Restore or maintain species-rich grassland along road/railway verges
Overall effectiveness category Evidence not assessed
Number of studies: 8
Background information and definitions
Grass verges beside transportation corridors may offer an opportunity to provide habitat patches in their own right, as well as to connect larger areas of grassland across the landscape (Munguira & Thomas 1992), although the butterfly community found in remnant habitat along road verges may differ from that found in large blocks of native habitat (Davis et al. 2008). However, roads pose a risk of mortality due to collision with vehicles, a risk which may be higher for smaller species (Skórka et al. 2013). Road verges with more native plant species have a greater abundance and species richness of native butterflies (Skórka et al. 2013, Leston & Koper 2016), although this can also increase the number of butterflies which fly into the road (Zielin et al. 2016). However, the number of individuals killed on the road may be lower next to verges which have a higher species richness of plants (Skórka et al. 2013). This suggests that restoring or maintaining species-rich grassland may provide an opportunity to conserve butterflies and moths along transportation corridors.
For studies on reducing collision risk on roads, see “Use infrastructure to reduce vehicle collision risk along roads”. For management options for road verges, see “Residential and commercial development – Alter mowing regimes on greenspaces and road verges” and “Pollution – Stop using herbicides on pavements and road verges”. For other studies on habitat connectivity, see “Habitat protection – Retain connectivity between habitat patches” and “Habitat restoration and creation – Restore or create habitat connectivity”.
Munguira M.L. & Thomas J.A. (1992) Use of road verges by butterfly and burnet populations, and the effect of roads on adult dispersal and mortality. Journal of Applied Ecology, 29, 316–329.
Davis J.D., Hendrix S.D., Debinski D.M. & Hemsley C.J. (2008) Butterfly, bee and forb community composition and cross-taxon incongruence in tallgrass prairie fragments. Journal of Insect Conservation, 12, 69–79.
Skórka P., Lenda M., Moroń D., Kalarus K. & Tryjanowski P. (2013) Factors affecting road mortality and the suitability of road verges for butterflies. Biological Conservation, 159, 148–157.
Leston L. & Koper N. (2016) Urban Rights-of-Way as Reservoirs for Tall-Grass Prairie Plants and Butterflies. Environmental Management, 57, 543–557.
Zielin S.B., Littlejohn J., de Rivera C.E., Smith W.P. & Jacobson S.L. (2016) Ecological investigations to select mitigation options to reduce vehicle-caused mortality of a threatened butterfly. Journal of Insect Conservation, 20, 845–854.
Supporting evidence from individual studies
A replicated, site comparison study in 1992–1996 in urban road verges in Baden-Württemberg, Germany (Schwenninger & Wolf-Schwenninger 1998) reported that road verges sown with native wildflowers had a greater species richness of butterflies and day-flying moths than verges with non-native vegetation. Results were not tested for statistical significance. Over four years, eight butterfly and moth species were recorded on two verges sown with wildflowers, compared to none on verges with non-native plants. Only one species, small white Pieris rapae, occurred every year in the sown verges. Two road verges (1,100–1,500 m2, up to 5–35 m wide) on busy roads in the centre of Stuttgart were sown with annual and biennial native wildflowers including white stonecrop Sedum album, common self-heal Prunella vulgaris, greater knapweed Centaurea scabiosa and wild carrot Daucus carota. For comparison, an unspecified number of vegetated road verges that contained non-native bearberry cotoneaster Cotoneaster dammeri, scarlet firethorn Pyracantha ccoccinea and cultivated roses were also surveyed. From April–August 1992–1994 and 1996, butterflies and day-flying moths were surveyed 6–10 times/year on each verge, and plants were occasionally searched for caterpillars.Study and other actions tested
A replicated, site comparison study in 1989 along 12 road verges in Dorset and Hampshire, UK (Munguira and Thomas 1999) found that there was higher butterfly and burnet moth abundance, species richness and diversity along verges containing more species’ larval food plants, and higher butterfly and burnet moth species richness and diversity, but not abundance, along verges with more plants in flower. There was higher abundance, species richness and diversity of butterflies/burnet moths along verges containing more plant species that represented larval food plants for more species. There was also higher species richness and diversity of butterflies/burnet moths along verges with more plants in flower, but this did not affect the total abundance of butterflies. Additionally, there was higher butterfly/burnet moth abundance and species richness, but not diversity, along wider verges. All data are presented as model results. Weekly from June–September 1989, butterflies and burnet moths were surveyed along a 100 x 2 m transect at each of 12 A-road verges in Dorset and Hampshire. Plants in flower were recorded along a 50 cm strip along the 100-m transect. A verge was considered to contain a species’ larval food plant if there were 30 examples of it at the site.Study and other actions tested
