A flatworm’s spread threatens the delicate balance of Macquarie Island
The invasive flatworm (Kontikia andersoni) is slowly spreading across Macquarie Island. Credit Kita Williams
Flatworm (Kontikia andersoni) in leaf litter. Credit: Melissa Houghton
Possible prey on Macquarie Island include spiders, rove beetles, flies, an endemic snail, a non-native slug, earthworms, and springtails. Credit: Kita Williams
The flatworm’s presence is strongly influenced by how wind interacts with the island’s topography. Credit: Justine Shaw
Kita Williams digging a pitfall trap for invertebrates on Macquarie Island in 2023. Source: Kita Williams
On a remote and windswept island halfway between Tasmania and Antarctica, a tiny predator is making a big impact.
Macquarie Island, a UNESCO World Heritage site renowned for its wild, natural beauty and large congregations of wildlife, is home to an underworld drama. Beneath the tussock grass and the megaherbs, an invasive predatory flatworm is quietly spreading, reshaping the island’s delicate invertebrate communities.
This underworld figure, Kontikia andersoni, was likely introduced during commercial sealing and penguin operations in the 19th century. While other non-native species introduced during this era, such as cats, rabbits, rats and mice, have been eradicated, many invertebrates—such as this flatworm—remain.
“Invasive species are a major threat to biodiversity on oceanic islands, and invasive predators can have particularly devastating impacts on native fauna and ecosystem functioning,” said SAEF PhD candidate Kita Williams from QUT, the lead author of the new study published in Ecology and Evolution.
Understanding the ecological impacts, habitat preferences, and distribution of the flatworm is critical to informing targeted conservation actions and ensuring the long-term protection of this region.
Flatworms can adapt their diet to local prey which helps them to spread across new regions. On Macquarie Island, they prey on native invertebrates—many of which are found nowhere else on Earth—and are currently spreading ~500 m per year, far exceeding predictions.
The researchers used data collected between 2015 and 2018 to understand the flatworm’s ecological impact and what environmental factors were driving this spread on Macquarie Island. The researchers analysed species co-occurrence data and existing literature on flatworm prey preferences, then compared invertebrate communities in areas with and without flatworms. They also used habitat suitability modelling to predict where the flatworm might spread next.
Their analysis revealed that wherever the flatworm was present, the surrounding invertebrate community changed. “Our findings indicate that the flatworm could affect the structure and flow of energy through the food web by increasing the predation pressure across all trophic levels where it occurs,” Kita explained.
One of the most heavily affected areas was a high-elevation site at 189 meters above sea level. There, the invertebrate community was the most distinct and showed the lowest species richness. “This indicates that the impact of the flatworm may be greater at high elevations, where species richness and abundance is generally lower, as predation is more intense upon the limited invertebrates,” Kita said.
The flatworm’s presence was most strongly influenced by slope and how wind patterns interact with the island’s topography. These factors affect soil and leaf litter moisture, which are critical for the flatworm’s survival.
The team used habitat suitability modelling to identify areas most at risk of future invasion. Their analysis pointed to vulnerable zones along the east coastline and north of Sellick Bay on the west coast—locations that can now be prioritised for monitoring and management. These models provide a powerful tool for biosecurity planning, helping conservationists get ahead of the flatworm’s steady advance.
Looking ahead, Kita emphasises the importance of invertebrate research in conservation planning, “Invertebrates are under-researched and under-represented in biodiversity monitoring and conservation projects worldwide, so it is important to fill in some of these knowledge gaps.”
By shedding light on how an inconspicuous predator can disrupt ecosystems, this research underscores the need for early detection and evidence-based responses that account for the impacts of even the smallest island invaders.
Read more
Williams, K.M., Waite, S., Houghton, M., Firn, J. & Shaw, J. (2025) A novel invertebrate predator on an oceanic island: Impacts and invasion dynamics of Kontikia andersoni on Macquarie Island. Ecology and Evolution. http://doi.org/10.1002/ece3.71663