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

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.

The effect of small-scale topography on patterns of endemism within islands

“…The pattern we observed between endemism and elevation may not be representative of the entire elevation gradient as our plots only reach 674m a.s.l., whereas the highest elevations on La Palma are in excess of 2000m. Using elevational belts, Steinbauer et al (2016Steinbauer et al ( , 2017 found an overall increase in the percentage of AEs with elevation on La Palma, but in a non-linear manner, with a slight dip in endemism at approximately 500m, consistent with our result. With regard to species richness, we find the same pattern: species richness decreases with elevation.…”

“…In our sample area, we found that exotic species make up only 4% of species. Previous work on La Palma found the proportion of exotics (non-natives) to peak at an elevation of 500m, after which there is a strong decrease with increasing elevation (Steinbauer et al 2017). As we sampled up to an elevation of 674m, the majority of our study area is located near the peak range for exotic species, indicating that exotics are largely excluded from the barrancos.…”

Spatiotemporal dynamics of plant diversity and endemism during primary succession on an oceanic‐volcanic island

“…An expert in bryophytes (Eduard Hertel, Ecological Botanical Garden, University of Bayreuth) was consulted for species not identified with certainty. On La Palma, both alien richness and endemic richness generally decrease with elevation (Steinbauer et al., ), while rare endemics aggregate at higher elevations (Irl et al., ).…”

An island view of endemic rarity—Environmental drivers and consequences for nature conservation

“…Climatic variables, especially mean annual temperature but also geographical constraints owing to decreasing area with elevation, govern endemic rarity on islands in general (Steinbauer, Irl, & Beierkuhnlein, 2013) but also on La Palma in particular (Steinbauer et al 2017). This climate dependency of endemic rarity results in a clear spatial pattern on the island.…”