The K’gari-Fraser Island bushfire is causing catastrophic damage. What can we expect when it’s all over?

Gabriel Conroy, University of the Sunshine Coast

K’gari (Fraser Island) has been burning for more than seven weeks and, so far, the fires have razed half of the World Heritage-listed island off the coast of Queensland. The devastation will become more pronounced in coming weeks, despite overnight rain.

Much of the commentary on these fires has focused on how these landscapes are “meant to burn”, and that (luckily) there have been no major fires in the fire-sensitive, rainforest-style ecosystems in the island’s centre.

However, the fact remains that a fire of this magnitude will alter the ecological balance on the island. Let’s explore why.

An uphill battle

K’gari is an incredibly biodiverse place, with more than 30 mammal species, 354 types of birds, 60 reptiles and 17 types of frogs.

For thousands of years, the Butchulla traditional owners maintained the island’s ecosystems with patch mosaic burning. The general principle behind patch mosaic burning is that by burning regularly and strategically, you create habitat niches that cater for a wide variety of generalist and specialist species, which favours biodiversity.

With an absence of this mode of burning during 130 years of logging on the island (ending in 1991), today’s environmental managers have faced an uphill battle to claw back the balance.

This — alongside tinder-dry conditions and large swathes of relatively inaccessible wilderness in the north — is why we unfortunately find ourselves in the situation where an incredibly widespread, intense fire has occurred.

These types of fires can irrevocably alter the nature of even fire-adapted ecosystems (like in the northern half of K’gari) and are likely to become more commonplace in our changing climate.

Let’s take two of K’gari’s rare plant species — the tiny wattle (Acacia baueri) and the much-loved Christmas bells (Blandfordia grandiflora) — as examples of why the Australian landscape’s need for fire isn’t straightforward, and requires patch burning.

These species rely on low intensity fire occurring every three to five years to regenerate and avoid local extinction.

However, other fire-adapted species that grow alongside them, such as Banksia robur, would struggle to withstand burning this frequently. Some may not even be able to reach reproductive maturity during that kind of time span.

Invertebrates: the island’s unheralded heroes

K’gari is famous for its wild population of dingoes, which undoubtedly will have suffered in these fires. We won’t know the full impact for these and many other species until the dust has settled.

But one of my greatest concerns is for the largely forgotten species propping up ecosystems: invertebrates. In normal circumstances, the island is teeming with highly abundant and diverse invertebrate life if you bother to look for it — and will undoubtedly bite you or keep you awake at night even if you don’t.

Herbivorous species, including insects, are the unheralded heroes that transfer a lot of the the energy generated by plants up through the food chain, for example, by providing food for predators like dingoes.

With 50% (and counting) of the island’s ecosystems already burnt in this fire, the amount of food available for herbivores has reduced. This means significantly less energy can be fed back up the food chain, affecting the entire ecosystem.

A dingo on the beach — Wild dingoes are an important feature of the World Heritage-listed island.
Shutterstock

When there’s nowhere to escape

On mainland Australia, birds, bugs and fast-moving animals like dingoes and wallabies often flee to safe habitats when fires occur, and then later recolonise fire-affected regions.

Although relatively close to the mainland, K’gari is a very long and narrow island, and because the entire northern end of the island has burnt, most terrestrial species have only a narrow interface through the central part of the island to try to escape. These lack of escape routes will likely exacerbate death rates of native fauna.

To make matters worse, the ecosystems to the north are markedly different to those in the centre of the island. So while we wait to see how the northern regions regenerate, the species that depend on them may have moved further south to find equivalent ecosystems.

Effectively, only 50% of the island now provides habitat and food sources for the entire island’s wildlife, and the remaining habitat is not always a like-for-like replacement.

How will it look when it ‘bounces back’?

When the fires have extinguished and plants begin to regenerate, a sea of green may convince people the ecosystems have bounced back marvellously from the fires. But in actual fact, they may have been irrevocably changed.

Certain species will flourish in the post-fire environment, but because of the fire’s widespread nature, it’s possible the composition of the vegetation will change.

Weeds, for example, love bare ground, and even native species can become “weedy” in nature and can dominate in disturbed areas. In ongoing research, our team has observed this in areas of the island impacted by sand mining in the 1970s, where there’s an overabundance of immature Acacia and Casuarina species.

My greatest hope is that the future of K’gari includes patch mosaic burning (and cultural burns) at a landscape scale.

If we want to ensure K’gari bounces back as much as possible, then we need to use these devastating bushfires as an opportunity to work collaboratively towards a common conservation goal.

The intent from key stakeholders is evident, with productive conversations already occurring between our team at the University of the Sunshine Coast, Queensland Parks and Wildlife Service and the Butchulla Aboriginal Corporation. For this to come to fruition, on-ground efforts will need to be well-resourced and supported by ongoing monitoring and research.

