Invasive Plants caused muddled thinking



The BBC picked up a story about “invasive plants” yesterday, but didn’t do a very good job of conveying its messages.

The paper just published in Proceedings of the National Academy of Sciences is from Chris Thomas’ group at the University of York. Basically the researchers looked at the occurrence of native and non-native (which they unhelpfully term “invasive) plants in Countryside Survey plots. They found no correlation between declines in native plant occurrences and occurrence of non-native plants; and concluded from this that non-native plants posed no threat to native plant status, and then, in quite a leap, concluded that non-native plants posed no threat to British biodiversity, or even global biodiversity.

The paper has serious weaknesses though; for example Perennial Ryegrass, one of the commonest species in the entire country, is treated as if it were a native plant, while Italian Ryegrass is treated as a non-native. Almost all of the Perennial Ryegrass in the UK originates from agricultural forms, specifically bred and sown: How is that native? Once established agricultural forms of grasses can be very difficult to shift, as anyone who has tried to restore a wildflower-rich grassland from a ryegrass field will know. Often the only way is to herbicide the ryegrass to get rid of it (or through repeated cultivation). The same applies to agricultural forms of white clover, again treated as a native species in the PNAS paper.

I’ve written before about the exaggerated claims of impending doom resulting from the presence of non-native plants – and was taken to task by some readers for suggesting that Himalayan Balsam wasn’t quite such an ecocidal maniac of a species as has been suggested. But there is evidence for HB that it has an impact in certain places in Britain, albeit not on native plants, but on invertebrate fauna.

Phil Brewin, the Somerset Levels hydrological expert also pointed out that where HB replaces grass on river banks, the banks can lose stability and become vulnerable to “shear forces, slips and frost damage”. Whether this in turns leads to negative impacts on nature is not clear, but it seems akin to me to the problems associated with Japanese Knotweed damaging the foundations of buildings.

One of the best ways to stop HB from colonising river banks is to ensure they continue to receive grazing, but this can also present challenges. The level of grazing, type of animals and seasonality of such grazing needed to control HB (without damaging river banks), may well not fit in with the needs of conventional livestock farmers, to ensure their animals continue to gain condition, weight or produce milk.

While HB clearly does cause problems, it also provides an important late summer nectar source for invertebrates, and this can be significant in landscapes now shorn of their native nectar sources.

In the BBC article,  Balsam was fingered for contributing to the decline of the Tansy Beetle, the argument being that HB has somehow ousted Tansy (on which the Tansy Beetle depends) from its riverbank homes. I find this argument dubious – the distribution of Tansy has not changed, according to the most recent BSBI Atlas; Tansy distribution is complicated by the fact that there was a native population, which has been supplemented over many centuries by escapes from gardens, as it was an important mediaeval medicinal herb. Tansy has a fairly catholic set of habitat preferences and pops in all sorts of places, not just on riverbanks and fens where the Tansy Beetle lives. It’s easy to blame the Balsam invader – but then how many of the Tansy populations on which the Tansy Beetle used to occur were garden escapes?

Perhaps the simplest answer to controlling Himalayan Balsam would be for Beavers to return to a catchment. Evidently they do eat HB. Beavers may consume several kilo’s of herbaceous vegetation a day through the summer, so a family of beavers in a smallish catchment with plentiful Himalayan Balsam could consume the entire population within a few years.  Beavers also eat macrophytes (free floating water plants), and could be a very effective way of removing problematic introduced species such as  Swamp Stonecrop and Parrot’s Feather from water bodies.

I think one of the reasons why Prof Thomas’ researchers found no effect of non-native plants on native populations is that the data set they analysed was far too coarse. The Countryside Survey sample sites were set in typical farmland, not the vanishingly small areas that are now sufficiently high quality habitat to support threatened plant species. It’s hardly surprising the researchers discovered the CS plots supported very few non-native species, as the CS plots only support very common plant species, whether they are native or not. What the CS plots did tell us was about the large-scale changes to the countryside that occurred over the period in which the Countryside Survey was running, although many of the changes that removed nature (especially the declining and threatened species) from our landscapes, had already happened by the time the CS started in 1978.

Does any of this further the debate around the “problem” of invasive non-native species? Perhaps. We need to think again how we view native and non-native. There is a  strong groundswell of support to bring back the Beaver, as shown by the public’s support for the Otter Beavers, and the way that public engagement forced the Government to back track on its goal of removing them.

