Science on an island sanctuary: Surveying Ashmore Reef Marine Park
At the very edge of Australia’s continental shelf there is an island sanctuary. Travel 630 km north of Broome, Western Australia, and you will find a few tiny islands in the Timor Sea. The islands and the surrounding coral reefs, seagrass meadows, sand flats and lagoons are part of one of Australia’s most remote marine parks: Ashmore Reef.
Looking after an ocean oasis
Ashmore Reef Marine Park is a haven for seabirds, turtles, dugongs and rich suite of species under and above the water. It has the most diverse marine fish fauna of any region in Western Australia. It is also thought to have the highest number of reef-building coral species (upwards of 255 species) of any area off the state’s coast. Each year around 100,000 birds breed on the islands, and many migratory shorebird species stop by on their long-haul journeys.
Despite its isolation — it’s closer to Indonesia than mainland Australia — there are changes happening on Ashmore Reef. Sea snakes were once abundant, with a population of about 40,000 from at least 13 species. But in recent decades they’ve almost disappeared, and we don’t know why. There’s also uncertainty about the impact of introduced species like tropical fire ants and certain introduced weeds.
How can management adapt to address threats to this ecosystem? And how can scientists mix old and new survey methods to best monitor this internationally important ecosystem?
To understand how life is fairing at Ashmore, in April and June 2019, Parks Australia brought together a team, including scientists from CSIRO and the University of Western Australia, to undertake one of the biggest ‘health checks’ of Ashmore Reef. Over two voyages, the team took a 60-hour round trip from Broome to Ashmore Reef, where it carried out a range of marine and terrestrial surveys.
Michelle Glover, from Parks Australia, is the marine park manager responsible for Ashmore Reef. When analysed, the survey data collected in 2019 will help shape how Parks Australia can manage and safeguard this biodiversity hotspot.
“Management of remote areas like Ashmore Reef Marine Park can be challenging, so science partnerships are a key aspect in understanding and protecting this special place. These surveys will give us a comprehensive measure of the state of the environment at Ashmore, from which we can measure the effectiveness of management intervention over time,” she said.
Unwelcome guests: surveying terrestrial introduced species
Ashmore Reef’s isolation is not enough to protect it. Over the years, introduced pests and weeds have made their way to the islands and may pose one of the biggest threats to the marine park’s biodiversity. But the exact impact of these non-native species on Ashmore Reef is poorly understood. Gaining insights into these potential pressures was a key focus of the field trips in 2019.
Across the islands, the science team focused their surveys of introduced organisms on three priorities: tropical fire ants, weeds, and the Asian house gecko. Insight generated on presence, abundance and impacts of these species will help to inform what management interventions are needed to minimise impacts of park values.
Tracing tropical fire ants
The tropical fire ant (Solenopsis geminata) gets its name from the burning sensation its powerful sting causes. It poses a serious threat to ecosystems and economies worldwide. An aggressive predator, they devour everything in their path, potentially putting seabird chicks and turtle hatchlings at risk.
Dr Ben Hoffman from CSIRO is an ant specialist. He led the surveys on tropical fire ant populations on the islands of Ashmore Reef.
“We found tropical fire ants across the Ashmore Islands, but they seem to be in different abundances and configurations. They’re very prominent: there are foraging trails that extend from the ant nest across the island. The trails can be extensive, up to 20 metres. I know, I’ve been stung plenty of times!” Dr Hoffman adds.
Despite their prominence, the team didn’t see clear signs of the ants having an impact on wildlife.
“We have photos of chicks beside ant nests and they’re not being harassed. I’m finding it very surprising that we haven’t seen that this trip. But we do know that it has happened in the past: we have photographic evidence [of ant impacts on wildlife] from Ashmore, and it’s well known from other parts of the world. The ants can kill hatchling birds and turtles, and even bite the webbing of bird’s feet,” Dr Hoffman explains.
The ant surveys will provide Parks Australia with more information to inform future management measures.
“We certainly have the capacity to eradicate the ant from the island. We’ve been doing eradication for many years on the Tiwi islands among other places. The problem is that the products we use elsewhere are general for invertebrates—so we could eradicate the ants, but it would have large scale non-target impacts on things like hermit crabs. We need new technology that is far more species-specific, that’s the holy grail for eradication programs. We’re hoping to develop this technology in the next decade.”
