This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky! You might also like the podcast Imagine This, a co-production between ABC KIDS listen and The Conversation, based on Curious Kids.
My name is Sanuki and I’m 8 years old. I live in Melbourne. My question is how do plastic bags harm our environment and sea life? – Sanuki, age 8, Melbourne.
Turtles (and other animals) may mistake plastic bags for food. Turtles like to eat jellyfish, and we think turtles eat the plastic bags because they resemble jellyfish.
When turtles eat plastic, it can block their intestinal system (their guts). Therefore, they can no longer eat properly, which can kill them. The plastics in their tummy may also leak chemicals into the turtle. We don’t know whether this causes long term problems for the turtle, but it’s probably not good for them.
Plastic bags can also smother corals and other seabed communities. When plastic bags end up in our oceans, animals (including seals, dolphins and seabirds) can get tangled up in them. An animal with a plastic bag around its neck will have trouble moving through the water, catching its prey or feeding, and escaping predators.
On land, plastic bags are an eyesore. They get stuck in trees, along fence lines, or as litter at our parks and beaches.
Many people don’t realise that plastic bags can also cause flooding. Previously in Ghana (in West Africa), plastic bags blocked storm water drains during a big rainstorm. This caused flooding so bad that people were killed.
Making plastic requires a lot of energy and work
Plastic bags can even be harmful before they are used. It takes a lot of resources and energy to create a plastic bag. A key ingredient is oil. As a fossil fuel, oil must be extracted from the ground. Do we want to use fossil fuel resources to make a product that is only used once (we call this a “single use plastic”)?
Many millions of barrels of oil are used to make plastic bags every year. A lot of energy is also used to make and transport plastic bags. It is better for the environment if we reduce our energy use.
Lately, lots of people recognise the impacts that plastic bags have, and they are working on alternatives. Many local and state governments have passed plastic bag bans here in Australia, which helps stop the use of single use plastic bags.
In fact, New South Wales is the only state in Australia where you can still get thin, single use plastic bags at the grocery store.
So, remind your parents to bring their reusable cloth bags whenever you go shopping. You just might save a turtle.
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But there’s a herbivore that also causes eco-catastrophe. Rabbits both compete with native animals for food and shelter and act as easy prey for abundant populations of cats and foxes. By over-grazing vegetation and reducing habitat complexity, they make hunting easier for introduced predators.
Food webs are complex. Because of this, once an invasive species is embedded in a food web, simply eradicating them without considering the potential knock-on effects to other species they interact with, could cause unintended and undesirable consequences. We modelled different rates of rabbit population reduction to assess what level of control might be best for aiding the conservation of native mammals and not causing negative outcomes.
Rabbit numbers boom and crash
Rabbits, famously, reproduce rapidly and can cope with a relatively high predation rate. This can cause “hyper-predation”, where rabbit-inflated cat and fox populations indirectly increase the predation pressure on native mammals. This is especially so when rabbit populations intermittently crash due to, for example, extreme environmental events (like severe and prolonged droughts) or disease. This causes predators to switch their diet and eat more native mammals.
This logically suggests that reducing rabbit numbers might thus help reduce cat and fox populations, by removing their abundant prey. Collectively this should benefit native plants and animals, including many threatened mammal species. However, ecosystem and pest management is a complex game.
When controlling rabbits we need to look beyond one or two species. We should consider the potential consequences for the entire ecological community, which ultimately depend on how changes in one species percolate through the network of ecological interactions between them.
Our new research, recently published in the Journal of Applied Ecology, set out to examine these questions in more detail. We consider other key players in Australia’s arid regions, such as kangaroos and dingoes, when looking at the effects of rabbit control on small native mammals. Our aim was to provide a better understanding of how changes in rabbit populations might affect other species via the food web.
We developed a multi-species ecological network model to describe and quantify how changing rabbit abundance can affect species on different feeding levels. In addition to rabbits, small native mammals, and mesopredators (cats and foxes), our model also considers apex predators (dingo) and large herbivores (kangaroo) as part of the Australian arid food web. This model allowed us to examine changes in predator-prey interactions (including potential prey switching and hyper-predation) and how these could affect the survival of native prey through time.
