Authors: Euan G Ritchie, Jannik Schultner, Dale G Nimmo, Joern Fischer, Jan Hanspach, Tobias Kuemmerle, Laura Kehoe and Ine Dorresteijn
Published in:Proceedings of the Royal Society B, volume 283, issue 1834 (July 2016)
A rapidly growing body of the literature reveals the important roles apex predators play in shaping the composition and functioning of ecological communities worldwide.
The principal effects of apex predators — namely herbivore and mesopredator population suppression — are often evident following their removal from environments, or their reintroduction, including rewilding initiatives. What remains less clear, however, is to what extent humans versus other apex predators affect ecosystems, how both interact across gradients of anthropogenic pressure and how such interactions can be affected by underlying bottom-up processes.
Such questions are critical to answer in the Anthropocene, where effective management of ecosystems and conservation of biodiversity requires a better understanding of how top-down and bottom-up processes vary according to anthropogenic influences…
Ritchie EG, Schultner J, Nimmo DG, Fischer J, Hanspach J, Kuemmerle T, Kehoe L, Dorresteijn I (2016) Crying wolf: limitations of predator–prey studies need not preclude their salient messages, Proceedings of the Royal Society B, 283:1834 PDFDOI
If you knew that there was zero percent chance of being eaten by a shark, would you swim more often? Rhetorical questions aside, the fear of being eaten has a profound influence on other animals too, and on the way they use marine environments.
Turtles, for example, fear being eaten by sharks and this restricts the movement and behaviour of entire populations. But when the fear of being eaten dissipates, we see that turtles eat more, breed more, and go wherever they please.
It might sound like turtle paradise, but in an article published today in Nature Climate Change we show that loss of ocean predators can have serious, cascading effects on oceanic carbon storage and, by extension, climate change.
When sea turtles aren’t being frightened by sharks, they consume more seagrass, and the Carbon stocks stored in it. Image credit Framk_am_Main via Flickr
Cascading effects
For a long time we’ve known that changes to the structure of food webs – particularly due to loss of top predators – can alter ecosystem function. This happens most notably in situations where loss of predators at the top of the food chain releases organisms lower in the food chain from top-down regulatory control. For instance, the loss of a predator may allow numbers of its prey to increase, which may eat more of their prey, and so on. This is known as “trophic downgrading”.
With the loss of some 90% of the ocean’s top predators, trophic downgrading has become all too common. This upsets ecosystems, but in our article we also report its effects on the capacity of the oceans to trap and store carbon.
This can occur in multiple ecosystems, with the most striking examples in the coastal zone. This is where the majority of the ocean’s carbon is stored, within seagrass, saltmarsh and mangrove ecosystems – commonly known as “blue carbon” ecosystems.
Blue carbon ecosystems capture and store carbon 40 times faster than tropical rainforests (such as the Amazon) and can store the carbon for thousands of years. This makes them one of the most effective carbon sinks on the planet. Despite occupying less that 1% of the sea floor, it is estimated that coastal blue carbon ecosystems sequester more than half the ocean’s carbon.
The carbon that blue carbon ecosystems store is bound within the bodies of plants and within the ground. When predators such as sharks and other large fish are removed from blue carbon ecosystems, resulting increases in plant-eating organisms can destroy the capacity of blue carbon habitats to sequester carbon.
For example, in seagrass meadows of Bermuda and Indonesia, less predation on herbivores has resulted in spectacular losses of vegetation, with removal of 90–100% of the above-ground vegetation.
Stop killing predators
Such losses of vegetation can also destabilise carbon that has been buried and accumulated over millions of years. For example, a 1.5-square-kilometre die-off of saltmarsh in Cape Cod, Massachusetts, caused by recreational overharvesting of predatory fish and crabs, freed around 248,000 tonnes of below-ground carbon.
If only 1% of the global area of blue carbon ecosystems were affected by trophic cascades as in the latter example, this could result in around 460 million tonnes of CO2 being released annually, which is equivalent to the annual CO2 emissions of around 97 million cars, or just a bit less than Australia’s current annual greenhouse gas emissions.
So what can be done? Stronger conservation efforts and modification of fishing regulations can help restore marine predator populations, and thereby help maintain the important indirect role that predators play in climate change mitigation.
It’s about restoring balance so that we have, for example, healthy and natural numbers of both sea turtles and sharks. Policy and management need to reflect this important realisation as a matter of urgency.
More than 100 million sharks may be killed in fisheries each year, but if we can grant these predators great protection they may just help to save us in return.
Authors: Thomas M Newsome, Guy-Anthony Ballard, Mathew S Crowther, Justin A Dellinger, Peter J S Fleming, Alistair S Glen, Aaron C Greenville, Chris N Johnson, Mike Letnic, Katherine E Moseby, Dale G Nimmo, Michael Paul Nelson, John L Read, William J Ripple, Euan G Ritchie, Carolyn R Shores, Arian D Wallach, Aaron J Wirsing and Christopher R Dickman.
Abstract
There is global interest in restoring populations of apex predators, both to conserve them and to harness their ecological services.
In Australia, reintroduction of dingoes (Canis dingo) has been proposed to help restore degraded rangelands. This proposal is based on theories and the results of studies suggesting that dingoes can suppress populations of prey (especially medium- and large-sized herbivores) and invasive predators such as red foxes (Vulpes vulpes) and feral cats (Felis catus) that prey on threatened native species. However, the idea of dingo reintroduction has met opposition, especially from scientists who query the dingo’s positive effects for some species or in some environments.
