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.
Over the past year the global media has been full of reports of catastrophic fires in California, the Mediterranean, Chile and elsewhere. One suggested reason for increases in catastrophic wildfires has been human-induced climate change. Higher temperatures, drier weather and windier conditions all increase the impact of fires.
While climate change indeed raises the risk of wildfires, our research shows that another way humans can change patterns of fire activity is by introducing flammable plants to new environments.
Plantations of highly flammable exotic species, such as pines and eucalypts, probably helped to fuel the recent catastrophic fires in Portugal and in Chile. In arid regions, such as parts of the US southwest, the introduction of exotic grasses has transformed shrublands, as fires increase in severity.
Invasive plants and fire
How do invasive plants change fire patterns? We burned species mixtures (aka “mixed grills”) on our plant barbecue to help find out.
One of the main ways flammable invasive plants can have long-lasting impacts on an ecosystem comes from positive fire-vegetation feedbacks. Such feedbacks can occur when a flammable weed invades a less fire-prone ecosystem. By changing the available fuel the invader makes fires more likely and often hotter.
If the invading species has characteristics that allow it to outcompete native species after a fire, then it will further dominate the ecosystem. Such traits include thick bark, the ability to resprout following fire, or seeds that survive burning. This invasion will likely lead to more fires, changing the species composition and function of the ecosystem in a “fire begets fire” cycle. Extreme examples of this dynamic are where flammable grasses or shrubs invade forests, leading to loss of the forest ecosystems.
We wanted to understand how invasive plants interact with other species when burned in combination. To explore the mechanisms underpinning such feedbacks, we examined how invasive plants might change the nature of a fire when burned together with native species.
We collected 70cm shoots of four globally invasive species (of both high and low flammability) and burned them in pairwise combinations with New Zealand native trees and shrubs to determine which characteristics of a fire could be attributed to the invasive plants.
We found that overall flammability was largely driven by the most flammable species in the mixture, showing how highly flammable weeds could set in motion fire-vegetation feedbacks.
We established that a greater difference in flammability between the two species led to a larger influence of the more flammable species on overall flammability. This outcome suggests weeds that are much more flammable than the invaded community can have larger impacts on fire patterns.
Importantly, we also showed the influence of the highly flammable species was independent of its biomass, meaning highly flammable weeds may impact community flammability even at low abundances.
When we looked closer at the different components of flammability (combustibility, ignitability, consumability and sustainability) we found some important nuances in our results.
While the maximum temperature reached in our burns (combustibility) and the ignition speed (ignitability) were both most influenced by the more flammable species, consumability (the amount of biomass burned) and sustainability (how long the fire burns) were equally influenced by both the more flammable and less flammable species.
In short, more flammable weeds will cause a fire to ignite more quickly and burn hotter.
However, less flammable species can reduce the duration of a fire compared to when a more flammable species is burnt alone. These results could have important ecological implications, as the longer a fire burns the more likely it is to kill plants: low-flammability plants could reduce this impact.
Managing weeds to reduce fire impacts
Even low abundances of highly flammable invasive weeds could set in motion positive fire-vegetation feedbacks that lead to drastic changes to ecosystems. If this result holds when our shoot-scale experiments are repeated using field trials, then land managers should work quickly to remove even small infestations of highly flammable species, such as gorse (Ulex europaeus) and prickly hakea (Hakea sericea).
Conversely, the role of low flammability plants in extinguishing fires further supports the suggestion that the strategic planting of such species across the landscape as “green firebreaks” could be a useful fire management tool.
In any case, our “mixed grill” study further highlights the role of exotic plants in fuelling hotter wildfires.
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.
Throughout history, humans have taken plants and animals with them as they travelled the world. Those that survived the journey to establish populations in the diaspora have found new opportunities as they integrate into new ecosystems.
These immigrant populations have come to be regarded as “invaders” and “aliens” that threaten pristine nature. But for many species, migration may just be a way to survive the global extinction crisis.
In our recently published study, we found that one of the Earth’s most imperilled group of species is hanging on in part thanks to introduced populations.
Megafauna – plant-eating terrestrial mammals weighing more than 100kg – have established in new and unexpected places. These “feral” populations are rewilding the world with unique and fascinating ecological functions that had been lost for thousands of years.
Today’s world of giants is only a shadow of its former glory. Around 50,000 years ago, giant kangaroos, rhino-like diprotodons, and other unimaginable animals were lost from Australia.
Formal conservation distribution maps show that much of Earth is empty of megafauna. But this is only a part of the picture.
Many megafauna are now found outside their historic native ranges. In fact, thanks to introduced populations, regional megafauna species richness is substantially higher today than at any other time during the past 10,000 years.
Worldwide introductions have increased the number of megafauna by 11% in Africa and Asia, by 33% in Europe, by 57% in North America, by 62% in South America, and by 100% in Australia.
Australia lost all of its native megafauna tens of thousands of years ago, but today has eight introduced megafauna species, including the world’s only wild population of dromedary camels.
These immigrant megafauna have found critical sanctuary. Overall, 64% of introduced megafauna species are either threatened, extinct, or declining in their native ranges.
Some megafauna have survived thanks to domestication and subsequent “feralisation”, forming a bridge between the wild pre-agricultural landscapes of the early Holocene almost 10,000 years ago, to the wild post-industrial ecosystems of the Anthropocene today.
