A recent study published in Ecology and Evolution documents a concerning phenomenon: the giant tortoises of western Santa Cruz in the Galapagos Islands are avoiding the forests of Cedrela odorata, an invasive species that interferes with their seasonal migratory routes.
This plant, introduced in the 1940s, has colonized over 1700 hectares and forms vegetative barriers that hinder the movement of these iconic reptiles.
Annual Migration: From Dry Heat to Lush Greenery
Every year, tortoises like Sebastián and Carlitos, weighing over 250 kilograms, migrate from the low and arid zones of Santa Cruz to the highlands and humid lands in search of food.
This migration, which can span kilometers, is vital to maintain their energy balance, body condition, and fertility. However, the expansion of cedar is blocking natural corridors, forcing the tortoises to deviate or stop.
Cedrela odorata: A Silent and Persistent Threat
Cedar is a successful invader due to its rapid growth, efficient dispersion, and allelopathic ability: it releases compounds that inhibit the germination of native species.
Its dense canopies block sunlight, reducing the understory vegetation. According to the study, Cedrela forests have 42% less richness of endemic species and 17% less diversity than native forests like those of Scalesia.
Cedrela odorata forests affect the migration of giant tortoises
Tortoises as Ecological Engineers
They dig, roll around, and disperse seeds, but now they also propagate invasive species.
Giant tortoises play a key role as landscape modifiers. By moving around, they disturb the soil, control plant species, and disperse seeds.
However, with over 1700 introduced species in the archipelago, they are also spreading invasive plants like guava (Psidium guajava), which has mixed effects on ecological dynamics.
Restoration and Control: Efforts by the Galapagos National Park
The Galapagos National Park carries out control actions in 60 priority hectares, where vulnerable species such as the petrel (Pterodroma phaeopygia) inhabit.
The team cuts cedars, applies targeted herbicides, and replants with native plants, planting up to 60,000 seedlings per year. However, lack of funding limits the scale of intervention, while the blackberry (Rubus niveus) rapidly invades cleared spaces.
A Balancing Act at Risk: Blocked Migration, Threatened Biodiversity
If the routes close, the tortoises will lose their ecological role and their population could decline.
Researchers warn that if cedar expansion continues, there could be a population reduction of Chelonoidis porteri, a species critically endangered according to the IUCN Red List.
This would imply the loss of their role as Galapagos gardeners, affecting the biodiversity and resilience of the island ecosystems.
