This article is the third in a series The Conversation Africa is running on invasive species.
Alien species have been introduced to Africa for a variety of reasons. They provide food, raw materials for industry, ornamental plants, recreation in the form of sport fishing, hunting and pets. Some that are highly valued have been moved around widely. And in some areas they now form prominent components of societies and ecosystems like the domestic cat for example.
Many alien species bring considerable benefits. But some have become invasive, causing a loss of biodiversity, changes to ecosystems, economic losses and, in some cases, even affecting people’s health.
The shrub Prosopis or mesquite is an example. It was introduced to South Africa to provide fodder, firewood and shade in arid parts of the country. But it’s also a major water user. And two trout species (S. trutta and O. mykiss) are used for recreational angling and commercial aquaculture. But they’ve also been implicated in having a negative effect on the environment.
Managing invasive species is therefore critical. In South Africa the movement and use of 552 listed invasive species are managed under the Biodiversity Act and regulations attached to it. But not all the species on the list are equally harmful. Several may in fact be relatively harmless.
All the listed species under these regulations require management. Given that the capacity is limited, regulations should arguably focus on priority species because not all are necessarily harmful to the extent that would justify spending large amounts of time and effort on keeping them under control.
The question then is: are there some species that could be removed from the list? In our recent study we set out to answer this question by classifying species as inconsequential, beneficial, destructive or conflict generating species. This was done by assessing the relative degree of benefit they brought and their negative effects.
Beneficial and harmful species
The classification was done by using a simple scoring system. It had two categories for the negatives (ecological and socio-economic) and two for the benefits (economic and intrinsic).
Inconsequential species: these make up 55% of the species listed under the act and in the regulations. They were associated with relatively low costs and low benefits to society. Species in this group had limited distribution or no known impact and were largely introduced as ornamentals or pets. Some examples include the eastern grey squirrel (Sciurus carolinensis), European perch (Perca fluviatilis), and the Père David’s Deer (Elaphurus davidianus).
Destructive species: these make up 29% of the list. They don’t bring substantial benefits to society or the environment, but they have a highly negative impact. Many were introduced accidentally and are regarded largely as pests and weeds. Examples include invasive rodents like the black rat (Rattus rattus) which causes damage to infrastructure and transmission of zoonotic diseases and pitch canker (Fusarium circinatum) a growing threat to pine plantations and forests worldwide.
Beneficial species: they make up 10% of the list and have clear social or environmental benefits. For example the jacaranda (Jacaranda mimosifolia) is an iconic tree species in the city of Pretoria where the species is regarded as part of the identity and “sense of place” of the city. Active management is not necessary or should only be done in particular cases.
Conflict-generating organisms: these can be either beneficial or destructive, depending on one’s perspective or what value is placed on them. They make up only 6% of the list. There’s huge disagreement about whether these species should be controlled, or how they should be controlled. Examples include woody plants introduced for forestry, erosion control, sand dune stabilisation, agriculture and as ornamentals. Acacias and pines are examples. Animal examples include species like the Himalayan tahr which was introduced to the Table Mountain National Park. The goat has been the focus of eradication attempts, despite strong opposition. It also includes species introduced for aquaculture like maroon and brown trout. Managing trout has been highly contentious with conflicting views about whether they pose a risk, or deliver a benefit. This has led to them being listed and delisted. The trout fraternity refuse to acknowledge that trout are invasive species and highlight the lack of scientific evidence of the risks they pose.
Finding common ground
We need to keep sight of the fact that there is general agreement on 94% of listed species. By identifying the small number that are generating the greatest tension, it’s more likely discussions can be held to reach common ground on regulation.
Most countries in Africa don’t have invasive species regulations. But there’s growing recognition that they’re needed. South Africa offers useful lessons on how this could be done.
The control of species listed under the country’s biodiversity act is compulsory. This means that plans to manage them have to be drawn up and implemented. But this doesn’t seem sensible given that not all are equally harmful and resources are limited. Our study suggests that some of the species currently regulated could be removed from the list.
Countries wanting to set up a system of managing invasive species could start by classifying a prospective list of candidates. Policymakers could then quickly bring out legislation against the most damaging and destructive ones. At the same time, discussions could be had on the ones that generate conflict with the aim of reaching consensus.
This would allow managers and regulators to focus on the most destructive species – as well as those that are at the centre of fierce disagreement.
This article is the first in a series The Conversation Africa is running on invasive species.
