From the tiniest cottage to the tallest skyscraper, the most important part of a building is its foundation.

It provides support for every other part of the structure — the walls, doors, windows, roofs. And, it offers stability in  the face of the elements.

Without one, a building would quickly crumble. But today we aren’t constructing architecture — we’re building ecosystems, whose foundations are plants! Across the globe, plants form the bedrock of these complex and interconnected webs of life.

Without them, there’d be no food, no fuel, no animals, no us. Hi! I’m Alexis and this is Crash Course Botany.

Now grab your hard hat, lace up your boots, and let’s get building! Get it? ‘Cause ecosystems are foundations…never mind, roll the intro. [THEME MUSIC] Picture an ecosystem as a network of life — a geographic area where living organisms and physical environment interact in glorious complexity. Just like buildings, ecosystems can come in all shapes and sizes, from a vast prairie to the miniature pond that forms in the center of a bromeliad plant.

I like to think of that as a “tiny home.” Dramatically different ecosystems can even bump up against each other — imagine a watery oasis in the middle of the Sahara Desert. And, they can be temporary. Vernal pools show up every spring but disappear in the heat of summer, just like the will to conquer my reading list on the beach.

I’ll finish War and Peace one of these days. So why are plants so important to these dynamic systems? Let’s think of it in terms of energy.

All organisms on Earth need energy to fuel the essential processes of life. In an ecosystem, the organisms that can harness energy to create that fuel are known as producers, or autotrophs. They take inaccessible forms of energy like sunlight, and convert them into usable fuel, like sugars.

You might recognize this process as photosynthesis. Plants are the autotrophs in their ecosystems thanks to their photosynthesizing superpowers — they convert about 60 billion tons of carbon per year to produce sugars, mostly to fuel their own processes. It’s not that plants love you or love you not— they’re just not thinking about you!

Thought Cafe, what have you done? Anyway, all usable energy enters an ecosystem through autotrophs, so I guess that makes them…[chuckles] power plants! AH!

Who threw Theo!? Sidenote: there are also some autotrophs that aren’t plants, like cyanobacteria and algae, which photosynthesize to keep aquatic ecosystems running smoothly. And the Sun isn’t the only source of energy that autotrophs can harness, either.

In deep, dark ocean ecosystems, some bacteria use chemicals like hydrogen sulfide as their energy supply instead. It’s wild down there! OK, so all other living things in an ecosystem that aren’t autotrophs are heterotrophs, or consumers —these are the walls, windows, and roofs that rely on the foundation of autotrophs.

They have to eat other organisms to get their fuel because they can’t create their own, as hard as they may try. [whimsical music plays] [Alexis grunts] [frustrated groan] Ugh! Nope, nothing. Animals, fungi, and many bacteria are heterotrophs.

And, actually, some parasitic plants are heterotrophs as well. Like Monotropa — with no green, chlorophyll- packed leaves to photosynthesize with, it hardly resembles a plant at all. Instead, it gets all of its energy from consuming its host, an underground fungus.

These guys are the funky additions to the building, like a turret or a bell tower… ooh! Or gargoyles. Heterotrophs come in a variety of types.

They can be herbivores like deer or caterpillars, which only eat autotrophs, or carnivores like lions or pythons, which eat herbivores and other carnivores. If they’re less picky, they can be omnivores like woodpeckers or us humans, which eat a little of everything… even if I choose to be a filthy vegan. And if they like to eat or break down already-dead organisms, we call them decomposers like millipedes or fungi.

All of these organisms are linked together in a food chain, which is essentially an ecosystem’s map of who eats who. You’ve probably learned about food chains before: squirrel eats acorn, fox eats squirrel, and so on. Simple.

But things are way more complex out in nature. A different way to think about it is as a food web, because in reality, lots of different species are eating each other in lots of interconnecting food chains, not one straight, uninterrupted line. It’s less like a sit-down meal and more like an ecosystem buffet turned all-out food fight.

Like in the tallgrass prairie ecosystem of the central US, there are over four hundred species of plants at the base of the food web. As its name suggests, grasses like big bluestem and switchgrass dominate the ecosystem. The grasses support scores of insects — butterflies, bees, and grasshoppers — as well as grazing herbivores like deer and bison.

Flocks of swallows, finches, and countless other birds feast on grass seeds and insects. And added to the mix are small rodents like rabbits and mice, which are kept in check by carnivores like the prairie kingsnake or the red-tailed hawk. But to tangle this web further, the hawks also eat the snakes, and the snakes sometimes eat each other.

You get the picture: no one’s waiting in line at this buffet! Finally, decomposers like vultures, bacteria, and fungi act like the bulldozers that demolish an old building to make something new. They break down all other members of the ecosystem that have died and return their nutrients to the soil, where they’ll be taken up by plants and reenter the food web once again.

The way those nutrients move between organisms and their environment is called a nutrient cycle. Nutrient cycles move in a circle, and plants are one stop on that roundabout route. Things like nitrogen, carbon, and oxygen move from nonliving parts of the environment —air, for example— to living things like plants and animals, and then back again to the environment.

