The aliens might be coming

Rhododendron x superponticum - beautifully strangling a lakeside at the Yorkshire Sculpture Park.

Whether it is kudzu vine strangling trees in Virginia, melaleuca throttling the Everglades or Himalayan balsam on the English river bank, invasive aliens are big news. Gardeners of course have a particular interest in them, as all too often they are responsible for the introduction and distribution of these plants in the first place, and in some regions, anything new in the garden is something of a potential Trojan horse.

A new book about invasives: plants and animals takes a nicely-balanced look. Where do camels belong? Its author, Ken Thompson is one of the best British writers on natural history. A retired member of staff at the University of Sheffield, he has worked on plant ecology for much of his life. I can't imagine a better person to keep a cool head on this topic. He is one of the key people behind the BUGS project which is looking at the relationship (or lack of it) between plant origin and wildlife in British gardens. He is particularly good at poking sacred cows with a science stick, and on occasion, as when he had a go at the fashionable pretences of permaculture, something of a cattle prod.

This is a very readable book and a balanced one. A key message is that many invasive alien stories have more bark than bite, that a species which may appear to be spreading may not be anything like as bad as it either appears to be, or the local press tell you it is. He does discuss some of the real horror stories, and does indeed recognise that aliens can be a very severe problem in some situations. Although if I lived somewhere where an alien species was causing real difficulties, I think I might feel the book didn't go quite far enough in recognising this. Some species really do destroy ecosystems. But, most don't and one of the valuable points of this book is pointing this out, and that over time many invasive plant species reduce in number, or start to get eaten by the local wildlife or infected by the local pathogens. 

One of the key points Ken raises is the cost and the impossibility of controlling many invasions. He discusses the whole new field of invasion biology. One can't help but get the feeling that there are a lot of people with a vested interest in keeping fears of invasives stoked up – a nice source of grant money for research/control etc. The implication is – don't throw money at things you can't do much about and keep it for things that either work or for battles that are really worth fighting. Invasives help keep journalists in business too, with lurid press stories often an opportunity for some covert racism – a topic which the book could have spent more time on. 

Being British and discussing invasives is a slightly odd position. Our flora has (like us) spread throughout the world, often aggressively. The turf-forming grasses of north-west Europe in particular – much of the native flora of western USA has been throttled by these aggressively spreading plants. Sounds like the effect of the European empires on global cultures. However the dense matt of growth these grasses form, and their ability to grow at low temperatures, does mean that it is very difficult to get invaded back. There isn't a single North American species which has become a problem here. The problems we have with our worst invasives (Japanese knotweed and Rhododendron x superponticum) are limited in geographical extent and pale into insignificance compared to the problems many other places face. Ken points out that some of our natives behave as invasives here, on home ground. As with my point about the nitrogen pollution fed nettles a few posts ago.

A recent trip to the US Pacific North West was an opportunity to appreciate just what a huge impact non-native and (in some eyes at any rate) invasive species make. It all depends on habitat. One of the wise points Ken makes in his book is that most invasive alien problems are in disturbed habitats. Any kind of succession process that starts taking a vegetation back towards what might be found naturally there will inevitably reduce their impact. Out in the open in the PNW you see a lot of European aliens, but few in the woods. In an area where thick conifer forest is the norm you could argue that any bit of open land is disturbed. Much grassland, in the Columbia River Gorge for example is populated by European turf grasses, and spattered with the European cornflower (Centaurea cyanus) and vetches, the native California poppy appearing in much lower quantities. It was almost uncanny how much of the open habitat was composed of 'interlopers'. But this kind of grassland is largely a human artifact, so should it surprise us that it is full of non-native species? None of which by the way was dominating anything else and incorporated many natives too. It all looked uncannily 'natural'. 

Eurasian vetch Vicia cracca and European grasses make a meadow in Oregon, but natives seem to join in too, but what was here before?

California Poppy - Eschscholzia californica in habitat with the far more common european cornflower, whose presence seems largely benign. This is south side Columbia river gorge, near Hood River.

Inevitably I think about what is escaping from my garden. Geranium x oxonianum 'Claridge Druce' is scattering itself into the hedge bottom. I have often seen this species escape. I suspect it will settle down to evolve into a true species and a widespread component of the British flora. With its parents from northern Spain and Italy, it is effectively a neighbour and can be expected to settle down decorously. Persicaria amplexicaulis has, somewhat surprisingly, taken up home in the hedge bottom too – funny as it never seeds in the garden. Its habit is much reduced compared to how it grows in the border and hardly has spread at all. Its scarlet spikes look oddly at home though. It is Senecio fuchsii which I think will inevitably take off. A tallish yellow daisy, it lights up woodland edge habitats from one of Europe to another, except that is did not get across the English Channel in time after the last ice age. A native really. In fact I think we can say that any north European plant is effectively a native for this reason, as they would almost certainly been here before the ice scraped everything off. The senecio seeds like crazy in the garden and will no doubt one day wander into the woods, where really it belongs. There is little in the wild flora that flowers in shade at this time, so pollinators will probably be really glad of it. Maybe it just wants to come back home.

Senecio fuchsii (S. nemorensis) good but slightly tatty as a garden plant, great in light shade.

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