Let’s say you’re travelling from Uganda to South Africa for business. You finally arrive at your hotel after a long day and decide to change before dinner. You unlock and unzip your luggage, but there’s something in your bag that you didn’t pack. As you reach for a clean shirt, a moth flies out. Did that come with you all the way from Uganda? It’ll be fine, right? Surely, something so small won’t cause any harm.
Species are intentionally or accidentally transported by humans between continents to regions where they are not native. With the help of humans or by natural means like flight, these alien species can also spread within continents.
Their spread within continents can be rapid, affecting both the ecology as well as societies and the economy. Unfortunately, it’s really challenging to prevent species from spreading. Given the vast amount of people and goods that are transported between and around continents they can easily be moved across oceans as well as between countries.
Many alien plants and animals have been introduced to Africa from other regions and then have spread from country to country, often having devastating effects.
Take the larger grain borer beetle, (Prostephanus truncatus) which is thought to have arrived on the continent in imported grain from Mexico and central America. The beetle was introduced to Tanzania before 1984, Togo before 1981 and Guinea before 1987. It then spread across the continent and within 20 years could be found further south in South Africa.
The beetle attacks crops such as maize and cassava, threatening food security and the livelihoods of the poor. Infestations often destroy maize that’s been stored by farmers, forcing them to buy maize as well as lose income they could have earned from selling any excess.
But alien species don’t just arrive from abroad. Many that are native to parts of Africa have also spread to countries on the continent where they are not native.
An example is the fish commonly known as the Mozambique tilapia (Oreochromis mossambicus) which is native to rivers on the east coast of southern Africa. Fishermen have transported the Mozambique tilapia to other areas and it is now found in river systems in southern and western South Africa and Namibia.
The Mozambique tilapia is a popular species for fishing but it can pose a threat to native fish and has been responsible for the disappearance of native species in some regions.
The spread of alien species within Africa is by no means a new thing. For instance, the bur clover (Medicago polymorpha), a plant from northern Africa, might have been accidentally transported by humans to South Africa as early as 760 AD.
A high and increasing threat
Recently a number of alien species have spread extremely rapidly across the continent, posing a particularly high threat to food security and livelihoods.
One is a caterpillar known as the fall armyworm (Spodoptera frugiperda). The species, native to the Americas, was first recorded in west and central Africa in early 2016 and then in South Africa in January 2017.
The moths of the armyworm are strong fliers and the species may have spread through flight to South Africa from other African countries. Although the species attacks a wide range of crops, it poses a particularly serious threat to grain farmers. It is extremely difficult to manage.
Another example is a wasp known as the bluegum chalcid (Leptocybe invasa), which is native to Australia. In 2000 it was detected in Israel and shortly afterwards it was reported in Uganda and Kenya. From there it spread rapidly to many African countries including Zimbabwe, Mozambique, and Tanzania and was finally detected in South Africa in 2007. The insect probably reached Israel on live plant material and spread into Africa the same way, or was carried by people travelling between countries.
The wasp causes swelling or growths on eucalyptus trees, which can lead to decreased growth and tree death. As eucalyptus trees are an important source of income and fuel, this species could have an impact on the livelihoods of locals in these countries.
Preventing the introduction and spread
Once a species is introduced to one African country it’s highly likely it will spread to others on the continent because borders checks are weak.
The introduction and spread of species could be reduced if countries introduced biosecurity systems. These are used extensively in countries like Australia and New Zealand and involve using technology to check for alien species when people and goods enter a country. In Australia this involves inspecting goods, vehicles and luggage before they enter the country.
But even these systems aren’t a guarantee that species won’t spread. African countries would need to work together and share information and skills. This would also allow countries to prepare for the arrival of species, and to draw up plans to reduce their impact.
This is a tall order. But as a country’s defence against alien species introductions is only as strong as that of its neighbours, such action would benefit all of the countries involved.
There are times when insecticides are needed (especially when pest populations are surging or the risk of disease is high) but you don’t have to reach for the spray every time. Here are five good reasons to avoid pesticides wherever possible, and live and let live.
1. Encourage the bees and butterflies, enjoy more fruits and flowers
Flowers and fruits are the focal points of even the smallest gardens, and many of our favourites rely on visits from insect pollinators. We all know about the benefits of European honey bees (Apis mellifera), but how about our “home grown” pollinators – our native bees, hover flies, beetles, moths and butterflies. All these species contribute to the pollination of our native plants and fruits and veggies.
