Abstract
Topography influences evolutionary and ecological processes by isolating populations and enhancing habitat diversity. While the effects of large-scale topography on patterns of species richness and endemism are increasingly well documented, the direct effect of local topography on endemism is less understood. This study compares different aspects of topographic isolation, namely the isolating effect of deep barrancos (ravines) and the effect of increasing isolation with elevation in influencing patterns of plant endemism within a topographically diverse oceanic island (La Palma, Canary Islands, Spain). We collected plant presence–absence data from 75 plots in 8 barrancos on the northern coast of La Palma, spanning an elevation gradient from 95 to 674m a.s.l. Using mixed-effects models, we assessed the effect of barranco depth and elevation on the percentage of single-island endemics, multi-island endemics, and archipelago endemics. We found that percent endemism was not significantly correlated with barranco depth and correlated negatively with elevation within barrancos (rather than the expected positive relationship). The topographic barriers associated with the deep island barrancos thus appear insufficient to drive speciation through isolation in oceanic island plants. The decrease in endemism with elevation contradicts findings by previous broader-scale studies and it may reflect local influences, such as high habitat heterogeneity at low elevations.
Highlights
Topography is a key factor influencing the evolutionary and ecological processes that generate and maintain the diversity of life on Earth (Irl et al 2015, Tukiainen et al.2017, Godinho & da Silva 2018)
We focus mainly on endemism defined at the archipelago level and differentiate between single-island endemics (SIEs – species that are endemic to La Palma) and multi-island endemics (MIEs – species which are endemic to the archipelago and found on at least one island other than La Palma; MIE + SIE = AE)
We documented a decrease in the percent endemism with increasing elevation, a result that opposed our first prediction derived from previous studies
Summary
Topography is a key factor influencing the evolutionary and ecological processes that generate and maintain the diversity of life on Earth (Irl et al 2015, Tukiainen et al.2017, Godinho & da Silva 2018) It influences species diversity via two main mechanisms. Barrancos (or ravines) are deep gorges with steep sides and very narrow beds They can harbour different, often milder, climates from their surroundings, while the barranco walls may represent extreme environments, which are rocky, extremely steep, and severely lacking in soil. This means that species at the bottoms of barrancos could be physically and ecologically isolated from equivalent environments (other barranco bottoms). This is true of the valleys in the Hengduan Mountain Region of China: they host a high diversity
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