Why we should love invasive species. Or not.July 8th 2015
There is a trendy argument doing the rounds – possibly based on a certain recent book about why invader species could be nature's salvation. I haven’t read the book, and I won’t name the author, or criticise it – because the argument put forward is no doubt more nuanced than portrayed in the headlines.
However, I have come across people genuinely arguing that ‘invasive species represent evolution’, that they are ‘better suited to the habitat than native species’ and have read headlines that suggest that we should learn to embrace all invasive non-native species.
It is these ideas that I am looking to challenge – particularly with regard to invasive plants.
Firstly, it’s important to distinguish between native- and non-native invasive species. Our native invasive plants (like bindweed, bramble, nettles and bracken) are very well adapted to the conditions that they live in, having developed traits which make them successful in our environment.
Although they are often hard to get rid of, tend to crowd out or strangle other plants and to form monoculture stands, they also generally support a variety of insect, bird and/or animal species which have evolved alongside them, providing food, or habitat, or both.
Non-native invasive plants have evolved to be successful in their native environments – but in most cases, they are not successful enough to become invasive in their native range. They are just one plant among many others, competing to survive under pressure from competition and more significantly from predation by insects, fungi, bacteria and viruses which exist in the environment they evolved in.
Once the plants are introduced to a new environment, they are often removed from the pressures of the predators in their native habitat, they effectively have a head-start on all the other plants around them. Native insects, animals and birds are rarely adapted to the habitats the plants provide and (as is the case with species like rhododendron), the plant soon becomes the only living thing in the area.
Of course, there are exceptions; in fact, many non-native species are affected by similar parasites to those found in their native range – which is a reason why they don’t go on to become invasive. Some of our native fauna can interact even with the most invasive of alien species – bees love Himalayan balsam, for example. Of course, this works in the plant’s favour – as without effective pollination to produce its seeds, this annual plant could not survive.
In the very long term, however, parasites will evolve to predate Japanese knotweed, giant hogweed and Himalayan balsam, and it’s conceivable that they could return to being very niche plants, rather than the widespread invaders that they are today.
Introducing non-native species is not evolution as we know it – and evolution will eventually catch up with these species – but in the meantime, the majority of them offer our native fauna little or nothing that isn’t already available from native plants, and they compete with our native flora to the point where valuable habitats and biodiversity are lost, as well as knock-on and trickle down effects on our waterways, embankments and things like flooding.
To argue that we should embrace the grey squirrel is not something I would particularly disagree with – although I wouldn’t oppose conservation efforts for red squirrels.
Humans (and probably pretty much everything else in the British eco-system) can probably adapt to what is essentially the like-for-like swap of one squirrel species for another. An evolutionary niche has been filled, and things are more-or-less as they were before.
That may or may not be the case for things like signal crayfish, and killer shrimp, which are able to predate on different species to their closest native relations. However, in these cases, it’s still conceivable that the food chain would be kept more-or-less in-tact – with predators and prey of the substitute species remaining in a position where they can all eventually develop and maintain fairly stable populations.
However, for zebra mussels or quagga mussels, which are capable of multiplying prodigiously, occupying large amounts of space and surface area and removing significant amounts of food which would otherwise have been available to a wide range of other species, we see a more problematic set of circumstances.
These circumstances are further enhanced where plant species are concerned, when physical space (soil) becomes the prime factor for competition. Invasive plant species are not “filling an ecological niche” by outperforming a single equivalent species; they are directly competing for space with all sorts of different native plants –which often have complicated interlocking relationships with the wider ecosystem. In this case, the successful invaders are often basically destroying habitats in favour of a monoculture which provide no food or shelter for native species.
Other native insects and animals further up the chain who relied on the aforementioned species are displaced, and the effect on the wider ecosystem is significant.
So when I see simplistic arguments (especially when they say invasive species are good) I am immediately sceptical. While humans may well be able to adapt in limited, short-term ways, we are heavily reliant on our existing ecosystem, and it’s a bit of a push to expect UK wildlife to undergo millions of years of evolution because of human activity – and it’s completely ignoring the issue of biodiversity loss, which is the principal problem with the most problematic invaders…