Fallen logs act as natural bridges over rivers in a tropical wet forest

Abstract

Streams and rivers are ubiquitous in tropical forests and can act as barriers for animal movement. This role as barriers can be modulated by connecting features like fallen logs. While logs are commonly observed across rivers, their role as natural bridges has rarely been highlighted. We used motion-activated cameras to characterize how mammals and terrestrial birds use logs across rivers and streams in the Osa peninsula, southern Costa Rica. We investigated factors that influence the frequency and probability of crossing and analyzed interactions like temporal avoidance/attraction or communication. We detected 20 terrestrial mammal species; on most detections (80%) animals used logs to get across the river. Crossing probability depended mostly on the species: species adapted to climbing like tamanduas (Tamandua mexicana), coatis (Nasua narica), and common opossums (Didelphis marsupialis) were more likely to cross than strictly terrestrial species like agoutis (Dasyprocta punctata) and pacas (Cuniculus paca). Animals also crossed logs that were wider, longer, and higher above the water. The diversity of species that used logs suggests they could be important drivers of movement, defining routes across the forest. We found evidence of temporal attraction between coatis, skunks (Conepatus semistriatus), and opossums, probably influenced by scent-marking, observed for 6 of 20 species, suggesting logs also act as inter and intra-specific communication sites. Our results provide evidence of the potential importance of logs and branches and suggest they could be critical features that influence daily movements and long-term space use of many terrestrial vertebrate species in tropical forests.<br/