A world of invaders

Invasive species have growing costs – not least for the environment. Ross Cuthbert reports

Alien invasions are happening all around us – not the extra-terrestrial kind, but from different regions of our own planet. Invasive species arrive, establish and spread outside their natural range. They present a growing environmental and economic problem worldwide.

Animals and plants evolved different characteristics to suit their particular part of the world. Distinct communities were separated by vast oceans and mountain ranges. Mixing these species was impossible over short timescales.

But human activities have eroded these barriers. With globalisation, we move species around the planet faster and in higher numbers than ever before. We cross oceans and continents within hours. That risks unique species compositions becoming a thing of the past as biodiversity is homogenised. In the last hundred years, thousands of invasive species have been established globally.

Invasions occur in many ways. Alien species arrive unintentionally in ships’ ballast water or by biofouling watercraft, or as hidden stowaways on aircraft or in cargo.

Other invaders stem from the exotic pet trade or are brought in for agriculture, aquaculture and forestry. With a 35 per cent increase in invasion rates expected in the next three decades, the scale of this problem will only rise.

Environmental impacts

Many alien species are harmless or do not successfully establish. Others survive, reproduce and spread. Impactful invasive species threaten entire ecosystems and cause trillions of dollars in economic harm. The ecological consequences can be enormous.

Invasive species are a leading cause of extinction. They eat native species and compete with them for food and habitat. They spread diseases native communities are not prepared for, killing off numerous native species around the world.

Islands are especially susceptible. Many have unique, endemic communities, unfamiliar with predators from abroad.

When the brown tree snake from Southeast Asia and Australia arrived on Guam in cargo by the mid-twentieth century it caused an ecological disaster. Within years, the snakes had eaten to extinction most of the island’s endemic bird, lizard and bat species.

Invaders have degraded the Caribbean’s once-vibrant coral reef habitats. Lionfish that arrived through aquarium releases have decimated the region’s reef fishes and crustaceans.

Europe’s native freshwater crayfish face extinction. Crayfish from North America have introduced crayfish plague, a foreign water mould. Unlike the North America species, European crayfish have no tolerance to it. European red squirrels suffered a similar fate, following arrival of squirrel pox with invasive grey squirrels from North America.

In North America, invasive kudzu vines from Asia smother native plant species. Growing up to 25 cm per day, they block sunlight under a blanket of leaves. Because native plant species cannot compete they die off.

Counting the costs

The environmental impacts of invasive species are unequivocal. Until recently, we paid less attention to invaders’ damaging economic impacts. But money talks – and money is tight. Environmental decisions become easier once we assign a price tag.

The figure was revealed last year. A new synthesis found that global economic damage from invasive species has topped US$1 trillion in recent years. As invasion rates rise, so do the costs. The most damaging invaders have cut yields in agriculture, forestry and fisheries. Others have spread deadly human diseases, creating a healthcare cost.

The costs can be eyewatering, running into hundreds of billions of dollars globally. Take invasive mosquitoes: yellow fever mosquitos from Africa and Asian tiger mosquitos have a taste for humans and are spreading dengue, yellow fever and Zika into new regions of the world. Prolific in urban areas, they breed in used tyres and wastelands.

Worldwide, invasive mosquitoes have cost at least US$150 billion dollars since 1970.

Rats cause massive damage costs in urban areas by spreading disease, spoil agricultural produce and cause fires by short-circuiting wires.

Ragweeds cause severe impacts through allergic reactions to pollen and compete with agricultural plants. Invasive insects eat farmers’ crops and decimate production values. Larvae of invasive moths and wood-boring insects destroy trees. Invasive mussels clog power stations’ water-intake pipes. Invasive plants guzzle water in arid regions and damage infrastructure.

Invader impacts can also make species vulnerable to climate change, itself a human-mediated threat to ecosystems and economies. And it can be challenging to predict how invasive species and climate change interact.

Global warming could boost invasion rates and worsen the impacts. Invasive species’ success rests on their ability to survive and reproduce in new environments. A warmer climate could benefit cold-blooded taxa such as invasive insects. They will disperse faster, have more offspring and wreak more damage.

