♪♪ NARRATOR: It's known as "America's Finest City."

Sand, surf, and superb weather attract people from around the world.

But we're not the first to enjoy it here.

♪♪ San Diego has more biodiversity than any county in the continental U.S. ♪♪ Humans have transformed San Diego's wild landscapes... ♪♪ ...but life here always finds a way... ♪♪ ...because San Diego isn't just "America's Finest City"... ♪♪ it's also... "America's Wildest."

♪♪ ♪♪ ♪♪ ♪♪ NARRATOR: San Diego is a land born of extremes.

[ Waves crashing, gulls calling ] To its west, the mighty Pacific.

To its east, the Sonoran Desert.

And in between... a profusion of life... ...adapted to a world where change is the only constant.

People have been part of this landscape for 12,000 years.

But just in the last century and a half, our numbers have grown 500-fold... and today's humans -- 3.3 million of us -- have reshaped San Diego, making life here more comfortable... more predictable... ♪♪ ...and nothing is more important to us than water.

♪♪ San Diego County's eastern mountains.

♪♪ For time immemorial, rain falling here has carved its way through an arid landscape to find the sea.

[ Stream gurgling ] ♪♪ Life made do with what little water the land could provide... ♪♪ ♪♪ ...but for some humans, it wasn't enough.

♪♪ Today, most of San Diego's rivers are dammed.

♪♪ And in a region with no large, natural lakes... ♪♪ ...these reservoirs haven't gone unnoticed.

[ Water lapping gently ] Western grebes, once only visitors here, have become year-round residents.

They've gathered this morning for a very particular reason.

It's too early to nest... but never too soon to be picky.

♪♪ Through the crowd, their eyes meet.

♪♪ They start by echoing each other's movements.

♪♪ ♪♪ [ Water splashing ] ♪♪ The male offers a gift... and the female accepts.

It's time to take this relationship to the next level.

♪♪ Wait for it.

♪♪ Not yet.

Now.

♪♪ ♪♪ ♪♪ The grebes' ritual demands strength, speed... and impeccable timing.

♪♪ It's not clear what, exactly, they're looking for... or even who's judging whom, but these two pass the test, taking the first, speedy steps towards raising a family together.

Below the dam, the river continues toward the sea, but some of its water takes a very different journey through pumps, pipes... and sprinklers, creating a lush oasis in the city.

♪♪ For California ground squirrels, San Diego's parks are a paradise.

[ Squirrel chittering ] There's plenty to eat, and humans keep predators like rattlesnakes and coyotes away.

The squirrels are more numerous today than ever.

But they're still...squirrelly.

When you're eight inches tall, dangers lurk around every corner -- some imagined... [ Squirrel squeaks ] ...and others very real.

[ Dog growls ] ♪♪ [ Dog whines ] ♪♪ [ Dog barks ] ♪♪ [ Dog growls ] ♪♪ ♪♪ [ Dog barks ] ♪♪ Safety, at last.

♪♪ Below the surface is a labyrinth of tunnels, constantly under construction.

♪♪ Wherever San Diego's squirrels flourish, their burrows are home to a menagerie of wildlife... ♪♪ ...a whole parallel universe right under our feet.

♪♪ ♪♪ But even the squirrels' elaborate earthworks pale next to ours... ♪♪ ...because humans have re-shaped this landscape not just to control water... but also other people.

♪♪ San Diego's southern border was once abstract -- an imaginary line through a single, connected ecosystem.

♪♪ Today, for anything with paws, hooves, or shoes... the border is all too real.

For desert species, the wall can block traditional migration routes, and even cut off access to water -- collateral damage in our fervor to defend our land.

But no wall is a match... for the sea.

[ Waves crashing ] ♪♪ Just offshore, ancient migrations are still underway.

♪♪ Winter brings ocean giants to San Diego's coast... ♪♪ ...and smaller species, like common dolphins, arrive from the north by the thousands.

♪♪ ♪♪ Predators are never far behind.

♪♪ ♪♪ ♪♪ This all-female pod of orcas normally hunts in Mexican waters... ...but San Diego's coastal abundance is worth the journey.

♪♪ A common dolphin won't keep the family fed for long... ...but there are more substantial meals closer to shore.

♪♪ Bottlenose dolphins are smart, fast, and acrobatic.

Catching one won't be easy... even for the ocean's top predator.

♪♪ ♪♪ The orcas isolate one dolphin from the pod.

Alone and outnumbered, the bottlenose needs a stroke of luck.

♪♪ Every leap is a chance to change direction... buying the dolphin precious seconds.