You can encourage these helpful pollinators by growing plants that flower at different times of the year (especially natives) and looking into sugar-water feeders or insect hotels.
2. Delight your decomposers, they’re like mini bulldozers
To break down leaf litter and other organic waste you need decomposers. Worms, beetles and slaters will munch through decaying vegetation, releasing nutrients into the soil that can be used by plants.
The problem is that urban soils are frequently disturbed and can contain high levels of heavy metals that affects decomposer communities. If there are fewer “bugs” in the soil, decomposition is slower – so we need to conserve our underground allies.
You can help them out with compost heaps and worm farms that can be dug into the ground. It’s also good to keep some areas of your lawn un-mowed, and to create areas of leaf litter. Keeping your garden well-watered will also help your underground ecosystems, but be mindful of water restrictions and encouraging mosquitoes.
3. An army of beneficial bugs can eat your pests
Mantises and dragonflies are just some of the hundreds of fascinating and beautiful bugs we are lucky to see around our homes. Many of these wonderful creatures are predators of mozzies, house flies and cockroaches, yet people are using broad-spectrum insecticides which kill these beneficial bugs alongside the pests.
It may sound counterproductive to stop using pesticides in order to control pests around the home, but that’s exactly what organic farmers do. By reducing pesticides you allow populations of natural enemies to thrive.
This form of pest control in growing in popularity because spraying can result in insecticide resistance. Fortunately, it’s easy to encourage these bugs: they go where their prey is. If you have a good range of insects in your yard, these helpful predators are probably also present.
4. Your garden will support more wildlife, both big and small
Spraying with broad-spectrum pesticides will kill off more than just insects and spiders – you’re also going after the animals that eat them. The more insects are around, the more birds, mammals, reptiles and frogs will thrive in your backyard.
Baiting for snails, for example, will deter the blue-tongue lizards that eat them, so cage your vegetables to protect them instead. Keeping your garden well-watered, and including waterbaths, will also encourage a balanced ecosystem (but change the waterbaths regularly).
5. You and your family be happier and healthier
Engaging with nature increases well-being and stimulates learning in children. Insects are a fantastic way to engage with nature, and where better to do this than in your own back yard! Observing and experimenting on insects is a wonderful teaching tool for everything from life cycles to the scientific method. It will also teach your kids to value nature and live sustainably.
Your backyard has a surprising impact on the broader health of your neighbourhood, and gardens can make significant contributions to local biodiversity. Insects are an important part of ecosystem conservation, and encouraging them will improve the health of your local environment (and probably your health and well-being too).
I am walking quietly through the forest. As I reach the edge of the trees there is a snort and a staccato of hoofbeats, and four horses materialise only metres in front of me: a foal, two mares and a dark stallion. The stallion, ears pricked, tosses his head and prances forward. As I crouch to pick up a branch, the stallion wheels and gallops off with the group. They hurdle an old stock fence, and almost as soon as their hoofs touch down, another big grey stallion comes towards them over the hill.
The next minutes are completely mesmerising. The two stallions fight, 50 metres from me. Dust hangs in the air around them, their screams echo off the hills, the impact of their hoof strikes reverberates in my belly. They rear, scream; snake heads out to bite, whirl and kick. Eventually, bleeding and bruised, the dark stallion breaks and runs. The grey makes a show of chasing, then canters back to the mares, arching his neck, prancing with lifted tail.
This is one of many times I have seen horses, called brumbies in Australia, in the mountains. While cross-country skiing in the south I have watched them in the snow – ragged manes flying, galloping through a mist of ice crystals – and many times while driving and bushwalking in both the north and south of Kosciuszko National Park. I have also watched them cantering in clouds of dust in central Australia, and grazing in the swamps of Kakadu. Each of these wild horse encounters has been deeply visceral and emotional, elemental expressions of life in dramatic and beautiful landscapes.
Horses are large, powerful and charismatic animals, and humans have ancient connections to them. Wild horses are dominant among the 13 species painted on the caves of Chauvet in France 30,000 years ago, and while there continues to be debate, archaeologists suggest evidence for horse domestication is at least 5,500 years old. And like the oldest human-animal relationship outside hunting – with dogs – the horse relationship is unique because we now mostly do not eat this animal.
