The rising threat of invasive species

Invasive species are considered one of the greatest threats to biodiversity, but controlling them is extremely challenging

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By Mark Rowe

Invasive species are the fifth greatest threat to global biodiversity, according to the UN Environment Programme (UNEP). While this places them some way behind the gravest threat, climate change, many scientists are troubled by a thought: what if the two combine, one reinforcing the other, creating an even greater danger? This isn’t a hypothetical question: those few scientists who’ve turned their attention to the potential for recombinant impacts are warning that this interaction is already well under way.

Invasive species – aggressive generalists that can thrive in a range of habitats – have long enjoyed the helping hand of humans. Either deliberately or unwittingly, the diaspora of European settlers in the 19th century and the acceleration of trade in the 20th century have introduced invasives to new ecosystems and helped them to overcome geographical barriers to their spread. Along the way, they’ve had an impact on native species and ecosystem functions, as well as economies and human health. Now invasives are hitching lifts on more frequent storms, floating on higher seas and thriving in warmer temperatures and in habitats where native species are under pressure from climate change. Sometimes, they reinforce climate change: invasive plants can accelerate the frequency and severity of wildfires caused by droughts, on which they, in turn, rely to propagate.

‘It’s a sad story. Climate change is reducing the resilience of habitats to the extent that it’s easy to get spreads of multiple species,’ says Max Zieren, a senior environmental specialist for UNEP, Asia and the Pacific. ‘Biomes are not in a healthy state to cope with invasives and we humans have already introduced so many species into so many places. We’re seeing whole habitats move, not just one species at a time, and it’s the species that are generalists that can adapt, that will thrive. We are effectively opening the door for invasives, which can thrive in a wide range of soils and precipitation.’

Yet for now, the interaction with invasive species is ‘the least understood’ of the impacts of climate change, according to Piero Genovesi, chair of the IUCN’s Invasive Species Specialist Group. ‘But there is clearly a tight link between the two,’ he adds. ‘It’s a huge threat, particularly for forests and freshwater species.’

While some species move aggressively, behaving in the ‘classic’ manner of an invasive species, others merely seek survival, relocating to higher latitudes and the temperatures in which they’ve evolved. Those in this latter category, according to Zieren, are referred to as ‘sleepers’ and, he says, ‘it’s likely that they are spreading like hell’.

According to a 2019 UN report on biodiversity and ecosystem services, records of alien species have increased by 40 per cent since 1980 and nearly 20 per cent of the Earth’s surface is at risk of plant and animal incursions. More than one-third of all introductions in the past 200 years have occurred after 1970 and the rate of introductions shows no sign of saturation. The number of established alien species is expected to increase by 36 per cent between 2005 and 2050.

Few habitats seem to be escaping unscathed from the twin incursions of climate change and invasives. ‘Arctic zones are becoming more boreal, boreal are becoming more temperate, temperate zones are becoming more sub-tropical,’ says Jane Uhd Jepsen, senior researcher at NINA, the Norwegian Institute for Nature Research.

Some biomes, such as temperate forests and freshwater systems, that currently have thermal barriers limiting the establishment of invasives are steadily becoming more suitable for alien species. Environmental selection for traits that enhance reproduction in warming climates will enable range expansion of some invasive species that can, in turn, step into niches vacated by species forced to head ever further towards the poles. ‘It’s often referred to as bush encroachment, where many of our trees and shrubs are expanding their range into savannah and grasslands,’ says Arne Witt, invasives coordinator for the Centre for Agriculture and Bioscience International.

Of 100 invasive species studied to see how they responded to climate change, Genovesi from the IUCN found that those primed to benefit were aquatic invertebrates and plants, microorganisms and terrestrial invertebrates. Insects, in particular, are able to respond rapidly to temperature changes and have a high capability for dispersal. Fishes, mammals and reptiles appear to have maintained stable ranges, while amphibians, birds and fungi seem under pressure and are even shrinking their ranges.

Perhaps this is why Zieren notes how little global response there has been to the issue. ‘Invasives and the way they are driven by climate change aren’t sexy,’ he says. ‘It’s like the spread of a slow disease, of climate change itself. It’s a steady, sure progression, not immediately visible until it becomes widespread.’

As Witt puts it: ‘It’s not like they appear overnight. It’s a gradual invasion – these guys are stealthy critters, slowly creeping up on you, and before you know it, a whole landscape has been transformed into a monoculture, a green desert. Then it’s too late – much of the damage has been done. They will spread and establish in every suitable nook and cranny.’

