Everywhere on Earth has its own wild species, which evolved in that place and are adapted to local conditions and to each other. They live in natural ecosystems in which every living thing has a certain role. But by accident or on purpose, people have moved many plant and animal species from wherever they naturally occurred to new locations.

Some of them take so well to their new homes that they are able to multiply and spread there, with dire effects. Perhaps they can grow faster or earlier in the year than local species, or steal their space and nutrients, or interbreed with them, or eat, strangle, poison or infect them. Introduced species that have impacts of this sort are known as ‘alien invasive species’ or ‘invasive non-native species’, or just ‘invasives’.

But whatever you call them, invasives are a menace. They threaten the health and integrity of nature reserves, botanic gardens, farms, forests and other ecosystems that we are trying to protect and manage. The result is that invasives are among the five most important threats to biodiversity world wide. The others are the conversion of natural ecosystems into plantations or suburbs, the over-exploitation of wild organisms by fishing, logging and hunting, the direct and indirect effects of climate change, and the alteration and pollution of natural habitats.

All five of these processes interact with one another. Over-exploited, damaged and polluted ecosystems can become so weak that they offer new opportunities to invasives, including weeds, pests and diseases. Also, climate change is shifting seasons and making conditions warmer, wetter or drier than they should be at different latitudes and altitudes. Invasives are able to exploit all such opportunities.

Scotland is a country where natural ecosystems have been massively disturbed by forest clearance, farming, muirburn, wetland drainage, grazing – you name it – for centuries. It also shares a long border with England, so Scotland is vulnerable to alien invasives that travel by road, foot or wind, and those helped by the northward spread of a new climate.

The Scottish government, now with added Green ministers – including Lorna Slater, the minister for biodiversity – recognises at least some of the invasive threat and has been funding anti-alien efforts, often in collaboration with charities such as the Scottish Wildlife Trust. On invasive mammals, they have been supporting efforts to slow the advance of pox-bearing American grey squirrels (Sciurus carolinensis) into the remaining heartlands of the native red (S. vulgaris), and have targeted the predatory American mink (Neogale vison) along several rivers.

For the plants, Scottish government (and charity) efforts have focused on a few prominent invasives: Himalayan balsam (Impatiens glanulifera), giant hogweed (Heracleum mantegazzianum), Japanese knotweed (Fallopia japonica), American skunk cabbage (Lysichiton americanus), and the pervasive Rhododendron ponticum, with its purple flower-patches now in forests across western Scotland. All these will be familiar as dangers to horticulturalists from the RHS, Woodland Trust and many other web-sites.

Meanwhile, in the north of the country, the Scottish Invasive Species Initiative is trying to keep track of alien invasions, sharing data with UK-wide efforts and contributing to Scottish and national strategies for biodiversity management. But the scale of the problem is actually much bigger that all this activity suggests. A case in point is the few-flowered leek (Allium paradoxum). These photos show them running rampant in April in Rosslyn Glen.

When I moved to Edinburgh at the start of 2020 I began to explore the city and its surrounding Lothian districts. I soon noticed that in nature reserves, woodlands and parks everywhere there were large areas of one single species instead of the normal diversity of native woodland plants. They were dreary, smelled vaguely of onions or chives, and were growing densely across the woodland floor, on river banks and islands, even up cliffs. The bluebells, primulas, snowdrops and wild garlics had simply been replaced.

I learned from the local flora that this plant was the few-flowered leek, that it had arrived from the Caucasus in the 19th century and had become quite widespread around Edinburgh by 2002. But it had clearly found its feet since then, perhaps helped by climate change as it seems to produce its few and ugly flowers earlier than its native victims. And it seems to spread easily by means of sticky bulbils that get picked up by passing people and animals.

I found that nature reserve managers and botanic gardeners were well aware of the threat, but that none of these specialists had enough resources to resist its spread. The ecology of the species had never been properly studied, so no one knew what its weaknesses might be. Control efforts, where they existed (such as in Dr Neil’s Garden at Duddingston), involved volunteers up-rooting plants by hand, but their effectiveness was unknown. Meanwhile the native woodland plants of Edinburgh and the Lothians are vanishing, and the public are unaware of the fact.

Alien invasives in Scotland deserve a much better-resourced strategy, informed by ecological and horticultural science and driven by public awareness and participation. It is clear that the few-flowered leek is a tricky opponent, robust, subtle and aggressive. Careful study will be needed to find out how best to defeat it.

It is also clear that this invasive alone can utterly transform the experience of strolling through a woodland in the spring, to the great loss of amenity, interest and beauty. Its impacts on the birds and insects that make these woodland ecosystems work are still simply unknown.

The decline of biodiversity in Scotland and everywhere else is going on largely in secret because few people are paying attention. Most wild species are unknown to science, and millions of them around the world are sliding quietly into extinction. Many will never even have the dignity of a name before they die. The progress of the few-flowered leek in Scotland stands for a whole process of global ecological decline, part of the climate extinction emergency that faces us all.

In Scotland there are at least a willing government and a huge number of people concerned about the environment. If the wonders of nature are to survive they must be educated and mobilised to pay attention and support action, even on the little things that are going on beneath our feet.

The educational potential of the horticultural community has a vital role to play in all this. Horticulturalists are on the front line, and should be taking every opportunity to raise awareness among public and politicians. They should also be supporting researchers and community groups against the threats to nature that are fast emerging. This is an emergency, and we should all act accordingly.

This article by Julian Caldecott was first published in Horticulture Week, 4 May 2022.