Large animals driven extinct by human hunters still affect ecosystems today

Below is an excerpt/paraphrased of Michael Marshall’s 14 August 2013 NewScientist Ecosystems still feel the pain of ancient extinctions, the abstract of the original Nature Geoscience article, and future losses of large animals will affect tropical forests in the future. Alice Friedemann ]

The large megafauna driven extinct by humans over 12,000 years ago still affects ecosystems today. Large animals have an out-sized effect on ecosystems because they range so widely, dispersing the nutrients in their dung across large areas, making the soil more fertile.

Chris Doughty at the University of Oxford showed that the loss of phosphorous when human hunters drove all the large herbivores extinct in South America is still affecting the Amazon basin today, by estimating how much phosphorus South America’s larger extinct animals would have moved 15,000 years ago, before their decline.

The model suggests that megafauna would have spread nutrients 50 times further than animals today do over the same time period. Or to put it another way, killing off the massive animals reduced the movement of nutrients by 98 per cent. This is because big animals move a disproportionately large amount of nutrients compared with small animals, they travel further in search of food, and they keep that food in their guts for longer.

Doughty compares big animals to the arteries that carry blood around the body. “When you get rid of big animals, it’s like severing the nutrient arteries.” He thinks the same thing has happened in North America, Europe and Australia, where most big animals have also been wiped out.

“The idea that herbivores redistribute nutrients is not new, but the scale of this thinking is much, much bigger,” says Tim Baker at the University of Leeds in the UK.

If Doughty is right, the Amazon is still changing in response to the extinction. His model predicts that nutrient distribution will get patchier for another 17,000 years, although the effect will probably be dwarfed by the impacts of deforestation and climate change in the short term, says Baker.

In the absence of massive herbivores, humans now dominate the movement of nutrients – but we do the opposite of what the extinct animals did. We spread fertiliser on small plots of productive farmland, and keep large animals like cows fenced in rather than letting them roam freely. “There are probably more nutrients because of people, but they are very poorly distributed,” says Doughty.


The legacy of the Pleistocene megafauna extinctions on nutrient availability in Amazonia

Christopher E. Doughty,    Adam Wolf    & Yadvinder Malhi

Nature Geoscience 6, 761–764 (2013) doi:10.1038/ngeo1895

Published online 11 August 2013

In the late Pleistocene, 97 genera of large animals went extinct, concentrated in the Americas and Australia (1). These extinctions had significant effects on ecosystem structure (2), seed dispersal (3) and land surface albedo (4). However, the impact of this dramatic extinction on ecosystem nutrient biogeochemistry, through the lateral transport of dung and bodies, has never been explored. Here we analyze this process using a novel mathematical framework that analyses this lateral transport as a diffusion-like process, and we demonstrate that large animals play a disproportionately large role in the horizontal transfer of nutrients across landscapes. For example, we estimate that the extinction of the Amazonian megafauna decreased the lateral flux of the limiting nutrient phosphorus by more than 98%, with similar, though less extreme, decreases in all continents outside of Africa. This resulted in strong decreases in phosphorus availability in eastern Amazonia away from fertile floodplains, a decline which may still be ongoing. The current P limitation in the Amazon basin may be partially a relic of an ecosystem without the functional connectivity it once had. We argue that the Pleistocene megafauna extinctions resulted in large and ongoing disruptions to terrestrial biogeochemical cycling at continental scales and increased nutrient heterogeneity globally.

1 Barnosky, A. D., Koch, P. L., Feranec, R. S., Wing, S. L. & Shabel, A. B. Assessing the causes of Late Pleistocene extinctions on the continents. Science 306, 70–75 (2004).

2 Gill, J. L., Williams, J. W., Jackson, S. T., Lininger, K. B. & Robinson, G. S. Pleistocene Megafaunal collapse, novel plant communities, and enhanced fire regimes in North America. Science 326, 1100–1103 (2009).

3 Janzen, D. H. & Martin, P. S. Neotropical anachronisms—the fruits the gomphotheres ate. Science 215, 19–27 (1982).

4 Doughty, C. E., Wolf, A. & Field, C. B. Biophysical feedbacks between the Pleistocene megafauna extinction and climate: The first human-induced global warming? Geophys. Res. Lett. 37, L15703 (2010).

The extinction of large animals from tropical forests could make climate change worse — according to researchers at the University of East Anglia.

New research published today in Science Advances reveals that a decline in fruit-eating animals such as large primates, tapirs and toucans could have a knock-on effect for tree species. This is because large animals disperse large seeded plant species often associated with large trees and high wood density — which are more effective at capturing and storing carbon dioxide from the atmosphere than smaller trees. Seed dispersal by large-bodied vertebrates is via the ingestion of viable seeds that pass through the digestive tract intact. Removing large animals from the ecosystem upsets the natural balance and leads to a loss of heavy-wooded large trees, which means that less CO2 can be locked away.

Prof Carlos Peres, from UEA’s School of Environmental Sciences, said: “Large birds and mammals provide almost all the seed dispersal services for large-seeded plants. Several large vertebrates are threatened by hunting, illegal trade and habitat loss. But the steep decline of the megafauna in overhunted tropical forest ecosystems can bring about large unforeseen impacts.  “We show that the decline and extinction of large animals will over time induces a decline in large hardwood trees. This in turn negatively affects the capacity of tropical forests to store carbon and therefore their potential to counter climate change.”

The research team studied data from more than 2,000 tree species in Brazil’s Atlantic Forest, and more than 800 animal species.  They found that frugivores which are not targeted by hunters — such as small birds, bats and marsupials — are only able to disperse small seeds, which are associated with small trees. Meanwhile large heavy-wooded trees, which can capture and store greater amounts of carbon, are associated with larger seeds. And these are only dispersed by large animals.

When we lose large frugivores we are losing dispersal and recruitment functions of large seeded trees and therefore, the composition of tropical forests changes. The result is a forest dominated by smaller trees with milder woods which stock less carbon.

‘Defaunation affects carbon storage in tropical forests’ is published in the journal Science Advances on Dec. 18, 2015

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