Meanwhile, native species may struggle, with climate change making it easier for new arrivals to displace them.

Invasive mosquito species such as the Asian tiger mosquito are already showing improved performance. Warmer climates in temperate areas allow them to reproduce faster, aiding their disease-carrying expansion across Europe.

Land-use changes also promote biological invasions. When humans disturb and degrade habitats, invasive species readily colonise. Invaders fill these vacant niches more efficiently than natives.

There are many examples of invasive plants opportunistically taking over cleared land after humans remove native species. Invasive ants are globally infamous for spreading into disturbed habitats.

Managing invaders

It is not all doom and gloom. We can manage invasive species, by preventing them arriving in the first place or eradicating them on detection.

This spring Wildlife and Countryside Link (WCL) urged the government to tighten restrictions on plant imports and called on gardeners to buy British to avoid accidentally introducing invasive species.

However, most countries ignore the invasion problem. They act too late, only once the impact becomes visible, if at all. A recent synthesis found that it takes 11 years on average to manage invasive species after they arrive.

Management delays are a big problem. By the time impacts are visible, invasive species may have established reproducing populations and are spreading. It is difficult — often impossible — to get rid of them at this stage.

Management spending by countries is relatively low. While damages of invasive species exceed US$1 trillion, management spending is more than ten times lower, at some US$100 billion. It shows we are not doing enough to reduce impacts.

Worse, only US$3 billion of the management bill is proactive, spent on prevention, surveillance and early eradication. These are measures that halt invasive species early – before they spread out of control.

Reactive management spending prevails. Delayed and deficient, actions such as control and eradication rarely succeed once an invader has been established. At best, they contain invasive species, preventing further spread. At worst, they can exacerbate the problem.

Invasive waterweeds, spread via fragmentation, are dispersed further by mechanical removal efforts. Classic biological control of invasive species using additional alien species has led to new, more severe invasions.

Prevention is better than cure. And it’s much cheaper. Preventing invasive species or acting immediately when we find them could save trillions of dollars in damages and long-term management spending.

There are parallels with infectious diseases. Covid-19 successfully prevented and contained at the pandemic’s earliest stages, would have wreaked far less economic and social disruption. In the same ways, invasive species prevention can save economies and ecosystems from profound impacts.

We must spend much more on management to prevent massive economic damages and invest that money more wisely. That means earlier, before invasive species become a problem.

Failing to prevent them has enabled thousands of invasive species to establish and cause impact. We need better biosecurity to prevent arrival and spread and thwart future damages.

We must co-ordinate our actions. Biological invasions are a global problem. One country’s poor biosecurity weakens others’ defences. Invaders spread into neighbouring countries from these hubs.

Countries need collaborative solutions and sensible, transboundary policies. As with climate change, no one country’s actions will solve this global challenge. That requires politicians and decision-makers to improve funding for prevention measures that reduce future invasions, including research into effective management practices.

Researchers and stakeholders must work to fill knowledge gaps and inequalities. We know a lot about costs of invasive species in North America, Oceania and Europe but relatively little in Africa, South America and Asia.

These gaps mean we grossly underestimate the damage. Our picture of the impacts is incomplete. We need more effort to report and publish impacts of invasions, ecological and economic. Biological invasions disproportionately impact developing countries. We need to build capacity here, to prevent and rapidly respond to invaders.

Success stories

We don’t have many examples of successful eradications of invaders once they have been established – but here is one; coypu eradication from the UK in the 1980s, which took co-ordinated and funded government trapping over a decade. The UK worked to mitigate future habitat damage by coypu burrowing and foraging on important crops.

Prevention and proactive measures have worked better. They often allow multiple species to be managed at once.

Eurasian aquatic species have heavily invaded the Laurentian Great Lakes in North America. Cargo ships from Europe dumped ballast water, used to stabilise empty ships, into these lakes. This discharged myriad damaging invaders; alien mussels, shrimps, jellyfish, even fish.

