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?