Synergy between roads and disturbance favourBromus tectorumL. invasion

Karina L Speziale,Maria N Lescano,Agustina Di Virgilio,Jorgelina Franzese,Gabriela Pirk

doi:10.7717/peerj.5529

Abstract

BackgroundGlobal change produces pervasive negative impacts on biodiversity worldwide. Land use change and biological invasions are two of the major drivers of global change that often coexist; however, the effects of their interaction on natural habitats have been little investigated. In particular, we aimed to analyse whether the invasion of an introduced grass (Bromus tectorum; cheatgrass) along roads verges and the disturbance level in the natural surrounding habitat interact to influence the degree of B. tectorum invasion in the latter habitats in north-western Patagonia.MethodsAlong six different roads, totalling approximately 370 km, we set two 50 m × 2 m sampling plots every 5 km (73 plots in total). One plot was placed parallel to the road (on the roadside) and the other one perpendicular to it, towards the interior of the natural surrounding habitat. In each plot, we estimated the B. tectorum plant density in 1 m2 subplots placed every 5 m. In the natural habitat, we registered the vegetation type (grassy steppe, shrub-steppe, shrubland, and wet-meadow) and the disturbance level (low, intermediate, and high). Disturbance level was visually categorized according to different signs of habitat degradation by anthropogenic use.ResultsB. tectorum density showed an exponential decay from roadsides towards the interior of natural habitats. The degree of B. tectorum invasion inside natural habitats was positively related to B. tectorum density on roadsides only when the disturbance level was low. Shrub-steppes, grassy steppes and shrublands showed similar mean density of B. tectorum. Wet-meadows had the lowest densities of B. tectorum. Intermediate and highly disturbed environments presented higher B. tectorum density than those areas with low disturbance.DiscussionOur study highlights the importance of the interaction between road verges and disturbance levels on B. tectorum invasion in natural habitats surrounding roads of north-western Patagonia, particularly evidencing its significance in the invasion onset. The importance of invasion in road verges depends on disturbance level, with better conserved environments being more resistant to invasion at low levels of B. tectorum density along road verges, but more susceptible to road verges invasion at higher levels of disturbance. All the habitats except wet-meadows were invaded at a similar degree by B. tectorum, which reflects its adaptability to multiple habitat conditions. Overall, our work showed that synergies among global change drivers impact native environments favouring the invasion of B. tectorum.

Highlights

In the Anthropocene era in which we are living, human activities impacting on biodiversity are so widespread that they are collectively known as anthropogenic global change (Vitousek et al, 1996; Sala et al, 2000)
We found that the density of B. tectorum in road verges interacted with the disturbance in the surrounding environment to determine its density in these latter habitats (Table 1)
We found that B. tectorum density decreases when we moved away from road verges towards the interior of surrounding environment (Table 1)

Summary

Introduction

In the Anthropocene era in which we are living, human activities impacting on biodiversity are so widespread that they are collectively known as anthropogenic global change (Vitousek et al, 1996; Sala et al, 2000). According to Sala and colleagues (2000) and Vitousek and colleagues (1996) two of the major global change drivers threatening biodiversity are currently land use change (e.g., habitat transformation into roads or grazing by domestic animals) and biological invasions (Vitousek et al, 1996; Dukes & Mooney, 1999; Sala et al, 2000). The degree of B. tectorum invasion inside natural habitats was positively related to B. tectorum density on roadsides only when the disturbance level was low. Our study highlights the importance of the interaction between road verges and disturbance levels on B. tectorum invasion in natural habitats surrounding roads of north-western Patagonia, evidencing its significance in the invasion onset. Our work showed that synergies among global change drivers impact native environments favouring the invasion of B. tectorum

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