Like dogs, horses now occur on every continent except Antarctica, and humans have been the primary agent for their dispersal. In North America, where the first true horses evolved and then died out, they were reintroduced by Columbus in 1493. Horses are the most recent of the main species humans domesticated, and the least different (with cats) from their wild counterparts.
Australia has the largest wild horse herd in the world, maybe 400,000 or more horses, spread across nearly every bioregion from the tropical north to the arid centre to the alpine areas. That sounds like a dramatically large number, but Australia also has around one million domestic horses, about 100 million cattle and sheep, maybe 20 million feral pigs and 25 million kangaroos. But the presence of wild horses here is deeply controversial.
Six thousand of these horses are in Kosciuszko National Park. Ongoing controversy around these wild horses encompasses debate about their impact and their cultural meaning. There is very little systematic research and a large amount of emotive and anecdotal argument, from both sides. There is circularity and self-referencing in government wild horse management plans, very little reference to studies from Australia and almost no peer-reviewed research on horse impacts in the Snowy Mountains, despite decades of argument that they cause environmental degradation.
And Kosciuszko is right next to Canberra and the Australian Capital Territory, which has the highest per capita horse-ownership of anywhere in Australia. Several enterprises run horse-trekking trips into the Snowy Mountains, often interacting with brumbies. The Dalgety and Corryong annual shows on the boundaries of the park highlight horse skills, including catching and gentling brumbies. In many places mountain cattle properties are increasingly using horses instead of motorbikes to handle stock.
The Kosciuszko wild horses are also tangled within the embedded idiosyncrasies and contradictions of the largest national park in New South Wales. Here there are protected populations of two species of invasive fish (brown and rainbow trout) that are demonstrably responsible for local extinctions of native fish and frog species; a gigantic hydro-electric scheme with dominant infrastructure across large areas of the park; and expanding ski resorts where it is possible to buy lodges. Much of the landscape that is now part of the park has a long history of summer grazing by sheep and cattle, with stockworkers’ huts scattered across the high country. This “wilderness” has been home to Aboriginal people for millennia, as well as well-known grazing grounds for more than a century.
These complexities and contradictions reflect our often unconscious modern propensity for hubris: we insist we are in charge of what happens on the planet, including in its “wild” places and “wild” species. Terms like “land management”, “natural resource management”, and “conservation management”, all reflect this assumption of superiority and control.
The United States has similar controversies over the management of mustangs across large areas of the west. New Zealand has the Kaimanawa horses, a special and isolated herd on army land. In both of those countries, as in Australia, there is a unique history of horse interactions with Indigenous communities. The great Native American horse cultures are well known and extraordinary, as Indians had no introduction to equestrian skills from the Spanish invaders, they learnt extremely quickly from scratch.
The first horses in New Zealand were a gift to Maori communities from missionary Samuel Marsden in 1814, and a Waitangi Tribunal Claim has been brought to protect the Kaimanawa horses as Maori taonga (treasures). Aboriginal stockmen and stockwomen were the mainstay of the pastoral industry all over Australia until the equal wage ruling of 1968 resulted in the wholesale expulsion of Aboriginal stockworkers in north and central Australia.
Peter Mitchell’s recent book Horse Nations uses that term to describe the people-animal relationship in certain Indigenous communities. Both Native American and Aboriginal cosmologies often place animals including horses, as their own “nations”, with whom they have a responsibility to respectfully interact.
The wild horses of the Australian Alps are arguably the strongest cultural icons. The enduring legacy of The Man from Snowy River, both the iconic Banjo Paterson poem and the 1980s film, but also the Silver Brumby series of novels by Elyne Mitchell, still in print after nearly 70 years, idealise the strength, beauty and spirit of wild mountain horses. At least one source suggests that “the man” from Paterson’s poem was in fact a young Aboriginal rider.
This is not at all implausible – there is much documentation, as well as strong oral histories, of Aboriginal men and women working stock on horseback across the Snowy Mountains. The Aboriginal mountain missions at Brungle and Delegate both have many stories of earlier generations working as stock riders and also mustering wild mountain horses. David Dixon, Ngarigo elder, says
Our old people were animal lovers. They would have had great respect for these powerful horse spirits. Our people have always been accepting of visitors to our lands and quite capable of adapting to change so that our visitors can also belong, and have their place.
While the iconic figure of the cowboy and stockman is masculine, amongst Aboriginal stockworkers women and girls were likely as common as men and boys. In contemporary times, women far outnumber men in equestrian participation, and brumby defenders are equally represented by men and women. Four Australian horsewomen generously shared their knowledge and skills in the research that backgrounds this essay.
