What impact do non native species have on local wildlife?

Will ring necked parakeets – now a common sight in west London – spread to the South West? And what species will suffer as a result?.

I need to come clean - I lead a double life; Monday to Friday I am the dweller of hedgerows, dipper of ponds, no rockpool is left unexplored, no stone left un-turned. No tree or bush is left un-scanned by my binoculars in the search Devon’s wildlife.

At the weekend I regress to an urban feral state, I retire to the city and to sport. Here I am, Saturday morning, well before kick-off at Twickenham with screeching calls of South Asian parakeets scything through the air above my head, some distance beneath the final flight paths of massive aircraft on approach to their Heathrow terminus.

It’s the final home fixture of the Six Nations for England and the captain Owen Farrell has been dropped to the bench. Massive news, but in my other world news broke this week of even more leviathan proportions.

The 2020 Plant Atlas has been published this week with the chilling revelation that non-native plants now outnumber our native wild species. Of the 3,445 different plant species recorded in fieldwork to research the atlas, 1,692 are native to Britain while 1,753 non-natives were found that have been deliberately or accidentally introduced into the wild by humans.

So what? I hear some of you cry. Let’s take a closer look.

Himalayan balsam is a pretty plant which is familiar to many of us who enjoy summertime riverside strolls. Its pastel pink blossoms are abuzz with bumblebees in late summer, clamouring to drink the plentiful, easily-collected nectar. So far so good, and the palatability of balsam for bees is often referred to as a positive for the plant and a reason to tolerate it in our countryside. So why then do I spend days of my summer and hundreds of hours of volunteer time removing it from those riverbanks I have responsibility for?

Himalayan balsam has no native insects which feed on its vegetation and is a rampant plant, able to spread at a very rapid rate. Left unchecked it quickly fills the riparian corridor, swamping native water plants and severely denuding the quality of the habitat. It’s not just plant species that suffer; bees preferentially forage from it meaning other plants go unfertilised. By outcompeting grasses, rushes and sedges along the riverbank water voles have nothing to feed on in balsam-dominated watercourses and are forced to move on in search of more hospitable habitat.

Rhododendron is a plant I go in to battle against every winter and I have developed a healthy loathing for it. A horticultural escapee, rhodi shares many traits with balsam, but it’s thick, chocking evergreen canopy so highly valued for garden greenery, effectively poisons other plants from growing around it and dominates an area absolutely. Next time you see a large rhododendron bush, or laurel for that matter, stick your head in and have a look at what lies beneath. For the amount of other organic life, the area may as well be concreted over.

And it’s not a problem reserved solely for plant species. Any time people decide to move something from its native area, where it has evolved over millions of years amongst the wider ecological assemblage, with their myriad checks and balances, problems almost always follow.

American mink are a mammal species closely related to stoats and polecats and, in their native North America, are a miracle of nature to be admired and valued. But their presence in Devon, the first county in the UK to record them as a breeding species in the 1950s, has been nothing short of catastrophic. There is no similar predator in this country with the same suite of abilities and so our native fauna has not evolved to cope with such a supreme hunter. Water birds and our old friend the water vole have all suffered huge population losses in the last 70 years due to this mammal and it is only the long-overdue return of otters to our rivers which is having a serious positive impact; otters, for whatever reason, will not tolerate mink in their territories and their smell alone is enough to make the mink move on.

So we return to my West London perch, sat in the stadium waiting for doors to open. The parakeets are still skimming overhead, long tails streaming out as they chase each other in an emerald blur. They are still a novel encounter for me and so I find myself smiling at their acrobatics, but what impact are they having? What food resource are they so effectively cornering? What hole-nesting species are they pushing aside as their population grows? Time will tell, but it’s unlikely to be a happy conclusion we draw.