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Invasive predators are eating the world's animals to extinction – and the worst is close to home
Invasive species are a threat to wildlife across the globe – and invasive, predatory mammals are particularly damaging.
Our research, recently published in Proceedings of the National Academy of Sciences, shows that these predators – cats, rats and foxes, but also house mice, possums and many others – have contributed to around 60% of bird, mammal and reptile extinctions. The worst offenders are feral cats, contributing to over 60 extinctions.
So how can we stop these mammals eating away at our threatened wildlife?
Counting the costOur study revealed that invasive predators are implicated in 87 bird, 45 mammal and 10 reptile extinctions — 58% of these groups’ contemporary extinctions worldwide.
Invasive predators also threaten 596 species classed as vulnerable, endangered or critically endangered on the International Union for the Conservation of Nature Red List. Combined, the affected species include 400 birds, 189 mammals and 149 reptiles.
Twenty-three of the critically endangered species are classed as “possibly extinct”, so the number of extinctions above is likely to be an underestimate.
Until now, these shocking statistics have been unknown, and the heavy toll of invasive predators on native biodiversity grossly underappreciated. Species extinctions attributed to invasive predators include the Hawaiian rail (Zapornia sandwichensis) and Australia’s lesser bilby (Macrotis leucura).
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We found that three canids (including the red fox and feral dogs), seven members of the weasel family or mustelids (such as stoats), five rodents, two primates, two mongooses, two marsupials and nine species from other families negatively impact threatened species. Some of these species, such as hedgehogs and brushtail possums, don’t immediately spring to mind as predators, yet they are known to prey on many threatened species.
Feral cats threaten the most species overall (430), including 63 that have become extinct. This equates to one-quarter of all bird, mammal and reptile extinctions – making the feral cat arguably the most damaging invasive species for animal biodiversity worldwide.
Five species of introduced rodent collectively threaten 420 species, including 75 extinctions. While we didn’t separate out the impacts of individual rodent species, previous work shows that black rats (Rattus rattus) threaten the greatest number of species, followed by brown rats (R. norvegicus) and Pacific rats (R. exulans).
The humble house mouse (Mus musculus) is another interesting case. Despite their small size, house mice have been recorded eating live chicks of albatrosses, petrels and shearwaters.
Other predators that threaten large numbers of species are the domestic dog (Canis familiaris), pig (Sus scrofa), small Indian mongoose (Herpestes auropunctatus), red fox (Vulpes vulpes) and stoat (Mustela erminea).
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Species found only on islands (insular endemics) account for 81% of the threatened species at risk from predators.
The isolation of many islands and a lack of natural predators mean that insular species are often naive about new predators and lack appropriate defensive responses. This makes them highly vulnerable to being eaten and in turn suffering rapid population decline or, worse, extinction. The high extinction rates of ground-dwelling birds in Hawaii and New Zealand — both of which lack native mammalian predators — are well-known examples.
Accordingly, the regions where the predators threatened the greatest number of species were all dominated by islands – Central America and the Caribbean, islands of the Pacific, the Madagascar region, New Zealand and Hawaii.
Conversely, the continental regions of North and South America, Europe, Africa and Asia contain comparatively few species threatened by invasive predators. While Australia is a continent, it is also an island, where large numbers of native birds and mammals are threatened by cats and foxes.
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Understanding and mitigating the impact of invasive mammal predators is essential for reducing the rate of global biodiversity loss.
Because most of the threatened species studied here live on islands, managing invasive predators on islands should be a global conservation priority. Invasive predators occur on hundreds of islands and predator control and eradication are costly exercises. Thus, it is important to prioritise island eradications based on feasibility, cost, likelihood of success and potential benefits.
On continents or large islands where eradications are difficult, other approaches are needed. This includes predator-proof fencing, top-predator restoration and conservation, lethal control, and maintenance of habitat structure.
Despite the shocking statistics we have revealed, there remain many unknowns. For example, only around 40% of reptile species have been assessed for the Red List, compared to 99% for birds and mammals. Very little is known about the impact of invasive predators on invertebrate species.
We expect that the number of species affected by invasive predators will climb as more knowledge becomes available.
This article was co-authored by Al Glen from Landcare Research, New Zealand.
![The Conversation](https://counter.theconversation.edu.au/content/64741/count.gif)
Tim Doherty has received funding from Earthwatch Institute Australia, Gunduwa Regional Conservation Association, Holsworth Wildlife Research Endowment, Edith Cowan University and Deakin University. Tim is affiliated with the Society for Conservation Biology (Oceania).
Chris Dickman receives funding from the Australian Research Council, the Long Term Ecological Research Network and the Threatened Species Recovery Hub of the National Environmental Science Programme.
Dale Nimmo receives funding from the Hermon Slade Foundation, the Department of Land, Water and Planning, the Department of Parks and Wildlife, Parks Victoria, and The Australian Academy of Science. He is affiliated with the Ecological Society of Australia.