Here, we ask ‘what is a feasible experimental design for assessing the role of dingoes in ecological restoration?’ We outline and propose a dingo reintroduction experiment — one that draws upon the existing dingo-proof fence—and identify an area suitable for this (Sturt National Park, western New South Wales).
Although challenging, this initiative would test whether dingoes can help restore Australia’s rangeland biodiversity, and potentially provide proof-of-concept for apex predator reintroductions globally.
Newsome TM, Ballard G, Crowther MS, Glen AS, Dellinger JA, Fleming PJS, Greenville AC, Johnson CN, Letnic M, Moseby KE, Nimmo DG, Nelson MP, Read JL, Ripple WJ, Ritchie EG, Shores CR, Wallach AD, Wirsing AJ, Dickman CR (2015) Resolving the value of the dingo in ecological restoration, Restoration EcologyPDFDOI
Feral cats are devastating our wildlife, so we need a long-term, sustainable solution. This is where Australia’s natural predators come in.
Feral cats are decimating Australian wildlife. Could the introduction of apex predators be part of the solution? Image credit Timo via Flickr.
A few moments on the internet will reveal that, as companion animals, cats are rivalled only by dogs. Our love affair with them is hardly surprising: they are elegant, graceful and affectionate animals. But they are also highly adaptable and successful hunters. Sadly our soft spot for them brings with it disastrous consequences for smaller wildlife species, particularly mammals, birds and reptiles.
Cats are wreaking havoc on our native wildlife, from northern Australia’s Kimberley to southern Australia’s Alps: nowhere is unaffected. Along with other threats such as habitat loss and fire, cats are pushing many native animals to the brink of extinction.
How serious is the cat problem? It’s been estimated that there may be as many as 15 million feral cats in Australia, each killing about five native animals per night: a nightly total of around 75 million native animals. And it’s not just feral cats doing the damage; that adorable Fluffy has a sinister side too. It often isn’t recognised that there is abundant and diverse native wildlife in and around many Australian cities and towns, including bandicoots, sugar gliders, quolls, koalas, parrots, water dragons and snakes. The development of remotely triggered camera traps has provided ample evidence that such wildlife frequently falls victim to domestic cats on their nightly excursions.
For a long time our other invasive predator of note, the European red fox, was seen as enemy number one to our wildlife, particularly mammals, but there’s a growing realisation that its many meals are eclipsed by those of cats, both feral and owned. Globally, cats are rated at 38th and red foxes at 99th on the IUCN’s list of 100 of the World’s Worst Invasive Alien Species.
And it’s not just cats’ appetites that are doing the damage. Cats are the primary host of the parasite Toxoplasma gondii, which can infect most mammals and birds, causing the potentially fatal disease toxoplasmosis (toxo). Even when not lethal, toxo can impact wildlife populations indirectly, through effects that include impaired vision, disorientation, loss of coordination, anorexia, lethargy, fever, skewed offspring sex ratios and abortion. Most bizarrely, animals infected with toxo are often attracted to the smell of cats and hence are more likely to be killed and eaten by them; a kind of parasite-driven mind control.
So what can we do to reduce the damage done by cats and mitigate this national conservation disaster? The most urgent needs are for vigorous education and awareness campaigns about the impacts of cats on wildlife, and tighter regulations and enforcement around responsible cat ownership. This should include mandatory desexing, the imposition of night-time curfews and containment of cats within pet owners’ properties at all times.
In addition, a number of on-ground solutions are already available or under consideration; there is no doubt that a range of measures will be essential. These include the development of cat-specific poison baits and release of a cat-specific virus. However cat control through disease and poison programs are unlikely to be effective long-term, as natural selection will favour disease-resistant and cautious cats which will result in rapid population bounce-back. After all, cats are known for their smarts, being more attracted to moving prey and suspicious of foreign objects. Under good conditions, furthermore, they can reproducequickly and bountifully.
Fencing can effectively exclude cats from habitats and provide wildlife refuges, and they play a critical role in holding the line for species such as bilbies, already teetering on the edge of extinction. However the use of fencing as a regular and ongoing strategy for wildlife conservation in our vast country will not only be logistically challenging (perhaps impossible) but also forbiddingly expensive. More fences will also result in it becoming harder and harder to see bilbies, bandicoots and many other native species in the wild. What a sad outcome for our nation and what an indictment of our allowing cats to reign supreme and unchecked!
Cue the predator
Why are cats able to run rampant? A significant reason is the lack of balance that now characterises our ecosystems. A crucial step is likely to be returning native top predators, in particular dingoes and Tasmanian devils, to landscapes, so that they can resume their important ecological roles. Scientific research (pdf) is strongly suggesting that dingoes not only kill cats but also instill fear in them, which means that they avoid areas and times where dingoes are active. Hence dingoes can provide a 24-hour-a-day, seven-day-a-week, cat control service.
If we want to conserve our iconic and globally unique wildlife then we need to work with, rather than against nature, and supporting more positive management of dingoes and returning Tasmanian devils to mainland Australia for their biodiversity benefits (pdf)would be one of the best and most cost effective things policy-makers could do now. Non-government conservation organisations such as the Australian Wildlife Conservancy are already leading the way.
In 1995 the USA and Canada worked together to reintroduce wolves to Yellowstone for their ecosystem benefits. The success of this bold step is now the stuff of legend. Isn’t it time we had our ‘Yellowstone moment’ and began restoring Australia’s ecosystems to some of their former glory?
Applied ecology and conservation research group 