They’re like the ultimate recycling program. But the circular movement of nutrients isn’t to be confused with the way that energy flows. Energy doesn’t move in a circle.

It goes in one direction, from the source —typically the Sun—to the autotrophs. The autotrophs transform it into usable energy, which is consumed by the heterotrophs. Along the way, a lot of energy is released into the atmosphere in the form of heat, which is generated from stuff like growth and movement.

There’s no getting that energy back to its previous form— we just count on the Sun to keep shining on Earth’s plants, and keep things moving. So yes, plants are connected  to Every. Living.

Thing. Nearly all members of an ecosystem ultimately get their energy from plants, no matter if they’re herbivores or carnivores. It’s just a matter of how directly that energy gets to you.

Whether it’s from corn on the cob, a chicken sandwich, or mushroom ragu, the energy that powers your life started with the Sun, was converted by a plant, and moved through the ecosystem ’til you gobbled it up — and the cycle continued. And as with a building, if something bad happens to the foundation, it can have some pretty serious consequences. So, conserving plants is essential for maintaining the health and stability of the whole ecosystem.

Let me show you what I mean in the Thought Bubble… For decades, an international team of researchers has been on a mission to save tomatoes. These botanists are interested in the wild variety native to the Atacama Desert of northern Chile, the driest desert in the world. These tomatoes are specially adapted to grow in conditions that would kill your average garden tomato.

Theo, you don’t need to see this! And, as autotrophs, they help support a whole web of life. From the bees that pollinate them to the mice and rats that eat their fruits, to the burrowing owls and South American gray foxes that eat the rodents, this ecosystem is dependent upon these wild tomatoes.

But human activity has disrupted this delicate balance. Farm animals are sent out to graze on the already-limited vegetation, and mining has made the already-poor soil even poorer. And as a result, the tomato population has started to decline — and the rest of the organisms in the food web may soon do the same.

There could be even broader consequences, too. These tomatoes have drought-resistant genes that could be used to breed plants in other ecosystems, especially those vulnerable to climate change. So, the research team has spent decades traveling across the Atacama, collecting tomato seeds to establish a seed bank that can be used for future restoration efforts — in this ecosystem and beyond.

In fact, the scientists are actively using the bank to breed genes for drought resistance into cultivated tomatoes— protecting food sources not only for desert rodents but for us. Okay, Theo, you can open your eyes now. Thanks, Thought Bubble!

In some cases, ecosystems have overlap in their food webs, so different species can fill in for one that’s been lost. But not always. Like, in North America’s Sonoran Desert, the iconic saguaro cactus has hundreds of other species depending on it for their survival.

This makes it a keystone species: a species that plays a disproportionately large role in its ecosystem. It’s like that block in a Jenga tower where if you pull it, a lot of other blocks are comin’ down with it. Saguaros make great homes for woodpeckers that like to make nest holes in their stems.

And when they vacate their nests after a year, other bird species move in. Meanwhile larger birds of prey perch on top of the saguaros to scout for food. In addition to their role as habitats, these cacti produce more nectar and pollen than any other flowering plant species.

Multiple species of birds, bees, and bats gorge on the floral feast and provide pollination services in return. And when the flowers produce fruit, still more animals line up for the buffet — lizards, rabbits, insects, bighorn sheep, even coyotes. And the saguaros produce this bounty during the driest and hottest time of the year, when other food is scarce.

But, saguaros have super high mortality rates — the vast majority of seedlings don’t make it. And if they do, it takes them fifty years to start producing flowers — talk about a late bloomer! This makes it tough for them to bounce back from  disturbances like weather  events or habitat destruction.

Which threatens not only the cacti themselves, but the whole ecosystem that relies on them. The good news? Understanding their role as a keystone species helps us bolster efforts to conserve saguaros and other species like them.

At the end of the day, whether an ecosystem  is built upon dozens of grass  species or a single cactus, plants are often the foundation that supports other links in the food web. From the driest deserts to the most humid tropics, the powers of plants keep the proverbial lights on, maintaining the crucial flow of energy throughout the systems of our world. Next time, we’re going global — we’ll see how ecosystems work together to form large biomes, and how they’re affected by one of our planet’s biggest challenges: climate change.

Our planet is home to so many unexplained phenomena -- from our ecosystems to what's happening in our own backyards. That's why you need to watch this fascinating new series over on PBS Terra called Untold Earth. The series unpacks the mysteries behind North America's most unique natural wonders.

Find out the secret behind Wyoming's multicolored Grand Prismatic Springs, and see where Mexico's Rincon de Parangueo crater really comes from. Check out the link in our description to deep-dive into these strange, yet amazing stories. Hey, before we go, let’s branch out!

What Hawaiian keystone species is currently under threat from a fungal infection? Go “tree” the answer in the comments! Thanks for watching this episode of Crash Course Botany which was filmed at the Damir Ferizović Studio and made in partnership with PBS Digital Studios and Nature.

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