Invasive plants will benefit from higher carbon dioxide levels and temperatures. Invasives, says Genovesi, may also reap the benefits that come with early blooming, such as shading out competitors and capturing a larger share of nutrients, water or pollinators. In laboratory trials, chromolaena, which is considered to be one of the worst weeds in Indonesia, benefitted significantly from increased CO2, increasing its height by 92 per cent and branch numbers by 325 per cent. In the real-world laboratory, in Australia, the invasive shrub Acacia nilotica is already thriving thanks to increases in water-use efficiency as a result of increased CO2 concentrations, allowing it to invade drier sites. In time, increased temperatures are expected to allow it to complete its reproductive life cycle in areas that are currently too cool for it to sustain populations.

No biome is safe

Changes in weather patterns and increasing temperatures enable invasives to take advantage of the struggles facing native species, according to Witt. ‘Increased disturbance reduces resistance in existing [flora and fauna] communities and creates enemy-free sites for the establishment of invasive plants. ‘Invasive species may be able to expand their range to higher altitudes, areas that are often the last refuges for endangered and threatened species because these areas have been less affected by human activities in the past,’ he says.

According to the IUCN, the biomes that are most vulnerable are temperate deciduous forest, warm mixed forest, temperate mixed forest and tropical forest and woodland. Numbers of invasive alien species tend to be higher in the Northern Hemisphere compared to the Southern and to decrease at lower latitudes, according to Genovesi. ‘Climate change is likely to cause drastic species range shifts,’ he says. ‘Given the harmful impacts these invasive species currently have on ecosystems, these species will likely dramatically influence the future of biodiversity.’

Genovesi has identified three future invasion hotspots: the eastern USA, northeastern Europe and southwest Australia, New Zealand, the Indonesian and Pacific islands. He believes that more than 60 invasive species could become established in each of these regions by the end of this century. The Arctic has historically been presumed a lower-risk region for biological invasions and a barrier to their global expansion due to limited access, harsh environmental conditions and inadequate food resources that hinder dispersal, survival, growth and/or reproduction for many species. Yet it appears that no biome or ecosystem will be exempt.

In one of the most vivid examples of climate change and invasives coming together, molluscs and other marine creatures are hitching a lift on the hulls of the cargo vessels that can now navigate the once-frozen passages of the High North. Such creatures have long done this, of course, but the concern is that not only do such creatures have a greater chance of survival in a journey from the eastern hemisphere to the west because of the shorter journeys, but multiple introductions simply improve a species’ odds of gaining a toehold along the way.

Yet invasives are now not only able to transit the Arctic but to establish themselves there. According to NINA, the region is now under unprecedented threat of biological invasions due to climate warming and increased human activity. Retreating sea ice is opening up suitable conditions for a range of more southern species with negative consequences for the native biota. Molluscs and fishes, for example, may spread from the Pacific across the Arctic to the Atlantic Ocean under warmer climates.

Blue mussels are still relatively rare in the high Arctic since they can’t survive exposure to air temperatures below –13°C. Yet they’ve become much more common in the Arctic in recent years. Historically, the Arctic has experienced such temperatures on at most 200 days a year, but since the 1990s, the number of days has fallen by 57 per cent in Greenland. The concern, according to the Arctic Research Centre at Aarhus University, Denmark, is that blue mussels and other invasives will outcompete rival native species for food.

In the North American Arctic, scientists have already seen the first signs of dwindling food for native eider ducks. Their preferred prey, Macoma calcarea, a type of mollusc, is being replaced by a poorer food source – a smaller mollusc with a thicker shell. Similarly, puffins in Norway have abandoned their eggs and allowed their young chicks to die following an invasion of mackerel, which eat the same food as the young puffins and cut off their food supply.

While scientific consensus has broadly anticipated a greening of the Arctic triggered by milder temperatures, studies now suggest that in sub-Arctic forests, the opposite is occurring: a ‘browning’ of the Arctic is being observed as the forests become less green. ‘We believe this may be related to increased pressure on the forest in certain areas,’ says Jepsen. ‘More-intense insect infestation, more forest fires and increased drought pressure are probably the three largest factors causing this sub-Arctic browning. We believe these factors may reinforce one another.’

Not-so healthy trees

Zieren has observed similar impacts in tropical rainforests to those described in the Arctic. ‘I look at what appear to be healthy trees in Indonesia, but it’s not the case. The huge trees are disappearing in places such as West Java. This is because the understorey is populated by invasive species and the big trees are outcompeted – they can’t survive.’

Yet it isn’t just charismatic hardwood trees that feel the pressure – huge areas of grassland also appear to be changing as a result of the interaction between climate change and invasives. In India, the IUCN threat level to Asian bird species such as the lesser florican, a small bustard with a distinctive twisted moustache, has been upgraded. The bird inhabits lowland grasslands in western and central India. Here, large areas of natural grassland are being converted to agricultural land and remaining patches are being degraded by overgrazing and invasive plants. Rainfall patterns are also being disrupted by climate change.

