Destruction without extinction: Long-term impacts of invasive tree in Galápagos

Background/Question/Methods A common belief in invasion ecology is that invasive species are a major threat to biodiversity, but there is little evidence yet that competition from an exotic plant species has led to the extinction of native plants. Adverse effects of invasive species may only become apparent over the course of an invasion and therefore long-term studies are needed. The red quinine tree (Cinchona pubescens), was introduced to the Galápagos Islands in the 1940s and today extends over at least 12 000 ha in the highlands of Santa Cruz Island. We measured species richness and percent cover of plant species in permanent plots in the highlands of Santa Cruz over seven years. Microclimate variables (photosynthetically active radiation, air- and soil temperature, humidity, and precipitation) were measured over two years underneath the Cinchona canopy and in the open canopy (intact native vegetation) to evaluate the effect of Cinchona on local environmental conditions.

Results/Conclusions The rapid pace of the Cinchona invasion was indicated by a more than doubling of percentage cover, a 4.6-fold increase in mean stand basal area and a 4-fold increase in the number of stems/ha over the study period. Photosynthetically active radiation was reduced by 87% under the Cinchona canopy while precipitation increased because of enhanced fog interception. Cinchona significantly decreased species diversity and the cover of most species by at least 50%. Endemic herbaceous species were more adversely affected than non-endemic native species. Stachys agraria, another invasive species, colonized bare ground that developed under the Cinchona canopy. The numbers of native, endemic and introduced species in the study area remained constant throughout the 7-year period. This study clearly established Cinchona pubescens as a habitat transformer. Despite the fact that no plant species has been lost completely from the study area so far, the introduction of the novel tree life form to a formerly treeless environment led to significant changes in stand structure and environmental conditions and to decreases in species diversity and cover. Such changes clearly conflict with conservation goals as set by the Convention on Biological Diversity (CBD).