The result is dysfunctional ecosystems — bereft of important pollinators, predators and scavengers — less able to support human or animal health, said Rebecca Shaw, the fund’s chief scientist.

But World Wildlife Fund officials emphasized one catastrophic consequence of the declines is now before our eyes. The coronavirus pandemic, caused by the sort of “spillover” of a zoonotic virus that is becoming more common as humans expand their footprint, should be viewed as “an SOS signal for the human enterprise,” the report says.

The report “provides unequivocal evidence that nature is unraveling and that our planet is flashing red warning signs,” wrote Marco Lambertini, director general of WWF International. “Covid-19 is a clear manifestation of our broken relationship with nature and highlights the deep interconnection between the health of both people and the planet.”

More than two-thirds of emerging infectious diseases in humans have origins in animals, and scientists say the virus that causes covid-19 probably originated in a horseshoe bat, though the virus’s precise path to people remains unknown. What’s clear is that animal viruses have more chances to hop to humans as we develop formerly wild spaces, fly around the globe, trade in wildlife and intensify livestock production.

“If you start deforesting and are grabbing those animals out of the forest and putting them on trains and putting them on trucks and boats, alive, with domestic animals and humans, you have all this viral mixing,” Shaw said. “We’re disrupting nature and creating the opportunity for these spillover events.”

Wildlife losses were particularly precipitous for freshwater species, which saw an average decline of 84 percent, in part because of the near disappearance of most wetlands that existed 500 years ago, the report said. The average drop in species monitored in Central and South America was an astonishing 94 percent, it said.

Populations of animals large and small are dwindling, and are changing differently in different regions, according to the report.

While tigers are on the rise in Nepal, the big cats’ numbers have fallen by more than 20 percent in Russia’s far east, where habitat loss, a decline in prey and poaching pose threats. Agriculture has caused gray partridges in the United Kingdom to plummet, while illegal hunting and mining have driven down the Grauer’s gorilla population in the Congo by 87 percent.

Several species of vibrantly speckled harlequin frogs in Colombia, Ecuador and Venezuela have faced “catastrophic” losses and extinctions, mostly attributable to a fungal disease that may have originated in East Asia in the 20th century and then shot around the planet with the boom in global trade and the exotic pet market.

Despite the declines, Shaw said urgent changes could blunt or even reverse some of the damage. Modeling carried out by the World Wildlife Fund and 50 partners as part of a two-year-old initiative called “Bending The Curve” found recovery will require a combination of increasing land conservation efforts and, crucially, overhauling food production to make it more sustainable and efficient and less wasteful. Those findings were described Thursday in the journal Nature.

The “Living Planet Report” describes wildlife population declines not only because they act as a barometer for ecosystem health, but also because they contain glimmers of hope, Shaw added.

“If you only measure threatened and endangered species, which has really been the focus for the conservation community for many, many years,” Shaw said, “you’re way too late.”

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