Gabriel Conroy, Environmental Management Program Coordinator, University of the Sunshine Coast

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Fraser Island Bushfire

Inskip beach collapse: just don’t call it a ‘sinkhole’

Stephen Fityus, University of Newcastle

As was widely reported in the media, at around 10pm last Saturday night, a “sinkhole” opened up at a beachfront campground on the Inskip peninsular.

The thing is, it almost certainly wasn’t a sinkhole.

Unanticipated ground collapses occur around the world from time to time, and these generally get labelled “sinkholes”, for want a more appropriate term. Yet “sinkhole” is poorly defined and often misused, generally referring to some type of geological phenomenon that causes localised ground surface collapse.

In its strict sense, a sinkhole occurs when there is movement of surface soil or rock downward to fill a cavity in the ground below it. Thankfully, open underground cavities are not so common in nature, and are limited to a few characteristic geological settings.

The classic manifestation of sinkholes is in karstic geological environments, such as the Nullabor Plains. These are where the percolation of groundwaters through limestones and dolomites over geological timescales causes them to dissolve, leading to the formation of underground cave systems.

Where the span of the caves becomes too great, or the overlying roof rocks are too thin to support themselves, these may collapse. This produces the stereotypical sinkholes such as those known from Guatemala, Florida, Louisiana, and parts of China.

Sinkholes can also arise from anthropogenic activity, such as mining and engineering works. Poorly backfilled or capped mine shafts may subside if the backfill collapses or is washed to deeper levels in the mine by inflowing water, such as occurred in the case of the Swansea “sinkhole” near Newcastle, New South Wales, in 2014.

Shallow tunnels can also collapse, leading to a hole or depression forming in the ground above. Small sinkholes can also occur above breaks in unpressurised wastewater pipes if soil from around the pipes is able to collapse into the pipe and be carried away with the flowing water.

Sandy straight

So how does any of this explain the Inskip beach “sinkhole”? Well, it doesn’t. And from the photographs and available geological information, it seems like the event at Inskip beach is not a sinkhole at all.

The Inskip beach area is not undermined, and not known for the occurrence of limestones in its bedrock. So its very unlikely that the missing sand has been swallowed into some deep hole in the sea floor.

To understand the likely reasons behind the Inskip event, it is necessary to understand the geological setting of the Inskip peninsular. For millions of years, the coastal river systems of New South Wales have generated vast quantities of clean quartz sand, which have been delivered to the ocean.

Some of this sand is pushed up to create some of the best sandy beaches in the world. Meanwhile, the excess (and there is a lot of it) is swept northward along the coast by ocean currents until it reaches a place where it can be deposited.

Through a complex combination of ocean current, ocean swell, coastal morphology and bathymetric factors, Fraser Island in Queensland – the largest sand island in the world – is the repository for much of this excess sand.

The situation is complicated by the Mary River, which discharges into the ocean at the same place. This means that Fraser Island is separated from the mainland by a channel, which allows the Mary River to discharge to the ocean, mainly northward through Hervey Bay.

The southern end of this channel, the “Great Sandy Straight”, forms an estuary at Tin Can Bay, which accommodates tidal flows inward and outward between the Inskip peninsular and Fraser Island. And this is the site of recent collapse event.

It might look like a sinkhole, but it’s something quite different.
AAP Image/Higgins Storm Chasing

Slippery sand

Tidal channels are dynamic environments, carrying sand backward and forward on a daily basis, depositing sand, and then scouring it out again when the channel becomes constricted. If sand is spilled into a pile, it forms a slope at a characteristic angle, referred to as the angle of repose.

If a slope is made any steeper than this, it is potentially unstable and prone to collapse. Sands deposited to form the submerged banks of the channel are flatter than, or equal to, the angle of repose and exist in a stable condition.

However, if the sandy banks of the channel are steepened through erosion in the bottom of the channel, then the over-steep submerged slope may become unstable, resulting in a submarine landslide. Such a slide, initiated at the toe of the slope, will effectively see the slope unravel, with slices of the slope progressively slumping into the space created by the slumping of the slice below.

This mechanism fits well with the situation at Inskip beach, both in terms of the geomorphological conditions and the reported characteristics of the beach collapse.

Will there be more events like this? At some time in the future, most likely. But when, where and how big are all questions that are difficult to quantify without site specific geotechnical and hydromorphological data. Coastal environments are dynamic, restless environments, and the risks of sudden changes are small, but ever-present.

Stephen Fityus, Professor in Geotechnical Engineering, University of Newcastle

This article was originally published on The Conversation. Read the original article.