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There is certainly no doubt the planet's nonhuman residents are paying the price of overpopulation and unsustainable lifestyles. The mere presence of humans will drive many other animals away, disrupting their feeding and breeding behavior.
That's why many naturalists now advocate for half the Earth being allocated to other species. It's an idea popularized by author and Harvard biologist E.O. Wilson, who proposed in his 2017 book Half-Earth that allocating half the planet's surface to the natural world would relieve 85 percent of species of the threat of extinction.
Bertulis, referring to this philosophy, calls himself "a half-Earther." He believes that with proper landscape management — including densification of cities — that California could revive its waning resources.
He believes, for example, that Californians have the know-how and technology to restore the massive salmon runs that shaped much of California's natural and cultural histories. The fish teemed in coastal waters, and scientists think at least 2 or 3 million adults — mainly Coho and Chinook — spawned every year in coastal creeks, streams and rivers. Salmon supported much of the marine and inland food web, and thousands of people relied on the nutritious fish as a staple food source and a trade item. In Oregon and Washington, the bounty was just as plentiful, if not more so.
But just about everywhere in the northern hemisphere where salmon range naturally, human population growth and economic development have correlated with a decline and disappearance of local runs — a pattern documented by biologist Robert Lackey, of Oregon State University, who has predicted near-extinction of salmon south of Canada.
"We squandered our true wealth," Bertulis said. And while we're not going to have grizzlies again, he believes salmon runs could be restored. "We have the tools to restore 100 percent of ecosystem function and historic abundance, even in the Bay Area."
But that's an optimist's viewpoint. Wackernagel believes humans are sucking the life out of the planet faster than the sun's incoming energy can replenish it. His Global Footprint Network concerns itself largely with measuring the rate at which that is occurring.
Right now, he said, humanity is consuming resources at a rate that it would take 1.75 Earth equivalents to sustain.
"This means we're drawing down our assets," he said.
The network's annual Earth Overshoot Day illustrates this concept, marking the day of the year by which the planet's humans have consumed all the resources that the planet is able to produce in a full year. This year, we will have consumed a full year's worth of food and energy by July 29 — the 2019 date of Earth Overshoot Day. Last year, it was on August 1. In fact, it has been arriving sooner and sooner almost every year. In the 1990s, it arrived in September.
If all the world's residents lived like Californians, Earth Overshoot Day would fall in late March. That's because Californians are using between five and eight times the biological resources that are available with the state, Wackernagel said. In his native Switzerland, the populace consumes 4.5 times the resources that could be produced inside the nation.
"A lot of these people say, 'We have money, we'll just buy what we need,'" Wackernagel said. "But in the end, not every nation can be a net importer, not every nation cannot be using more than what they have."
Currently, on average, they are, and it's a problem many people argue outweighs the problem of population. But on average, Wackernagel said, per capita global consumption rates haven't changed much since the 1970s. That means the increased consumption rate of the planet is largely being driven by population growth.
'At Some Point There Are Limits'
The Population Bomb is widely viewed today as little more than a curiosity, reflective of a brief moment of social panic. Forecasts warning that 4 billion people on the planet would cause calamitous environmental collapse were obviously way off the mark. Still, Ehrlich's warnings shouldn't be shelved forever.
"When a stand of trees is planted too densely for its environment, at some point there's going to be a massive die-off," said David Keller, a former Petaluma city councilman who has advocated for various environmental causes. He said an even worse event resembling the flu pandemic that infected a third of the planet's population in 1918 and killed 50 million could strike again.
"There are carrying capacities," Keller said. "We see it in animal populations over and over and over again — you exceed the carrying capacity, and populations collapse." With humans, he said, the effects may be "increases in diseases, accidents, anxiety or migration away — at some point there are limits to what a habitat can support."
In the past 12,000 years of agriculturally based society, the human population has grown about 2,000-fold, from an estimated 4 million to 7.3 billion today.
Mid-range forecasts from the United Nations show the population stabilizing at 11 or 12 billion in about 100 years. Higher-end estimates suggest a peak at 16 billion. Another U.N. projection, though, suggests a peak at 8.8 billion by 2050 and then a steady decline, dipping back to the present level by 2100.