Saving Christmas Island: Sarah Wilson's Fight Against Yellow Crazy Ants for the Love of Red Crabs

Drawn by its reputation as one of the most biodiverse places on the planet, our expedition team set out to explore the remote Christmas Island. This tiny green speck in the Indian Ocean lies 489 kilometres south of Jakarta, Indonesia, and 2,600 kilometres northwest of mainland Australia. Closer to Asia than to its official home country, Christmas Island is a fascinating intersection of culture, geology, and ecology.

The island sits atop a sea mount that gradually rose from the ocean, forming a tropical jungle thriving on an ancient reef. In the dense forest, you can find slabs of fossilized coral—tangible remnants of its geological history. Life here owes much to the birds that arrived over the course of centuries, carrying seeds and depositing phosphate-rich guano—bird droppings—that fertilized the land and allowed the jungle to flourish.

Today, 63% of the island is protected as a National Park, a sanctuary for its rare and iconic wildlife, particularly the red crabs that form the backbone of its ecosystem. These crabs play an unparalleled role in maintaining the health of the forest, making their protection vital to the island’s future. During our visit, we met Sarah Wilson, the coordinator of the Yellow Crazy Ant Program, who is at the forefront of the park’s conservation efforts. From combating invasive ant supercolonies to protecting the red crabs, Sarah’s work exemplifies the innovation, resilience, and dedication needed to preserve this extraordinary place.

(Sarah Wilson tells us about what it’s like working at Christmas Island National Park. Photo Credit: Adam Moore)

The Draw of Christmas Island

“I didn’t come here intending to work. I came for the adventure—the promise of pristine diving, an untouched jungle, and a place that felt truly wild. It delivered on all fronts. This isn’t a polished, tourist-ready destination; it’s raw, rugged, and unapologetically itself. I fell in love with the island, its untamed beauty, and the community of people who call it home. Somewhere along the way, I met the right people and found my place on the Parks team.

Now, as part of this incredible group, every day feels like a privilege. Whether it’s an office day or time spent out in the field, I’m constantly in awe of this place and its people. The team’s dedication to protecting nature and preserving the environment is inspiring. Christmas Island has a way of drawing you in and making it hard to leave.

The Red Crab Migration

One of the main reasons it’s hard to leave Christmas Island is because of the annual red crab migration, which I’ve witnessed in its fullest force over the years. Over 100 million red crabs, each about the size of a dinner plate, move en masse from the forest to the ocean, surging across roads, scaling rocks, and navigating human-made structures like crab bridges and underground crossings. Their mission? To spawn the next generation in the ocean, ensuring the survival of a species vital to the island’s ecosystem.

The migration begins with the first heavy rains of the wet season, typically in October or November, which act as a catalyst for the crabs. The males lead the charge, digging burrows along the coastline to prepare for mating. Afterward, the females remain in these burrows to lay and guard tens of thousands of eggs. When the full moon and high tide align, the females emerge to release their eggs into the water, turning the shoreline into a spectacle of millions of tiny larvae swirling in the ocean. These larvae develop into juvenile crabs and make the long, treacherous journey back to the forest.

(A single red crab getting ready for its great migration. Photo Credit: Adam Moore)

The migration is essential to the island’s environment because red crabs are the “gardeners” of Christmas Island—keeping the forest floor clear by eating fallen leaves and saplings, which prevents overgrowth. Their movement stirs the soil, aerating it and promoting plant growth. Without them, the jungle’s delicate balance would collapse. Protecting these crabs is synonymous with protecting the island itself, and that responsibility guides much of the conservation work here.

This year, we’re anticipating one of the largest migrations in a decade, with the population moving in waves. Witnessing it firsthand is exhilarating, humbling, and occasionally challenging. Roads become blanketed with crabs, and we carry rakes or blowers in our cars to gently clear the way without harming them. Sometimes, we simply have to wait and let the crabs pass—it’s their island, after all. These migrations remind us that the crabs are the original inhabitants here, and everything we do is in their service. 

Threats to the Red Crabs

From what I’ve seen, and what I’ve been told by the locals who predate my arrival here, is that climate change has made the red crab migration and spawning events increasingly unpredictable and perilous. In recent years, the triggering rains have become inconsistent—starting and stopping, or sometimes delayed entirely. This leaves the crabs vulnerable to exhaustion as they rely on rainfall for moisture and hydration during the long trek. Without rain, many risk dying before they can reach the shoreline.

(The cliffs of Christmas Island. Photo Credit: Adam Moore)

Once the migration begins, it’s a carefully choreographed cycle that really can’t handle much interruption. The males lead the way, traveling from the island’s interior to the coast, where they dig burrows and mate with the females. After mating, the males return to the forest, leaving the females behind to nurture their eggs in the burrows. About 12-14 days later, guided by the lunar phase, the females emerge to release their eggs into the ocean during the early morning high tide. It’s a delicate process, and if ocean currents push the baby crabs too far from the island, they may never make it back—lost at sea or consumed by predators. Changes in ocean conditions can be truly deadly for the tiny baby crabs. 

Infrastructure plays a vital role in protecting the crabs during migration, and our team works tirelessly to ensure their safety. We’ve established over 40 underpasses and even a crab bridge, enabling crabs to bypass roads and avoid collisions with vehicles. Temporary barricades and road closures are implemented throughout the park during peak migration times. Each morning and afternoon, we patrol the roads, raking crabs to safety and collecting data to monitor their progress. These efforts, which began in the 1980s when crab mortality from human activity was alarmingly high, have significantly reduced fatalities, which once ranged from 700,000 to a million crabs annually.

