Why we simply must have predators

ALERT member John Terborgh is a scientist of enormous stature, whose many accomplishments include a rare MacArthur 'Genius' Award.  Here he tells us why predators are so crucial for the Earth -- a lesson with big implications for understanding nature and our future:

Amur Leopard -- just 70 left in the wild today.

Amur Leopard -- just 70 left in the wild today.

Forty-five years ago, three leading ecologists asked a question of child-like simplicity, “Why is the world green?”

We take a green world for granted, yet this deceptively simple question goes to the very heart of how ecosystems work.

The world is green, the trio argued, because predators limit the numbers of herbivores, thereby protecting plants and allowing them to flourish.

At the time this idea was floated, other ecologists were busy discovering that plants manufacture a potent arsenal of chemical compounds to deter herbivores -- everything from deadly toxins to chemicals that make them hard to digest. 

Far from being helpless, these other ecologists argued, plants actively defend themselves, and this keeps herbivore numbers down.  Under this scenario, predators aren't really needed to keep the world green.  

Who is right?  In theory, it should be simple to find out: just remove the predators from an ecosystem and see what happens. 

Wolves -- widely persecuted but trying to make a comeback in parts of North America and Europe.

Wolves -- widely persecuted but trying to make a comeback in parts of North America and Europe.

But it's one thing to remove little predators such as insects and spiders.  Getting rid of big predators, such as wolves, lions, or jaguars, is a far taller task.  To perform such an experiment properly, one would need to fence off huge areas, some with predators and prey, some with prey alone, and others with neither. 

The cost of such an ambitious experiment would be so high that, to this day, nobody has tried it.  As a result, ecologists are still arguing about why the world is green.

An Accidental Experiment

But purely by accident, an unplanned experiment in Venezuela created the right conditions to test the 'Green World' hypothesis.  There, a massive expanse of forest flooded by a hydroelectric dam created hundreds of artificial islands.  The smallest islands were barely the size of a tennis court; the biggest, at hundreds of hectares, would span a dozen large golf courses.

My students, colleagues, and I studied these islands for 14 years.  And what we found is enormously exciting -- and scary.

The largest islands had enough habitat to support both predators and their prey.  But as island size declined, fewer and fewer species of predator remained.  Once below 10 hectares in size -- equivalent to about 20 football fields -- the predators vanished entirely. 

The Harpy -- king of eagles.

The Harpy -- king of eagles.

At this point you have an ecosystem with a few species of herbivores -- particularly howler monkeys, an iguana, a type of tortoise, and leaf-cutter ants -- but nothing to eat them.

With little to keep herbivores in check, did these islands stay gloriously green, or collapse ecologically?  

Ecological Collapse

What we observed was ecological chaos.  Herbivores attacked foliage in all parts of the forest. The first plants to be killed were seedlings, cut up and carried away by leaf-cutter ants, while howler monkeys and iguanas defoliated trees and vines in the canopy.

Without predators, leaf-cutter ants can decimate a forest.

Without predators, leaf-cutter ants can decimate a forest.

Small saplings were the next to disappear under the herbivore onslaught, followed by larger saplings and woody vines.  Finally, after a decade or more, big canopy trees began to die, standing leafless, ghost-like.

By the end of our study, the once-verdant forest was a degraded tangle of shrubs and vines -- a pathetic vestige of their original diversity.

And while herbivores had won, they ultimately brought about their own self-destruction.  In the end, the decimated islands barely sustained any life at all.

The trio of ecologists -- the architects of the 'Green World' hypothesis -- were right.  We need predators to keep our planet verdant and healthy, and to maintain biodiversity.

Alternative States

The 'alternative state' of a predator-free island is alarming.  Equally worrisome is that we can get there in a variety of ways.  Killing off predators is one way -- and we humans are very good at doing that.

Another way is flooding an ecosystem with nutrients, a process called "eutrophication".  This can happen, for example, if we carelessly use farming fertilizers, which then leach into waterways and other ecosystems.  Biodiversity depends on a balance, and tip that balance too far one way and nutrient-loving species dominate while excluding many other species.