But how long ago should we look to consider whether a species is worthy of return, or indeed being valued as an authentic member of Britain’s nature. The poor humble rabbit, vilified as a pest, shot gassed and poisoned as vermin, was a native mammal of Britain during previous interglacials.  Rhododendron ponticum and Water Fern, both regarded as serious problems for native wildlife which need to be eradicated, were native species. And Arable Weeds, which were all introduced from the Near East from the Neolithic onwards, are regarded as high priorities for conservation.

Where is the rationale for arguing that a species that has been extinct for 500 years is worthy of restitution, while those that were extinct for a few hundred thousand but returned by human agency, must be extirpated?

About Miles King

UK conservation professional, writing about nature, politics, life. All views are my own and not my employers. I don't write on behalf of anybody else.
This entry was posted in alien invasive species, Beavers, himalayan balsam, invasive species and tagged , , , . Bookmark the permalink.

11 Responses to Invasive Plants caused muddled thinking

  1. The rationale is surely all about the relatively new adverse impacts happening now, on what are now just relic sem-natural fragments of previously widespread natural habitats – ie it’s all about the here and now and an eye to the future. I would like to think we are saving what precious little we have left now as a storehouse for recreating larger areas of semi-natural habitat on into the future!

  2. Mark Fisher says:

    I think the authors pretty much explain in the Discussion section why they wouldn’t attribute changes in native plant cover to affects of non-natives:

    “Using repeat census field survey data for British plants from 1990 and 2007, we find that the sum total of area changes of native plant species is an order of magnitude greater than the changes to the abundances of non-native species, indicating that native, rather than non-native, plant species dominate vegetation changes. This strong influence of native species arises because there are more native plant species (85% of the 531 plant species recorded in at least one site in both surveys), and they tend to be more widespread (Figs. 1A and 2A), rather than because there were any fundamental differences in the population trajectories of plants that arrived in Britain at different times in the past. These same quadrats only detected 81 (<5%) non-native plant species present in both survey years, of a total of 1,728 non-native plant species in the flora (36), emphasizing that most non-native species remain too localized to have national-scale effects on other species."

    They don't even mention HB, although it forms part of their data set. The seed leaves of HB are appearing in my local woodland, and I will be "weeding" then as usual!

  3. Mark Fisher says:

    They don’t distinguish whether the CS plots for Lolium perenne were agricultural locations or semi-natural, or if you could distinguish. How would you tell if a CG2, CG3 etc had only native ryegrass? As you say, ryegrass and white clover contributed a lot of CS in the data.

    • Miles King says:

      native ryegrass is pretty uncommon these days but also can be difficult to separate out from cultivars. wild white clover is also much less common than cultivars; there is no indication that these have been treated separately in this analysis.

    • Miles King says:

      most lolium and white clover in the countryside are cultivars. I don’t know whether CS differentiated between wild and cultivated forms of these two. Cultivars of white clover and Lolium can be very persistent in swards even after fertiliser application has ceased. Last year I saw a Lolium cultivar acting in a very invasive manner, colonising shingle along the Solent.

  4. Interesting comment about the variation of tansy plants! However, from experience of surveying River Ouse north and south of York for Tansy Beetles suggests that impatiens does compete with tansy eventually ousting it. See ‘Ecology of the Tansy Beetle’ Chapman, Sivell, Oxford et al (2006) Naturalist 13, p. 53.

    From experience of practical management of sites the tansy, being a relatively late developer, needs protection from grazing and competing vegetation inc cow parsley for the plants to thrive and beetles to survive. The sites are largely along water courses where the seed (and that of impatiens) travels, and gets trampled in by cattle. Erosion also affects the plants (and beetles) chances of survival.

    If we didn’t clear impatiens and other competing vegetation by hand at Rawcliffe Meadows we’d have many fewer beetles –

  5. Miles King says:

    thanks for your comment. I can see that, with Tansy Beetle restricted to a tiny number of sites now, there might be very local factors which lead to HB affecting Tansy populations. But what factors are leading to increases in Himalayan Balsam? And are these the same factors that are causing the Tansy population to decline?

    As Buglife say, Tansy Beetle was widespread across the UK before the dramatic decline of the Beetle. As HB has only increased substantially in recent years, HB cannot have been the cause of the decline.

    • Is anyone sure of when the beetle started to decline (I’ll check the literature again)? When Buglife say it was widespread that was probably in Victorian times.

      Agreed there are other factors but locally (in York) HB does affect the tansy and hence beetle populations. HB can be a great pollen resource but in this instance it has to go – however it continues to come down the catchments and recolonize so is a pest…

      In last few years I’m starting to see HB appear on top of Howardian Hills – how did it get there – obviously not waterborne?

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