In the mid 1970s, I worked as a ranger in Kosciuszko National Park. In those days rangering was a seat-of-the-pants enterprise: we used to buy at least part of our uniforms out of our own money because the issued items were so inadequate, we taught ourselves to cross-country ski, we drank socially with the brumby-runners and other people from the surrounding rural communities.
In many places rangers were and are intimately part of the community, not seen as “public servants”. There is a complex and interesting relationship between university-educated national parks staff and local rural workers with deeply embodied knowledge and skills, with rangers acknowledging that they need the skills of these locals to carry out much animal-related work in the parks, including trapping and mustering wild horses. Recent proposals to helicopter shoot large numbers of wild horses in Kosciuszko would potentially sever this link. Helicopter shooting requires specific marksmanship skills not common in rural communities.
While we debate how to reduce our wild horse numbers, other countries are working to re-establish wild horse herds in Europe and Asia. It is often argued that domestication saved horses (and many other species) from extinction, aiding their establishment all over the planet while their wild ancestors diminished or disappeared. Creating populations of newly wild species is termed both “rewilding’ and ”de-domestication“, and there are numerous and increasing examples around the world. Some of these proposals include the reestablishment of species long extinct, or their ecological equivalents.
In the period increasingly accepted as the Anthropocene, species are both declining and flourishing. Domesticated species have been moved all over the world; other introduced species flourish in new landscapes, and many of these are escaped or released domesticates. In the oceans, as large predators have declined all the cephalopods (octopus, squid and cuttlefish) are increasing. Highly specialised species that evolved on isolated islands have declined precipitously, while generalist species are flourishing.
Global conservation management attempts to work against both of these trends: we attempt to suppress populations of flourishing species, while supporting or increasing populations of declining ones, including through translocations and captive breeding programs. These activities call into question the nature of nature in the 21st century: what is the “wild” in all this management and manipulation?
In these questions, the lives and cosmologies of Indigenous peoples, and the lives of other species, offer us serious teachings. The agency and intelligence of animals, the increasing discoveries of distinct cultures amongst animal populations, the agency of planetary systems in continually reorganising around changing inputs, all stand against the modern human insistence on control, stability and stasis.
While hiking mountain grasslands looking for wild horse bands, I have several times come across horse skeletons whitening in the sunlight, their energy and power transmuted back into the source from which new lives will spring. In a world where human societies are increasingly narcissistic, where our dominant concern is ourselves, recognising the agency and intelligence of other species can be deeply humbling.
Perhaps our task is to harmonise ourselves with these old and new environments, not continually attempt to “manage” them into some other state that we in our hubris think is more desirable, whether ecologically, economically or culturally.
Thanks to Adrienne Corradini, Jen Owens, Blaire Carlon and Tonia Gray for improving my understanding of horse and brumby issues.
What if there was a humane, targeted way to wipe out alien pest species such as mice, rats and rabbits, by turning their own genes on themselves so they can no longer reproduce and their population collapses?
Gene drives – a technique that involves deliberately spreading a faulty gene throughout a population – promises to do exactly that.
Our study, published today in the journal Proceedings of the Royal Society B, suggests that under certain circumstances, genome editing could work.
Good and bad genes
The simplest way to construct a gene drive aimed at suppressing a pest population is to identify a gene that is essential for the pest species’ reproduction or embryonic development. A new DNA sequence – the gene-drive “cassette” – is then inserted into that gene to disrupt its function, creating a faulty version (or “allele”) of that gene.
Typically, faulty alleles would not spread through populations, because the evolutionary fitness of individuals carrying them is reduced, meaning they will be less likely than non-faulty alleles to be passed on to the next generation. But the newly developed CRISPR gene-editing technology can cheat natural selection by creating gene-drive sequences that are much more likely to be passed on to the next generation.
Here’s how the trick works. The gene-drive cassette contains the genetic information to make two new products: an enzyme that cuts DNA, and a molecule called a guide RNA. These products act together as a tiny pair of molecular scissors that cuts the second (normal) copy of the target gene.
To fix the cut, the cell uses the gene drive sequence as a repair template. This results in a copy of the gene drive (and therefore the faulty gene) on both chromosomes.
This process is called “homing” and, when switched on in the egg- or sperm-producing cells of an animal, it should guarantee that almost all of their offspring inherit the gene-drive sequence.