A replicated, paired, site comparison study in 1998 in 12 road verges in Iowa, USA (Ries et al. 2001) found that restored roadside prairies where herbicide application was restricted had a higher abundance and species richness of habitat-sensitive butterflies than verges dominated by non-native weeds or grasses with no herbicide restrictions. On restored roadside prairies, both the abundance (2.3 individuals/plot) and species richness (1.6 species/plot) of habitat-sensitive butterflies was higher than on roadsides dominated by non-native weeds (abundance: 1.4 individuals/plot; richness: 0.9 species/plot) or grasses (0.5 individuals/plot; 0.7 species/plot), and not significantly different from remnant prairies (1.6 individuals/plot; 1.7 species/plot). In addition, mortality risk was lower on prairie or weedy road verges than on non-native grass verges (data presented as model results). On eight well-established, restored prairie road verges (>0.5 km long) and four native (never ploughed) prairie verges dominated by native prairie vegetation, the use of herbicides was restricted. Roadside vegetation (>6 m wide) within 1.6 km of the 12 prairies was classified as “weedy” (>20% non-native legumes) or “grassy” (dominated by non-native grasses). From June–August 1998, butterflies were surveyed nine times in 1–3 plots/habitat (restored prairie, native prairie, weedy, grassy) at each of 12 sites. Plots were 50 × 5 m, >50 m apart and >500 m from a different verge habitat. In addition, three plots in each of four native prairie remnants (2–16 ha) were surveyed. Roadkill butterflies were surveyed six times along both road edges next to each plot.Study and other actions tested
A replicated, site comparison study in 2010 in 60 road verges in southern Poland (Skórka et al. 2013) found that less frequently or later mown road verges had fewer individuals and a lower species diversity of dead butterflies than more frequently or earlier mown verges, and wide road verges attracted a higher abundance and species richness of butterflies than narrower road verges. Both the number of individual butterflies and number of species killed by traffic were lower on verges mown less frequently, or later in the summer, than on more frequently or earlier mown verges (data presented as model results). In addition, both the abundance and species richness of butterflies on wider road verges was higher than on narrow verges, and higher on verges with a greater species richness of plants (data presented as model results). Sixty roads, >2 km apart, with verges of similar width and vegetation on each side, were selected. Between roads, verges differed in the frequency and timing at which they were mown. From April–September 2010, butterflies were surveyed 12 times on two 100-m transects along each side of each road. Dead butterflies were collected from the asphalt and the first metre of verge next to the road. Live butterflies were counted within 2.5 m of the road edge.Study and other actions tested
A replicated, site comparison study in 2002–2003 along 51 road verges in South Karelia, Finland (Saarinen et al 2005) found that meadow-specialist moth, but not butterfly, species richness was higher along road verges with higher plant species richness, but plant species richness did not affect meadow-specialist butterfly or moth abundance. Richness of all butterfly and moth species and meadow-specialist butterfly species, and abundance of all individuals and meadow-specialist butterflies and moths were not affected by plant species richness, but meadow-specialist moth species richness was higher along verges with higher plant species richness. In additional, total species richness and species richness and abundance of meadow-specialist moths were not affected by the abundance of nectar-bearing plants, but abundance of all butterflies and moths, and meadow-specialist butterfly species richness and abundance, were higher along verges with a higher abundance of nectar-bearing plants. No raw data was provided. In June–August 2002–2003, butterflies and day-flying moths were surveyed weekly along thirteen 250-m transects on 51 verges of highways, urban roads and rural roads on the route from Lappeenranta to Imatra in south-eastern Finland. Plant species were recorded monthly June–August in ten 1-m2 quadrats every 25 m along transects and the number of plants in flower was converted to an index of nectar abundance.Study and other actions tested
