Natural canopy bridges effectively mitigate tropical forest fragmentation for arboreal mammals

Abstract

Linear infrastructure development and resulting habitat fragmentation are expanding in Neotropical forests, and arboreal mammals may be disproportionately impacted by these linear habitat clearings. Maintaining canopy connectivity through preservation of connecting branches (i.e. natural canopy bridges) may help mitigate that impact. Using camera traps, we evaluated crossing rates of a pipeline right-of-way in a control area with no bridges and in a test area where 13 bridges were left by the pipeline construction company. Monitoring all canopy crossing points for a year (7,102 canopy camera nights), we confirmed bridge use by 25 mammal species from 12 families. With bridge use beginning immediately after exposure and increasing over time, use rates were over two orders of magnitude higher than on the ground. We also found a positive relationship between a bridge's use rate and the number of species that used it, suggesting well-used bridges benefit multiple species. Data suggest bridge use may be related to a combination of bridge branch connectivity, multiple connections, connectivity to adjacent forest, and foliage cover. Given the high use rate and minimal cost, we recommend all linear infrastructure projects in forests with arboreal mammal populations include canopy bridges.

Figure 1

The six species that most frequently used the 13 natural canopy bridges over the pipeline clearing: (a) Aotus nigriceps, (b) Potos flavus, (c) Coendou ichillus, (d) Caluromys lanatus, (e) Bassaricyon alleni, and (f) Hadrosciurus spadiceus.

Figure 2

Overall crossing rate for all connections (events/100 trap nights) in each of the 13 natural canopy bridges over the natural gas pipeline clearing, and the total number of species registered in each bridge. The connection score (1–5, see text) and the number of connections (1–4) are indicated above each pair of columns.

Figure 3

Events per 100 trap nights in all functioning connections of the 10 bridges that were still standing through August 2013 (September 2012 and 2013 were not included because not all camera traps were in place; N = 2,781 events; R ² = 0.63, p < 0.05).

Figure 4

Map of the study area, including the location of natural canopy bridges (green bars), the Bridge Zone (yellow line, 5.2 km), and the No Bridge Zone (red line, 4.0 km) along the pipeline right-of-way (black outline). Map created by T.G. in ArcMap 10.3 (Environmental Systems Resource Institute, ArcMap 10.2 ESRI, Redlands California, www.esri.com).

Figure 5

TG climbing a canopy bridge over a recently cleared natural gas pipeline in the Lower Urubamba Region of Peru to place and program (inset) camera traps.