As the gene-drive sequence spreads, mating between carriers becomes more likely, producing offspring that possess two faulty alleles and are therefore sterile or fail to develop past the embryonic stage.
Will it work?
Initial attempts to develop suppression drives will likely focus on invasive species with rapid life cycles that allow gene drives to spread rapidly. House mice are an obvious candidate because they have lots of offspring, they have been studied in great detail by biologists, and have colonised vast areas of the world, including islands.
In our study we developed a mathematical model to predict whether gene drives can realistically be used to eradicate invasive mice from islands.
Our results show that this strategy can work. We predict that a single introduction of just 100 mice carrying a gene drive could eradicate a population of 50,000 mice within four to five years.
But it will only work if the process of genetic homing – which acts to overcome natural selection – functions as planned.
Experiments with non-vertebrate species show that homing can fail in some circumstances. For example, the DNA break can be repaired by an alternative mechanism that stitches the broken DNA sequence back together without copying the gene-drive template. This also destroys the DNA sequence targeted by the guide RNA, producing a “resistance allele” that can never receive the gene drive.
A recent study in mosquitos estimated that resistance alleles were formed in at least 2% of homing attempts. Our simulation experiments for mice confirm this presents a serious problem.
After accounting for low failure rates during homing, the creation and spread of resistance alleles allowed the modelled populations to rebound after an initial decline in abundance. Imperfect homing therefore threatens the ability of gene drives to eradicate or even suppress pest populations.
One potential solution to this problem is to encode multiple guide RNAs within the gene-drive cassette, each targeting a different DNA sequence. This should reduce homing failure rates by allowing “multiple shots on goal”, and avoiding the creation of resistance alleles in more cases.
To wipe out a population of 200,000 mice living on an island, we calculate that the gene-drive sequences would need to contain at least three different guide RNA sequences, to avoid the mice ultimately getting the better of our attempts to eradicate them.
From hype to reality
Are gene drives a hyperdrive to pest control, or just hype? Part of the answer will come from experiments with gene drives on laboratory mice (with appropriate containment). That will help to provide crucial data to inform the debate about their possible deployment.
We also need more sophisticated computer modelling to predict the impacts on non-target populations if introduced gene drives were to spread beyond the populations targeted for management. Using simulation, it will be possible to test the performance and safety of different gene-drive strategies, including strategies that involve multiple drives operating on multiple genes.
The giant African land snail is a poster child of a global epidemic: the threat of invasive species. The snails are native to coastal East Africa, but are now found across Asia, the Pacific and the Americas – in fact, almost all tropical mainlands and islands except mainland Australia.
Yet, despite their fearsome reputation, our research on Christmas Island’s invasive snail population suggests the risk they pose to native ecosystems has been greatly exaggerated.
Giant African land snails certainly have the classic characteristics of a successful invader: they can thrive in lots of different places; survive on a broad diet; reach reproductive age quickly; and produce more than 1,000 eggs in a lifetime. Add it all together and you have a species recognised as among the worst invaders in the world.
In research recently published in the journal Austral Ecology, we tested these assumptions by investigating giant African land snails living in native rainforest on Christmas Island.
Giant African land snails have spread through Christmas Island with the help of another invasive species: the yellow crazy ant.
Until these ants showed up, abundant native red land crabs ate the giant snails before they could gain a foothold in the rainforest. Unfortunately, yellow crazy ants have completely exterminated the crabs in some parts of the island, allowing the snails to flourish.
We predicted that the snails, which eat a broad range of food, would have a significant impact on leaf litter and seedling survival.
However, our evidence didn’t support this at all. Using several different approaches – including a field experiment, lab experiment and observational study – we found giant African land snails were pretty much just eating a few dead leaves and little else.
We almost couldn’t distinguish between leaf litter removal by the snails compared to natural decomposition. They were eating leaf litter, but not a lot of it.
We saw almost no impact on seedling survival, and the snails were almost never seen eating live foliage. In one lab trial, we attempted to feed snails an exclusive diet of fresh leaves, but so many of these snails died that we had to cut the experiment short. Perhaps common Christmas Island plants just aren’t palatable.
It’s possible that the giant African land snails are causing other problems on Christmas Island. In Florida, for example, they carry parasites that are a risk to human health. But for the key ecological processes we investigated, the snails do not create the kind of disturbance we would assume from their large numbers.
