Plant invasion and speciation along elevational gradients on the oceanic island La Palma, Canary Islands

Juana María González‐Mancebo,Raquel Hernández‐Hernández,Sebastian Hopfenmüller,Yohannes Kidane,Manuel J Steinbauer,Frank T Breiner,Anke Jentsch,Severin D H Irl,Carl Beierkuhnlein

doi:10.1002/ece3.2640

Abstract

Ecosystems that provide environmental opportunities but are poor in species and functional richness generally support speciation as well as invasion processes. These processes are expected not to be equally effective along elevational gradients due to specific ecological, spatial, and anthropogenic filters, thus controlling the dispersal and establishment of species. Here, we investigate speciation and invasion processes along elevational gradients. We assess the vascular plant species richness as well as the number and percentage of endemic species and non‐native species systematically along three elevational gradients covering large parts of the climatic range of La Palma, Canary Islands. Species richness was negatively correlated with elevation, while the percentage of Canary endemic species showed a positive relationship. However, the percentage of Canary–Madeira endemics did not show a relationship with elevation. Non‐native species richness (indicating invasion) peaked at 500 m elevation and showed a consistent decline until about 1,200 m elevation. Above that limit, no non‐native species were present in the studied elevational gradients. Ecological, anthropogenic, and spatial filters control richness, diversification, and invasion with elevation. With increase in elevation, richness decreases due to species–area relationships. Ecological limitations of native ruderal species related to anthropogenic pressure are in line with the absence of non‐native species from high elevations indicating directional ecological filtering. Increase in ecological isolation with elevation drives diversification and thus increased percentages of Canary endemics. The best preserved eastern transect, including mature laurel forests, is an exception. The high percentage of Canary–Madeira endemics indicates the cloud forest's environmental uniqueness—and thus ecological isolation—beyond the Macaronesian islands.

Highlights

Elevational gradients on isolated mountains pose a unique opportunity to study important ecological processes, including evolutionary dynamics (Steinbauer et al, 2016) and plant invasion (Alexander et al, 2011; Daehler, 2005)
Islands tend to be more prone to invasion than areas with higher degrees of connectivity on the continent due to low species richness, functional diversity, and reduced competitiveness of species compared to those of the mainland (Sol, 2000)
Recent studies suggest that diversification, as indicated by two frequently used indices, increases with elevation—a pattern which has large-scale support for islands as well as continental mountains (Steinbauer et al, 2016)

Summary

| INTRODUCTION

Elevational gradients on isolated mountains (especially on oceanic high-elevation islands) pose a unique opportunity to study important ecological processes, including evolutionary dynamics (Steinbauer et al, 2016) and plant invasion (Alexander et al, 2011; Daehler, 2005). Recent studies suggest that diversification, as indicated by two frequently used indices (i.e., the percentage of single-island endemic species and the percentage of archipelago endemics species; Emerson & Kolm, 2005; Whittaker et al, 2007), increases with elevation—a pattern which has large-scale support for islands as well as continental mountains (Steinbauer et al, 2016) This trend can be modified on an intra-insular level by other environmental influences such as different aspects of climate The contrary is to be expected, if the extreme environmental conditions at high elevations pose a more effective filter for non- native species than the already bypassed geographical isolation

| MATERIALS AND METHODS

Findings

| DISCUSSION