Ecosystems as arenas for endemism

The classic theoretical approach of island biogeography explains species richnes by the rate of establishment for new species and by the rate of extinction of established populations. These rates are modified through filters such as distance to source populations and area as a proxy for ressource availability. Further theoretical advancements added variables such as topography, vectors, climate, and dynamics such as disturbances and the geomorphological history of islands. Long-term evolutionary processes resulting in speciation and endemism are mostly addressed in relation to entire islands. Islands are generally characterized by a high proportion of endemic species. Like species richness, endemismis is influenced by the area, age, elevation, climate and isolation of an island.

Here, i present a differentiated concept that takes into account the fact that evolution always happens in company. Even when isolated from ancestor populations, island populations develop in the matrix of communities and ecosystems, which are not equally distributed accross the surface. Many endemic species are restricted a one or a few habitats, interacting with the species assemblages in these sites. In consequence, the area and isolation of such habitats needs to be considered when species-area relationships are applied to endemic plant species. Based on data from the Canary Islands, where the flora was updated in terms of completeness and accepted taxa, and based on an updated map of major ecosystems for the archipelago, all species were linked with islands and ecosystems. The combination of these large data sets made it possible to identify the correlations of Single Island Endemic and Multi-Island Endemic plant species with the (surface-corrected) area of ecosystems. While forest and scrubland ecosystems performed a clear correlation between island-specific area and endemism, this was not the case for open ecosystems such as rocks, where interspecific competition is low.

Damp laurel forest with many Multi-Island Endemic plant species (La Palma)