Severin D. H. Irl, Manuel J. Steinbauer, Lilith Epperlein, David E. V. Harter, Anke Jentsch, Susanne Pätz, Christian Wohlfahrt, Carl Beierkuhnlein
Published in Biodiversity and Conservation, 23, 2273-2287, 2014
ABSTRACT
Roadsides are habitats with very specific environmental conditions, often substantially differing from their natural surroundings. However, roads can have a positive effect on local vascular plant species richness. Endemic species on oceanic islands are considered to be less disturbance-adapted than native non-endemics and thus should be negatively affected by roads. Islands provide optimal conditions for testing this, as they possess a large share of clearly defined endemic species. This study focuses on a comparison of endemic plant species in roadside and surrounding communities and the interacting effects of elevation, vegetation type and trade wind-induced precipitation differences. We applied 96 circular plots with 50 m radius along two elevational gradients on the eastern (humid) and western (dry) slope of La Palma, Canary Islands, ranging from 100 m to 2400 m. Interestingly, we found roads to have a significant positive effect on endemic richness and the percentage of endemics as well as the same tendency for plant species richness after correcting for elevation and precipitation. Endemic species turnover was relatively high. The opening of cliffs during construction and, not to be overlooked, the protection from disturbances such as fire and omnipresent introduced herbivores (mainly rabbits or goats) probably leads to a positive effect of roads on endemics. In addition, many endemics might profit from species-specific dispersal capabilities well suited for roadside conditions. However, we do not argue for the use or even construction of roads for nature conservation but suggest protecting existing endemic populations because natural areas have a higher conservation value.
Keywords
Road ecology; disturbance; vascular plant species; endemic richness; elevational gradient; Canary Islands
INTRODUCTION
Roadsides offer a specific composition of plant species and specific moisture, temperature, nutrient and disturbance regimes, which substantially differ from the habitats in the landscapes in their vicinity (Spellerberg 1998; Trombulak & Frissell 2000). In steep mountainous areas, where roads are cut into slopes (Wemple & Jones 2003), this is presumably of particular relevance. Besides having effects on the environmental conditions in their direct surroundings, roads can influence these conditions in natural communities in varying distances from the road verge (Spellerberg 1998). This effect, however, was found to decrease significantly with increasing distance to the road (Watkins et al. 2003).
Oceanic islands of volcanic origin exhibit high rates of endemism due to their geographic isolation and adaptive radiation (Whittaker & Fernández-Palacios 2007). Therefore, compared to their total surface area, islands contribute in a disproportionally substantial manner to global biodiversity (Kreft et al. 2008).
This is especially true for oceanic archipelagos such as the Canary Islands (Reyes-Betancort et al. 2008, Steinbauer & Beierkuhnlein 2010). Endemic species are not distributed randomly throughout an island (Steinbauer et al. 2012; 2013a). Often these species are linked to natural vegetation communities
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belonging to the zonal vegetation or are confined to habitats that are characteristic for the island topography and microclimate (e.g. volcanic outcrops and cliffs; del Arco-Aguilar et al. 2010). In consequence, one would expect that anthropogenic habitats do not offer adequate conditions for endemic species (Rotholz & Mandelik 2013). However, as roads are correlated with artificial outcrops and low competition pressure, their ruderal habitats may satisfy the needs of endemic species.
It has been widely shown that roads can have a positive effect on plant species richness (e.g.
Forman & Alexander 1998). The occurrence of non-native species has been found to be mainly responsible for this (Arévalo et al. 2005) as well as the opening of canopy in forested ecosystems, leading to the establishment of herbaceous species (Watkins et al. 2003). However, to our knowledge a single comparative assessment exists focusing on comparing the percentage of endemic plant species between roadside and surrounding communities (Rotholz & Mandelik 2013). The authors found a significantly lower percentage of endemics in roadside communities than in the adjacent continental Mediterranean shrub land communities owing to high disturbance intensities and frequencies associated with roads. The primary aim of the presented study is to fill this research gap and to assess the effect of roads on endemic species.
The aspect of island habitats (i.e. the location of the habitat in relation to the trade winds) has been shown to influence roadside plant species richness on Tenerife (Arteaga et al. 2009). However, endemic richness is not affected by precipitation differences (Arteaga et al. 2009). Also, there is a tendency towards floristically homogenized roadside communities (i.e. a reduced beta-diversity between communities) (Arévalo et al. 2010). This, however, has mainly to do with non-native and widespread native species in roadside communities along an elevational gradient. So far it has not been tested how endemic species composition responds to roads, especially with regard to the differing environmental conditions between roadside and surrounding communities.
La Palma (Canary Islands) offers steep elevational and environmental gradients spanning from subtropical coastal to treeless summit habitats and from humid evergreen laurel forest and Canary Pine forests to semi-arid succulent scrub (del Arco-Aguilar et al. 2010, Garzón-Machado et al. 2013). In general, the rate of endemic species increases with elevation (Steinbauer et al. 2012, 2013a). Owing to the steep relief, many archipelago as well as single-island endemic species on La Palma are adapted to rupicolous habitats (e.g. species of the genera Aeonium, Aichryson, Crambe, Micromeria, Monanthes, Sonchus, Teline, and Tolpis; Santos-Guerra 1983). The paved mountain road from near Santa Cruz de La Palma (at sea level on the Eastern side) to the highest point of the island (Roque de los Muchachos; 2426 m a.s.l.) and down again to the coast below Santo Domingo de Garafía on the North-western side offers an excellent opportunity to study roadside habitats and endemic species (Fig. 1 and Fig. 2), by following the main precipitation gradient on the island.
Here, we compare roadside and surrounding communities along two elevational transects on the Western and Eastern side of the island spanning more than 2000 meters in elevation. First, we estimate the effect of roads on plant species richness, endemic richness, percentage of endemics and endemic species composition by comparing roadside communities to their surrounding counterparts. Secondly,
we test the effect of elevation, island side (as a proxy for precipitation differences) and vegetation type (i.e. forest vs. non-forest) on the mentioned indices and how these environmental factors interact with habitat type (i.e. roadside or surrounding community). In general, we expect roads to have a positive effect on plant species richness. However, according to the existing knowledge, roads should negatively impact endemic species richness and the relative contribution of endemics to the communities, respectively.
Figure 1. Map of the Canary Islands, La Palma and the study area. Tile a) shows the position of the Canary Islands and La Palma in the Eastern Atlantic Ocean. A digital elevation map of La Palma including the mountain road sampled in this study is displayed in tile b). A 3D view of the Northern part of the island is used to present the roadside and surrounding plots on the eastern and western transect in tile c). Note that the perspective has changed to roughly the Northeast. Vertical exaggeration in 3D view: x2. Figure created in ArcMap 10 and ArcScene 10 (both ESRI Inc., 2010). highest and steepest part of the island (Carracedo et al. 2002). Cool oceanic currents produce a relatively