Drivers of Rarity in Protected Areas on an Oceanic Island

Carl Beierkuhnlein (1), Severin Irl (1), Felix M. Medina Manuel (2), Antonello Provenzale (3), Anke Jentsch (1)

(1) University of Bayreuth, Bayreuth, Germany (2) Servicio de Medio Ambiente, Cabildo Insular de La Palma, Santa Cruz de La Palma, Canary Islands, Spain (3) Institute of Geosciences and Earth Resources (IGG), National Research Council (CNR), Pisa, Italy

One aim of protected areas such as national parks or man and biosphere reserves is to protect rare species that often exhibit small population sizes. Oceanic islands, like the ones forming the Canary Island Archipelago, are hosting a large number of endemic species that do not exist elsewhere. These unique biota need particular protection, because losing local populations would also mean to lose these species totally for this planet. However, rarity is scale-dependent. It can vary over several orders of magnitude.

Within limited regions and within protected areas, respectively, there is a need for planning and management to identify and understand the major drivers of rarity. Abiotic constraints of habitats and populations play an important role but the contribution of specific site conditions to explain rarity patterns needs clarification. On the island of La Palma (Canary Islands, Spain), where different categories of Protected Areas exist and partly overlap, we conducted a study in order to assess the main abiotic drivers of endemic rarity. Furthermore, we aim to determine how well existing Protected Areas cover or even coincide with hotspots of endemic rarity. Finally, we developed a new approach that can also be applied in other cases by introducing and evaluating a new hypervolume-based rarity estimator. This method can provide a better understanding of the environmental niches of spatially strongly restricted rare species.

Figure 1. Aeonium nobile, a single-island endemic plant species that occurs only on the island of La Palma. There, this species is focused on steep slopes in low to intermediate altitude. Locally, large populations are formed, but these are strongly fragmented and isolated even within the island.

The island of La Palma is a focal research area of the ECOPOTENTIAL project. Here, a series of prestudies have been carried out and an excellent database as well for Single-Island Endemics (SIE) as for Archipelago Endemics (AE) was built up by the Biogeography lab at the University of Bayreuth, in recent years. We recorded these endemic vascular plant species in 1,212 plots covering the entire island. Only few species that have small remnant populations in inaccessible steep rocks were not considered. However, these species seem to be well-protected anyhow because of the extreme isolation of their habitats.

We calculated endemic rarity (corrected range-rarity richness for endemics) using a rarity estimation approach based on kernel density estimations (hypervolume approach). We performed a sensitivity analysis based on multiple linear regressions and relative importance estimations of environmental drivers to estimate the performance of the hypervolume-based rarity estimation compared to standard methods (occurrence frequency, convex hulls, alpha hulls).

This study found that climatic traits such as mean annual temperature, climatic rarity, precipitation variability best explained archipelago endemic (AE) and single-island endemic (SIE) rarity over the entire island. The existing PAs covered the majority of AE and SIE rarity, especially national and natural parks as well as the Natura 2000 sites. In our study system, hypervolumes performed better than standard measures of range size.

Both AE and SIE rarity on La Palma show a clear spatial pattern, with hotspots of endemic rarity found at high elevations and in rare climates, presumably owing to geographical and climatic constraints and possibly anthropogenic pressure (e.g., land use, introduced herbivores, fire). Areas of high rarity estimates coincide with the distribution and extent of PAs on La Palma, especially since the recent addition of the Natura 2000 sites.

In the near future, this study that is based on in-situ Earth Observation data, will be linked with remote sensing in order to provide an efficient approach to monitor future changes. Climatic changes and invasive species can have strong impacts on native ecosystems and species on oceanic islands, where species richness is poor and adaptation to competition, herbivory and predation is low. The effort of recording detailed and comprehensive in-situ data can hardly be repeated in due course. In consequence, remote sensing tools that are closely linked with real-life conditions on the ground and particularly to highly threatened biota of islands are urgently needed.

Citation: Irl, S., Schweiger, A., Medina, F. M., Fernández-Palacios, J. M., Harter, D., Jentsch, A., Provenzale, A., Steinbauer, M. J., Beierkuhnlein, C. (2017). An island view of endemic rarity – environmental drivers and consequences for nature conservation. Diversity and Distributions, 23(10), 1132-1142, doi:10.1111/ddi.12605.

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