How Mammals Change Litter and Soil Cycling in Brazil's Atlantic Forest

Soil is a continuously running biochemical factory. When leaves, branches, fruits, and seeds fall to the forest floor, they enter the process of decomposition. Nutrients are returned to the soil, taken up again by plants, and eventually become the foundation for the next round of forest growth. For decades, studies of this cycle have focused mainly on plants, invertebrates, bacteria, and fungi. Yet mammals moving through the forest are also quietly shaping the speed and direction of nutrient flow. As they walk, forage, trample the ground, deposit urine and feces, and disturb the soil, they can alter the chemical properties of the litter layer and the soil beneath it, influencing nutrient cycling across the forest.

A research team chose Carlos Botelho State Park, in the Atlantic Forest of Brazil, as the site for this study. This forest receives high rainfall and remains humid for much of the year, but its soils are often highly weathered, acidic, and relatively nutrient-poor. Under these conditions, litter decomposition and the rapid recycling of nutrients are especially important for maintaining forest productivity. To examine the role of mammals in this system, the researchers used a 14-year mammal exclusion experiment with paired open and closed plots. The closed plots were surrounded by wire fencing that prevented medium- and large-sized mammals weighing more than 1 kg from entering, while the open plots remained accessible to all animal species. This design allowed the researchers to compare how litter and soil differed when medium- and large-bodied ground-dwelling mammals were present or excluded.

Study site. (A) Geographic location of the Atlantic Forest of Brazil; (B) closed plot; (C) open plot（Image source：Ribeiro LG et al. (2026)， CC BY 4.0 ）

Over the course of one year, the researchers used camera traps to monitor ground-dwelling mammals. They then calculated a mammalian biomass index based on each species' mean body mass, the number of photographic records, and camera-trap effort. At the same time, they repeatedly collected litter and topsoil samples during both the dry and wet seasons. In the litter, they measured litter biomass, litter compositional diversity, lignin, cellulose, nitrogen, phosphorus, potassium, calcium, magnesium, organic matter, and carbon. In the soil, they analyzed pH, nutrient concentrations, bulk density, microbial biomass carbon, basal respiration, and urease activity.

The strongest overall patterns came from seasonality. The dry and wet seasons substantially changed litter and soil properties, and spatial heterogeneity among plot pairs was also clear. This is consistent with the nature of tropical forests, where microtopography, parent material, drainage, root systems, and microbial communities can cause soil nutrients to vary sharply even across short distances. Still, despite this complexity, the researchers detected several clear signals. Where mammalian biomass was higher, litter diversity was higher, litter lignin content was lower, litter potassium content was higher, and soil pH was also higher. Large frugivores made up a major part of this biomass, especially the white-lipped peccary (Tayassu pecari), the red brocket deer (Mazama rufa), and the lowland tapir (Tapirus terrestris).

The most likely explanation for these changes is physical disturbance. As these mammals forage on the forest floor, they move litter, disturb the soil, trample plant remains, and break apart layers of accumulated organic material. Once litter is fragmented, more of its surface comes into contact with the soil, giving microbes and decomposers easier access to the material. Lignin is one of the more resistant components of plant cell walls, and it usually slows decomposition. The observed reduction in litter lignin suggests that these mammals may help litter enter a more decomposable state more quickly. The increase in litter diversity may also reflect the way mammals mix different materials together. Leaves, twigs, fruit peels, seed remains, and partially decomposed organic matter no longer remain as separate, undisturbed layers; instead, the forest floor becomes a constantly rearranged organic mixture.

The increase in litter potassium is also meaningful. Potassium is relatively easy to release from plant tissues. When mammals break litter into smaller pieces and increase contact between moist soil and plant debris, potassium may be released more readily from the litter material and become more available for uptake by plants. The increase in soil pH suggests that animal activity may also alter the acid–base environment of the soil. Because the tropical soils in the study area are already highly weathered, acidic, and nutrient-limited, even a modest shift in pH may influence microbial processes and nutrient availability.

Conceptual diagram of the effects of ground-dwelling mammals on litter and soil. In open plots, where mammals have free access, ground-dwelling mammals alter forest floor conditions through foraging, trampling, and soil bioturbation. These activities can fragment litter and increase litter–soil contact, promoting decomposer activity and organic matter decomposition, potentially reducing litter biomass, organic matter, and lignin content (A), while facilitating the release of nutrients such as carbon, nitrogen, phosphorus, and potassium from plant residues (B). Soil disturbance and trampling may also improve soil aeration, water infiltration, and soil mixing, further influencing soil nutrient status as well as microbial biomass and activity (C). Seasonal variation between the dry and wet seasons can modulate these processes; therefore, the effects of mammals on litter decomposition, nutrient mineralization, and soil microbial processes should be understood within a seasonal context（Image source：Ribeiro LG et al. (2026)， CC BY 4.0 ）

Many other variables did not show a clear mammalian effect, including total litter biomass, organic matter, nitrogen, carbon, several soil nutrients, and microbial indicators. This suggests that ground-dwelling mammals do not completely rewrite soil chemistry. Their role is more precise: they appear to push nutrient cycling at certain key points, especially by changing litter quality, increasing litter mixing, promoting the release of some nutrients, and altering soil pH.

This study also points to an important conservation message. If white-lipped peccaries, lowland tapirs, and other large mammals disappear, forests may lose more than seed dispersers. They may also lose animals that disturb litter, mix organic matter, and help shape the chemical environment of the soil. Such changes may alter the speed of nutrient recycling and weaken the connections among plants, belowground microbes, and soil chemistry. In ecosystems such as the Atlantic Forest, where habitats are already highly fragmented and many large mammals are threatened, defaunation may have consequences that extend far beyond the animals themselves.

Author: Shui-Ye You

Reference:

Ribeiro LG et al. (2026). Mammals' zoogeochemical effects change litter and soil biogeochemistry in a tropical rainforest. Ecological Monographs.