6 days ago
Even 'pretty' invasives are not welcome for very good reasons
Himalayan balsam is a beautiful flowering plant, introduced into ornamental gardens for its vigorous growth and abundance of bright pink, bell-shaped flowers through the summer.
But having escaped in to the wild, Himalayan balsam has now conquered huge swathes of river banks, parks, gardens, and waysides, leaving no room for native wild plants or their associated butterflies, hoverflies, moths and more.
One of the especially problematic aspects of Himalayan balsam is how it dies back each autumn, leaving only bare soil in its wake. This leaves river banks vulnerable to erosion — excess sediment enters waterways, clogging up riverbed habitats and the spawning beds for salmon and trout, causing significant problems for aquatic wildlife.
This is just one example of how ecological and economic damage can reverberate through ecosystems when invasive species become established.
In order to be officially categorised as an invasive species, a plant or animal must be 'alien' to the territory, capable of establishing viable populations, and have significant impacts on biodiversity, ecosystem services and human health or economic activity. According to the National Biodiversity Data Centre, there are 340 potentially damaging invasive species here in Ireland, spanning both terrestrial and aquatic plants and animals. Of these, about 60 are 'high impact' invasive alien species.
High impact invasives include rhododendron ponticum, cherry laurel, and fallow and sika deer — all of which cause significant harm in woodland habitats. Japanese knotweed is a well-known high-impact invasive, a massive headache and economic burden for homeowners, local authorities and others. Brown rats are a high impact invasive species too, a danger for ground-nesting birds and especially for coastal colonies of seabirds whose eggs and chicks can be obliterated when rats arrive.
Chinese mitten crab (Eriocheir-sinensis). Picture: AQUENS Ltd/
Potentially imminent arrivals such as the oak processionary moth, the Chinese mitten crab and coypu are on the national watchlist.
Preventing the spread of invasive species is hugely challenging, especially getting information out to people who are unknowingly spreading them.
Montbretia
I have neighbours who transplanted montbretia in to their boundary hedge, recognising it as a plant that grows easily and brightens up byways through late summer with its profusion of tall fronds carrying gorgeous orange tubular flowers. They simply didn't realise that montbretia spreads uncontrollably, taking over verges and even whole gardens within a few years. It is an alien invasive of the highest order, a man-made hybrid plant that smothers diversity and leaves no room for others.
In this island of lakes and rivers, aquatic invasives are especially problematic
Waterway users are often the unknowing vectors of problematic invasives. Zebra mussels, arriving originally via the ballast water of ships, have now established themselves all over the Shannon and Erne waterways, displacing native invertebrates from their range. Several species of invasive waterweed, escaped from garden ponds, have been spread by recreational boaters who may forget to clean out the engines properly when they move the boat from one waterway to another.
Freshwater crayfish, a species that looks a little like a mini-lobster, are a staple food for our native otters and other aquatic animals. Several of their populations have been wiped out in recent years due to outbreaks of crayfish plague, a fungal infection spread by imported spores that are transported on canoes, angling equipment and suchlike. For this, regional campaigns to limit the spread of crayfish plague urge waterways users to always 'check, clean, and dry' all equipment.
When enough people are aware of the threats posed by invasive species, and can identify at least some of them, managing their spread becomes more feasible. There are lots of actions that individuals can take to help prevent their spread. Last week I went to meet with a group of volunteers in County Clare, part of the 'Field Good Fridays' initiative run by the Common Knowledge Centre — a not-for profit social enterprise. The volunteers were spending the day removing invasive Himalayan balsam from the banks of a stream that crosses the site, learning some of the skills to bring back to their communities and tackle invasives there.
First, there were protocols to follow. As with most invasive species, knowing the correct procedures for their removal is crucial, lest the interventions turn out to be counterproductive. Himalayan balsam, for example, has explosive seed-pods. When ripe, these seed pods fire out seeds far and wide as soon as anything brushes off them. Hence the plant can only be tackled before the seed pods develop later in the summer. Once briefed, the team of volunteers broke the stems beneath the lowest leaf node, preventing the flowers from developing into seed pods and thus limiting their spread.
Oak processionary moth
One invasive that hasn't managed to get a foothold yet in Ireland is the oak processionary moth. Caterpillars of the oak processionary moth can strip oak trees almost bare of their leaves, and are a hazard to humans too because the caterpillars' hairs can cause nasty skin rash or even conjunctivitis.
Oak processionary (Thaumetopoea processionea) caterpillars on the move on a tree
It was seen in a park in Dún Laoghaire in 2020, duly reported, and quickly dealt with by the authorities. It was seen again in 2023 in a housing estate near the Phoenix Park in Dublin. Members of the public who spotted the characteristic procession of the hairy, grey caterpillars reported them immediately to the authorities, and the Department of Agriculture, Food, and the Marine promptly dealt with the invasion, eradicating the caterpillars' nests and assessing a three-kilometre buffer zone for spread.
As far as we know, these colonies have all been removed and the threat of an influx of these harmful invasives has been kept at bay, for now. It is likely that the infestations came in on sapling trees from abroad, which is another reason why tree planting schemes should always seek to source homegrown saplings from native stock.
Coypu in Cork
Coypu are an invasive rodent native to South America, now spreading across Europe. Measuring one metre from head to tail tip, coypu live in and around rivers, lakes, ponds and drainage ditches, where they chomp through huge quantities of vegetation and impact aquatic birds by decreasing nesting habitat, as well as causing economic damage by eating crops. Coypu have been seen and subsequently removed from both the Curaheen River in Cork city and the River Lee. Efforts to ascertain whether coypu are still present in and around Cork city are ongoing, with no known sightings since 2017. If you think you see a coypu, report it urgently to
To help prevent the spread of invasive species, there are a few simple steps that everyone can follow. Never dump garden plants, including pond or aquarium plants, in the wild. If you have invasive plants such as montbretia, cherry laurel, rhododendron, or gunnera in your garden, get informed about the best way to eradicate them, then take action. If you are someone who enjoys fishing or getting out in boats or canoes, learn the codes of conduct to keep aquatic invasive species in check, especially the procedures of 'check, clean, dry'.
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