The US and Canada have worked effectively to address this. In 2006 and 2008, they enforced ballast-water regulations that required vessels to conduct open ocean flushing before entering the Laurentian Great Lakes.

The idea was that high sea salt levels would kill alien taxa before the ships entered the freshwater lakes. Rates of invasion of the Great Lakes have since fallen by 85 per cent, to the lowest levels in two centuries.

In New Zealand, a rapid response successfully removed red imported fire ants that inflict painful bites and damage electrical equipment. Rapid removals have saved hundreds of millions of dollars in impacts per year in New Zealand.

In the UK, rapid responses have blocked the spread of Asian hornets that could decimate honeybee populations – critical pollinators. The UK combined citizen science and rapid communications to help specialist teams to locate and destroy the hornets’ nests.

Ireland used similar rapid responses to eradicate chub, an invasive freshwater fish, using electric fishing crews, tank-boats and land-based support to eradicate the fish before they spread.

What this shows

These examples show that well-resourced and co-ordinated management can prevent and eradicate invasive species before they establish, spread and cause impact. We can act to prevent future invasions. Doing so will save trillions in costs, supports biodiversity conservation and promotes sustainable development.

Better biosecurity, surveillance, dispersal modelling and impact quantifications will equip us to deal with future invasions. We can use expert elicitation via horizon-scanning exercises to predict and prioritise future invaders.

Providing funding for rapid-response teams could allow us to eradicate invaders quickly once we detect them. But we also need to raise awareness among the general public. Understanding the threats from invading species will empower citizens to report new invasions through citizen-science schemes.

Co-operation at all levels and efficient investment will enable us to better manage invasive species and reduce the damage they cause. And doing that will leave our ecosystems healthier and our economies more prosperous.

Dr Ross Cuthbert is a Leverhulme early-career Fellow at Queen’s University Belfast and a research associate at the South African Institute for Aquatic Biodiversity. He completed a PhD in invasive species and pest management and previously held a Humboldt Postdoctoral Fellowship at GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany

BOX: THE UK’S LEAST WANTED

Established invasive species already damaging UK wildlife and the economy include:

Oak processionary moth

Damages oak trees; a risk to human health. Arrived in egg form in the canopy of trees for planting in the early 2000s

Spanish slug

The super slug arrived here in potted plants and salad leaves around 2010 and is now found UK-wide. It’s a voracious eater of crops, wild and garden plants but is too slimy for hedgehogs and birds to eat and resistant to poisons

Red lily beetle

Recorded in the UK in 1939, probably a hitchhiker on imported lilies. Highly destructive to lilies and fritillary butterflies

Harlequin ladybird

Accidentally introduced through plant imports, consumes native ladybirds. Hibernates in people’s homes

New Zealand and Australian flatworms

Introduced in potted plants and now firmly established. Eats native earthworms, reducing populations by up to a fifth, affecting wildlife food chains and soil fertility

Phytophthora austrocedri

A fungus-like pathogen that arrived through the plant trade is attacking and killing native juniper, a rare tree that provides a habitat to goldcrests and is used to flavour Scottish gin.

Future threats include:

Asian hornet

A bee-eating wasp that is destroying bee populations in France. More than 20 have been sighted in the UK; could cost the UK £7.6 million a year in damage and control costs

The New Guinea flatworm

Already found in France and named one of the world’s 100 worst invasive alien species, it has destroyed populations of native snails across the Indo-Pacific

Argentine ant

Eliminates native ant species, harms ecosystems and can damage wiring. Spotted in London and Birmingham but not yet known to be established

Emerald ash borer

A beautiful, destructive wood-boring beetle native to East Asia that is spreading across Europe, having damaged billions of North American ash trees

• Two-leaf water milfoil

Introduced to Europe through the aquarium and water gardening trades and spotted in the UK since the 1940s but not yet established. It forms dense underwater mats that suffocate native wildlife and affect fishers, swimmers and boating.

Source: Wildlife and Countryside Link, May 2022

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