In the USA, purple loosestrife – a European import widely planted during the 19th century for medicinal use – blooms 24 days earlier in some areas than it did a century ago and so is expanding its range and, in effect, over-staying its welcome. In the UK, where purple loosestrife has a long-established evolutionary niche, it can be an attractive plant forming clumps of dense strands and is an important food plant for butterflies and moths. In the USA, it chokes wetland habitats by crowding out cattails and other marsh plants that serve as food, shelter and nesting material for wildlife.

Grasses are exceptionally successful world travellers, particularly in livestock-based economies, as the UK is discovering. Of the 580 species of alien grass in the British Isles, 430 are believed to have been brought here in imported wool, 95 in imported grains and seeds, and only 55 as horticultural introductions.

Food and farming impacts

It has long been recognised that invasive species impose huge costs on farming and wider economies. A report by CABI puts the total economic costs of invasive plants and animals in the USA at US$137 billion and in Southeast Asia at US$33.5 billion a year. Unsurprisingly, the greatest burden falls on developing countries, which have high levels of subsistence and smallholder farming, and often lack the capacity to prevent and manage biological invasions. This, in turn, leads to altered foodweb dynamics and decreasing productivity. In South Africa, 15 per cent of the grass genera and 12 per cent of grass species are now naturalised aliens.

In the Horn of Africa (principally Ethiopia), the invasive grass Prosopis juliflora has taken over millions of hectares of grass savannah that was formerly livestock or grazing country. The government in Ethiopia has put this high on its agenda and organised a nationwide campaign to reduce the spread of invasives in a changing climate. ‘Millions of hectares of grasses are now a feature on savannahs. Imagine the reaction if that spread across agricultural land in Europe,’ says Zieren.

In the future, as temperatures continue to rise, many introduced crop pests will become bigger problems and many pests that were relatively benign may suddenly become a serious threat. ‘With increased temperatures, an insect pest may now have two or three generations in any one cropping season, when previously it may only have had one,’ says Witt, ‘so we are heading towards interesting times.’

Then there’s the farming response to climate change. New varieties of pasture plants that grow quickly and can cope with varying weather conditions are being developed, either through GM technology or crossbreeding. Yet, as Witt points out, these are exactly the kinds of traits that can make a species aggressive and may inadvertently facilitate further invasion.

Time for a rethink?

Clearly there’s a distinction to be made between species that grow and reproduce quickly and spread aggressively, with potential to cause harm – the classic ‘invasive’ – and those that are given a greater opportunity to move thanks to climate change. Native species are likely to shift their geographic ranges into novel habitats and some may come to be seen as invasive. As climate change pushes species farther north, south and higher, is it time to rethink what we mean by an invasive species? Are the impacts of a species forced to relocate by a warming planet as bad as those of an introduced species?

Zieren has little time for this argument, believing it may be one for philosophers. ‘It’s pretty academic,’ he shrugs. ‘It doesn’t matter if a species comes from another continent or it migrates from a border zone. It’s still an invasive, it’s still having an impact. It’s the same problem.’

According to Jepsen, range expansions by native species can be as much a game changer in natural ecosystems as alien invasions. Her own observations have identified how climate change is expanding the outbreak range of moth species in the forests of Fennoscandia (the Kola Peninsula, Karelia, Finland and the Scandinavian Peninsula). ‘Climatic zones are shifting and eventually biological zones will, too. ‘Some biotic changes are rapid,’ she says, ‘others are slow and there are others that we can anticipate will happen.’

Historically, the autumnal moth has been the sole occupant of its niche in these forests, but it has recently been joined by the winter moth, which has moved its outbreak range northwards. The combination of large outbreaks of both moth species can damage birch trees and have wider impacts, says Jepsen. ‘There are knockon effects. The moths also eat other kinds of vegetation, such as the dwarf bilberry, which reindeer and grouse eat. You get species [of insect] that had one generation a year now able to have two because things are warmer.’

However, Genovesi’s view is that attention should be focused on aggressive non-native species. ‘Are species that move because of climate change part of the problem? My answer is “no”,’ he says. ‘That is happening a lot but they are simply finding more suitable habitats.’

He compares the differing impacts of two arrivals into Italy. ‘The golden jackal was a species of southern Italy but has moved north with warmer temperatures – that’s a natural expansion,’ he says. ‘In contrast, we have seen the Asian bedbug, Halyomorpha halys, come in from China and cause huge damage – €350 million a year – to crops. We’ve had to introduce a wasp from Japan to control it.’

New ecosystems

Climate change makes the task of keeping invasives under control harder and, to date, international efforts, says Zieren, are ‘like placing a drop of water on a hot plate’. The issue is simply not high enough on the political agenda, he says, ‘even though it’s cost-effective to tackle invasive species before they become even more of a problem through climate change. Invasives are already costing us billions of dollars a year.’

