Canopy bridges: preventing and mitigating anthropogenic impacts on mantled howler monkeys (Alouatta palliata palliata) in Costa Rica

Increasing levels of anthropogenic modification, degradation, and fragmentation of arboreal mammal habitats substantially influence wildlife movement and behavior. In Costa Rica, there has been a surge in infrastructure development in the past decade. This infrastructure puts wildlife at high risk of mortality from electrocution, vehicle collisions, falls, and attacks by domestic animals. To prevent and mitigate the mortality of arboreal animals, canopy bridges have been implemented in several parts of the country. In Playa Hermosa, Guanacaste, a coastal community, we implemented a canopy bridge project for mantled howler monkeys (Alouatta palliata palliata). For the first year of our study (2015-2016), we performed monthly three-day censuses and follows of all howler monkey groups. After the first five months of data collection, we identified 20 high-use arboreal pathways that would benefit from reinforcement with canopy bridges and installed camera traps in the pathways to verify their use outside of sampling periods. After use was verified, we installed canopy bridges at those 20 sites. We continued to identify high-use paths via group follows, ad libitum sightings from community members, and annual group censuses between 2018 and 2021 and installed an additional 91 bridges (N = 111 total). We also developed a system for community reporting of howler monkey injuries and fatalities and advocated for better insulation of electric cables, with 8341 m insulated between 2015 and 2021. Between 2015 and 2021, we saw the number of monkey groups grow from five to seven, group home ranges grow, the population grow from 59 to 99, and fatalities decrease from five to one annually. We conclude that the bridges likely contribute to howler monkey population growth via decreased fatalities and safer access to feeding resources and mates. Costa Rica is currently adopting legislation to make connectivity mitigation measures for wildlife mandatory in infrastructure development, making it a global leader.