Destruction without extinction: long‐term impacts of an invasive tree species on Galápagos highland vegetation

Abstract

Summary 1. 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 any native plant species at the landscape scale. However, effects of invasive species at community and ecosystem levels can severely compromise conservation goals. 2. Our model species, the red quinine tree (Cinchona pubescens), was introduced to the Galápagos Islands in the 1940s and today extends over at least 11 000 ha in the highlands of Santa Cruz Island. It is also invasive on other oceanic islands. 3. We adopted a long‐term approach, analysing permanent plots in the Fern‐Sedge vegetation zone over 7 years, to test for impacts of C. pubescens density on resident plant species composition and on microclimate variables. We also tested whether the C. pubescens invasion facilitated the invasion of other species. 4. The rapid pace of the C. pubescens 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−1 in 7 years. 5. Photosynthetically active radiation was reduced by 87% under the C. pubescens canopy while precipitation increased because of enhanced fog interception. 6. Cinchona pubescens 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 C. pubescens canopy. 7. The numbers of native, endemic and introduced species in the study area remained constant throughout the 7‐year period. 8. Synthesis. This study clearly established C. pubescens as a habitat transformer, although its average cover did not exceed 20%. 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.