We found that removing rabbits at rates between 30-40% appeared to benefit small mammals. This is approximately the rate at which rabbits are currently managed in Australia using biocontrol agents (introduced diseases).
Rabbit control in Australia typically involves a “press and pulse” approach. Rabbit populations are suppressed via biocontrol (press) and periods of warren destruction and poisoning (pulse). Finding that reducing rabbit populations by around 40% seems most beneficial to small mammals is important, as it informs how and when we combine these strategies.
The 40% rate corresponds well with the disease-induced (press) mortality rate in rabbit populations due to rabbit haemorrhagic disease and myxomatosis. These are the primary biocontrol agents used in arid Australia to control rabbit populations.
Our study supports rabbit-reduction strategies that involve sustained “press” control, that kill a moderate portion of a rabbit population, with less frequent removal at higher proportions of the population.
To effectively manage invasive species, it’s important to focus on entire communities. Targeting single species might not be enough – every animal exists within a complex web of interactions.
There has been much focus by the current government on controlling feral cats, as a way to conserve many of Australia’s unique and threatened mammal species.
However, more focus could be devoted to protecting habitat cover and complexity, by reducing the land clearing and over-grazing that makes hunting easier. We can also manage rabbits sensibly to reduce competition for resources, and indirectly control cats and foxes.
Australia’s landscapes are dotted with mature eucalypts that were standing well before Captain Cook sailed into Botany Bay. These old trees were once revered as an icon of the unique Australian landscape, but they’re rapidly becoming collateral damage from population growth. Mature eucalypts are routinely removed to make way for new suburbs.
This has a considerable impact on our native fauna. Unless society is prepared to recognise the value of our pre-European eucalypts, urban growth will continue to irrevocably change our unique Australian landscape and the wildlife it supports.
In urban landscapes, many consider large and old eucalypts a dangerous nuisance that drop limbs, crack footpaths and occupy space that could be used for housing. But when we remove these trees they are effectively lost forever. It takes at least 100-200 years before a eucalypt reaches ecological maturity.
As trees mature, their branches become large and begin to grow horizontally rather than vertically, which is more attractive to many birds as perches and platforms where they can construct a nest.
Wildlife also use cavities inside ageing eucalypts. These are formed as the heartwood – the dead wood in the centre – decays. When a limb breaks it exposes cavities where the heartwood once occurred.
This is such a ubiquitous process in our forests that around 300 of Australia’s vertebrate species, such as possums, owls, ducks, parrots and bats, have evolved to use these cavities as exclusive places to roost or nest.
Mature trees also support high concentrations of food for animals that feed on nectar, such as honeyeaters, or seed, such as parrots.
One study found that the number of native birds in an urban park or open space declines by half with the loss of every five mature eucalypts.
How can we keep old trees?
Decaying heartwood in older eucalypts leads to some large branches falling. This is when most eucalypts are removed from urban areas. So we remove trees at the exact point in time when they become more attractive to wildlife.
A well-trained arborist knows that old — or even dead — eucalypts don’t need to be removed to make them safe. A tree is only dangerous if it has what arborists call a target. Unless there is a path, road or structure under a tree, then the probability of something or someone being struck by a falling branch is often below the threshold of acceptable risk.
Progressive arborists first focus on eliminating targets. For example, they might plant shrubs around the base of dead or rapidly ageing trees to minimise pedestrian traffic, rather than eliminating trees.
Where targets can’t be managed, trimming trees can remove branches that have a high risk of falling. Trees can also be structurally supported (braced) to remain stable. Such trees remain suitable as habitat for many native species.
How to design around trees
The removal of mature eucalypts is, in part, due to urban developers not considering these trees early in the planning process.
I have worked with one developer on the outskirts of Canberra to identify important trees. The developer then planned around, rather than in spite of, these trees.
The outcome has been around 80% of mature trees have been retained. This is much greater than the proportion of mature trees retained in other new urban developments in Canberra.
Australia’s population is projected to double in 50 years, so our suburbs will continue to infill and expand. This will result in the continued loss of our mature eucalypts unless our approach to planning changes.
We collated information from about 100 previous local studies of cats’ diets across Australia. These studies involved teasing apart the contents of more than 10,000 samples of faeces or stomachs from cats collected as part of management programs.
