How does roadside vegetation management affect the diversity of vascular plants and invertebrates? A systematic review
With appropriate management, based on vegetation removal that reverses late-successional vegetation stages, roadsides can support high levels of biodiversity. However, current recommendations for roadside management to conserve or restore biodiversity are largely based on research on non-roadside grassland habitats, and much of the evidence on how roadside management practices affect biodiversity is found in non-peer-reviewed grey literature. Therefore, based on suggestions from key stakeholders and an initial systematic map that identified 97 studies on how biodiversity is affected by vegetation removal on roadsides, we conducted a full systematic review of the effects on plant and invertebrate diversity of disturbance-based maintenance of roadsides.
The review was restricted to effects of non-chemical interventions such as mowing, burning, grazing and mechanical shrub removal. Studies were selected from the systematic map and from an updated search for more recent literature using a priori eligibility criteria. Relevant articles were subject to critical appraisal of clarity and susceptibility to bias, and studies with low or unclear validity were excluded from the review. Data on species richness, species diversity and abundance of functional groups were extracted together with metadata on site properties and other potential effect modifiers. Results from the 54 included studies were summarised in a narrative synthesis, and impacts of mowing practices on the total species richness and diversity of plants and on the abundance of forbs, graminoids and woody plants were quantitatively analysed using t tests of study-level effect ratios.
Nearly all of the 54 studies included in the review were conducted in Europe (29) or North America (24). The vast majority of studies (48) examined impacts of mowing. Effects on vascular plants were reported in 51 studies, whereas 8 of the studies reported on invertebrates. Quantitative analysis of plant species richness and species diversity showed that mowing effects were dependent on the interplay between mowing frequency and hay removal. Thus, there were no statistically significant overall effects of mowing vs. no mowing, frequency of mowing, timing of mowing or hay removal. However, species richness was higher in roadsides mowed once or twice per year with hay removal than in unmown roadsides, and positively affected by mowing twice compared to once per year. Similar, but less pronounced, effects were found for plant species diversity. In addition, mowing had a negative impact on woody plant abundance, and increased mowing frequency had a negative impact on graminoid abundance. The few studies on invertebrates showed effects that diverged across taxon groups, and there was not enough data for quantitative analysis of these results.
The results provide evidence on the effects of mowing on plant species richness. To increase plant species richness, roadsides should be mowed each year, preferably twice per year, and hay should be removed after each cutting. The review also identifies large knowledge gaps concerning roadside management and its effects on biodiversity, especially regarding invertebrates. Hence, this systematic review provides not only a valuable basis for evidence-based management but also guidance for future research on this topic, essential to inform management of road networks for biodiversity conservation.
Biodiversity, Burning, Grazing, Invertebrates, Mowing, Species diversity, Species richness, Vegetation removal
Roadsides have been acknowledged as potential substitutes for semi-natural grasslands and other open habitats with high biodiversity, many of which are now declining. Current recommendations for roadside management to promote conservation of biodiversity are largely based on studies of plants in meadows or pastures, although such areas often differ from roadsides in terms of environmental conditions and disturbance regimes. Stakeholders in Sweden have emphasised the need for more targeted guidelines for roadside management, based on actual roadside studies. We recently performed a systematic mapping of the evidence on how roadside management affects biodiversity and the dispersal of species. Through this process, we found 98 studies on how the richness or abundance of species on roadsides is affected by management such as regular mowing, burning, grazing or selective mechanical removal of plants. Since all of these interventions entail removal of plant biomass, they are comparable. Most of the studies recorded management effects on vascular plants, but there were 14 investigations of insects and other invertebrates. We now intend to proceed with a full systematic review of how maintenance or restoration of roadsides based on non-chemical vegetation removal affects the diversity of vascular plants and invertebrates.
Most of the evidence on which the proposed systematic review is to be based will be selected from the systematic map. To identify more recently published literature on the topic of the review, we will perform a search update using a subset of the search terms applied for the systematic map. The criteria for inclusion of studies will be the same as for the map, except that interventions and outcomes will be restricted to those relevant to the review. Relevant studies will be subject to critical appraisal and categorised as having high or low validity for the review. Studies with low validity will be excluded from the review. Utilisable data on outcomes, interventions and other potential effect modifiers will be extracted from included articles. A narrative synthesis will describe the validity and findings of all studies in the review. Where a sufficient number of studies report similar outcome types, meta-analysis will be conducted.