If life within the biosphere was a big Jenga tower, each block would be a species.

So this one is a… tarantula. And this one is a… saguaro cactus.

This one...this one's a... [sniffs] skunk, this is definitely a skunk. It’s normal for a block to be removed now and then. But it’s hard to tell exactly how that’ll affect the other blocks.

If the tower falls —or, over three-quarters of the species get wiped out— we call this a mass extinction. Mass extinctions like this have happened five times in Earth’s distant past, millions of years ago. And in each case, a huge, dramatic, devastating event was probably involved, like an asteroid impact or a volcanic eruption.

Or a…wobbly table. WOBBLY TABLE. There we go, all right, okay. —sometimes it takes a few wobbles, ya know?

But many scientists think we’re on the brink of a sixth mass extinction right now. Species are vanishing at about 1,000 times the typical rate. Only this time, there’s no asteroid.

Instead: a no less dramatic, way more complex force is at play in both helpful and destructive ways: humans. Hi! I'm Dr.

Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology. 1, 2, 3, 4... [Sammy sings a meow-ified version of THEME MUSIC] [Sammy beatboxing] REMIX! [more beatboxing] [clears throat] It’s tough to admit, being a human myself, but we humans are causing or contributing to some of our planet’s worst losses. But we're also in a unique position to stop, or even reverse them. Conservation biology is the science that focuses on understanding and combating threats to life on Earth - sort of like real-life superheroes.

It has the goal of preserving the many varieties of Earth’s species, or what scientists call its biodiversity. And we’re not just talking about individual species, biodiversity also refers to a diversity of ecosystems. Like, you know how in Star Wars, every planet only ever seems to have a single environment?

Like, you’ve got the desert planets or the ice planets or the jungle planets. Well, here on Earth, conservation biologists like to make sure we have lots of different ecosystems to support lots of different types of life. Biodiversity also describes genetic diversity, the range of trait variations within a species.

High genetic diversity increases the chances that a population will survive challenges that come their way. Kind of like when you're getting ready for a camping trip, you need to be prepared for all kinds of situations. If you only have a sleeping bag and a bear shows up, [bear roars] that might not be great.

But if you only have bear spray and a torrential downpour comes through, that won’t be good either. You need a whole checklist of items. In the same way, a population with all kinds of gear — or, diverse genes — will be better able to adapt to new threats.

But unfortunately, for all the good that people have brought into the world, like poetry and pizza, we’ve also done stuff that has removed blocks from life’s Jenga tower - both directly and indirectly. We’ve altered habitats worldwide and split the remaining ones into smaller pieces. We’ve given organisms rides to new places where they’ve proceeded to eat, or outcompete, the locals, becoming “invasive species.” With our industries and our fossil fuels, we’ve introduced pollution and climate change.

And often, we’ve just taken more than our fair share of Earth’s natural resources. Those changes ripple out to affect all kinds of biological processes. When a species disappears, that can disrupt how other species interact with each other and their ecosystems.

Or when a swamp gets turned into an amusement park, that changes how much food and habitat is available for all the living things that called it home. The loss of certain species can even interrupt how chemicals and energy cycle through the biosphere. Like, without enough pooping elephants around, important chemicals like nitrogen and phosphorus aren’t circulated as well.

Which in turn means fewer nutrients for plants, and fewer plants means less food for other animals. Basically, with every block removed, it’s becoming way harder for life’s big Jenga tower to stay upright. And there are many ways losses like this impact our daily lives, even when we can’t see them.

Each time we lose a species, we’re losing more blocks in that Jenga game we’re all playing. Biodiversity represents the capacity to solve life’s problems in unique ways. Humans can often use lessons we learn from other organisms to solve our own problems.

Like for example, some of the tech used to make digital screens vibrant while using minimal energy came from studying the brilliance of butterfly wings. But if those creatures didn’t exist, we’d lose access to lifehacks that we don’t even know about yet! If we lose enough species, then entire ecosystems can collapse and all of the benefits that come with them, like the wetlands that prevent floods and protect coasts from hurricanes, or vegetation that prevents landslides, or mangroves that shelter fish.

The consequences of mass extinctions impact whole communities of people, and yet they don’t affect all communities equally. That's because the environment includes people, and people are complicated. In societies all around the world, people have tended to organize and govern themselves in unfair and unjust ways, often based on categories like race.

When legacies of racist policy intersect with damage to the environment and disproportionately affect some groups over others, it creates what's called environmental injustices. Like, in Louisiana, there’s a stretch of the Mississippi River that’s home to primarily Black communities, and dozens of petrochemical plants, which release toxic chemicals into the air. Over the years, it has earned the nickname “Cancer Alley,” because compared to the average American, people living there are 50 times more likely to develop cancer.

Indigenous communities in Ecuador have faced similarly high cancer rates, likely due to contamination from oil exploration. In San Quintín, Mexico, migrant farm-workers face widespread health problems and contaminated drinking water, steeped in agricultural chemicals from the same fields where they’re employed. So, what negatively affects the environment also affects the people living there — often in unjust ways that stem from how our societies function more broadly.

But it goes both ways. When you improve an ecosystem, you can also improve people’s lives. Let's pay a visit to the Theater of Life.