We calculate that an average feral cat kills 225 reptiles per year, so the total feral cat population kills 596 million reptiles per year. This tally will vary significantly from year to year, because the cat population in inland Australia fluctuates widely between drought and rainy years.
We also estimated that the average pet cat kills 14 reptiles per year. That means that Australia’s 3.9 million pet cats kill 53 million reptiles in total each year. However, there is much less firm evidence to quantify the impact of pet cats, mainly because it is much more straightforward to catch and autopsy feral cats to see what they have been eating, compared with pet cats.
According to our study, cats have been known to kill 258 different Australian reptiles (snakes, lizards and turtles – but not crocodiles!), including 11 threatened species.
The cat autopsies revealed that some cats binge on reptiles, with many cases of individual cats having killed and consumed more than 20 individual lizards within the previous 24 hours. One cat’s stomach was found to contain no less than 40 lizards.
Such intensive predation probably puts severe pressure on local populations of some reptile species. There is now substantial evidence that cats are a primary cause of the ongoing decline of some threatened Australian reptile species, such as the Great Desert Skink.
By our estimate, the average Australian feral cat kills four times more lizards than the average free-roaming cat in the United States (which kills 59 individuals per year). But there are many more such cats in the US (between 30 million and 80 million), so the total toll on reptiles is likely similar.
The conservation of the Australian reptile fauna has been accorded lower public profile than that of many other groups. However, a recent international program has nearly completed an assessment of the conservation status of every one of Australia’s roughly 1,000 lizard and snake species.
Our research provides yet more evidence of the harm that cats are wreaking on Australia’s native wildlife. It underlines the need for more effective and strategic control of Australia’s feral cats, and for more responsible ownership of pet cats.
Pet cats that are allowed to roam will kill reptiles, birds and other small animals. Preventing pet cats from roaming will help the cats live longer and healthier lives – not to mention saving the lives of wildlife.
The authors acknowledge the contribution of Russell Palmer, Glenn Edwards, Alex Nankivell, John Read and Dani Stokeld to this research.
Antarctica’s ice sheets could totally collapse if the world’s fossil fuels are burnt off, according to a recent climate change simulation. While we are unlikely to see such a dramatic event any time soon, we are already observing big changes and it’s worth considering what the worst case scenario might look like for the continent’s ecosystems. How long before Antarctica turns into grassy tundra?
For now, life thrives mostly at the very edge of the continent – it’s driven by the plankton-rich Southern Ocean and clustered around seasonally ice-free areas of coastal land. The interior might be sparsely inhabited, but the continent is not as barren as many think. There are around 110 native species of moss and two flowering plants, the Antarctic hairgrass and pearlwort. These plants have flourished along the relatively mild Antarctic Peninsula in recent decades. However they can’t go much further – they already occur at almost the most southern suitable ice-free ground.
With ice-caps and glaciers receding already in the Peninsula region, native land plants and animals are benefiting from more easily available liquid water. Already we are starting to see increased populations, greater areas occupied and faster growth rates, consequences only expected to increase – everything is currently limited by the extreme physical environment.
It may eventually prove too warm for some native species, but the bigger issue in upcoming decades and centuries will be whether new and currently “non-native” species will arrive that are stronger competitors than the native organisms.
Native polar species are inherently weak competitors, as they have evolved in an environment where surviving the cold, dry conditions is the overriding selective pressure rather than competition from other biological sources. If humans (or other wildlife expanding their range southwards) bring new competitors and diseases to Antarctica, that may pose a very grave risk to the existing biodiversity. Some native species would likely be pushed into the remaining more extreme regions where they can avoid competition and continue to rely on their inherent stress tolerance abilities.
We usually split the process of natural colonisation – which applies even today in Antarctica – and that of movement of “alien” species by human agency. The best available data for the Antarctic region come from some sub-Antarctic islands, where it appears humans have been responsible for many more successful colonisations than nature. In fact, over the recent centuries of human contact with the region we have introduced 200-300 species compared to just two or three known natural colonisations.
