Original articleInvasiveness of Galenia pubescens (Aizoaceae): A new threat to Mediterranean-climate coastal ecosystems
Introduction
Plant invasions are one of the most important threats to coastal ecosystems and native flora, especially in Mediterranean-temperate ecosystems (Rundel et al., 1998, Sala et al., 2000). The current rhythm of global tourism and trade greatly increases the risk and rate of new introductions (Williamson, 1996). However, only a small fraction of introduced species succeed in establishing themselves, and even fewer become widespread and recognized for their impact to natural ecosystems (Smith et al., 1999). Early identification of the invasive species is key for environmental management. Resources available for management of invasive species are often limited, however, costs derived from invasive weed control increase as the invader spreads and eradication becomes expensive and often unfeasible (Hobbs and Humphries, 1995). Effective management involves the prevention of the large-scale spreading of the invaders. However, invasion impacts often go undetected until the invasion is already widespread and often, convincing evidences at the early stages of invasions may be needed to justify the investment.
Different attributes may be used to predict the success of a species to become invasive. Residence time (time since introduction) (Richardson and Pyšek, 2006), taxonomic ties or known invasiveness in similar scenarios can shed some light on the potential invasiveness of an exotic plant. Among biological features of a potential plant-invader, growth rate, flowering phenology, allelopathy, and specific leaf area have been pointed out as relevant contributors to invasion success (e.g., Callaway and Ascheloug, 2000, Lake and Leishman, 2004, Pyšek et al., 2004, Sans et al., 2004). For example, species that flower over a longer time period may have a greater total reproductive output and hence a greater colonization potential (Baker, 1974). Longer flowering periods may also provoke competition for pollination via pollinator preference and improper pollen transfer (Moragues and Traveset, 2005). Seed production is another feature to be considered. More seeds result in a greater number of individual colonization opportunities. These attributes can then be used to assess the invasiveness of exotic plant species and thus support decision making and prioritization of actions.
Blanket or carpet weed, Galenia pubescens (Ecklon and Zeyher) Druce (Aizoaceae) is a prostrate perennial species native to South Africa (Arnold and De Wet, 1993), where it colonizes mainly inland locations with karoo vegetation and coastal areas (altitudes = 15–1830 m) Sandy, silty or rocky soils seem to be invariably colonized (Germishuizen and Meyer, 2003, García-de-Lomas et al., 2009). G. pubescens has been introduced either accidentally or intentionally to different parts of the world. Currently, G. pubescens has been found naturalized in similar (but remote) climatic areas, specifically in the Southern coast of Australia (Prescott and Venning, 1984), California (Ross, 1994), Central Chile (Leuenberger and Eggli, 2002, González et al., 2008), Israel (Greuter and Raus, 2007) and Southern Spain (see Material and methods section). Despite its increasing prevalence, information about this plant is very scarce. This species is not included in national weed lists (Sanz et al., 2004, Dana et al., 2005), and to our knowledge, no studies have been previously reported concerning its potential to become an invasive weed. Previous data suggest that G. pubescens is an opportunistic species, mainly colonizing highly disturbed environments, but the ability to colonize coastal natural habitats such as sand dunes and salty wetlands requires the assessment of invasiveness and potential environmental impacts (García-de-Lomas et al., 2009).
In this study, we assessed the invasiveness of G. pubescens based on stochastic and biological factors such as (1) residence time (time since introduction), (2) growth type, (3) growth rate, (4) flowering phenology and (5) seed production. The analysis of the incipient impacts in natural ecosystems was proposed as an additional test to discern the invasiveness of the species. We assessed changes in (6) native species richness, (7) Shannon's diversity index, (8) the composition of plant functional types, and (9) light availability. These attributes were evaluated in two different coastal ecosystems: salty wetlands and dunes. Finally, some of these attributes were compared with those reported for two highly-impacting invaders in the Mediterranean basin: Carpobrotus spp., and Mesembryanthemum crystallinum. Specifically, the following questions were addressed: (1) What are the main features that may predict the invasion success of G. pubescens?, (2) Does the analysis of incipient impacts suggest an environmental threat in natural ecosystems?, (3) What habitats are more vulnerable to invasion?, and (4) Is G. pubescens a potential invader?
Section snippets
Study area and species description
This study was carried out in the Andalusian region (Southern Spain) (surface = 87 600 km2). The area is characterized by a Mediterranean climate, but shows marked rainfall and elevation gradients (e.g., annual rainfall ranges from 170 to 1800 mm). Within this region, G. pubescens currently is restricted to three coastal areas, in Huelva (Verloove and Sánchez-Gullón, 2008), Cádiz (García-de-Lomas et al., 2009) and Málaga (Casimiro-Soriguer and Pérez-Latorre, 2008) provinces (Fig. 1). Those
Residence time
Based on the date of the first citation for G. pubescens (Molesworth, 1976), a minimum residence time of 40 years may be established for this species in Southern Spain. The current area of extension is 15 804 ha.
Growth type
G. pubescens was the only species forming dense prostrate mats in dunes and wetlands. Lotus creticus L. (Fabaceae) and Mesembryanthemum nodiflorum are also prostrate species, but these are annual or biennial species, respectively, and form very sparse mats. On the other hand,
What are the main features that may predict the invasion success of G. pubescens?
The invasiveness of G. pubescens was evaluated by testing stochastic and biological features, and by assessing the incipient impacts in coastal natural ecosystems. The small seeds produced by G. pubescens are a feature commonly found in invasive plants (Kolar and Lodge, 2001) that may contribute to its rapid spread along transport corridors (Hansen and Clevenger, 2005, Gabbard and Fowler, 2007). In fact, road borders and other human-disturbed environments are usual areas where this invader is
Acknowledgements
This research was supported by the Andalusian Regional Government (Consejería de Innovación, Ciencia y Empresa, Junta de Andalucia) through the project P06/RNM/02030. We thank F. Bravo and J. M. López from the Cadiz Bay Natural Park for permission to carry out experiments.
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