Tag Archives: invasive non-native species

Riversearch June 2016: Alien invader – Japanese Knotweed

The main challenge with my Riversearch survey in June was seeing the river – since my previous survey, the plants along the riverbank. have shot up. There were stinging nettles taller than me, and inpenetrable thickets of bramble blocking me from getting close to the river in many places. Still, I did manage the occassional glimpse of the river – enough to see that, though the river level was normal, it was still quite turbid.

Intriguing holes in the riverbank

There weren’t any particularly exciting wildlife sightings to report, although I did spot some intriguing holes.


I did see a couple of invasive non-native species –  Himalayan balsam, as usual, and a probable sighting of Japanese knotweed. This is the first time I have spotted Japanese knotweed along by the river, and I had to use binoculars from the opposite bank to see it, so I’m not 100% sure about my identification. But I’ve shared the photos with the wildlife trust, who seem to think it is knotweed. I don’t know if it’s new here, or if I spotted it this time and missed it previously because I was doing my stretch in the opposite direction to normal. Anyway, that’s now been reported to the National Trust, who own the land, so hopefully they’ll be able to sort it out swiftly.

Japanese knotweed?

Japanese knotweed doesn’t look particularly startling (unlike Giant Hogweed), but it can be a big problem, spreading quickly and hard to get rid of. In urban areas it can grow up through patios or conservatory floors, so it’s not something you want in your garden. In the countryside it can quickly overwhelm native species.


Alien invader taking over River Mole

The day had been sweltering and, even in the relative cool of early evening, people were picnicking on the river bank and kids splashing in the water. They had no idea that just metres away lurked an alien invader, laying siege to the River Mole, and threatening to take over completely.

Like Scarlet Johannson in Under the Skin, this alien’s danger was disguised by a pretty face. The pink flowers of Himalayan Balsom distract from its invasive nature. The plant can grow up to 2.5m tall, and forms dense swathes. When the seed pods are ready they explode dramatically, firing seeds into the river, where it can be swept for miles downstream, before settling on a new stretch of river bank.

Himalayan Balsom
Himalayan Balsom

As well as crowding out other species of plant, the shallow roots of Himalayan Balsom can cause erosion of riverbanks. Thick stands of it may also increase the risk of flooding.

Luckily, although it spreads quickly, it’s easy to pull up. Further downstream in Leatherhead volunteers with Surrey Wildlife Trust have done quite a lot of clearance work. But because it spreads downstream so easily, it’s hard to eradicate it permanently from a small stretch of river.

Himalayan Balsom is one of the non-native invasive species I look out for during my Riversearch surveys. And unfortunately I found quite a bit on my latest survey.

Once again the river bank has changed dramatically with the seasons. This time much of the river was inaccessible, as shoulder high nettles formed a barrier I wasn’t willing to test in light trousers and short sleeves.

The river level was low, and apart from the Himalayan Balsom there was little new to report. I did find a rather intriguing stash of fruit between some tree roots. Any idea what could have hoarded that?

Hoard of fruit hidden under tree roots in river bank
Whose secret stash is this?

Alien invaders 1: Canada Geese

Canada GeeseEach day I walk through the park, past the mill pond, and nod good morning to one particular goose. He’s quite distinctive, with the white marks on his face. He’s the patriarch of the community of Canada Geese on the mill pond. While his many goslings are growing up, he tirelessly watches over them, protecting them. (Unlike the ducks, which take a rather more laissez faire approach to parenting, letting their tiny ducklings zoom off or get left behind.)

Over the years I’ve grown affectionate towards him – you’ve got to admire that kind of devotion. A couple of years ago a tree fell right on top of the nest his mate was sitting on, yet miraculously she and the eggs survived, and together they brought up the large brood. I was rooting them on, each morning anxiously counting the number of goslings to make sure they were still surviving.


So it came as more of a surprise to me than it should have, when I learned that they count as an invasive non-native species. I mean, the clue’s in their name: Canada Geese. But I still find it hard to think of them in the same category as Japanese knotweed. According to the GB Non-native species secretariat, an invasive non-native species is any non-native animal or plant that has the ability to spread causing damage to the environment, the economy, our health and the way we live.

So, are Canada Geese a problem? They’ve lived in the UK since they were first brought from North America to St James Park back in the 17th century. But their numbers have increased hugely over the last 60 years, going from 4000 in 1953 to around 89,000 in 2000. They are viewed by many as pests – they make a lot of mess on footpaths in parks, and there are concerns they may spread salmonella to cattle. Canada Geese who are nesting or looking after young can be aggressive towards people (which can be a problem as they seem to like living in public parks, bringing them into close contact with people, particularly children). Their droppings may increase the nutrient content of water, which reduces oxygen content for fish. And around airports there have been problems with damage to planes and people when they collide with Canada Geese. The main issue is they are so numerous…

In fact, in some places the eggs of Canada Geese are treated so they will not hatch, to try and prevent them from expanding even more. So while I was rooting on each of the goslings, should have been hoping the reverse? I try and be fairly unsentimental about my love for wildlife. But I can’t yet find it in my heart to feel anything other than affection for at least this one particular Canada Goose, alien invader though he may be.