Invasive plants don't cause extinctions a premature conclusion
Who cares if alien species invade and displace our local flora? It's nature in action, aided by one of its more troublesome products (us). Or perhaps it's a necessary evil if our natural world is to adapt to accelerated climate change: plants will need to move and we can't be too precious about what should grow where.
Either way there is certainly a backlash against what can be caricatured as the puritanical view that natural ecosystems are sacred and man-induced movement of plants is evil. As always, the answer is not black and white and it would help if we had more evidence.
In a recent early edition of PNAS (Proceedings of the National Academy of Sciences of the United State of America) there is an interesting scientific review of this topic by two North American botanists, Benjamin Gilbert and Jonathan Levine.
Their starting point is the contention that 'competition from introduced organisms has driven remarkably few plants species to extinction'. After some nifty modelling of what might happen on serpentine soils in California, a habitat supporting lots of rare and threatened plants but also home now to a wide range of invasive species, they conclude this is at best a premature conclusion.
It's well know that native species in an 'invaded ecosystem' end up in what are called refugia, places where they can survive and thrive, potentially. However these refugia become disconnected from other pockets of native vegetation and may become quite small. Gilbert and Levine found that as these refugia decreased in size there was likely to be less seed production and increased local extinction. Extinction is also more likely because there are less opportunities for the species to disperse into new areas.
Because of the relatively short time since large-scale invasions have occurred in most parts of the world it may be too early detect the real impact of weedy plants on native habitats. It may be 'in the order of hundreds of years' before the changes are obvious.
The nuances are different, I think, between places like the UK where gardening in its broadest sense (including hedgerows, clearing forests) has radically changed the landscape, and Australia where there are vast tracts of landscape with relatively* few human interventions (the most dramatic until recent centuries being the use of fire by Aboriginal communities).
We might also look at the time scales involved. In the UK there is already many hundreds of years of intervention, meaning the extinctions and impacts may already be manifest. In Australia the fire impacts go back tens of thousands of years so no shortage of time for them to influence the flora, as they have clearly done. So in both examples the cat is probably out of the bag. But then what about the more intensive clearing and weed dispersal going on in Australia today? That has a history of only a couple of hundred years at most. The extinctions in this case may be ahead of us.
Then there is the climate change argument. If we value particular species or communities we may have to assist them into new habitats where inevitably they will, if they thrive, send other species into refugia...and so on. Which species are important and which aren't? That old chestnut.
It's certainly a complex business. I would recommend a cautious approach. Where something is likely to result in a change that is difficult or effectively impossible to reverse or the benefits of that something are small compared to the potential damage or it would be easy to not do it or to stop it, then don't do it. Sometimes, however, we may want to manipulate things a little to look after something particularly precious to us as humans, or better still, Earth itself. Sometimes.
*Since posting this story, one of my Sydney correspondents, Rick Kemp, has pointed out quite rightly that vast areas of Australia have been transformed by agriculture and land clearing. The term 'relatively' doesn't really explain what I mean here, which is simply that there are still areas in Australia - albeit mostly those of little economic value (as Rick reminded me) - that we can call wilderness. These areas experiencing only a light touch by humans and largely absent from the UK. Here humans have been crawling over an manipulating, often in very beautiful ways, almost every inch of the land. My Canberra colleague Jim Croft was more concerned with my apparent championing of individual species conservation at the expense of habitats and systems. On that score I should say that my personal view is that we looking after individual species is driven by social not scientific values. That may be OK but we need to be transparent about our reasons, and careful, if we go down that road.