In a forthcoming paper from Heinke Jäger and colleagues in the Journal of Ecology, Cinchona pubescens invasions on the Galápagos Islands were monitored in long-term plots for more than seven years. What they found was that there was a four-fold increase in Cinchona density as the invasion progressed and that this increase had measurable effects on native species abundance. While they did not observe any native extirpations in their plots, native densities decreased by at least 50%. Of the greatest concern was that Island endemics appear to the most susceptible to this invasion.
What these results show is that, while there were not any observed extinctions, there were serious deleterious changes to native diversity. Further, the native species, and especially the endemics, are now more susceptible to other invasions or disturbances due to their lower abundances. The impact of exotic invaders may not be readily apparent but may be a major contributor to increased extinction risk.
Jäger, H., Kowarik, I., & Tye, A. (2009). Destruction without extinction: long-term impacts of an invasive tree species on Galápagos highland vegetation Journal of Ecology DOI: 10.1111/j.1365-2745.2009.01578.x
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