Reduced predation by arthropods and higher herbivory in burned Amazonian forests

Biodiversity losses have increased in tropical forests due to fire-related
disturbances. As landscape fragmentation and climate change increase,
fires will become more frequent and widespread across tropical rain
forests worldwide, with important implications for forest dynamics by
altering plant-animal interactions. Here we tested the hypothesis that
recurrent fires in tropical rain forests change bottom-up and top-down
forces controlling the abundance of insect herbivores, which in turn
increases herbivory. To quantify herbivory, we collected 50 leaves per
tree of five species in burned and unburned experimental plots (N = 75) in
southeastern Amazonian forests. We measured leaf nitrogen content and leaf
thickness of tree leaves as bottom-up factors that could explain
differences in herbivory; we measured predation pressure on model
caterpillars and estimated the abundance of predatory ants as top-down
factors. We found higher herbivory in burned than in unburned forests, as
well as lower predator attacks in caterpillar models and lower abundance
of predatory ants. Leaf nitrogen content did not vary across treatments.
Birds attacked model caterpillars more frequently in burned than in
unburned forests, and leaf thickness was higher in burned forests, but
these factors together were not enough to offset the higher herbivory in
burned plots. Fire degrades tropical forests not only by killing trees and
altering their structure and community dynamics, but also by reducing
predatory arthropods and disrupting predator-prey interactions, which
triggers increased herbivory. These indirect impacts of recurrent fires
probably contribute to further alter forest structure, functioning, and to
decrease forest regeneration in Amazonian forests.