Increase of island endemism with altitude – speciation processes on oceanic islands
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Understanding speciation on oceanic islands is a major topic in current research on island biogeography. Within this context, it is not an easy task to diff erentiate between the infl uence of elevation as an indicator for habitat diversity and island age as an indicator for the time available for diversifi cation. One reason for this is that erosion processes reduce the elevation of islands over time. In addition, the geographic distance to source ecosystems might diff er among habitats, which could lead to habitat-specifi c reduction of species immigration, niche occupation and diversifi cation. We used the percentage of single island endemic species (pSIE) in fi ve diff erent zonal ecosystems (distributed in altitude) on the Canary Islands as an indicator for diversifi cation. We tested whether diversifi cation increases with altitude due to a greater ecological isolation of high elevation ecosystems on oceanic islands under the assumption of a low elevation source region on the mainland. In addition we tested whether the ‘ hump-shaped ’ (unimodal) relationship between pSIE and island age as well as the linear relationship between species richness and pSIE is consistent across spatial scales. We also analyse a potential infl uence of island area and habitat area. We found that pSIE increases with elevation. Th e relations between species richness as well as age with pSIE are consistent across scales. We conclude that high elevation ecosystems are ecologically isolated. Surprisingly, the altitudinal belt with the strongest human infl uences has the highest values of pSIE. We successfully transfer the ‘ general dynamic theory of island biogeography ’ to the ecosystem scale, which provides multiple opportunities for future studies. With this approach we fi nd that the eff ects of elevation on diversifi cation can be separated from those of island age.