Landscapes in Southeast Asia once thought to stifle biological evolution may instead stoke its fires.
Karst ecosystems have been referred to as arks of biodiversity, a term that highlights their biological richness but also implies they merely preserve ancient lineages. These landscapes, with their isolated caves, cliffs and sinkholes, were thought to shelter species from extinction without contributing much to evolution.
But the discovery over the past several years of nearly 200 gecko species in such regions reveals that karsts are far from stagnant. “They’re not museums, but centers of speciation,” says evolutionary biologist Lee Grismer of La Sierra University in Riverside, Calif.
When Grismer first explored Myanmar’s karst landscapes in 2017, the richness of life hidden within the limestone towers and caves left him stunned. During a 19-day expedition, these ancient rock formations, rising abruptly from the surrounding farmland, revealed geckos so distinct and unexpected that his team identified 12 new species.
Since then, Grismer and his colleagues have ventured into similar formations across Southeast Asia, delving into the evolutionary secrets they harbor. In early 2024, an expedition to western Cambodia uncovered three new species of bent-toed geckos and a slender gecko — all detailed in upcoming papers — bringing the number of gecko species he has described to around 185. “The biodiversity in these landscapes is just off the charts,” Grismer says.
The gecko discoveries highlight this dynamism. Many karst-dwelling geckos belong to Cyrtodactylus, the third largest vertebrate genus in the world with close to 400 species described so far. Geckos of this genus discovered by Grismer and his team are among the most recently evolved members of their groups. They exhibit unique adaptations, such as elongated limbs, larger eyes and flatter heads, that enable them to cling to sheer rock faces, much like expert climbers.
L. Lee Grismer
Grismer likens the karst formations to islands in an archipelago. Each formation, he says, serves as an evolutionary microcosm, producing species entirely distinct from neighboring karsts. “Species are coming from completely different species groups and different times throughout history.”
The true extent of gecko diversity in the karsts remains unknown. Grismer and his colleagues have surveyed only about 20 percent of the formations in western Cambodia, and he plans to return there and to Myanmar in 2025. “It wouldn’t surprise me if there are another 200 species out there.”