How do alien plants distribute along roads on oceanic islands? A case study in Tenerife, Canary Islands

Abstract

Islands are paradigms of the pervasive spread of alien plants, but little work has been done assessing pattern and cause of the distribution of such plants in relation to roads on oceanic islands. We studied richness, composition, and distribution of alien plants and compared them with native species along roads on Tenerife (Canary Islands). We studied a single road transect that sampled two contrasting wind-facing aspects (leeward versus windward) and ran from coastal Euphorbia scrubland through thermophilous scrubland to Makaronesian laurel forest at the top of a mountainous massif. We evaluated the effects of elevation, aspect, distance to urban nuclei, and several road-edge features (including road-edge width and management-implying disturbance intensity), using regression models, analysis of variance, and multivariate ordination methods. Richness of both endemics and native nonendemics was explained by elevation (related to well-defined vegetation belts), steepness of the edge slope, and cover of rocky ground. Despite a short elevational gradient (0-650 m), we found clear altitudinal zonation by biogeographic origin of both nonendemic natives and aliens, and altitudinal distribution of aliens followed the same zonation as that of natives. Alien species' richness was related to management intensity determining edge disturbance, roadedge width, and distance to the nearest urban nuclei (propagule sources). Different variables explained distribution patterns of natives, endemics, and aliens along roadsides on leeward and windward aspects. Altitude and aspect also had a strong influence on the frequency of life strategies (woody species, annuals and biennial/perennial herbs) of roadside plant communities. Due to harsher environmental filters operating on the leeward aspect, alien species were distributed along the altitudinal gradient in apparent consistency with general biogeographical affinities. Tropical/subtropical taxa showed exponential decrease with increasing elevation, Mediterranean taxa showed a unimodal response (i.e., maximum richness at mid elevation, minimum at the extremes of the gradient), and temperate taxa showed linear increase with elevation. Native but nonendemic species followed analogous trends to those of aliens. This suggests climatic matching as a prerequisite for successful invasion of this topographically complex island. Other road traits, such as edge width, slope steepness, soil cover, and road-edge disturbance intensity may play a complementary role, at a more local scale, to shape the distribution of alien plants on these island roads.

Distributional patterns of endemic, native and alien species along a roadside elevation gradient in Tenerife, Canary Islands

“…Species were classifi ed into three groups based on their status: native of Tenerife and endemic of Canary Islands (hereafter called endemic), native to Tenerife but more largely distributed beyond the Canarian archipelago (native) and introduced to Tenerife (alien) -following Izquierdo et al (2004). Most of the alien species recorded in Tenerife are of Mediterranean origin (Arteaga et al 2009), and their introduction is probably related to the long history of human activity and landscape transformation at mid-elevation areas that has occurred since the prehistoric times.…”

“…A peak in species richness was observed at intermediate elevation for both native and endemic species. This is likely due to the correlation of the elevation gradient with other environmental gradients, which are known to drive the distribution of plants (e.g., Fernández-Palacios 1992, Arteaga et al 2009). In particular, variables such as high humidity (derived from mean annual precipitation plus trade wind interception), low thermal stress, and high productivity tend to reach a maximum at intermediate elevations in the Canary Islands (Arévalo et al 2005).…”

“…In this paper, we examined patterns in species richness for three groups of species, namely endemic, native, and alien species, using as a test area the island of Tenerife (Canary Islands, Spain). In the Canary archipelago, the road network has increased dramatically over the past four decades, so that these islands are among the European regions with the highest road density (Arteaga et al 2009), with paved road occupying 3% of the surface of Tenerife island . Changes in environmental conditions along elevation gradients represent the major determinant of patterns in plant species communities (Arévalo and Fernández-Palacios 2005, Whittaker and Fernández-Palacios 2007, consistent with patterns observed for other oceanic islands (Whittaker and Fernández-Palacios 2007).…”

Scaling properties of rainfall and desert dust in the Canary Islands

“…Predominant trade winds interact with the relief of the islands to generate mesoscale and microscale meteorological phenomena, leading to a large variability of microclimates. As result, the islands show a great biodiversity characterised by a large variety of species classified as tropical, subtropical, arid, semi-arid or Mediterranean (Arteaga et al, 2009;Zachow et al, 2008) and containing a great proportion of endemic ones. Among other important factors, the cold Canary Current is responsible for inducing atmospheric stability and, therefore, reduces the probability of development of subtropical systems and the occurrence of heavy winds or rainfall associated with them.…”

The role of bioclimatic origin, residence time and habitat context in shaping non-native plant distributions along an altitudinal gradient

“…The spread of non-native plants in mountainous regions has usually been studied along roads (Alexander et al 2009b;Arteaga et al 2009;Sullivan et al 2009;Wilson et al 1992); this is appropriate not only for practical reasons, but because roads are important dispersal corridors (Christen and Matlack 2009;Johnston and Johnston 2004;Lilley and Vellend 2009) and roadsides are usually disturbed habitats (Christen and Matlack 2006;Forman et al 2003) which favour the establishment of non-native species (Gelbard and Belnap 2003). In addition, with the exception of climate, the most relevant abiotic (e.g.…”