Penguins, seals and flying seabirds move between islands and the Antarctic Peninsula, so there is potential for some natural colonisation. Vagrant birds are regularly observed across the sub-Antarctic and even along the Peninsula, some of which have colonised successfully (such as the starlings, redpolls and mallard ducks on Macquarie Island).
Migrants such as skuas and gulls, which spend time on land at both ends of their migration, could be important natural vectors of transfer for invertebrates, plant seeds and spores, and microbes into an ice-free Antarctica. Importantly, bird colonies also fertilise surrounding rock and soil with faeces, eggshells and carcasses. Plant and animal life flourishes near seabird colonies, encouraged by this enrichment.
However it can be tough to predict what Antarctic melt would mean for individual species, never mind entire ecosystems. Take penguins, for instance – they have already survived previous inter-glacial retreats, but at reduced population sizes. This time round it is likely that Adélie and emperor penguins who are more dependent upon sea ice would decline, while less ice-dependent species such as gentoos and chinstraps might benefit. Indeed, there is already some evidence that emperors are struggling (although also that they may be adapting and learning to emigrate).
However the fact fish-eating gentoo penguins are increasing on the Peninsula while Adélies and chinstraps (both krill eaters) aren’t doing so well suggests prey availability can be more to blame than ice cover. Figuring out the impact of large-scale environmental change at ecosystem or food-web level is hard – it’s a complex process that will no doubt throw up some unexpected results.
The sub-Antarctic islands are full of examples of such unexpected impacts. Pigs, dogs, cats, sheep, reindeer and rabbits have all been intentionally introduced in the past, with often devastating effects. Rats and mice were introduced to South Georgia and other islands accidentally by sealers and whalers, for instance, and have decimated seabird populations. A recent eradication campaign appears to have been successful and pipits, ducks and small seabirds are showing some immediate signs of recovery.
The removal of non-native cats from Macquarie and Marion Islands has similarly helped native burrowing seabirds, although responses in such ecosystems can be far more complex and unpredictable – the removal of cats from Macquarie also led to increase in the introduced rabbit population, and considerably increased damage to sensitive native vegetation.
Antarctic biodiversity is far more complex than widely assumed, with up to 15 distinct biogeographic regions that have been evolutionarily isolated for many millions of years. Humans present the greatest threat, not only of introducing new species, but also of moving “native” species between regions within Antarctica. This could be even more damaging, as these native species would already be pre-adapted to polar life.
Visitors to Antarctica are subject to increasingly strict biosecurity measures but accidental introductions continue to occur, often through food shipments for scientists. Changes in sea and land ice affect access to new areas, so we can only expect plant and invertebrate invasions to increase unless biosecurity becomes more effective.
While cost issues may be raised, it is worth remembering that prevention will always be better – and cheaper – than subsequent control and eradication, even if such action is possible.
Many of Australia’s mammals spend their entire lives imprisoned, glimpsing the outside world through tall chain-link fences and high-voltage wires. There are dozens of these enclosures across Australia. Many are remote, standing alone in the endless expanse of inland Australia, but others are on the outskirts of our largest cities – Melbourne, Perth, Canberra.
Every year there are more of them, the imprisoned population growing, while the wild populations outside dwindle. These are Australia’s conservation fences.
The captives within our conservation fences are adorable – floppy-eared bilbies, tiny hare-wallabies, long-tongued numbats – and they all share an extreme susceptibility to introduced predators. At least 68 native mammal species cannot exist in the wild if either foxes or cats are present. Many of these species once numbered in the millions, ranging from the woodlands of Queensland to the deserts of Western Australia, but predation has driven them to the brink of extinction.
Fences offer these species a future in the wild, and conservation groups have risen to the challenge. Last week, the Australian Wildlife Conservancy completed a new cat-proof fence in their Newhaven Sanctuary, the largest conservation fence ever constructed.
Fences are extraordinarily successful
Make no mistake, these conservation fences work. Species that wilt at the sight of a fox, that have been exterminated from every corner of the Australian mainland, will explode in numbers behind fences. Along with offshore islands, inside these fences are the only places in Australia where these species can prosper – a few hundred square kilometres of safety, surrounded by 7.6 million lethal square kilometres.
Environmentalists have never particularly liked fences. Rather than hide behind walls, they repeatedly took the fight to the cats and foxes on the outside.
