Rewilding

The Crucial Role of Predators

A New Perspective on Ecology

Scientists have recently begun to understand the vital role played by top predators in ecosystems and the profound impacts that occur when those predators are wiped out. Now, researchers are citing new evidence that shows the importance of lions, wolves, sharks, and other creatures at the top of the food chain.

Just as the world's lions, tigers, and bears are disappearing worldwide, a scientific consensus is emerging that they are critical to ecosystem function, exerting control over smaller predators, prey, and the plant world. Studies of predation — a so-called top-down force in nature — have always run a weak second to ecology's traditional focus, which holds that the foundation of life springs from bottom-up processes enabled by plants capturing energy from the sun. While no one disputes the importance of photosynthesis and nutrient cycling, experts on predation have become increasingly convinced that ecosystems are ruled from the top.

ABeginning with aquatic experiments, they have amassed considerable evidence of damage done to food chains by predator removal and have extended such studies to land: Predation may be as consequential, if not more so, than bottom-up forces. With a comprehensive new book (Trophic Cascades) and a major Science review published this summer, these specialists present the case that our persecution of predators menaces the marine and terrestrial ecosystems that produce food, hold human and zoonotic diseases in abeyance, and stabilize climate.

Using such terms as deep anxiety and grave concern to signal their alarm, the authors contend that the loss of large animals, and apex predators in particular, constitutes humanity's most pervasive influence on the environment. It amounts, they argue, to a global decapitation of the systems that support life on Earth.

These are hardly new ideas: Both publications catalogue decades of work examining the power of predators. Charles Elton, an Oxford ecologist, first conceptualized food webs in the 1920s, speculating that wolf removal would unleash hordes of deer, a notion that weighed on Aldo Leopold's mind as he compared the consequences of wolf-extirpation in German forests to still-thriving, intact systems in Mexico's Sierra Madre Mountains.

These insights gave rise to the 1960s green world hypothesis, which held that plants prevail because predators hold herbivores in check. Profound food chain effects — caused by adding or removing top species — are now known as trophic cascades. In a classic 1966 experiment, biologist Robert Paine removed the purple seastar, Pisaster ochraceus — a voracious mussel-feeder — from an area of coastline in Washington state. Their predator gone, mussels sprouted like corn in Kansas, crowding out algae, chitons, and limpets, replacing biodiversity with monoculture.

Corroborating evidence multiplied. Less than a decade after Pisaster, marine ecologists James Estes and John Palmisano reached the astonishing and widely reported conclusion that hunting of sea otters had caused the collapse of kelp forests around the Aleutian Islands. While the cat was away, the prey (sea urchins) stripped the larder bare. When otters returned, they regulated urchins, allowing luxuriant regrowth of biodiverse kelp communities. Around islands farther out to sea, where the mammals had not reestablished themselves, urchin barrens remained.

The Science review this summer and other recent research have highlighted the cost of cascades in other marine systems. Extirpation of great sharks along the eastern seaboard caused an irruption of rays and the collapse of a century-old scallop fishery, a glimpse of the future as shark populations crash worldwide. Overfishing of cod, a top predator of lobster and sea urchins, upended the coastal North Atlantic, producing hyper-abundant lobster and a market glut in the Gulf of Maine, as well as an urchin boom-and-bust cycle off Nova Scotia, where urchins have been periodically wiped out by disease.

Yet, as data from aquatic systems proliferated, skeptics suggested that top-down forces might be all wet — limited to marine or freshwater systems, with a dearth of evidence for cascades in terrestrial systems.

Where was that evidence? Designing experiments to reveal cascades on land, across large-scales and over long time periods, seemed nearly impossible. So many ecosystems had already been irreparably altered that predator-related effects — including damage done to food chains, so-called trophic downgrading — could not be measured with certainty. Long-term trials teasing out wide-ranging interactions among predators and other species promised to be unwieldy and expensive.

Nonetheless, startling revelations continued to crop up. In a Venezuelan valley flooded by construction of a dam in the 1980s, Duke University ecologist John Terborgh and his students documented the strange perturbations that afflicted the islands of Lago Guri. Top predators — jaguar, mountain lion, harpy eagle — fled rising waters. Multiplying out of control, howler monkeys went mad as their numbers soared and the plants they ate increased toxins in self-defense. Some islands were cloaked in thorns as leaf-cutter ants — undeterred by armadillos or other predators — starved the soil of nutrients by carrying every leaf down to their lairs.

