Pristine Tropical Dry Forests found in the Galápagos

The Galápagos Islands are world famous for their fauna, with the endemic Darwin’s finches playing a key role in the discovery and popularization of the Theory of Evolution, and the Galápagos tortoises constituting one of the best examples of gigantism on islands. The unique biota of the islands is protected in a National Park that covers >97% of the archipelago’s land area. However, the flors of this World Heritage Site has received comparatively little attention.

Dr. Gonzalo Rivas-Torres in dry forest on the island of Santa Cruz during field work. Photo: Thomas W. Gillespie.

Tropical dry forests are forests receiving 500-2000 mm of precipitation per year with a pronounced dry season of 4-7 months and are among the most threatened forests in the world. The Galápagos are believed to have the best protected and largest extent of tropical dry forest in the Pacific. However, these forests had never been studied in detail until recently when Thomas Gillespie of the University of California, Los Angeles, and colleagues surveyed tropical dry forests on the three largest islands in the Galápagos.

What they found offers great hope for tropical dry forests in the Pacific, which are highly threatened ecosystems and remaining fragments are often degraded and threatened by invasive species. The extent of tropical dry forests in the Galápagos has remained stable over the last decade and no woody invasive species were encountered. This makes the tropical dry forests of the Galápagos one of the most pristine globally. Furthermore, these forests were found to be unique and half of the species enumerated were endemic to the archipelago. Another unique feature compared to dry forest in other parts of the Pacific are the lower densities of stems and canopy heights, which is likely related to the low mean annual precipitation (less than 520 mm).

Tropical dry forest on San Cristobal island with Darwin’s cotton (or Darwin’s hibiscus) displaying yellow flowers in the understorey. Photo: Thomas W. Gillespie.

Therefore, the Galápagos hold some positive news for the tropical dry forest biome, which is believed to have less than 1% of the original extent remaining globally and is increasingly threatened by invasive species. In fact, the well-protected, invasive-free and still-extensive forests of the Galápagos Islands hold a unique exemplar of a healthy tropical dry forest ecosystem. They also are proof that the flora of the Galápagos is as unique as its well-known fauna.

Further Reading: Gillespie TW, Keppel G, Robinson CM and Rivas-Torres G (2020) Dry forests of the Galápagos: a comparative assessment of a World Heritage Site. Pacific Conservation Biology, online early.

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