surely the good folk of the Wildlife Trust and the NNSS are not making it all up for no good reason; there must be a problem. For sure, where L. americanus grows it, like any plant, native or not, must have displaced something else. It grows rapidly with large leave that can shade out smaller plants so where it gets established it can form mono-species stands. But this only happens in quite restricted areas since it requires very wet, boggy land. There is no way that it is going to spread away from the bogs. So we came to the issue of biodiversity. Other things being equal, adding a species to a habitat increases biodiversity. A decrease only occurs if it replaces and excludes the existing species. And we really should think about the area we are talking about. If an invasive species establishes a stand that excludes others over a few square metres or tens of square metres that might not indicate a lowering of biodiversity, if the pre-existing plants are thriving in their own stand a little way off.  That is, after all, how many native plants operate. Think nettles or great willow herb.
What we need is evidence that the establishment of L. americana has put in jeopardy the existence of other species in the overall area of study.  It may have done but I've not found the evidence reported in the scientific literature and so think it is best not to jump to conclusions.
Biodiversity is, of course, not just about the plants, but involves the whole ecosystem. Where is the study that describes which insects benefit from L. americana, if any? Certainly the smell of the Skunk Cabbage is popularly considered to attract flies and beetles. What is the ecosystem-wide impact of the introduction of such an attractive (to beetles) new addition?

Scientific knowledge is always provisional, nothing is ever proven, and it may be that the escape of an invasive species is deleterious to the environment, but very often it is not. There may be a bit of elbowing and shuffling around before a new dynamic sort of stability is achieved but do let us avoid jumping to conclusions on the basis of hearsay and prejudice and ensure that our policies and actions are evidence-based.

P.S. Legislation: American skunk cabbage is not actually listed under Schedule 9 of the Wildlife and Countryside Act in England and Wales (2010) as an offence to plant or otherwise cause to grow in the wild.

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