Herbivores, poop and the nutrient cycle

Ecosystems are intricate webs shaped and sustained by all their living elements – and you can’t underestimate the role of animal dung as a valuable, even critical, source of nutrient cycling and distribution.

But not all dung is the same, according to an international study published in the journal Proceedings of the National Academy of Sciences.

It found that droppings of megaherbivores, such as elephants, rhinos and hippos, contain a lower phosphorus-to-nitrogen ratio than that of smaller herbivores, which the authors say could have broad impacts, including plant nutrient availability and carbon sequestration.

And this is part of a larger web. The researchers found that the distribution of different sized animals – and their steamy deposits – is affected by environmental factors such as food availability and predation.

The study is part of an ongoing exploration into the ecosystem impact of megaherbivores compared to their smaller counterparts, says lead author Elizabeth Le Roux, from South Africa’s Nelson Mandela University.

“There are several reasons to suspect that they should differ from smaller herbivores in moving nutrients across a landscape,” she explains. They are less vulnerable to predation, tend to move over greater distances and live longer.

The team reasoned that because larger animals need nutrients in different ratios to smaller animals – they have bigger bones and thus need and retain more phosphorus relative to nitrogen – this would likely be reflected in their droppings.

To investigate, they chose patches of a grassland savanna in Hluhluwe-iMfolozi park with a diverse range of mammal herbivores ranging from the lightweight impala (Aepyceros melampus) to the massive white rhino (Ceratotherium simum).

The patches had a variable spread of rainfall and landcover – representing food availability and predator ambush opportunity.

Monitoring which animals spent time on each patch showed these environmental factors impacted the average size of herbivores in an ecosystem.

Although these factors impacted their distributions, it didn’t alter the total amount of faecal droppings in each area. However, as predicted, it did change its nutrient profile, which the authors say could have far-reaching consequences.

“This shift in nitrogen/phosphorus ratio can in turn feed back into the plant community,” they write, “potentially altering plant productivity, plant species composition, decomposition dynamics, and many other aspects of ecosystem functioning.”

It can also give insights into the ecological legacies of past megafaunal extinctions, says le Roux – a risk that large modern-day herbivores face.

“Today we view these megaherbivores… as a uniquely African characteristic, apart from a few places still in Asia,” she says. “But 10,000 years ago, species as big as these and bigger were living practically all over the world.”

“[T]he loss of these large animals may have systematically altered nutrient distributions on continental scales, and what we are measuring in terms of nutrient availability and animal nutrient movement may have been very different in the past.”

The Royal Institution of Australia has an Education resource based on this article. You can access it here.