Managing invasive species

6 February 2023


A population of African green monkeys deemed invasive living on a Caribbean island has recently come under the spotlight.

Authorities in Sint Maarten, the Dutch-administered part of the island which the Netherlands shares with France, are planning to use culling to control the population by targeting at least 450 of the feral monkeys,

Born Free’s Head of Policy, Dr Mark Jones

Green monkeys, who were thought to have been introduced onto the island as exotic pets by European settlers as long ago as the 17th century, have been living free and breeding there since at least the 1970s. The authorities say that if the population continues to grow, it could threaten native bird populations and food security on the island.

This sad situation reflects the ever-growing problem of so-called ‘invasive alien species’, which the International Union for the Conservation of Nature (IUCN) defines as “animals, plants or other organisms that are introduced by humans, either intentionally or accidentally, into places outside of their natural range, negatively impacting native biodiversity, ecosystem services or human economy and well-being.

More than 17,000 invasive species of animals and plants have been identified globally. They are considered to be one of the main causes of the precipitous and unprecedented crisis that nature faces. More than a million species are thought to be at risk of extinction, with changes in land and sea use, direct exploitation of organisms, climate change, pollution, and invasive alien species identified as the main drivers

While many of these invasive species were introduced accidentally, others have been deliberately released. Whether purposeful or accidental, the rate at which species are being shunted across the globe has exploded over recent decades. Thanks to our global travel infrastructure and an exponential increase in international trade, the breeding and transportation of wild animals for consumption, commercial, research use, and as exotic pets is rampant. Global warming is also forcing some species to move out of their traditional ranges and into areas where they may cause problems for the native wildlife.

At Born Free, we recognise the need to address the invasive species issue. However, this cannot be used as an excuse to employ methods that unnecessarily compromise animal welfare.

Here in England and Wales, the Government lists 30 animals and 36 plants as invasive species of concern. Among them are some well-known species that have become widespread, such as grey squirrels, muntjac deer, Egyptian geese, and several species of terrapin.

Invasive species can affect native wildlife in a number of ways. They might predate or out-compete their native counterparts. Some may breed with closely related native species, diluting their genetic integrity. Others may introduce novel diseases against which native species have little or no protection. Squirrel pox in the UK, which arrived from North America in grey squirrels carrying the virus, is lethal to native red squirrels, and is just one example. The chytrid fungus, which has devastated many indigenous populations of amphibians around the world, may have been spread via the international trade in frogs as exotic pets or for research.

By no means will all non-native species become invasive if they are released into new habitats. However, those that do can cause havoc. The damage they cause, and the price of trying to control them, was estimated in 2014 to cost the European Union alone some €12 billion annually, a figure that has surely increased since then. Island habitats, with their often unique and highly specialised wildlife, are considered to be particularly vulnerable.

Some invasive species can also cause problems for people, through aggression, by spreading zoonotic diseases (diseases that can be passed from animals to people), or by damaging property.

At a time when the crisis facing nature and biodiversity is gaining increasing international attention, tackling invasive species has become a priority for conservationists and governments.

Sadly however, although non-native animals that end up becoming invasive do so through no fault of their own, they are often ‘controlled’ using lethal culling methods which are designed to eliminate them for the lowest cost. This can result in immense animal suffering, and indiscriminate methods of killing can further damage non-target native species or domestic animals, while often proving ineffective at tackling the problem.

At Born Free, we recognise the need to address the invasive species issue. However, this cannot be used as an excuse to employ methods that unnecessarily compromise animal welfare.

First and foremost, the vast international trade in wild animals must be brought under control, in order to prevent non-native species from being introduced and potentially becoming invasive in the first place. It is far, far cheaper and more effective to prevent the importation of potentially invasive species than it is to tackle the problem once they have become established.

Second, when considering how to deal with situations where non-native species have already become established, criteria must be followed to ensure any actions taken are effective, proportionate, and humane. To this end, an important set of principles was developed by a group of scientists and published in 2017 in the academic journal Conservation Biology as the “International consensus principles for ethical wildlife control.”

The scientists determined that efforts to control wildlife should begin wherever possible by altering the human practices that cause the problem in the first place; be justified by evidence that significant harm is being caused to people, property, livelihoods, ecosystems, and/or other animals; have measurable outcome-based objectives that are clear, achievable, monitored, and adaptive; predictably minimize animal welfare harms to the fewest number of animals; be informed by community values as well as scientific, technical, and practical information; be integrated into plans for systematic long-term management; and be based on the specifics of the situation rather than negative labels (such as ‘pest’) so often applied to the target species.

Born Free endorses these principles and encourages decision-makers and wildlife managers at every level to adopt them.

As for the green monkeys on Sint Maarten, the proposed cull could result in serious suffering not only to those being directly targeted, but also to wider family members and social groups. Born Free representatives have joined with 75 international experts in a letter to the island’s authorities, urging them to withdraw the decision to cull the animals, and instead consider a humane neutering programme to maintain the population at manageable levels, while at the same time developing methods to mitigate the problems they are believed to cause. The experts have also offered to help neighbouring islands in the region that may be experiencing similar problems.

In this way, Sint Maarten could lead the way on humane primate population management in the Caribbean region and set a precedent for others to follow.