Loss of biggest predators is seen as a threat that might rival climate change

The faltering fortunes of the world’s 31 largest carnivores are having ripple effects far beyond simple predator-prey relationships, setting off cascades of ecological change that could prove as significant a threat as climate change to earthly life as we now know it.

That’s the sobering conclusion of a paper published last Friday in the journal Science on the subject, “Status and Ecological Effects of the World’s Largest Carnivores.”

The present-day situation and general outlook for these species is not so great, in the view of the paper’s international team of authors, led by William J. Ripple of Oregon State University at Corvallis. Populations are shrinking, range is contracting, the pressures of habitat loss and persecution are increasing.

More disturbing is that the ecological ramifications of their continuing decline is still, at this rather late date, not well understood — even for the seven species about which the most is known: African lions and leopards, gray wolves, dingoes, Eurasian lynx, pumas and sea otters.

It may be obvious that as species decline, so must their delivery of “ecosystem services.” But defining and measuring those services is very much a work in progress.

Take wolves, a species whose struggles are especially well known to many Minnesotans and which is, according to this paper, the most thoroughly studied of all the world’s large carnivores.

More than just wolves, moose and fir

We know, of course, that when there are fewer wolves there are bound to be more deer in most places and also, in a few places like Isle Royale, more moose. On the island, more moose means  more pressure on balsam fir; everywhere else, it means more deer wrecking crops and residential gardens. Also, more car wreckage.

But this is only the beginning, the new paper attests, because scientists are learning that falling populations of wolves, leopards, pumas and other top predators set off  “trophic cascades” of disturbance — long chains of cause and effect across multiple levels of ecosystem structure that we tend of think of as functionally independent, though of course they aren’t.

For just one intriguing example, falling populations of pumas could promote increased flooding or even rerouting of streams, because these particular predators “appear to influence processes affecting terrestrial and aquatic species, including hydrophytic plants, wildflowers, amphibians,  lizards, and butterflies. Their presence may also help to stabilize stream banks and channels.”

Unexpected effects of trophic cascades and various taxa and processes include changes to bird, mammal, invertebrate, and herpetofauna abundance or richness; subsidies to scavengers; altered disease dynamics; carbon sequestration; modified stream morphology; and crop damage. 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.

Wait a minute. Carnivores driving carbon sequestration? Yep, that’s what they say:

[T]he decline of large carnivores in western North America was followed by a decline in hardwood tree recruitment in riparian areas of over two orders of magnitude. In northern North America, gray wolves limiting moose populations may be responsible for increased net ecosystem uptake of carbon due to decreased browsing and increased net primary productivity.

Likewise the presence of sea otters and nearshore environments suppresses sea urchins, allowing macroalgael kelp to thrive, thereby increasing ecosystem carbon production and storage by one to two  orders of magnitude.

Top of the food webs

There are 245 land-dwelling species in the mammalian order Carnivora, and another 30-odd that live in marine environments, including seals and walruses.

Among the shared characteristics of these species, the authors say, is a quality of being “naturally rare because of their position at the top of the food web. These are also some of the world’s most revered and iconic species. Ironically, they are also some of the most threatened.”

Because of their large size, long lives, large appetites and tendency to roam over considerable territory, these top-of-the-web creatures also come into especially intense conflict with humans — especially humans who hunt large game or raise livestock, which inclines them to see large carnivores as competition or economic threat.

Thus the same factors that make carnivores appealing and fearsome to us also make them vulnerable to human persecution. This only compounds habitat losses driven by agriculture, deforestation and urbanization, resulting in considerable pressure on carnivore populations around the world.

For the 31 species examined in this paper — the largest mammalian carnivores on Earth, with average adult weights above 15 kilograms, or about 33 pounds — the researchers find that things in general aren’t going so well:

• Nearly two-thirds are listed as endangered, threatened or vulnerable by the International Union for the Conservation of Nature.
• More than three-quarters are undergoing continuing population declines, sometimes in spite of conservation efforts.
• For the 17 species whose range is reliably known, the territory occupied today is less than one-half its historical size.

Again, taking the gray wolf as an example:

• Although populations and range have been rising somewhat in the 48 contiguous states, thanks to four decades of federal protection, the wolves’ range worldwide is only about two-thirds of its historic size. (In the U.S. outside Alaska, despite the intensive recovery efforts, wolves occupy only about 15 percent of their traditional territory.)
• Since the 1940s, biologists have documented U.S. deer populations mushrooming to six times their previous densities in areas where wolf predation had been eliminated — a surge too big to be susceptible to solution by the other most important predator of whitetails and mule deer. That would be men (and women) in blaze orange.
• But the problems are reversible, as proved by the reintroduction of wolves to the Yellowstone ecosystem in the mid 1990s, “making this park one of the most predator-rich areas in North American [and triggering] various direct and indirect effects” that benefit ecosystem health.

Lions, leopards and baboons

Perhaps we should be grateful that we’ve been spared the problems associated with declining populations of lions and leopards in Africa, where leopards have lost two-thirds of their former range and lions more than four-fifths, leading to surges in the population of olive baboons.

Increases in baboons correlated with accelerated declines in small ungulates and primates. Among large mammals, baboons pose the greatest threat to livestock and crops and they use many of the same sources of animal protein and plant foods as humans in sub-Saharan Africa. In some areas, baboon raids in agricultural fields require families to keep children out of school so they can help guard planted crops.

And from Australia comes the findings of an unintended, continental-scale experiment that followed construction of a 5,500-kilometer-long “dingo-proof fence.”

The most significant and well-understood effects of dingoes are in the control of populations of native herbivores, introduced herbivores and the exotic mesopredator the red fox. The suppression of these species by dingoes reduces total herbivory and predation pressure, in turn benefiting plant communities and smaller native prey …

Overall, the suppression of dingoes probably contributed to the endangerment and extinction of small marsupials and rodents over much of the continent.

These are not problems for which there is no solution. For a success story, consider efforts in Finland to promote the recovery and conservation of Eurasian lynx.

The resulting increases in lynx populations “were accompanied by a decline in red fox abundance of the commencement recovery in the abundance of forest grouse and mountain hare. Moreover, where lynx density had recovered to ‘ecologically effective’ levels, the controlling effects of links on red foxes and prey increased with ecosystem productivity. …”

And though the species lay outside the boundaries of their paper, the authors point to success created by the Global Tiger Initiative, “which is coordinating local, national, and international tiger conservation policy across �� all 13 tiger range countries, with funding from the World Bank.

“These 13 countries and partners have moved well beyond words to accomplishments on the ground, including securing funding, establishing new tiger reserves, passing laws and tiger conservation, creating high-level commissions to improve wildlife law enforcement, addressing habitat loss and fragmentation, providing connectivity [of habitat] and more.”

Tigers are in trouble, too. They number about 3,000 now, down from 100,000 at the turn of the last century, and exist in an area about one-fifth the size of their historical range.

But at least they’re moving in the right direction now.

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For further reading:

• The Science paper is available to subscribers and to those who purchase pay-per-view rights here.
• I saw some fine coverage of the paper in several publications and broadcast outlets, especially pieces presented by National Public Radio, NBC News, the BBC and the UK Guardian.