A photo angled from above of a small white bird and a white egg in a nest of twigs.
A one-day-old harpy eagle chick in Paranaíta, Mato Grosso, Brazil, waits for food. New research investigated how harpy eagle droppings influence the flow of nutrients around nest trees. Credit: Everton Miranda/SouthWild.com
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The Amazon rain forest is home to a contradiction: an extremely biodiverse rain forest thriving on nutrient-poor soils. Through much of the region, low levels of nutrients such as phosphorus and nitrogen in the soil limit plant productivity.

Unlike plants, animals move around, shifting nutrients as they do. In a recent study, scientists found that harpy eagles bring nutrients to their nests, creating localized hot spots and potentially affecting soil biogeochemistry.

A Jaguar’s Weight in Nutrients

Harpy eagles are apex predators in the rain forests of South and Central America, hunting for prey such as sloths, monkeys, and other birds in the forest canopy. The eagles often pair for life and rear one or two chicks at a time in large platform nests built 30–50 meters (100–160 feet) above the ground. Chicks spend their first 1–2 years in the nest, during which time their parents bring prey for them to eat.

Researchers installed camera traps at harpy nest sites and found that each eagle pair transports more than 100 kilograms (220 pounds) of prey to their nest each year—that’s more than an entire jaguar’s weight in carcasses.

“We wanted to find out whether harpy eagles could be increasing the presence of nutrients in soils and vegetation around their nest trees,” said ecologist Everton Miranda from the Peregrine Fund. Miranda is the lead author of the study.

From Death, New Life

A photo of a person holding a large gray and white bird. Both the bird and the person are looking at the camera.
Everton Miranda, here with a harpy eagle, led the new study. Credit: Everton Miranda

Most of the prey brought to the nest is eaten by the parents and chicks, but leftovers often fall to the ground, along with lots of eagle poop. The combination of carcasses and excrement introduces scarce and vital nutrients such as phosphorus around nest trees, and plants are quick to take advantage.

Using a rope chainsaw, Miranda and his colleagues collected leaves from the rain forest understory and canopy near nest trees as well as trees themselves. They also collected leaves from areas far from nest trees and found that nutrients, including nitrogen and phosphorus, were much more abundant in the understory and canopy near harpy eagle nests than in the vegetation far from nest trees. Levels of nitrogen were 87% higher, and levels of phosphorus were 142% higher surrounding nest trees.

An Uncertain Path from Eagle to Tree

A photo of a black, gray, and white bird perched on a branch with a carcass in its talons.
Harpy eagles are apex predators that bring prey, including sloths and other birds, to their nests. Credit: cuatrok77/flickr, CC BY-SA 2.0

It’s not surprising that the nutrients being brought in by harpy eagles are ending up in the flora around their nest trees, said Chris Doughty, an ecologist at Northern Arizona University who was not involved in the study. “I think the study makes a convincing argument that nutrients from harpy eagle kills and droppings are making it into the vegetation around their nest trees,” he said.

But how these nutrients are making it into the trees is not quite clear. Scientists understand very little about how nutrients cycle through an ecosystem, especially in diverse tropical forests, Doughty said.

One route could be through the soil. But surprisingly, the researchers did not find elevated nutrient levels in soils around nest trees. When he initially saw soil data from the nest trees, Miranda nearly concluded that harpy eagles had no significant effects on local nutrient cycles. Then, he read the tree leaves and found the missing nutrients in the plant life around nest trees.

Nutrients Flow…

Biologically important elements like phosphorus can cycle incredibly rapidly through ecosystems.

One caveat to keep in mind is that the soil measurements in the study were made at a single time point, said Julia Monk, a community and ecosystem ecologist at the University of California, Berkeley, who was not involved with the study. “It’s possible that capturing soil nutrient influx and outflow over longer periods of time…could help us really understand what’s going on.”

Doughty agreed that even if the nutrients are passing through the soil en route to the trees, one-time measurements may not capture their presence in the soils. “Biologically important elements like phosphorus can cycle incredibly rapidly through ecosystems,” he said. “Oftentimes, it can seem like they are not even really entering the soil because they are being taken up so rapidly by [fungi] and roots.”

…Through Leaf and Bark?

But some of the nutrients from harpy eagle leavings could be taken up directly by the vegetation around nest trees through leaves or bark before reaching the soil. Eagle poop often sprays the foliage beneath the nest, according to Miranda, which could allow the plants to intercept at least part of the nutrient input from harpy eagles before it reaches the soil.

“The loss of these eagles could have profound effects on Amazon biodiversity.”

Monk agreed that the vegetation around nest trees could be directly absorbing some of the nutrients, but she also said that some nutrients could reach the soil and then be absorbed by the trees. “The two ideas are not mutually exclusive,” she said.

Even though it’s unclear how nutrients are getting from harpy eagle kills and droppings to trees, “the data are quite convincing that the transfer is happening,” Doughty said.

These findings highlight the important role harpy eagles play in the Amazon ecosystem: By concentrating nutrients in a relatively small area, the birds increase heterogeneity in the rain forest landscape. “Environmental heterogeneity is one of the main predictors of biodiversity,” Miranda said. “The loss of these eagles could have profound effects on Amazon biodiversity.”

—Adityarup Chakravorty (chakravo@gmail.com), Science Writer

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Citation: Chakravorty, A. (2023), Harpy eagles concentrate precious nutrients in the Amazon, Eos, 104, https://doi.org/10.1029/2023EO230183. Published on 10 May 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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