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Invasive Spanish cedar on the Galapagos Islands could be impeding the seasonal migration of the Galapagos Tortoise—islands’ biggest herbivores. Giant reptiles navigate around toxic forests during their seasonal movements.
Western Santa Cruz tortoises (Chelonoidis porteri), a species of Galapagos tortoise considered critically endangered by the International Union for Conservation of Nature, are partial seasonal migrants. They spend the rainy season in lowland areas, when a burst of precipitation stimulates fast-growing, nutritious vegetation. Once the dry season begins, many adult tortoises migrate up to the highlands, where year-round rain offers reliable food that is less nutritious than the lowlands during the rainy season but better than the sparse pickings of the now arid lowlands. Tortoises travel an average of about 20 kilometers to complete these annual treks.
People brought Spanish cedar (Cedrela odorata) from mainland Latin America to Santa Cruz—the most populous island on the Galapagos Archipelago—in the 1940s to provide a source of timber. “It’s relatively fast-growing—it’s nice, easy-to-work wood,” Stephen Blake, an assistant professor in biology at Saint Louis University said.
The problem is the cedar has toxic compounds, which it uses to poison competitor plants around it—this resistance to pests in part makes it popular for products like cigar boxes. “It’s quite toxic, and it’s relatively insect-proof,” Blake said. “Native and endemic species tend not to do well in Cedrela forest.”
Anecdotally, researchers noticed that tortoises tended to avoid forests made up of Spanish cedars. In research presented at the 2024 TWS Conference in Baltimore, Maryland, Blake and his colleagues tracked tortoises on Santa Cruz to see how they were reacting to cedar forests during migration. The study was published in Ecology and Evolution in 2024.
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