Vortrag, Association of American Geographers (AAG) Annual Meeting 2012, New York, USA: 24.02.2012 - 28.02.2012
In general, the two options for filling empty niches, diversification of established taxa and invasion of novel species, are not equally important along elevational and environmental gradients on oceanic islands. Thus, species richness should decrease with elevation, due to a decrease in available habitat diversity and in connectivity with other habitats. Furthermore, differences in precipitation are expected to modify both processes on an infra-insular level. On La Palma (Canary Islands) vegetation survey transects following the elevational gradient were implemented on the arid and humid island side. Here, species diversity is indicated by different species richness endemism measures and diversification is represented by the percentage of these measures. Invasion indices are calculated in the same way. Species diversity and invasion generally expressed negative correlations with elevation, while diversification increased. The humid transect exhibited a steeper slope for most indices compared to the arid one. Ecological and geographical filters effectively reduce total species richness with increasing elevation, yet adaptation and specialization increase. The upper limit of agricultural land use, reduced anthropogenic disturbances and the focus on generally low disturbed high-elevation communities are correlated with a 1000 m threshold for non-natives. Species diversity and invasion diverge at low to mid elevations, probably due to strong differences in moisture availability, yet diversification is to a certain degree independent from moisture availability. We conclude that the underlying environmental drivers influence both diversification and invasion processes. This leads to a promising linkage between the approaches used in island biogeography and invasion biology.