Earth's growing population: 'A direct affront to our own survival'

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Listen: What Earth's growing population means for our planet's future .

The population of planet Earth reached 8 billion people late last year. By the year 2100, we're headed for 2 billion more.

What does that mean for us and our planet?

Elizabeth Hadly is a professor of biology at Stanford University, and director of the Jasper Ridge Biological Preserve in California's Santa Cruz Mountains.

For four decades, she's been an eyewitness to dramatic changes in the plant and animal kingdoms caused by human beings.

ELIZABETH HADLY: Remember all those insects you used to have on your windshield when you drove across the West? Gosh, they're not there anymore. Why? So I did a lot of work in Yellowstone driving back and forth from either here or Berkeley, wherever I was living at the time. And sometimes I would have to stop more frequently to clean my windshield than I did to fill up with gas. And no more. I can drive across the country, which I just did, by the way, and not clean my windshield. And that's horrifying to me.

So in my 40-some years of doing this, I have seen fewer large game animals like rhinos, like grizzly bears, like bighorn sheep, like whales. I have seen fewer fish in the sea. And the fish that I have caught, I like to fly fish, the fish that I have personally handled in some of these amazing streams in the American West are smaller and harder to find. Why do we have a loss of almost 70% of the wild animal populations in the world?

Why is this happening? And the bottom line is that there are so many people and we are consuming so much of the biomass of the planet, we're kind of harboring it for ourselves and for our domesticated animals. I learned this from a farmer. If you look at a ranch where there's a lot of cow patties, you know, they're drugging their animals big time. And those cow patties are going to stay there for a long time because they're basically sterilizing their soil. If you look at another ranch where they're not applying so many of those pesticides, there's not as many cow patties.

And it's because the dung beetles and the worms and everything that kind of helps decompose is working at it. So even if you don't care about the ethical considerations of eliminating species on the Tree of Life, it's important to realize that we're actually doing the culling. The great acceleration is a compilation of data that shows it's not just the increase in human population size, but it's the increase in our consumption. And it's not just per capita consumption that stays the same. And so therefore, you just add more people and you increase consumption that way. We're actually consuming more than we used to.

There's only so much solar radiation that reaches the earth, and that solar radiation for millions of years is the only thing that's kind of governed biomass. And now, of course, you know, we're getting solar energy from fossil fuels and we're adding to that. And what we're doing is we are now replacing all wild biomass with the biomass we choose. And the problem is that's not sustainable.

And there are all sorts of ways that it could unravel. But the one I'm the most concerned about is that as we lose biomass, as we lose biodiversity, the biomass of diversity, we're losing our potential solutions. And if we're losing populations of species, the way all of the data suggests, more than 60% of them in the last few decades, then what that means is it's a direct affront to our own survival. They are the canary in the coal mine, and we're living in the coal mine, too.