Their tactics have been diverse, innovative and brutal. Managers have rained bullets from helicopters and poison baits from planes. They have set cunning snares and traps, mimicked the smell and sound of their enemies, and have turned landscapes to ash with wildfire.
Nothing has worked for the most threatened marsupials. Some of the largest and most expensive management campaigns in Australian conservation history have ended in exhaustion and stalemate, and with a retreat back behind the fences.
Fences were once a source of vehement debate in conservation circles. Should they be permanent? Are fenced populations wild or captive? Should they contribute to species’ conservation status?
These arguments have effectively been abandoned. Scientific studies and painful experience has proven fences and offshore islands to be the only reliable method of protecting predator-threatened species http://www.wildliferesearchmanagement.com.au/Final%20Report_0609.pdf. In place of these debates, conservation organisations and governments have turned to more practical questions of fence height, electric wire voltage and skirt depth.
So now, on average, Australians are building a new fence every year, some of them truly enormous. The just-completed fence at Newhaven encloses a staggering 10,000 hectares of red sand and spinifex. By the time the project is complete, this fence will be home to 11 different threatened mammal species.
And Australia is not alone: around the world, from New Zealand to Hawaii to South Africa, an archipelago of fences is emerging from an ocean of predators. It is one of the great achievements of modern conservation and has already averted the extinction of critically endangered species. Although it’s much smaller than our network of protected areas, it offers refuge to species that are long-gone from our national parks and wilderness areas.
A troubling pattern
However, in recent years a concerning pattern has begun to emerge. While the number and size of fences continue to increase, the number of new species being protected has stalled. In fact, the last five fences haven’t included any new species – they have only offered additional protection to species that were already protected behind existing fences https://www.nature.com/articles/s41559-017-0456-4.
As an example, the first two marsupials planned for introduction behind the Newhaven fence will be the mala (Lagorchestes hirsutus) and the burrowing bettong (Bettongia lesueur). These two species undeniably deserve more protection. Both are highly susceptible to foxes and cats and will derive tremendous benefit from the protection of this enormous fence. However, both species are already found elsewhere behind fences (four different fences for burrowing bettongs). Meanwhile, yet-to-be-published research from the National Environmental Science Program has found 41 other species that are desperately vulnerable to introduced predators are not protected by any fence.
This problem is not new to conservation. In the 1990s, Australian researchers suddenly realised that our national park system was failing to protect the full range of Australian ecosystems. Despite our best efforts, we had created a system of reserves that was biased towards mountainous landscapes and deserts, and away from the fertile valley floors. The solution was to create new national parks using systematic and mathematical methods.
This discovery – the theory of systematic conservation planning – revolutionised global conservation. In 2018, conservation fences need their own systematic revolution.
Unfortunately, the national park system had natural advantages that fences lack. The vast majority of Australia’s protected areas belong to the state and federal governments. The centralised nature of the protected area network is perfect for systematic thinking and top-down optimisation – picture the Soviet Union’s Politburo. In contrast, the conservation fencing sector is diverse and decentralised – picture the third day of Woodstock.
Fences are built by governments at the state, federal and municipal levels, by multimillion-dollar NGOs like the Australia Wildlife Conservancy, by tiny local environmentalist groups and by profit-making corporations. This diversity is a fundamental strength of the fence network, giving it access to a spectrum of funding and ideas. But it makes it almost impossible to plan in a systematic manner. You can’t tell a small bilby conservation group in western Queensland that they should protect the central Australian rock-rat instead (Zyzomys pedunculatus). It doesn’t necessarily matter to them that bilbies are already protected behind four different fences and the rock-rat has none.
While conservation science tries to work this problem out, new and larger fences will continue to be built at an accelerating rate into the foreseeable future. True, the absence of coordination will make mathematicians break their slide rules, but each fence will do its job. The furry denizens will hop, and scurry, and bounce around, heedless of their precarious safety.
And for us, from the outside looking in, these fences will help us forget the parlous state of Australian marsupial conservation. It will be possible to forget our record-breaking rate of extinctions, to forget the empty forests and deserts, and to imagine what a bushwalk might have been like before Europeans unleashed foxes and cats onto Australia.