Euan Ritchie receives funding from Pozible, the Australia and Pacific Science and Hermon Slade Foundations, and the Australian Research Council. Euan Ritchie is affiliated with the Ecological Society of Australia and the Australian Mammal Society.
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The silencing of the seas: how our oceans are going quiet
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Despite appearances, the oceans are far from silent places. If you dunk your head underwater you’ll hear a cacophony of sounds from wildlife great and small, crashing waves, and even rain. And it’s louder still for creatures attuned to these sounds.
However, humans are changing these ocean soundscapes. Our recent research showed that changes caused by people, from ocean acidification to pollution, are silencing the seas' natural noises. (We’re also filling the oceans with human noise).
This is bad news for the species that depend on these noises to find their way.
Ocean soundscapesAll over the world you can hear a lively crackling sound made by thousands of snapping shrimp that live along coastlines.
These common shrimp, often referred to as pistol shrimp, have a large claw that they can close with such force that a cavitation bubble is formed. As this bubble implodes on itself a loud snap is created – like a pistol shot – which can be heard over long distances.
In fact, snapping shrimp are the loudest marine invertebrates, and second only to the noisest marine animals, which are sperm whales! Snapping shrimp are found all over the world, including in coral reefs, kelp forests, seagrass beds and mangroves.
Other types of animals create ocean noise too. Urchins and parrotfish make clearly audible chomping sounds as they scrape algae off rocks. Many fish are frequent and loud talkers and make an array of sounds such as chirps, burps, whistles, knocks and so on. They use these to mark out their territory, during fights and to locate mates.
These biological sounds, together with those from rain, crashing waves and seismic activities, form the so-called underwater soundscape.
Learn more about marine soundscapes watching this video.Sounds that are emitted from temperate and tropical reefs are loud and quite constant. As such these sounds form a reliable source of information for animals, particularly for navigation.
Most animals in the sea let go of their fertilised eggs without providing any parental care. As these eggs hatch, small babies (larvae) are dispersed by ocean currents. Growing up away from coastal areas provides a safer place with fewer predators.
However, after growing for a few weeks or months in the open ocean, it is time for these young animals to return to the coast to find a home. How do they find their way in the vast and uniform open ocean? Sounds and odours from coastal habitats are key cues that allow marine animals to find their new homes and replenish adult populations.
Going quietHumans are increasingly dominating the physical and chemical environment. We are altering the carbon cycle through the burning of fossil fuels and the nitrogen cycle by extracting vast amounts of nitrogen for food production and releasing it as waste. Large amounts of this carbon and nitrogen liberation end up in the ocean.
About one-third of the carbon dioxide that humans emit into the atmosphere dissolves in the ocean, leading to increased seawater acidity (or ocean acidification). This is an obvious problem for animals that produce a calcium carbonate shell or skeleton (such as corals, some plankton, and snails). Remarkably, ocean acidification also alters the behaviour of many animals by messing up their brain functioning.
Earlier studies (see also here) have shown that ocean acidification can change the response of fish larvae to settlement habitat sounds by deterring them rather than attracting them.
Learn more about the effects of ocean acidification on fish behaviour watching this animation video.Two of our recent studies (see also here) showed that ocean acidification not only affects sound reception, but also the sounds that ocean ecosystems produce. If we don’t reduce greenhouse gas emissions, rocky reefs could be much quieter in 2100 than now. And snapping shrimps are the reason.
Coastal discharge of nutrients from sewage plants and catchment runoff also degrades kelp forests and seagrass beds. These coasts are more silent than their healthy counterparts.
In many parts of the world, kelp forests, seagrass beds and coral reefs have been replaced by carpets of turf-forming or mat-forming algae. These weedy types of algae have much lower diversity of species and provide less shelter and feeding opportunities for shrimps and other noisy animals.
Degraded habitat means fewer animals, which means less noise. For larvae that use sound as a navigational cue, this means that fewer larvae will be able to successfully locate their home. And fewer returning larvae means less replenishment of fish stocks.
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Climate change and ocean acidification act at global scales and are difficult to stop in the short term. In contrast, nutrient pollution is a local stressor, which makes it more manageable.
Various options exist for local communities to reduce nutrient pollution of coastal areas. These include improved sewage treatment, restoration of coastal vegetation (such as mangroves) and swamps that extract sediment and nutrients from stormwater runoff, and decreasing the use of rivers as outlets for polluted waters.
Reducing the impacts of nutrient pollution on coastal ecosystems makes these systems more robust and provides them with increased resilience to cope with the impacts of ocean warming and acidification.
![The Conversation](https://counter.theconversation.edu.au/content/65157/count.gif)
Ivan Nagelkerken receives funding from The Australian Research Council.
Sean Connell receives funding from The Australian Research Council.
Tullio Rossi owns the YouTube channel where the linked videos are hosted.