Depressing 'alternative states' can arise for other reasons too, such as disrupting natural fire regimes or introducing exotic species that fundamentally change ecosystems.

But for me, the widespread decimation of predators is the most worrisome way that we are disrupting the natural world.  For that reason, I strongly support efforts to 're-wild' ecosystems -- to reintroduce big carnivores such as wolves, grizzy bears, and tigers to regions where they once held sway.  

Arctic fox in the summer.

Arctic fox in the summer.

Rewilding sizable parts of the world will not be easy.  There are many who will object -- out of fear or ignorance or potential risks to their livelihoods. 

But the world needs predators.  I have been studying nature for a long time, and one thing I have learned is that biodiversity utterly depends on them.

Bringing the science we need to maturity and explaining it to policymakers and the public is a vital goal for our next generation of conservationists.


Unconventional-gas mining: Are we grabbing a tiger by the tail?

In Australia, as elsewhere, huge efforts are being focused on exploiting unconventional-gas deposits.  Australian ecologist professor Steve Turton explains what it's all about and why it should be making us nervous. 

We hear a lot these days about unconventional gas.  What is it? 

Unconventional gas includes coal-seam gas, shale gas, and tight gas.  There's a heck of a lot of it on Earth but it's found in complex geological systems and can be devilishly difficult to extract. 

In its 2013 report, the U.S. Energy Administration estimated that recoverable supplies of shale gas totaled some 188 trillion cubic meters worldwide.  Known reserves occur in 41 different countries. 

And there's a lot of coal-seam gas too -- about 143 trillion cubic meters worldwide, according to a 2006 study.

This sounds like good news for an energy-hungry world, so what's the downside? 

For starters, like many forms of mining, unconventional-gas extraction has been implicated in a suite of nasty environmental impacts.  Some of these were highlighted in an independent report commissioned by the Australian Council of Environmental Deans and Directors, which focuses largely on coal-seam gas exploitation in Australia.

Why worry about coal-seam gas?  For starters, there are major surface impacts, with native vegetation being bulldozed for often-dense networks of roads and drilling platforms.  Fragmentation of forests is a common result, with potentially serious impacts on biodiversity. 

In farming regions, coal-seam gas operations can have large impacts on agriculture and forestry operations.  They can also pollute surface waters and have largely unknown impacts on vital groundwater resources.

Unconventional-gas production often competes with existing land uses, such as agriculture, plantation forestry, and urban areas, leading to heated conflicts among different users.  The Lock the Gate Alliance in Australia resulted from a conflict between farmers and coal-seam gas companies.  Expect wicked environmental problems to arise for irrigated agriculture and intensive grazing as well.

A big problem is getting rid of dirty water.  For coal-seam gas extraction, high-pressure water is pumped into wells to hydraulically crack ("frack") gas-bearing strata, releasing the gas.  The gas is pumped to the surface along with a lot of dirty water, salt, and chemicals liberated by the fracking process. 

Disposing of all this dirty water is a big problem.  Heavy rains can cause containment ponds to overflow, releasing the dirty water into nearby waterways.  

Compared with the surface risks of unconventional-gas extraction, we know much less about how groundwater is affected when mining shale gas and coal-seam gas.  A key worry is this: Could underground aquifers vital for irrigation and human uses be contaminated? 

What is the way forward for unconventional gas?   There is little doubt that production of unconventional gas poses a risk to biodiversity and groundwater.  We also know the industry often encroaches into high-value landscapes, competing with biodiversity conservation, the production of food and fiber, and sometimes even moving into urban or peri-urban areas.

Because the planet needs energy, it's clear that unconventional gas is not going to go away.  Managing its exploration and production will require a holistic landscape approach, taking into account cumulative impacts and strategic assessment frameworks

Where there are key unknowns –- as often occurs when one is dealing with groundwater –- the precautionary principle should be applied.  If we're uncertain about the environmental impacts that might arise, the wisest choice is simply not to drill.

Otherwise the tiger we're desperately trying to hang onto might just turn around and bite us.