A global examination on the differential impacts of roadsides on native vs. exotic and weedy plant species

Roads cause multiple ecological impacts by altering biotic and abiotic conditions that affect ecosystem structure and functioning from local to regional scales. Roadside plant assemblages are driven by the interplay between propagule arrival and seedling establishment, which are highly influenced by roadside characteristics, landscape configuration and road management and maintenance practices. Multiple plant species with different growth forms use roadsides as temporary or permanent habitats and/or dispersal corridors. Over the past seven decades, numerous studies have documented how roadsides affect the distribution and dispersal of plant species; however, a comprehensive review of those is lacking in the literature. Here, we addressed this issue and found 1,098 studies that met all the study eligibility criteria. Over half of the studies reported the effects of roadsides on the distribution and dispersal of exotic/weed species only, of which all but one indicated a positive effect, and around three quarters of the studies that considered native species showed a positive effect of roadsides on such species. Only 10 studies corroborated the use of roadsides as linear corridors by some native species to expand their distributional ranges. Generally, roadsides had the most positive effects for native species in landscapes that are greatly impacted by intensive agriculture, urbanization and forest management, while negative effects were concentrated in areas that have not been highly altered by humans. Furthermore, studies regarding exotic/weed species were concentrated within a few regions of the world. We conclude that roadsides are key to maintain native biodiversity, especially in highly modified anthropogenic landscapes, but also serve as landscape corridors for exotic species. (C) 2019 The Authors. Published by Elsevier B.V.