Mining operations near migration routes also require close collaboration. We work with mining companies to minimize disturbances during peak crab activity, and the local community is highly cooperative, understanding the importance of preserving this iconic species. While climate change and human activity are some of the biggest threats to the red crabs, there’s one other major, major one: the invasive yellow crazy ant supercolonies. 

(Introducing the yellow crazy ant. Photo Credit: Adam Moore)

The Yellow Crazy Ant Invasion

One of the biggest challenges we face on Christmas Island is managing the invasive yellow crazy ants, which pose a grave threat to the red crabs and, by extension, the entire island ecosystem. These ants have a diabolical strategy: they farm yellow scale insects for sustenance, extracting a substance that allows them to produce formic acid. The ants then spray this acid like a fine mist to attack red crabs, blinding and incapacitating them. Once the crabs are defenseless, the ants consume them.

Without enough red crabs, the jungle becomes overgrown, impacting countless other species that depend on this unique environment. Protecting the crabs is critical not only because they’re found only on Christmas Island but also because their role in maintaining the health of the island is irreplaceable.

Make it stand out

(Where there’s super colonies of ants, there are no red crabs. Photo Credit: Adam Moore)

The yellow crazy ants, native to, we think, India and other tropical areas, likely arrived on Christmas Island via trade routes, perhaps carried by boats or coconuts. They’ve been documented here since 1936, but it wasn’t until 2001 that we began implementing large-scale interventions to control their spread. These ants are extraordinary colonizers and have created massive “supercolonies” across the island. The largest supercolony I’ve found this year spans 22 hectares, with 156 hectares of active colonies identified in total. These colonies are essentially death traps for crabs—any crab that ventures into one is almost certain to die.

The impact is staggering. While we estimate the island is home to that staggering number I told you before—100 million red crabs—the number of those killed by the ants is impossible to quantify. The devastation is heartbreaking, but our efforts to control the yellow crazy ants are relentless. 

Leading the Ant Control Team

For about four to five months each year, my team and I head into the field, walking in single file with radios and GPS devices to track the ants. We call it an “emu bob” formation, methodically covering the ground to locate the ant colonies. Some days, we’re out there for eight to ten hours straight, often not seeing each other for long stretches as we focus on the task. It’s tough work, but the data we collect is invaluable.

(Sarah leads her team on long treks through the forest. Photo Credit: Adam Moore)

Once we identify a supercolony, we log its location into ArcGIS software, creating detailed maps of the infestations. These maps are then shared with Yamaha Australia, who provide us with specialized drones for aerial baiting. The drones, capable of carrying up to 40 liters of bait, fly 20 to 30 meters above the canopy, slowly releasing a sticky, fipronil-based bait. This bait is highly toxic to ants but carefully targeted to minimize harm to other species. We only deploy bait in areas where the ants have already wiped out other wildlife, ensuring we’re as precise and responsible as possible.

Another critical tool in our arsenal is the micro-wasp, introduced in 2016 as a biocontrol measure. These tiny wasps target the yellow scale insects that the crazy ants farm for sustenance. The wasps lay their eggs inside the scale insects, effectively killing them and cutting off the ants’ primary food source. This disrupts the ants’ ability to produce formic acid, which they use to attack and kill crabs. In areas where the micro-wasps are thriving, we’ve observed a significant reduction in ant aggression, creating a healthier balance between ants, crabs, and the broader ecosystem.

(There’s a need to mitigate the impacts of the yellow crazy ants. Photo Credit: Adam Moore)

The combination of aerial baiting and biocontrol has put us in one of the best positions since efforts began in 2001. It’s rewarding to see these strategies making a tangible difference, and I’m always thrilled when we can avoid baiting thanks to the success of the micro-wasps. Leading this team is demanding but incredibly fulfilling. Every day, we’re making strides in restoring balance.

The Future of the National Park

The ultimate goal of the National Park is, in many ways, to make our work obsolete. For the invasive species team, this means eradicating threats like yellow crazy ant supercolonies. Success would mean reaching a point where these invasive species are no longer a danger to the ecosystem, allowing the island to thrive without constant intervention. It’s a unique mindset—this isn’t a job you take for long-term security. Instead, we’re working toward a healthier, self-sustaining environment that no longer needs us in the same way.

Other teams share this vision of sustainability. The rehabilitation team is restoring land damaged by mining, replanting trees, and recreating forests that were lost. Meanwhile, our reptile team is focused on rewilding species like the blue-tailed skink, which is extinct in the wild. The goal is to ensure they not only survive but thrive in their natural habitats once again. Preventing extinctions is central to everything we do, especially on an island like Christmas, where so many species are found nowhere else on earth.

(Christmas Island’s endemic species use both land and sea. Photo Credit: Adam Moore)

The work itself is challenging—scaling cliffs, navigating dense jungle, and spending long days in the field tracking ants or surveying crabs. Even the office work can be demanding, coordinating logistics and reporting on a place as wild and unpredictable as this. But every effort feels worth it when we see the results. Just recently, we observed one of the strongest red crab migrations in years, thanks to our progress in controlling the yellow crazy ant population. Watching females release their eggs into a safer environment, knowing our work has made this possible, is deeply rewarding. It’s moments like these that remind us why we’re here.

Christmas Island is a place unlike any other, with raw beauty, immense biodiversity, and conservation challenges that keep us driven every day. Seeing the progress we’ve made—and imagining what this island can become—gives us hope that our work will leave a lasting and vital impact.”

Words by: Andi Cross | Photography by: Adam Moore | Support from: Christmas Island National Park

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SHE Changes Climate collaborates with the Edges of Earth Expedition, a woman-led team dedicated to highlighting impactful stories from the environmental frontlines. This partnership focuses on amplifying the voices of women who are pioneering positive change in some of the world’s most vulnerable coastal and marine environments, many of whose stories have gone untold.