In 1995, the terrestrial camp landed an extraordinary boon as Yellowstone National Park gave William Ripple, director of Oregon State University's Trophic Cascades Program, the chance to study top-down forcing in action. Ripple watched in amazement as the wolf's return to Yellowstone — an ecosystem where elk had had the browse of the place for 75 years — gave willow and other trees the chance to take hold along stream banks, cooling water temperatures for trout and encouraging the return of beaver, whose ponds host long-absent amphibians and songbirds. Yellowstone proved that damage to a terrestrial food web could be reversed and an ecosystem restored with the return of a single species. It is a sobering lesson for the eastern U.S., where the explosion of white-tailed deer has eradicated hemlock, a keystone species in once-biodiverse hardwood forests.

Yet despite such developments, researchers of trophic cascades have despaired of securing the money and means to examine predator removal in large-scale, long-term trials on land. Some have dealt with constrictions by adopting a more manageable, meadow-sized scale. In a three-year experiment, ecologist Oswald Schmitz of the Yale School of Forestry & Environmental Studies found that even the tiniest of predators (spiders) exercise a more significant top-down influence on plants than bottom-up factors. The type of predation — active versus ambush hunting — also appears to be consequential, affecting the composition of plant communities and nitrogen levels. Spiders that hunt actively reduce grasshopper density, allowing grass and goldenrod to dominate other plants and increasing available nitrogen. Ambush hunting has an opposite effect, forcing grasshoppers, which would rather feed on grass, to shelter in goldenrod, yielding a more diverse plant community and less nitrogen. Taken together, Schmitz says, it's the richness of the functional role of predators that becomes important to conserve.

Estes and Terborgh, editors of Trophic Cascades, question whether spiders and grasshoppers will convince anyone that orcas, great white sharks, wolves, tigers, and jaguars are important. But Schmitz, who grew up north of Toronto where wolf-hunting was a way of life, thinks the process is underway: Piece by piece, it's taken 20 years to accumulate the evidence, and the culmination is in that Science paper — that the world is driven by predators as well as nutrients. We have to pay attention to their health and well-being if we want a healthy ecosystem. Simply eliminating them because we want more prey or because we don't think they're important is very misguided.

Indeed, the Science review presses the trophic case into new territory, extending predation's impact to human health. A reduction in lion and leopard populations in Ghana has led to an explosion of olive baboons. The release of such mesopredators — mid-sized carnivores such as cats or raccoons that run rampant without control — has wreaked havoc around local villages, where baboons attack livestock, damage crops, and spread intestinal parasites to the human population.

In the Science paper, the authors call for a paradigm shift in ecology. Scientists and land-managers, they argue, must adopt top-down forcing as a given if there is to be any real hope of understanding and managing the workings of nature.

In Trophic Cascades, Terborgh and Estes go farther, criticizing national science agencies for failing to fund research on predator removal in terrestrial systems, accusing them of clinging to old views and retarding progress while ecosystems are undermined. The idea that plants are affected by the things that eat them, Estes says dryly, has not been widely appreciated.

But Alan James Tessier, program director of the National Science Foundation's Environmental Biology Division, disagrees, asserting that the agency has funded much research into top-down processes. It's ridiculous to talk only about top-down or bottom-up control, said Tessier. Both are happening all the time.

In science, new ideas are rightly met with skepticism, if not denials and dismissals. But as the consequences of predator loss become increasingly measurable and predictable, they implicitly call for a reassessment of our ancient foes. Estes is as reluctant as any scientist to weigh in on the wolf wars, but his frustration is clear. That's not the way we should be behaving as a species, he says.

Could rewilding help to tackle climate change?

Trophic rewilding refers to the restoration of dwindling or recently lost animal populations. (This is opposed to Pleistocene rewilding, which is concerned with reintroducing animals that have long been extinct in an area, such as the idea of returning mammoths to Siberia.)

It is argued that reintroducing large herbivores, such as elephants, sloths and tapirs, could help countries to lower their emissions for several reasons.

Large herbivores were once a common sight in all of Earth's habitable continents. However, hundreds of years of intensive livestock farming has contributed to steep declines in native herbivores and large increases in cattle. Today, there are around 1.5bn cows on Earth.

The replacement of native animals with cattle has caused steep rises in emissions of methane – a greenhouse gas that is 34 times more potent than CO² over a 100-year period.

This is because cows are ruminants – meaning that they have specialised stomachs capable of digesting tough and fibrous material, such as grass through fermentation. The digestive process causes cows to belch out high amounts of methane.

Native herbivores, on the other hand, can have much smaller methane footprints. Many large herbivores, including rhinos, elephants and camels, are hindgut fermenters – meaning that they have a simple stomach and carry out fermentation of food in the large intestine.

This type of digestion produces much smaller amounts of methane, says study lead author Prof Joris Cromsigt, a researcher of megafauna (large animals) from the Swedish University of Agricultural Sciences.

