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Top predator decline affects ecosystems everywhere

For the first time, a team of 14 ecological researchers from America, Australia, Italy and Sweden have shown that large carnivores are necessary for the maintenance of biodiversity and ecosystem functions in their environments.

Yet the carnivores all face serious threats and are experiencing massive declines in their populations and geographic ranges around the world. The researchers say this is having serious impacts on other species.

“Promoting tolerance and coexistence with large carnivores is a crucial societal challenge that will ultimately determine the fate of Earth’s largest carnivores and all that depends upon them, including humans,” they write in a report on the research published in Science.

The research team analysed the effects of threats such as habitat loss, human persecution and reduced prey on the world’s 31 largest mammalian carnivores. The species studied included lions, tigers, African wild dogs, leopards, cheetahs, wolves, lynx, otters, bears, hyenas and dingoes – spanning all continents except Antarctica.

The team found that more than three quarters of the 31 large carnivores were in decline and that 17 species occupied less than half of their historical distributions.

The hotspots

‘Hotspots’ of carnivore decline are South East Asia, Southern and East Africa, and the Amazon, where several large carnivores are declining. In the developed world there are now few places where large carnivores remain.

Seven carnivore species in particular were shown to have profound effects on the environment and cause what is known as ‘trophic cascades’ – a ripple effect where one species’ influence spreads through multiple levels of a food web.

This effect is most well known for African lions, leopards, the Eurasian lynx, cougars, gray wolves, sea otters and Australian dingoes. The latter animal greatly reduces kangaroo and red fox numbers, which in turn reduces grazing of vegetation and predation of native animals, helping to conserve and protect biodiversity.

In coastal North America, sea otters keep sea urchin numbers in check, which helps maintain kelp forests and benefits other marine species dependent on this habitat. But in this case otters might also offer a defence against climate change because healthy kelp forests can grow rapidly and store large amounts of carbon.

The researchers say that in Africa, a decrease in lions and leopards has coincided with a dramatic increase in olive baboons, which threaten farm crops and livestock, and spread intestinal worms. Baboons even impact on education when children have to stay home to defend their farms from raids.

“Clearly predators have far-reaching ecological, economic and social benefits that are grossly underappreciated. There is no doubt predators pose challenges too, such as wolves attacking livestock. But education and new management practices offer ways forward – for instance, we could use guardian animals to protect livestock from predators.”

The Science report notes that the largest terrestrial species of carnivores are wide-ranging and rare because of their positions at the top of food webs. They are some of the world’s most admired mammals and, ironically, some of the most imperilled.

Population declines

Most have experienced substantial population declines and range contractions throughout the world during the past two centuries.

Because of the high metabolic demands that come with being warm-blooded and of a large body size, the carnivores often require large prey and expansive habitats.

But these requirements often bring them into conflict with humans and livestock. This, in addition to human intolerance, renders them vulnerable to extinction, the researchers say:

“Large carnivores face enormous threats that have caused massive declines in their populations and geographic ranges, including habitat loss and degradation, persecution, utilisation, and depletion of prey.”

Yet large carnivores deliver economic and ecosystem services via direct and indirect pathways that help maintain mammal, avian, invertebrate and fauna abundance or richness. Further, they affect other ecosystem processes and conditions, such as scavenger subsidies, disease dynamics, carbon storage, stream morphology and crop production.

“The maintenance or recovery of ecologically effective densities of large carnivores is an important tool for maintaining the structure and function of diverse ecosystems,” the researchers write.

“Current ecological knowledge indicates that large carnivores are necessary for the maintenance of biodiversity and ecosystem function [but] human actions cannot fully replace the role of large carnivores.

“Additionally, the future of increasing human resource demands and changing climate will affect biodiversity and ecosystem resiliency. These facts, combined with the importance of resilient ecosystems, indicate that large carnivores and their habitats should be maintained and restored wherever possible.”

The researchers argue that preventing the extinction of the carnivore species and the loss of their “irreplaceable ecological function and importance requires novel, bold and deliberate actions”.

They call for a Global Large Carnivore Initiative to coordinate local, national, and international research, conservation and policy.

“Together we call on governments to end policies and management practices that are responsible for the ongoing persecution and loss of predators from our planet. We need an international initiative that aims to conserve large predators and promote their coexistence with people.”
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