Wild cattle, for example, are descendants of the extinct aurochs. Meanwhile, the wild camels of Australia have brought back a species extinct in the wild for thousands of years. Likewise, the vast majority of the world’s wild horses and wild donkeys are feral.
There have been global calls to rewild the world, but rewilding has already been happening, often with little intention and in unexpected ways.
A small population of wild hippopotamuses has recently established in South America. The nicknamed “cocaine hippos” are the offspring of animals who escaped the abandoned hacienda of Colombian drug lord Pablo Escobar.
By insisting that only idealised pre-human ecosystems are worth conserving, we overlook the fact that these emerging new forms of wilderness are not only common but critical to the survival of many existing ecosystems.
Megafauna are Earth’s tree-breakers, wood-eaters, hole-diggers, trailblazers, wallowers, nutrient-movers, and seed-carriers. By consuming coarse, fibrous plant matter they drive nutrient cycles that enrich soils, restructure plant communities, and help other species to survive.
The wide wanderings of megafauna move nutrients uphill that would otherwise wash downstream and into the oceans. These animals can be thought of as “nutrient pumps” that help maintain soil fertility. Megafauna also sustain communities of scavengers and predators.
In North America, we have found that introduced wild donkeys, locally known as “burros”, dig wells more than a metre deep to reach groundwater. At least 31 species use these wells, and in certain conditions they become nurseries for germinating trees.
The removal of donkeys and other introduced megafauna to protect desert springs in North America and Australia seems to have led to an exuberant growth of wetland vegetation that constricted open water habitat, dried some springs, and ultimately resulted in the extinction of native fish. Ironically, land managers now simulate megafauna by manually removing vegetation.
It is likely that introduced megafauna are doing much more that remains unknown because we have yet to accept these organisms as having ecological value.
Living in a feral world
Like any other species, the presence of megafauna benefits some species while challenging others. Introduced megafauna can put huge pressure on plant communities, but this is also true of native megafauna.
Whether we consider the ecological roles of introduced species like burros and brumbies as desirable or not depends primarily on our own values. But one thing is certain: no species operates in isolation.
Visions of protected dingoes hunting introduced donkeys and Sambar deer in Australia, or protected wolves hunting introduced Oryx and horses in the American West, can give us a new perspective on conserving both native and introduced species.
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 word “pigeon” evokes thoughts of gentle cooing, fluttering in rafters, and poo-encrusted statues. The species responsible for the encrustation is deeply familiar to us, having ridden waves of European expansionism to inhabit every continent, including Australia. First domesticated thousands of years ago, urban pigeons have turned feral again.
Less familiar are the native species that are not your stereotypical pigeons: a posse of pointy-headed crested pigeons in a suburban park, or a flock of topknot pigeons feeding in a camphor laurel.
The future of Australia’s native pigeons however, may depend on our domestic pigeons. Australia’s domestic pigeon population — both feral and captive – is large and interconnected by frequent local and interstate movements. Pigeon racing, for example, involves releasing captive birds hundreds of kilometres from their homes only so they may find their way back. While most birds do navigate home, up to 20% will not return, of which some will join feral pigeon populations. Birds are also traded across the country and illegally from overseas. These movements, together with poor biosecurity practices, mean that captive pigeons can and do mingle with feral domestic pigeons.
And here’s a paradox. Could Australia’s feral domestic pigeons become the vector for a dramatic decline of columbids – native species on which Australian ecosystems rely?
Emerging viral epidemics
In recent years, two notable infectious diseases have been found to affect our captive domestic pigeons: the pigeon paramyxovirus type 1 (PPMV1) and a new strain of the pigeon rotavirus (G18P). These diseases are notable because in captive domestic flocks they are both spectacularly lethal and difficult to control.
PPMV1, although likely to have originated overseas, is now endemic in Australia. This virus has jumped from captive to feral domestic pigeon populations on several occasions, but fortunately has yet to establish in feral populations.
The movements of captive pigeons, and their contact with their feral counterparts, can be the route through which virulent and lethal diseases – such as the PPMV1 and the G18P – may spread to Australia’s native columbids.
What have we got to lose?
Fortunately, neither PPMV1 nor G18P has crossed over to Australia’s native columbids. We can’t say how likely this is, or how serious the consequences would be, because we have not previously observed such viral infections among our native pigeons.
If the viruses prove equally lethal to native columbids as they are to domestic pigeons, we could see catastrophic population declines across numerous columbid species in Australia over a short period of time.
Should these viruses spread (via feral domestic pigeons), the control and containment of losses among our native pigeon species would be near impossible. Such a nightmare scenario can only be avoided by predicting if and how these viruses might “spill over” into wild columbids so that we can prevent this in the first place.
Protecting our pigeons
Agricultural poultry is routinely screened to check their vulnerability to threats like the PPMV1 and G18P. Such screening is an appropriate response to protect our agricultural industry.
For our native pigeons and doves however, no such similar testing is planned. Based on progress in veterinary vaccine development and advancements in understanding of feral pigeon control, the knowledge and technology required to mitigate this threat should be relatively inexpensive. The threat for these species can be actively managed, now, by improving our biosecurity and vaccination programs for captive domestic pigeons, and eradicating feral domestic pigeons.
The protection of our native columbids however, ultimately relies on valuing their ecosystem functions in the first place.