The escalating global problem of alien species invasions — shedding light on the neglected

Alien frogs, geckos, lizards, snakes, and turtles — they have been and will continue coming unnoticed to invade and wreak havoc. In our study, we have shed light on who they are, where they are coming from and entering, and how often they are invading.

A gecko dead and frying on your car engine? — Nauseating.

A frog in your salad? — Disgusting.

A snake! crawling out of the plane's luggage compartment! — Mayhem.

Everyone has a relatable story of how pestilent critters, like insects, spiders, frogs, lizards, and snakes, have made their day eventful, for the better or worse. Oftentimes even, these nuisances end up buzzing the news or starring in blockbuster horror films.

In a  study we published in Biological Conservation, we found that these nuisances are not rare nor local, but are, in fact, the heart of the escalating global problem of alien species invasions.

It is a problem that brings undesirable environmental impacts and economic burdens. And it is a problem that has been and continues to pass under our noses, and we still have not fully figured out how to stop them.

Alien invasions — It’s all because of us

Over the past century, we have increasingly transported tens of thousands of species — animals, plants, fungi, and microbes — to areas where they do not naturally occur or are alien.

We deliberately transported many of them for food, companionship, sports, landscape improvement, and pest control. But we also accidentally transported many that stowed away on cargo and luggage, hitchhiked on ships, aeroplanes, and cars, or contaminated traded goods.

In some cases, alien species manage to breach, multiply, and spread beyond our control — at this stage, they are classified as invasive.

Invasive alien species wreak havoc on native ecosystems, bringing about crippling economic burdens.

A classic example of how humans assisted the global invasion of aliens is the presence of European animals and plants in far corners of the world. In the 19th Century, nostalgic Englishmen formed Acclimatization Societies in North America, Australia, New Zealand, and Europe. This global movement deliberately introduced proper animals and plants to diversify native fauna or, accurately to their intention, beautify their surroundings.

Some of the societies' lasting legacies are the global invasion of the House Sparrow, which outcompete many native birds. Or the invasion of the European Red Fox in Australia and the European Hedgehogs in New Zealand, which kill millions of native wildlife ever year.

A historic intergovernmental blunder committed about a century ago has since spiralled into one of the most well-known environmental catastrophes today — the cane toad's global invasion. In the 1930s, Ill-informed agriculturists set loose cane toads in over a hundred territories worldwide. They hoped the voracious American frog would biocontrol beetles devastating sugar cane plantations.  

But what promised to get rid of sugar cane pests turned out to be an ultimate pest to the environment! In its march across northern regions of Australia, the cane toad has left behind a massacre of millions of native wildlife, such as goannas, quolls, snakes and even crocodiles.

Deceased freshwater crocodile in a massacre of cane toads. A freshwater crocodile skeleton next to a cane toad. Image by Ruchira Somaweera/CSIRO
Deceased freshwater crocodile in a massacre of cane toads. A freshwater crocodile skeleton next to a cane toad. Image by Ruchira Somaweera/CSIRO

And finally, a textbook example of the detrimental environmental impacts of alien species invasions is the Brown Tree Snake in Guam. During World War II, military movement in the Pacific brought hitchhiking Brown Tree Snakes from Southeast Asia to Guam. The Brown Tree Snake has since eaten to extinction 10 of the 12 original forest birds of the island. The remaining two are functionally extinct. The extinction of many endemic birds in Guam is an unrecognised collateral damage of war.


Still an escalating global problem

Invasive alien species are currently the leading culprit of modern-day extinctions and the grim state of global biodiversity. They cost world countries up to US$160 billion yearly in environmental losses and management expenditures.

More worryingly, the scale and magnitude of alien species invasions, as well as their impacts and cost, continue to expand and escalate dramatically.

Having recognised the severe impacts of invasive alien species, in the recently concluded 2022 United Nations Biodiversity Conference (COP 15), world countries agreed to reduce alien species introductions and establishment by 50% by the end of the decade (Target 6).

These and many other international agreements hope to address the pandemic inadequacy, if not lack thereof, of policies and management responses against ongoing and future alien species invasions.

Notably, the preeminent lesson learned after decades of managing alien species invasions is prevention is better than cure. Eradicating and controlling alien species after their arrival is difficult and expensive, if not impossible. Thereby, preventing the future entry of alien species is the most crucial, pragmatic, and effective strategy.

There are now existing intergovernmental policies regulating the international trade of exotic pets. Similarly, stringent frameworks have been adapted globally in assessing the potential risks of biological control agents. Albeit needing more rigorous implementation, these policies and frameworks have markedly reduced alien species' introductions through these deliberate routes.

Nonetheless, recent studies have consistently shown that the current and future alien species invasions are, and will, be dominated by alien species accidentally transported and introduced as stowaways and contaminants.