Drawing a comparison with other environmental initiatives, Zieren points to the resources now poured into tackling plastic pollution. ‘We need a global agreement and to make the economic case,’ he says. ‘That will give access to resources. It’s not new science – New Zealand and South Africa, among others, have been taking out invasives for years. We don’t have to relearn this.

The difficulty is that there is no drop-in approach that fits all scenarios. ‘A lot of these species, you cannot eradicate. It has become impossible – you can only suppress them,’ says Zieren. ‘You need localised chemical control, spraying on individual trees to take out most invasive species. But it’s labour intensive, because you can’t spread chemicals too widely as you would harm the habitat you want to protect. It’s a multiple-year effort.’

Jepsen points out that the task becomes even more complex when dealing with native species that are merely expanding their range as temperatures warm up. ‘That’s more tricky than dealing with invading non-native species,’ she says. ‘Invasions of high-risk species quickly make it onto national agendas. If you have a new invasive and you discover it early enough, then you might combat it. But we don’t have a similar system in place to systematically monitor rapid range expansion. In the case of range-expanding moth species [for example], there might be mitigations, to make forests less susceptible but there’s actually very little we can do.’ Where commercial forestry is involved, Jepsen believes that the appetite exists to plough resources into mitigation, but that this will probably not be the case for the vast northern boreal birch forests.

Genovesi is a little more optimistic, taking the view that many invasive species don’t cause significant problems and that measures can be targeted at the real ‘door- knockers’ as he describes those species that walk, swim or fly straight into new habitats and take over. ‘With horizon scanning, we can identify which pests may cause us the greatest problems and prepare accordingly,’ he says.

One straw to clutch at is that, in time, natural predator–prey relationships may re-establish themselves, albeit a few degrees closer to the Poles. ‘Maybe species such as the winter moth are being so successful in their expanded range because their expansion has, for a time, outpaced some of their natural enemies,’ says Jepsen. ‘Their success may be transitory.’

As the relationship between climate change, invasive species and species on the move unfolds, we may eventually see the development of new ecological niches, which might resemble a soup of boreal, Arctic and temperate habitats. ‘We might not have a historical precedent for the new ecosystems that will emerge,’ says Jepsen. ‘It’s a tremendous challenge because we lack a historical reference.’

Although Genovesi is unsure just how things will play out, what’s clear, he says, is that ‘we won’t keep the same evolutionary trajectory and dynamics. The warming temperatures may see a rapid increase in the arrival of pests. It may become more and more serious – we may see the loss of natural forests and changes to ecosystems on a scale that could be catastrophic. Non-native fish species could totally destroy our freshwater systems.’

People have short memories, however, which may dampen public clamour for action. Genovesi points out that perceptions change quickly. ‘In Italy, we have this beautiful landscape of cypress trees, everyone loves it, yet it is not natural – they were introduced.’ The same can be said of the UK’s Lake District, which is anything but ‘wild’ or ‘natural’.

Zieren is struck by the way in which what ostensibly appears to be a healthy, attractive landscape, is anything but. ‘You look at some stretches of savannah and they appear attractive but they are not, they are full of invasives and vulnerable to climate change. People don’t realise that the natural habitat has already been replaced. Things can look very green and functional but the species diversity has already dropped.’

Talk in recent years has been of the Anthropocene, an era where humanity has dominated and changed the Earth. Witt cautions that we may be headed towards another era, which he bleakly calls the Homogynene, ‘a period where the whole planet will be dominated by a few species.’ He wearily notes how ‘the perception among most of the public is that if it’s green, it’s good. There is also a failure among international donors and governments to fully appreciate the impacts of, especially, invasive alien plants.’

Yet few of those who’ve been first to identify the link between invasives and climate change believe all is lost. ‘I am the eternal optimist,’ says Witt. ‘We can get on top of the problem if we embrace all of the tools at our disposal.’

A combination of urgent action and long-term strategic thinking is required, argues Zieren. ‘Invasives already cost economies billions of pounds a year,’ he says, ‘with impacts ranging across a vast spectrum, from the cost of crop pest control to Japanese knotweed affecting house prices. We have to struggle like hell, do habitat restoration and implement chemical control.’

In essence, this would entail a bigger focus and funding for prevention, early detection and rapid response, says Witt. Drones could be used to manage invasive plants in hard-to-reach places and host-specific diseases could be introduced into invasive fish populations. Although controversial, gene drives, whereby a genetic modification is designed to spread through a population and essentially ‘knock out’ the species, are another option.

A more basic approach can be just as effective, Witt believes. ‘If every individual on this planet removed one invasive alien plant every day, the way people pick up plastic on the beach, we could make a real difference. It really is low-hanging fruit,’ he says. ‘I mean, what can we really do to prevent a neighbouring country or even a factory down the road from polluting the atmosphere and contributing to climate change? Not much. But we can all pull out a weed.’