Meet Dr. Wangari Maathai, an ecologist who understood the connection between a person’s health and their environment. By 1971, her thirst for knowledge led to her becoming the first woman in East Africa to earn a doctoral degree.

When she returned to her home country of Kenya, she saw how it had been stripped of its forests, its soil had been eroded, and its water had become scarce. People in rural Kenya needed firewood and clean drinking water. Maathai believed she knew the solution: plant more trees.

So Maathai recruited many Kenyan women and got to work, growing seeds into seedlings, and seedlings into trees. Little by little, their trees transformed into forests. And as they shared their knowledge of how tree planting benefited communities, their efforts gained momentum.

Maathai called it the Green Belt Movement. The movement began as a way to bring much-needed resources to people in rural Kenya. But Maathai also saw how planting trees reduced deforestation in the area, preserving precious ecosystems.

Despite resistance from those in power who did not want to see women organizing, the Green Belt Movement spread. For thousands of women, it offered income, community, and a way of meeting basic needs. And restoring forests transformed the landscape —even bringing back water sources that had run dry.

The Green Belt Movement has since spread to other countries in Africa. And over 51 million trees have been planted in Kenya alone. Maathai worked in the realm of restoration ecology, which supports returning a habitat to its previous state.

That can mean lots of things —like, removing barriers so a stream flows as it once did, or helping a cornfield become a tallgrass prairie again. Restoration ecology is a really important part of conservation. It can improve water quality, revive degraded soils, and support species that we’re in danger of losing.

But there are lots of other ways conservation happens, and lots of other people that make it happen, too. Like engineers, activists, and even architects. For example, many species are threatened when their habitats get sliced and diced by highways and neighborhoods.

So building corridors, like tunnels or land bridges, can enable animals to move more easily between patches of habitat, keeping populations from becoming genetically isolated from each other. Isolated populations tend to lose genetic diversity, which we know makes a population more vulnerable to potential threats. Conservation can also involve protecting biodiversity hotspots, relatively small habitats that house a wide variety of species.

Like the Atlantic Forest of Brazil, Argentina, and Paraguay, which is home to over 8,000 plants that don’t exist anywhere else. Other conservationists help endangered species breed in captivity, with the ultimate goal of reintroducing them into the wild. That work demands close attention to genetic diversity so that the species stay viable.

And sometimes, it requires strong puppeteering skills, like the biologists who raised baby California condors ‘round-the-clock using hand-puppets. Seriously, hand-puppets! Conservation also happens at the governmental level through regulation —by making rules and policies about how we use natural resources, to be sure we’re not logging, mining, fishing, or hunting more than an ecosystem can handle.

And when all these conservation efforts come together, they can alter a species’ fate —and an ecosystem. Let’s head to the Thought Bubble… In 1926, the last wolf pack in Yellowstone National Park was killed —part of a campaign to get rid of them overall. Many people said “good riddance,” fueled by fear and fairy tales of big bad wolves.

But those people didn’t necessarily consider how removing that piece from Yellowstone’s ecological puzzle would lead to other consequences. The park’s elk population boomed, feasting on willow and aspen trees. With fewer trees, beavers didn’t have enough materials for dams.

And without dams and trees, riverbanks eroded and water temperatures rose. Through all of this, biologists were just starting to understand the interconnectedness of life, and some began to wonder how the park might change if the wolves were reintroduced. But wolves have long been associated with dangerous villains, so it wasn’t easy to get everyone on board with putting them back.

It took decades of work and many changed minds, but in 1995, Yellowstone rolled out the welcome mat, bringing some of the wolves back into the park. As the wolves’ numbers have grown, they’ve helped reduce the elk population. Some scientists say that has already helped trees, beavers, and even rivers recover.

But Yellowstone is a really complicated, ever-changing puzzle and others believe it hasn’t quite been solved yet. Today, there are nearly 100 wolves roaming the park, re-establishing their place in the intricate puzzle we call life. And one thing’s for sure, we still have a lot to learn from what happens next.

Thanks, Thought Bubble! So, it’s hard to predict how removing a species will affect an ecosystem, but putting a species back can be really complicated, too —in ways biologists are still working to understand. Returning wolves to Yellowstone took decades of effort, collaboration, and persuasion: the same ingredients that have kept other species from going extinct, such as bald eagles, the Karner blue butterfly, and Arabian oryxes.

There are a lot of ways conservation can directly benefit us. But many would also argue that there’s just intrinsic worth in a moose or a mangrove that has nothing to do with us at all. Like, wolves just deserve to be here, same as you and me, regardless of their value to us.

These effects of biodiversity loss are complicated, and removing one Jenga block can sometimes topple the whole tower. But conservation showcases just how complex people are, too, in our capacity for both wreaking havoc and making repairs. Conservation aims to preserve biodiversity in all its forms.

And that work is happening across the world —through habitat restoration, captive breeding programs, wildlife corridors, and in backyards near you. In our next episode, we’re going to start a new, exciting chapter in our series. That’s right, I’m talking about evolution y’all!

I’ll see you then! This series was produced in collaboration with HHMI BioInteractive. If you’re an educator, visit BioInteractive.org/CrashCourse for classroom resources and professional development related to the topics covered in this course.

Thanks for watching this episode of Crash Course Biology, which was filmed at our studio in Indianapolis, Indiana, and was made with the help of all these nice people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.