And to this end, we have yet to comprehensively understand who, when, where, how, and how often alien species are accidentally transported and introduced, limiting us from responding effectively. This is especially the case for alien species neglected by biosecurity due to their inapparent economic impacts, despite the many studies proving otherwise.


Global flows of neglected aliens

 In a  study we recently published in Biological Conservation, we provided new evidence and perspectives on the global flows of accidentally transported alien species — who, when where, how and how often alien species are accidentally transported worldwide.

 We did this by analysing Aotearoa New Zealand's Ministry for Primary Industries's records of intercepted alien amphibians and reptiles. 

Alien amphibians and reptiles are the predominant and, thereby, the best model of accidentally transported neglected alien species. Nonetheless, this database is the most comprehensive database of its kind, and is inarguably our best, if not our only, window to understand the global flows of accidentally transported neglected alien species.



An exceptional diversity of amphibians and reptiles (243 different species) have been accidentally transported to New Zealand. This is more than twice the diversity of that in Australia — the only other information we know on the matter.

These include alien frogs that are currently invading Aotearoa New Zealand, such as the southern brown tree frog (Litoria ewingii), the green and golden bell frog (Ranoidea aurea), the growling grass frog (Ranoidea raniformis), and the delicate skink (Lampropholis delicata).

Some were historically deliberately introduced and successfully eradicated in Aotearoa New Zealand, such as the common toad (Bufo bufo), the natterjack toad (Epidalea calamita), the European common frog (Rana temporaria), and the green tree frog (Litoria caerulea).

And most worryingly, these include some of the most destructive invasive alien species, such as the cane toad (Rhinella marina), the brown tree snake (Boiga irregularis), and the green iguana (Iguana iguana).

Notably, we found that the variety of intercepted aliens changed over time — those that were intercepted in the early 2000s were different from those intercepted in recent years.




Alien amphibians and reptiles came from all parts of the world, with the top sources being major trade partners such as Australia (some 25%) and countries in Asia and the Pacific.

We emphasise that countries in Asia and the Pacific are not only important recipients but, as we found out, are also important sources of alien species. The third highest source of amphibians and reptiles for New Zealand was Fiji, albeit the island country's minor contribution to New Zealand's trade.

About half of all accidentally transported alien amphibians and reptiles were intercepted in Aukland. But still, we found that seaports and airports across the country are at high risk.


When and how often?

Since the beginning of the 21st century, there have been >2400 interceptions of accidentally transported alien amphibians and reptiles. That's about nine alien amphibians and reptiles intercepted each month.

Although dwarfed by the number of intercepted alien insects, this is much higher than previously expected (five times the interception records on alien vertebrates in Australia), more so magnitudes higher than natural rates of amphibian and reptile dispersal.

Interestingly, we showed how the number of intercepted aliens coincides with cycles of global economic booms and recessions. These findings are concrete support for the link between globalisation and the global flows of alien species.



Alien amphibians and reptiles stowed away, hitchhiked, and contaminated various vehicles, equipment, luggage, and goods and their packaging. These include cars, helicopters, yachts, luggage, potted plants, fruits, and vegetables, to the more random fish hooks, fibre-glass moulds, sports equipment, plastic Christmas trees, and bird seeds.

But overall, they stowed away on people's luggage and machinery, vehicles, and equipment more frequently than any other way.

This is very different from how economically-significant alien insects have been accidentally transported in New Zealand and other parts of the world, which mainly contaminate goods.


A harrowing future 

Our study provided new evidence and perspectives on the global flows of alien species.

We showed that we have completely underestimated how common and diverse neglected alien species are accidentally transported to New Zealand and, by extension, any part of the world.

All countries can be sources of neglected alien species, and they arrive at virtually all possible entry points.

And finally, neglected alien species tend to stow away on luggage and transport vehicles. This contrasts with economically significant alien species, which tend to contaminate goods.

Considering the pandemic inadequacy, or lack thereof, of prevention strategies, our findings suggest that an immensely high and dynamic diversity of accidentally transported neglected alien species have and will continue to be freely introduced worldwide.

This, in turn, suggests the continued progression of the escalating and expanding global problem of alien species invasions.

Such a harrowing future is inevitable unless jurisdictions worldwide commit to their international commitments to strengthen their biosecurity to curb alien species invasions.

And on this final note, as our study has shown, there is no silver bullet. The global flows of neglected alien species contrast with that of economically-significant alien species. A one-fits-all biosecurity strategy may be insufficient in stopping the escalating global problem of alien species invasions.

What's next?

I will be capping off my PhD thesis with a final chapter that aims to forecast future invaders and invasions. I will forecast future invaders — predict global candidate of species with invasion syndromes — using a #nextGen life-history trait-based quantitative risk assessment framework. And I will forecast their invasions using a a #nextGen Species Distribution Modelling framework that can address environmental non-equilibrium and climatic niche non-conservatism.

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Life Sciences > Biological Sciences > Ecology