Should we worry about non-native plants?
Environet UK Ltd
Leaders in the effective management and removal of invasive plants through eco-innovation.
Our industry specialises in removing non-native invasive plants. But should all non-native plants be treated as a problem because they affect and dilute our native ecology, or do we need to embrace plants from further afield that thrive in our ever-changing climate that are more resilient and may become the “native ecology of the future”?
In this article, we explore what we mean by non-native, before going on to what we believe to be the real issue: how invasive a species is, what may come with that species, and why this is a problem for the biosphere.
What is a non-native plant?
At first glance, this seems like a simple question to answer – however, there are two different date markers widely accepted, which causes a “grey area” of established species in the UK that are considered “native” but in fact, have their roots elsewhere. We have a long history of movement and trade in plants dating back centuries albeit limited until around 1500AD, which is commonly thought of as the dawn of true international trade. But there’s lots of evidence of human movement of plants and animals before that so – in scientific terms, it’s more common to think of true native species as those present at the end of the last ice age when the UK lost its land bridge to the continent.
Putting aside this disagreement over “cut-off” dates, all non-native species have one thing in common – that is they have been introduced either intentionally or accidentally by humans to a new location, as opposed to a “New” species which refers to the natural shifting of species distribution over time in response to changing conditions, or movement without human intervention.
Native plants have co-evolved with other native species, including animals, fungi, and microorganisms, forming intricate ecological relationships, and playing important roles in the functioning of their respective ecosystems. They are typically well-suited to their native environment and often provide essential habitat and resources for native wildlife. Non-native species on the other hand often lack these relationships, but survive here based on us already having, or indeed creating similar environmental conditions to their native range.
A plant’s ability to simply survive, or thrive, underpins what is probably the most important consideration – how aggressive, or invasive it is in the host environment.
What makes a plant invasive?
In order to be classed as an invasive species, a plant must negatively impact other organisms or environments, usually typified by dominance in a given environment, out-competing other flora (so there ends up being a disproportionate amount of one species) resulting in a lack of balance and overall reduction in biodiversity and speed of spread – maybe through seed distribution or fast-growing roots.
These characteristics are often a result of a plant being translocated out of its native environment where it has evolved in an intricate food web – (controlled by predation, soil condition, climate, competition, and disease) into a habitat where one or more of these factors are absent. ?But – NOT all non-native plants are invasive – and, in a world where truly “wild” habitats are ever scarcer, many native plants (aka weeds!) are equally invasive!
What are the problems caused by non-native plants?
Problems caused by non-native, and particularly invasive plants, are far-reaching and must not be underestimated in an environment that is already under intense pressure from agriculture and climate change. While the effects of slow-spreading species may be negligible, the problems are magnified in those more invasive cultivars.
Species invasions are one of the biggest causes of ecosystem change. More than half of UK native plants are in decline, while many introduced species are expanding their populations.
- Decline in “unique” habitats - As we allow more species to be introduced, we lose the “uniqueness” of our habitats, dubbed the “McWildlife” phenomenon. ?
- Diversion of pollinators - Many non-native plants serve as excellent nectar resources for pollinators like bees (e.g. Himalayan balsam). While bees are targeting the readily available nectar from these alien invaders however, they are neglecting our smaller, harder-to-reach plants – which then miss out on pollination – affecting their survival success.
- Reduction in local biodiversity. Where non-native species outcompete native plants, their numbers are reduced or even wiped out in localised areas. Furthermore, the other plants, fungi, and animals associated with that missing species also decline or disappear.
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- Some species, like rhododendrons, can effectively poison the surrounding soil, reducing microbial and insect populations around the plants. It has also been attributed to poisoning of bees.
- Others, like the annual Himalayan balsam, leave vast areas of bare soil on riverbanks during winter, leading to erosion and flooding that would otherwise be prevented by the roots of perennial plants.
- Some tree species like Sitka spruce – although commercially important in the UK, cover vast swathes of land, yet support little native ecology. Incorrectly managed, they also encroach on native heath and moorland.
- Invasive plants not only disrupt plants and animals, but they also impact our built environment. These pioneer species are fast-growing, hard to get rid of, and often have adaptations that mean they can grow in the most challenging locations.
- Japanese knotweed, bamboo, and buddleia cause millions of pounds of damage to UK buildings and infrastructure each year. The increasing prevalence of our native Field Horsetail is also becoming problematic.
- The importation of plants carries a risk that may result in far greater damage than the plants themselves, and that is the introduction and spread of pests and diseases. The battle in mainland Europe with the Xylella bacteria is a devastating example. The bacteria, originally from the South Americas, spread by spittlebugs, is responsible for the death of over 21 million olive trees in Italy over a few short years. ?
- Managing weeds in agricultural systems has long been a challenge for farmers. In an industry where there are increasing pressures from climate change, pests and diseases, coupled with the desire to reduce our dependency on chemicals - the added threat of invasive plants and their corresponding pathogens is an unwelcome one.
How might non-native plants be helpful?
The perceived value of non-native plants has varied wildly over human history. As we have already established, plants were imported and traded over centuries, as sources of food, fuel, and materials with empires rising and falling as a result. Because the pendulum has now swung in the opposite direction, with the introduction of non-native plants largely seen as damaging, we mustn't underestimate what role they may play in our uncertain future.
As climate changes, habitats can be dramatically affected, resulting in declines in native plant populations. We may find, that in order to sustain pollinator populations and diverse, functional ecosystems, we will once more value the introduction of non-invasive, non-native plants. It has already been shown that some non-native plant species increase biodiversity within defined habitats.
Research carried out in 2023 showed that there are now as many species of non-native plants in the UK as there are native ones, and the variety of UK flora growing in the “wild” has increased by 100% in the past 100 years. While many ecologists, botanists, and invasive plant specialists would argue that the current list of “Invasive” species falls massively short – we’re certain that they would not condone adding every non-native species to the list!
Lastly, there is also the argument that non-native host environments may become important in saving some plant species. This year scientists discovered that Giant Redwood trees were thriving in the UK. These enormous trees number around half a million here and can live for over 2000 years. Climate change has been putting increasing pressure on the species in California, where numbers have dwindled to around 80,000 mature trees. This is an interesting example of a globally important species that is arguably being conserved in a non-native environment.
So, should we look to eliminate all non-native plants from the UK?
In short, no – by far the most important assessment criteria is whether the plant has the capacity to be invasive and whether the species harbours any natural pests or diseases that might affect our native biology. There is now recognition on a national level that prevention is by far the best management strategy. Organisations such as CABI & NNSS have already started to monitor the distribution and impact of plant species that are introduced, and complete risk assessments, based on their likely impacts. They are also looking at what is not yet here, but maybe our European neighbours are struggling with and lobbying for action before the species get here. Treating species as guilty until proven innocent, in a similar way to New Zealand and Australia may prove an effective way to manage the future ecology of our fragile island.
We also need to take collective responsibility to preserve wild spaces and protect them as far as possible from the introduction of non-native plants, especially those that pose a high risk of altering the balance of habitat and eliminating as far as possible invasive species from these spaces.