Nacho Villar, Claudia Paz, Valesca Zipparro, Sergio Nazareth, Leticia Bulascoschi, Elisabeth S. Bakker and Mauro Galetti
Tropical forests play a critical role in the global nitrogen cycle. These forests are populated by many animals that feed mostly on fruits, including some of the largest mammals such as tapirs and peccaries. Now a new study in the Atlantic Forest of Brazil has found that these so called “large frugivores” and their hyper-dominant fruiting resources regulate the nitrogen cycle in these highly threatened yet critically important forests. The findings suggest that extinctions of large tropical frugivores may not only affect global carbon and nutrient cycling dynamics, but also have important implications for current understanding of plant-herbivore interactions and how biodiversity is maintained in tropical forests.
Nitrogen is a key essential ingredient for life, found in every single living thing. Large herbivores that feed on grasses and leaves, such as elephants, wildebeest, rhinos, moose, deer, and domestic sheep and cattle are known to strongly affect the way nitrogen flows through nature, aka The Nitrogen Cycle. In doing so, they shape the structure, dynamics and diversity of ecosystems. But most large herbivores in tropical forests feed mostly on a different plant tissue: fruit. Until now, whether the role of such large fruit-loving animals was comparable to their savannah, boreal forest or temperate grassland comrades was a mystery to science.
A replicated long-term exclosure experiment in Brazil demonstrates for the first time strong effects of large frugivores on the nitrogen cycle. We compared different aspects of the nitrogen cycle in soils of open plots – where frugivores such as tapirs, peccaries, deer and other large frugivores had access- and paired closed plots from which these animals were excluded. We also inspected how these effects varied along a gradient in palm abundance, since palms are a key fruiting resource in many tropical forests. Results were stunning: we found that on open plots ammonium was 95% higher. We also found that this nutrient increased with palm abundance. Rates of nitrification, a fundamental microbial process that streamlines nitrogen availability to plants, also increased with palms, but only in plots where large frugivores had access. Such findings lead us to introduce the concept of “fruiting lawns”, nitrogen cycling hotspots where large frugivores induce a positive feedback on fruit production in areas with abundant palms and/or fruit-bearing trees.
The study brings some additional surprises. Against the common conception that nitrogen is homogenously abundant in tropical forests, results showed that nitrogen availability varied by two orders of magnitude among different plot locations, but large frugivores reduced such spatial variability. This suggests that these animals transport and redistribute nitrogen across large tracts of tropical forests, enriching nutrient-poor patches, which could boost plant productivity. The study also reinforces the notion that some palms act as foundation species in Neotropical forests, structuring ecosystem processes through their trophic interaction with frugivores.