Climate change: how elephants help pump planet-warming carbon underground
Climate change: how elephants help pump planet-warming carbon underground
Jeppe Aagaard Kristensen, The Conversation
November 18, 2021
See link
https://theconversation.com/climate-change-how-elephants-help-pump-planet-warming-carbon-underground-170415
for photos.
Imagine you’re in a hot air balloon flying over an African savanna in the
late growing season. Below, herds of elephants, zebras, wildebeests and
rhinos roam a mosaic landscape dotted with lonesome trees and daubs of
woodland on a canvas of yellow-brown grass. The hungry and rowdy herbivores
are eating and trampling the vegetation that stores carbon and keeps it
from heating the atmosphere.
You’d be forgiven for thinking that their voracious appetites and
blundering steps might be disturbing and releasing the carbon stored in
this ecosystem in much the same way wildfires do. But, incredibly, the way
herbivores disturb the landscape actually helps it lock up more carbon in
durable stores that are difficult to reach. In a new review which compiled
evidence from lots of different studies, we uncovered how large herbivores
could help slow climate change this way.
Forests are often evoked as the ultimate vessels for storing carbon. But
carbon in the bark and leaves of trees is vulnerable to logging, pests and
fires which can unleash decades of accumulated carbon in a matter of hours.
Even in healthy forests, most of the carbon stored in vegetation above
ground is decomposed and recycled to the atmosphere as greenhouse gas in
less than a century.
Meanwhile, the soil beneath savannas and grasslands where trees are sparse
but herbivores are abundant can guard carbon for thousands and even tens of
thousands of years in hard-to-reach underground pools. So how is this
possible?
Research from 2009 showed how wildebeests returning to the Serengeti
savanna of east Africa after a virus tore through their populations led to
fewer catastrophic wildfires. Because the wildebeests trampled and ate the
vegetation that had been fuelling the fires in their absence, their return
in the 1960s allowed the plants to gradually reestablish and recover their
pre-epidemic abundance, along with the carbon the landscape’s soil was
storing.
This may sound counter-intuitive, but large herbivores and seasonal fires
are natural elements of grassland ecosystems. Without wildebeests keeping
on top of all that fuel, small fires turn into catastrophic blazes and
consume everything.
But does it really matter for the climate whether herbivores or fires
consume the vegetation? If you assume that 100% of the carbon in vegetation
is released to the atmosphere as greenhouse gasses from fires or elephant
digestion, then it shouldn’t.
But this isn’t what really happens. For one thing, the burned carbon that
remains in the soil after a fire is resistant to decomposition by microbes,
and often called black carbon.
Meanwhile, up to half of the plant material eaten by large herbivores is
excreted as dung and urine, which tends to be easier for decomposers in the
soil (such as beetles, earthworms, fungi and bacteria) to break down
compared to plant litter like dead leaves and fallen logs. Scientists once
assumed that plant material eaten by microbes or animals was lost from the
soil. But recent discoveries suggest this picture is far too simple.
While some of what’s eaten by decomposers is released as CO₂ to the
atmosphere, most of it enters persistent soil pools of carbon. An effective
way of forming long-term stores of carbon underground then is to feed the
soil with easily decomposable organic material.
Large animals seem adept at reorganising where ecosystems store carbon,
directing a larger fraction towards persistent and stable reservoirs
underground. This shows how valuable intact wildlife communities can be,
and should urge us to protect the few remaining herbivore-rich ecosystems
on Earth, such as the African savanna. We may restore so much more by
restoring nature’s four-legged ecosystem engineers in the places they have
been lost.