Do we have good data on the ecological impact of all invasive species? Of course not. Should we still try to control all abundant invasive species even if we don’t have evidence they are causing harm? That’s a more difficult question.
The precautionary principle drives much of the thinking behind the management of invasive species, including the giant African land snail. The cost of doing nothing is potentially very high, so it’s safest to assume invasive species are having an effect (especially when they exist in high numbers).
But we should also be working hard to test these assumptions. Proper monitoring and experiments give us a true picture of the risks of action (or inaction).
In reality, the giant African land snail is more the poster child of our own knee-jerk reaction to abundant invaders.
Humans and their canine companions share many close bonds. Wolves (Canis lupus) were the first animal domesticated by people, some time between 15,000 and 50,000 years ago.
There are now an estimated 1 billion domestic dogs across their near-global distribution.
Domestic dogs include feral and free-ranging animals (such as village and camp dogs), as well as those that are owned by and completely dependent on humans (pet dogs).
Our latest research reveals that the ecological “pawprint” of domestic dogs is much greater than previously realised.
Using the IUCN Red List of Threatened Species, we counted how many species are negatively affected by dogs, assessed the prevalence of different types of impacts, and identified regions with the greatest number of affected species.
Dogs are third-most-damaging mammal
We found that dogs are implicated in the extinction of at least 11 species, including the Hawaiian Rail and the Tonga Ground Skink. Dogs are also a known or potential threat to 188 threatened species worldwide: 96 mammal, 78 bird, 22 reptile and three amphibian species. This includes 30 critically endangered species, two of which are classed as “possibly extinct”.
Even though dogs have an almost global distribution, the threatened species they are known to affect are concentrated in certain parts of the globe. South-East Asia, South America, Central America and the Caribbean each contain 28 to 30 threatened species impacted by dogs. Other hotspots include Australia, Micro/Mela/Polynesia and the remainder of Asia.
Lethal and non-lethal impacts
Predation was the most commonly reported impact of dogs on wildlife. The typically omnivorous diet of dogs means they have strong potential to affect a diversity of species. For instance, dogs killed at least 19 endangered Kagu (a ground-dwelling bird) in New Caledonia in 14 weeks. Threatened species with small population sizes are particularly vulnerable to such intense bouts of predation.
Aside from simply killing animals, dogs can harm wildlife in other ways, such as by spreading disease, interbreeding with other canids, competing for resources such as food or shelter, and causing disturbances by chasing or harassment. For example, contact with domestic dogs increases disease risk for endangered African Wild Dogs in Kenya.
Part of the problem is that when wild animals perceive dogs as a threat, they may change their behaviour to avoid them. One study near Sydney found that dog walking in parklands and national parks reduced the abundance and species richness of birds, even when dogs were restrained on leads.
None of the Red List assessments mentioned such indirect risk effects, which suggests that their frequency is likely to be much higher than reported.
For example, in Australia, the closely related dingo (Canis dingo) can suppress populations of introduced predators such as red foxes (Vulpes vulpes), and in doing so can benefit smaller native prey. It is possible that domestic dogs could perform similar ecological roles in some situations.
An emerging and exciting conservation role for dogs is their growing use as “guardian animals” for wildlife, with the remarkable story of Oddball being the most well known.
Managing the problem
Dogs not only interact with wildlife, but can also attack and spread disease to humans, livestock and other domestic animals. As such, managing the problem requires looking at ecological, cultural and social perspectives.
Some of the regions with high numbers of species threatened by dogs are also hotspots for urbanisation and road building, which make it easier for dogs to access the habitats of threatened species. Urban development increases food waste, which feeds higher numbers of dogs. As dogs expand into new areas, the number of species they impact is likely to grow.
We can protect wildlife by integrating human health and animal welfare objectives into dog management. Vaccination and desexing campaigns can reduce disease risk and overpopulation problems. We should also focus on responsible dog ownership, removing dogs without owners, and reducing access to food waste.
Given the close relationship between humans and dogs, community engagement should form the basis of any management program. More research is needed to get a better picture of the scale of the problem, and of how dogs interact with other threats such as habitat loss. Such actions are critically important for ensuring the conservation of wildlife threatened by dogs around the world.
This article was co-authored by Dr Al Glen from Landcare Research, New Zealand and Dr Abi Vanak from the Ashoka Trust for Research in Ecology and the Environment, India. These institutions had no role in the design or funding of this research.