However, to date, there has been no research comparing the methane footprints of different large herbivores and cattle, he tells Carbon Brief:

Little empirical data is out there for wildlife. The largest emissions, per individual animal, come from large ruminants, such as cattle, and similar wild species, such as African buffalo or American bison. The least methane is, in principle, produced by the large non-ruminants, such as rhino, elephant and equid [horse] species.

Research released in 2017 estimated that, over the past 1,000 years, the replacement of wildlife with cattle in Africa has caused methane emissions to more than double from 3.4m tonnes a year to 8.9m tonnes a year.

The reintroduction of large herbivores could also boost the carbon storage of forests, Cromsigt says. This is because large herbivores act as long-distance seed dispersers for large fruit trees. In fact, research suggests that many large tree species in South America (such as the jicaro tree, shown below) may have evolved giant fruit in order to entice now-extinct large herbivores, such as the giant ground sloth.

Without large herbivores, numbers of the tallest and woodiest trees – which also store the most carbon – have declined. Additionally, research shows that the loss of living herbivores, such as American tapirs and African and Asian forest elephants, could cause the world's tropical forests to lose between 2% and 12% of their stored carbon.

Mitigating Wildfires

In many parts of the world, climate change is likely to be making wildfires more severe. This is because warming has led to higher summer temperatures and, in some regions, less summer rainfall – creating the dry, tinderbox conditions that enable fires to quickly spread.

Some large herbivores are thought to play a role in suppressing wildfires, according to a paper. One reason for this is because large herbivores eat fallen leaves and vegetation, which would otherwise act as fuel in a fire. Second, herbivores eat some plants, but leave others, which can change the makeup of plants in a forest. This can mean that zones of low and high flammability are interspersed in arrangements that could impede the spread of landscape fires, the authors say. Third, some animals alter their forest landscapes by leaving trails or digging holes. The bare patches of land left behind from these activities can act as fire breaks, the researchers say:

The physical scale of these features can be substantial. For example, in a montane vegetation complex in Tasmania, 10% or more of the ground surface was covered by animal paths, mostly created by medium-sized macropods [kangaroos and wallabies] and wombats.

Evidence suggests that, in some parts of the world, the reintroduction of large herbivores appears to have had an impact on wildfire intensity, the researchers say.

For example, earlier research shows that the reintroduction of white rhino to Hluhluwe iMfolozi National Park in South Africa could have impacted the severity of wildfires. White rhinos are thought to be efficient fire suppressors because they graze on tall grass, which can help fires spread between trees. During an experiment, researchers removed white rhinos from some parts of the park and measured changes to grass height and wildfire size in areas both with and without rhinos. The research found that fires were significantly larger in sites where rhinos were removed.

These results suggest that reintroducing large herbivores could help to suppress wildfires in other parts of the world, such as in California, says study lead author Prof Christopher Johnson, an ecologist from the University of Tasmania. He tells Carbon Brief:

There is evidence that climate change is increasing the risk of destructive fires in many parts of the world. Those same changes mean that traditional strategies of fire suppression and prevention work less well. Large herbivores – and some other animals – can stabilise fire regimes, and potentially make fire control more tractable and effective.

Rewilding Europe

Rewilding is a progressive approach to conservation. It's about letting nature take care of itself, enabling natural processes to shape land and sea, repair damaged ecosystems and restore degraded landscapes. Through rewilding, wildlife's natural rhythms create wilder, more biodiverse habitats.

Rewilding is about:

Nature's own ways

Nature knows best when it comes to survival and self-governance.

We can give it a helping hand by creating the right conditions – by removing dykes and dams to free up rivers, by stopping active management of wildlife populations, by allowing natural forest regeneration, and by reintroducing species that have disappeared as a result of man's actions.

Then we should step back and let nature manage itself.

Bringing back wildlife

European wildlife species have strongly declined, even in our wildest areas. Some of them have even gone extinct, while they play a critically important ecological role. Rewilding works to restore lost species guilds by giving them space to thrive, by population enhancement, and by reintroducing key native species.

Ensuring wellbeing

When nature is healthy, we are healthier too. We rely on the natural world for water, food and air. There is a growing realisation that connecting with wild nature makes us feel good and keeps us mentally and physically well.

Rewilding is about reconnecting a modern society – both rural and urban – with wilder nature. We invite people to experience and live in these new, rewilded landscapes.

Delivering for the future

There is no defined end point for rewilding. The aim is to support nature-driven processes, which in turn will bring about wilder nature. This takes time and space. Rewilding is about moving up a scale of wildness, where every step moving up this scale is seen as progress.

If we create and protect areas where rewilding can take place, both people and wildlife will benefit in the long term.