Canopy bridges are an economical mitigation reducing the road barrier effect for three of four species of monkeys in Diani, Kenya

Abstract

For primates, canopy bridges can reduce the road barrier effect. Yet little information exists to predict species bridge use. We examined bridge use across a 9 km suburban road in Diani, Kenya, in three survey years (Nbridges: 21 = 2004, 27 = 2011, 29 = 2020) by four sympatric species of monkeys. The asphalt road is 6 m wide with a 50 km/h speed limit. Roadside observers recorded ground () and bridge () crossings, crossing direction, and traffic volume. Colobus (Colobus angolensis palliatus), Sykes’ monkeys (Cercopithecus mitis albogularis), and vervets (Chlorocebus pygerythrus hilgerti) used the bridges while baboons (Papio cynocephalus cynocephalus) rarely did. Crossing rates (Sykes’>vervet>colobus>baboon) did not fit our predictions based on species’ attributes of stratum preference (arboreal>terrestrial) or body mass (small>large), while the interaction between these attributes was more informative. Crossings were bidirectional. Colobus crossed bridges during higher traffic volumes than on the ground, whereas we found the opposite for vervets. Sykes’ monkeys crossed at similar traffic volumes on the ground and bridges. The mean annual bridge cost was USD 157, deriving a cost per crossing as < USD 0.10, though it undervalues the savings in ecosystem services, tourism benefits, and contributions to protecting colobus, a vulnerable species. While we consider this highly economical, funders and road engineers will ultimately determine if it is so.

An effect of canopy bridges on monkey‐vehicle collision hotspots: Spatial and spatiotemporal analyses

“…Of the four species of monkeys sympatrically occurring in Diani, three use canopy bridges (Cunneyworth et al, 2022): Peters' Angolan colobus (colobus), Zanzibar Sykes' monkey (Sykes' monkey), and Hilgert's vervet (vervet). These species have varying attributes: colobus are arboreal folivores (Anderson, 2005; Dunham, 2017), Sykes' monkeys are arboreal frugivores (Takahashi et al, 2019), and vervets are semiterrestrial omnivores (Cheney et al, 1988; Rose, 1979).…”

“…Diani's canopy bridges effectively reduce the road barrier effect for colobus, Sykes' monkeys, and vervets but in a species‐specific way (Cunneyworth et al, 2022). The effect was considerable for colobus because, at higher vehicle volumes, colobus almost exclusively crossed the road using the bridges rather than crossing on the ground.…”

A new window into canopy bridges as a mitigation strategy for arboreal mammals

“…2). Many studies continue to use a twodimensional horizontal lattice design, either without or with a crisscross X reinforcement (Cunneyworth et al, 2022;Monticelli et al, 2022), like the one developed by Teixeira and colleagues (2013) nearly a decade ago (fig. 2f).…”

“…When installing canopy bridges, cost and maintenance can be a big concern and even inhibitory for implementation. Several papers in the special issue provide valuable information on the cost of the canopy bridges studied (Azofeifa Rojas and Gregory, 2022;Cunneyworth et al, 2022;Flatt et al, 2022;Minato et al, 2022). Azofeifa Rojas and Gregory (2022) also provide information on the maintenance schedule used to keep bridges functional.…”

“…1 and 3; Thailand: Aggimarangsee et Saralamba et al, 2022;Mexico: Hidalgo-Mihart et al, 2022;Bangladesh: Maria et al, 2022;Singapore: Ow et al, 2022;Argentina: Raño et al, 2022;Malaysia: Yap et al, 2022). The special issue includes two papers from Africa (Cunneyworth et al, 2022;Linden et al, 2022) but given the high number of arboreal species on the continent, it still remains under-represented in the world of canopy bridge research.…”