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.
Note: This article was updated on 20 September 2021 to include a section on the findings (also see this article).
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 from the shallow waters here, 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. The insights generated from analysing the vast survey data collected in 2019 has generated a prioritised roadmap for 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 have given 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 non-native 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 behaviour on this trip. But we do know that it has happened on Ashmore Reef in the past: we have photographic evidence [of ant impacts on wildlife], 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 have provided Parks Australia with more information to inform what management measures to implement next.
“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 generally 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). There are a number of large patches of buffel grass at Ashmore, as well as other Cenchrus species. These are 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 native species and 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 identification and abundance surveys then allowed patterns to be understood across all parts of the islands.
From this data, researchers have created the first high-resolution maps of Ashmore Reef’s terrain, plants and plant communities.
“We’re hoping our vegetation surveys will ensure that we can appropriately deal with any weeds that are causing problems so that future island management here ends up with enduring, effective results,” 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.
They worked with the CSIRO vegetation team to use drones for surveying birds, alongside traditional ground survey techniques of spotting scopes, binoculars and cameras. Drones allow scientists to conduct less-intrusive bird surveys with greater accuracy and improved time efficiency. With this data, the scientists were able to map where birds were 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 environmental DNA (or 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 sampled these communities 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.
What did we find at Ashmore Reef?
Overall, the surveys have shown that Ashmore Reef is healthy but is vulnerable to a range of pressures.
The marine surveys found that coral cover was highly variable across the habitats surveyed but were very high in some areas. Other good news showed that at the time of the survey, there was no coral bleaching or disease or signs of stress or major recent disturbance observed. A total of 365 species of fish were observed and were strongly influenced by the reef zone. Sharks were rare but this finding was consistent with previous studies and similar to other areas that are open to fishing.
Eighteen species of Holothurians (sea cucumbers) were recorded and at least one species is locally extinct. The other species are at very low levels and many of these have not recovered from being exploited in the mid- 1980s. Giant clams and another smaller species of clam were observed in low densities but indicated a slow recovery from previous overfishing.
The seagrass beds at Ashmore Reef are very widespread, but the cover is generally low. However, the surveys indicated that the seagrass beds are critical habitat providing food for green turtles and dugongs, stabilising sediments and creating habitat and nursey areas for marine organisms despite being sparse.
Despite an extensive search effort, the survey showed a significant decline in sea snakes at Ashmore Reef with only one olive sea snake (Aipysurus laevis) observed on the survey. The cause of this decline remains unknown.
The terrestrial surveys found that the park continues to support significant numbers of seabirds and that populations are generally increasing in both number and the area of their breeding territories across the islands. While this is good news, significant decline in shrub abundance and health was observed, which could well be related, at least in part, from the pressure of seabirds that use these shrubs for nesting.
Across the four main islands, 35 distinct vegetation communities were identified, establishing the most detailed understanding of vegetation patterns on the island and creating a robust baseline for establishing future management success. Of concern is that the islands have eight non-native plant species that have naturalised. This includes four Cenchrus species (including buffel grass), which along with beach caltrop (Tribulus cistoides), represent a real threat to these island ecosystems.
Considering how to mitigate the impacts of the non-native species on the Ashmore islands needs to also factor in the resident tropical fire ants (Solenopsis geminata), the Asian house gecko (Hemidactylus frenatus) and rodents (both rats and mice have been observed there in the past), all of which can interact to threaten the native plants and animals.
To ensure that Ashmore Reef remains resilient to current and predicted future threats, regular detailed monitoring will be important. This will help us gain a better understanding on how the habitats and species are interacting are changing over time, and, in turn, how these ecosystems are responding to management.