Patterns and Disturbance-Induced Drivers of Plant Diversity and Endemism on High Elevation Islands
In the seven manuscripts presented in this dissertation I contribute to our understanding of the drivers of plant diversity, endemism and speciation by using empirical, experimental and theoretical approaches. I introduce and define the concept of high elevation islands (HEI), which – in my opinion – are ideal research objects to address ecological and biogeographic questions. I intend to bridge the gap between island biogeography (i.e. by focusing on the global scale) and island ecology (i.e. by addressing the within-island scale). As a model HEI the mountainous island of La Palma (Canary Islands) is used and all field-related research was conducted there. HEIs are found in all major oceans from mid to low latitudes. Characteristic features are elevational range reaching more than 1000m a.s.l. and possessing ecosystems ranging from coastal to alpine systems. The alpine treeline, as the borderline between forest and treeless alpine systems, is therefore fundamental in describing HEIs. Interestingly, I demonstrate that globally treeline elevation on HEIs, which is generally lower on HEIs than on the mainland, is mainly driven by island elevation, and not, as expected, by latitude. Another characteristic feature of HEIs is the phenomenon described by the elevation-driven ecological isolation hypothesis, which suggests increasing speciation rates with elevation due to geographical and environmental isolation. Although island climates are generally considered to be buffered by the surrounding ocean, the reviewed literature indicates that global climate change poses a considerable threat to HEIs, especially to systems adapted to climatic stability (e.g. cloud or laurel forests) and alpine ecosystems. La Palma is a typical trade wind-dominated subtropical HEI hosting a variety of different environmental gradients, subsequent vegetation zones and a rich endemic flora. Topography and climate (including different measures of precipitation variability) express varying importance in explaining the distribution of species richness, endemic richness and endemicity (i.e. floristic uniqueness) on La Palma. Besides environmental gradients, I show that island ecological processes and patterns on La Palma are in a large measure shaped by human-mediated disturbances. Harsh environmental conditions, a high degree of endemism and the presence of several introduced herbivores, especially the European rabbit Oryctolagus cuniculus and the feral goat Capra hircus, characterize the high elevation ecosystem of La Palma. In a 11-yr exclosure experiment I am able to show that introduced herbivores have likely led to a vegetation shift in the high elevation ecosystem, which changed from a diverse shrub community to the mono-dominance of a single shrub species (Adenocarpus viscosus subsp. spartioides). In addition, introduced herbivores selectively browse rare endemics (some even on the brink of extinction) and reduce endemic seedling establishment to nearly zero, making a recuperation of the natural system impossible without substantial herbivore control measures and conservation efforts. Although fire frequencies have increased due to human interference, fire seems to have a positive effect on species richness and seedling establishment in the high elevation ecosystem. Contrary to our expectations, roads have a positive effect on endemic species on La Palma. Many rupicolous endemics profit from roadside cliffs because these cliffs function as ‘safe sites’ and protect them from introduced herbivores and fire. As a result of this dissertation several intriguing research questions have arisen in HEI science. These questions focus on within-island patterns of plant species diversity and especially endemism, the novel field of disturbance-driven island ecology, and global and macroecological patterns. HEI science is a promising research field with the potential to substantially advance our knowledge of ecology and biogeography in the future.
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