Large Mammals Are Quietly Engineering the Soil Beneath Brazil's Atlantic Forest

Large mammals such as tapirs, peccaries, agoutis and deer alter the chemical composition of leaf litter and soil in Brazil's Atlantic Forest, increasing nutrient availability and likely soil fertility, according to a study published in the journal Ecological Monographs by researchers at the Center for Research on Biodiversity and Climate Change (CBioClima), based at São Paulo State University's Institute of Biosciences.

"Most of the mammalian biomass in contiguous areas of the Atlantic Forest consists of peccaries, native wild pigs that live in herds that can exceed 100 individuals," said lead author Letícia Gonçalves Ribeiro. "They arrive in an area and spend a lot of time trampling and rooting around in the ground, searching for fallen fruits and seeds, as well as defecating and urinating," which influences nutrient cycling and litter diversity, she said.

Since 2009, researchers have compared soil and litter in 15-square-metre forest plots left open to roaming mammals against fenced plots where the animals were excluded, using camera traps to estimate mammal biomass. The current study analysed 10 open and 10 closed plots in Carlos Botelho State Park. Soil was less acidic where mammals were present, and levels of aluminum, which harms plants at high concentrations, were lower. Leaf litter in mammal-occupied plots also had less lignin, a molecule that resists decomposition, because trampling breaks litter into smaller, more evenly distributed pieces.

"Our research has demonstrated, with increasing certainty, how large herbivores play a vital role in forests," said Mauro Galetti, the IB-UNESP professor who coordinated the study. "It's precisely those animals — which are most targeted by hunters — that act as ecosystem engineers, influencing everything from the composition of plants in the landscape to the chemistry of the soil." In a newer phase of the research, Ribeiro is finding that plots with large mammals also have a higher abundance of predatory nematodes, microscopic soil worms whose presence indicates a healthier ecosystem because it requires every other trophic level to be present too.