One of Australia’s worst weeds
The sandy islands of Ashmore Reef are home to grasses, herbs, shrubs and the occasional small tree. As part of the survey work CSIRO scientists, led by Dr Bruce Webber, described these plant communities with a level of detail never before captured.
“Not all of the plants on Ashmore’s islands got here by themselves,” he explains. “Some of them were brought by people, including one of Australia’s worst weeds—buffel grass (Cenchrus ciliaris). You can see large areas of buffel grass across Ashmore. It’s a threat to native plants and to the habitat of nesting seabirds.”
The team used remotely-piloted aircraft systems (drones) to map, measure and identify the vegetation, including all of the introduced weeds. A useful finding was that the drones were less disruptive to seabirds, compared to walking across the island. Initial surveys used larger drones with laser-mapping (LIDAR) technology, providing vegetation structure and capturing the underlying island topography for the first time. Smaller drones were then flown with high resolution cameras, allowing researchers to zoom in for fine-scale data on plants, and an understanding of community composition across the islands. Complementary traditional abundance surveys then allowed patterns to be understood across all parts of the islands.
From this data, researchers will create the first high-resolution maps of Ashmore Reef’s terrain, plants and plant communities.
“We’re hoping our vegetation surveys will ensure that future weed management here ends up with effective results, so we can get rid of any weeds that are causing problems,” said Dr Webber.
The Asian house gecko
The Asian house gecko (Hemidactylus frenatus) is the only lizard found on the Ashmore reef islands. It has been naturalised at Ashmore Reef since at least 1990, but its impact is unknown. Surveys in 1995 only reported a single individual in a malaise trap. But surveys in 2001 found the species was ‘abundant’ on one of the Ashmore Reef islands. The gecko has been listed as a potential pest species requiring management at Ashmore Reef, but nothing is known about its abundance or impact on the islands.
Dr Ru Somaweera, a herpetologist from the CSIRO, led the reptile surveys.
“We’re not sure of the origin of Asian house geckos on Ashmore—they’re very wide-spread in Asia. Part of our work here is to get some genetic material during our surveys [tail tips were obtained from five individuals for future genetic analyses] to see if we can pinpoint where they may have been introduced from,” he explained.
New technologies on land and sea: drones and eDNA
On Ashmore Reef, new technology is helping researchers and marine park managers to gather data more efficiently, with fewer environmental impacts.
Ashmore Reef is internationally recognised for its birdlife. Hundreds, sometimes thousands of birds, cram on to the three tiny islands to nest. The bird surveys were led by Dr Belinda Cannell from the University of Western Australia and Dr Chris Surman from Halfmoon Biosciences. They drew on new technology to avoid disrupting nesting birds. Drones were used alongside with traditional ground survey techniques of spotting scopes, binoculars and cameras. Drones allows scientists to conduct less-intrusive bird surveys more time efficiently. With this data, scientists will map where birds are nesting and how they are using the islands.
The marine science team, led by Dr John Keesing from the CSIRO, also made use of new survey techniques. Sea water contains eDNA, the naturally shed genetic remnants of the biodiversity it supports. Reading eDNA means we can identify species living in an area, from plankton to sharks, and detect rare or cryptic species often missed in traditional visual surveys by divers. Sampling eDNA was trialled as a complementary method to monitor marine biodiversity at Ashmore Reef.
“We could haul them [the marine species] out of the water, go diving or spend hundreds of hours watching underwater video footage,” CSIRO marine biologist Dr Cindy Bessey said. “But instead we’re using eDNA.”
The team focused their eDNA surveys on fish and sea snakes in the reefs and the lagoons of the marine park. Cindy and her colleagues pumped and filtered eDNA from almost 750 kilograms of seawater. They sampled about 110 sites using this method; at the same time, divers counted fishes and invertebrates and took photos underwater.
Our capacity to work on the blue frontier, in places like Ashmore Reef, is continually improving with the advent of new technologies, scientific innovations and collaboration. Through this work, Parks Australia now has a robust baseline to understand future change, and to adapt their management accordingly. This project will bring new insights, understandings and ideas for Ashmore Reef Marine Park, ensuring we’re in the best possible place to safeguard this ocean oasis into the future.