A replicated, site comparison study in 2012 along three highways and in three prairies in Oklahoma, USA (Mueller & Baum 2014) found that monarch Danaus plexippus caterpillars living on managed road verges had a similar number of parasites to caterpillars on lightly managed prairies. There was no significant difference in the rate of infection with parasitic flies Lespesia archippivora or protists Ophryocystis elektroscirrha between caterpillars from road verges (L. archippivora: 28%; O. elektroscirrha: 28% infected) and prairies (L. archippivora: 42%; O. elektroscirrha: 24% infected). A total of 47 caterpillars were collected from road verges, and 76 from prairies. Between April and October 2012, the verges of three highways were mown once or twice and sprayed with herbicide, and three managed prairies (0.07–0.19 km2) were mown once or not at all. From April–May and September–October 2012, every milkweed Asclepias viridis plant in three 1,000 × 40 m survey areas (2 km apart) along each highway, and on the whole area of each prairie, was inspected 2–4 times/week and all large monarch caterpillars were collected and reared in the lab. The number of L. archippivora parasites which emerged from caterpillars or pupae were counted, and adult monarchs were checked for the presence of O. elektroscirrha spores.Study and other actions tested
A replicated, site comparison study in 2007–2008 along 52 road verges and power lines (collectively “transmission lines”) in Manitoba, Canada (Leston & Koper 2016) found that managed grassland along transmission lines had a similar overall abundance and species richness of butterflies to remnant prairie fragments, but there were differences for individual species. The abundance and species richness of native butterflies along transmission lines (10–14 individuals/visit; 21–32 species) was statistically similar to remnant tall-grass prairie fragments (12 individuals/visit; 20 species). The abundance of native skippers (Hesperiidae) was higher on transmission lines cut once/year without haying (0.8 individuals/visit) than on native prairie (0.12 indiviuals/visit). The abundance of northern pearl crescent Phyciodes morpheus and pearl crescent Phyciodes tharos was higher on unmown transmission lines (2.7 individuals/visit) than on remnant prairie (0.4 individuals/visit), but lower on transmission lines mown twice/year (0.1 individuals/visit). There were also fewer fritillaries on transmission lines mown twice/year (0.1 individuals/visit) than on prairie remnants (0.5 individuals/visit). The abundance of monarch Danaus plexippus on all transmission lines (1–2 individuals/visit) was lower than on remnant prairie (5 individuals/visit). See paper for other species results. Fifty-two road verges and power lines (>30 m wide, >400 m long) were managed in one of four ways: 21 were neither mown nor sprayed with herbicide, but some trees were removed; 10 were mown once/year with cuttings left on site and sprayed infrequently with herbicide; seven were mown once/year with cuttings baled and removed with no spraying; 14 were mown twice/year with cuttings left on site and sprayed frequently with herbicide. Four similarly-sized remnant tall-grass prairie fragments (two urban, two rural) were managed by prescribed burning on a >3-year rotation. From 15 June–15 August 2007–2008, butterflies were surveyed on one 400- or 500-m transect at each site 2–4 times/year.Study and other actions tested
A replicated, site comparison study in 2015 on 30 roads in Delaware, Maryland and Pennsylvania, USA (Keilsohn et al. 2018) found that more butterflies and other insects including moths were killed on roads with meadow verges than on roads bordered by mown, non-native grasses or woods, and mortality was higher on roads with habitat in the central reservation. The number of dead butterflies on roads with meadow verges (2.9–10.0 individuals/site) was higher than on roads with mown grass verges (1.0–3.9 individuals/site) or wooded verges (0.3–0.8 individuals/site). In addition, the number of dead butterflies was higher on roads with habitat in the central reservation (0.8–10.0 individuals/site) than on roads with no habitat in the central reservation (0.3–2.9 individuals/site). The results for other insects, including moths, were similar (see paper for details). Thirty road sections, 200 m long, >400 m apart, with speed limits between 70–105 km/h and high traffic volumes, were classified to three habitat categories: meadow verges dominated by wildflowers and tall grass; frequently mown, short, non-native grass verges; and wooded verges dominated by trees and shrubs. Roads were further split by the presence or absence of a vegetated central reservation. In June 2015, all dead insects were initially removed from the road edge on both sides of the road. From June–July 2015, all dead insects were collected from the road edge five times, at weekly intervals, and identified to species.Study and other actions tested