♪♪ But orcas are quick learners, and luck has a habit of running out.

♪♪ ♪♪ [ Water churning ] In San Diego, these violent clashes between predator and prey happen just off shore... where a crowded coastline meets the planet's greatest ocean wilderness.

[ Waves crashing ] ♪♪ Winter waves -- the biggest of the year -- draw surfers to San Diego's best breaks.

♪♪ ♪♪ Signs of the ocean's power pile up just about everywhere... ...except here... one little beach, sheltered from the surf.

This concrete breakwater was built in 1931 to create a safe beach for children, but waterfront property is in high demand.

[ Seal barks ] Pacific harbor seals gather here every winter.

It's a perfect place for a nap in the sun... if you can just find a spot.

♪♪ But this female isn't here to nap.

She's on a much more urgent mission.

[ Seal groaning ] ♪♪ ♪♪ ♪♪ Just minutes after birth, her pup is ready for his very first swim.

At least, he thinks so.

♪♪ ♪♪ Maybe not just yet.

♪♪ Back on the beach, he eats as much as Mom will let him.

With milk on his mind, he hardly notices he's being watched.

The beach is closed for pupping season... but people still flock here to watch the seals... argue about whose beach this really is... and occasionally get into fistfights over it all.

Oblivious to the debate, Mom and pup settle in for a nap on what is, at least for now, their beach.

[ Waves crashing ] [ Thunder rumbling ] Late-winter storms are a good reminder... that for all we've changed San Diego... ...we still don't control the weather.

[ Thunder rumbles ] ♪♪ The storm pushes inland, bringing rain to San Diego's desert.

♪♪ ♪♪ This winter's been unusually wet... setting the stage for a rare and spectacular event.

♪♪ As the soil warms and the days grow longer, the desert begins to transform.

♪♪ ♪♪ ♪♪ ♪♪ Locals call it a "superbloom."

For a few short weeks, you can see the colors from space.

♪♪ ♪♪ In years like this one, the superbloom comes to the city, too.

♪♪ But San Diego's urban superbloom -- thanks to the water we pump into the city -- can last for months.

Ornamental plants from distant shores join native flowers in a cacophony of color.

And people aren't the only ones who enjoy all the flowers.

[ Wings flittering ] ♪♪ A male Anna's hummingbird -- one of four species thriving in San Diego's parks and suburbs.

♪♪ But it isn't just the urban superbloom that keeps the hummingbirds coming back.

♪♪ For these hungry birds, sugar water will do just as well as the real thing... [ Wings flittering ] ...so a hummingbird feeder is a year-round attraction.

♪♪ With all the flowers and feeders, you'd think there was plenty of nectar to go around, but that's not how they see it.

♪♪ ♪♪ ♪♪ The G-forces in these maneuvers put "Top Gun" to shame.

♪♪ ♪♪ Maybe tomorrow's the day they'll finally sort it out.

But today, the garden is an ideal place for a female Allen's hummingbird to raise a family.

Right from the start, this is her responsibility -- male hummingbirds don't help with parenting.

[ Birds chirping ] ♪♪ Her nest is a tiny cup of grass, lichen, and plant fibers... ...held together by spiderwebs.

♪♪ Nesting so close to people can come with some surprises... ♪♪ ...but soon, her human neighbors just fade into the background.

♪♪ With babies on the way, she needs to get some rest while she can.

Spring is in the air all over San Diego, and if you're young and looking for love, Pacific Beach is the place to do it.

♪♪ But the scene here is positively Puritan... next to what's about to happen on the beach.

[ Waves crashing ] ♪♪ A fish out of water?

But this male California grunion beached himself on purpose.

He's a "scout," and he's checking out the venue.

♪♪ For most of the year, grunion live mysterious lives out at sea.

But each spring, they come ashore to spawn.

♪♪ The scouts are easily spooked... ♪♪ ...but the spring's highest tide is just minutes away.

Offshore, the drive to spawn is becoming irresistible.

On some unspoken cue, the grunion charge en masse... ♪♪ ♪♪ ...riding the waves to shore... ♪♪ ...and then flopping onward under their own power.

♪♪ Soon, the beach is a mass of squirming silver.

♪♪ [ Waves crashing ] The safest place for grunion eggs is beneath the sand.

A female begins to dig herself in... and right on cue... ...she's surrounded by eager males, ready to fertilize her 3,000 eggs.

♪♪ Some won't get the chance.

Birds normally active during the day seem to know when a grunion run is coming.

♪♪ ♪♪ The females full of eggs are a special treat.

♪♪ But there are so many grunion... the birds don't make much of a difference.

Thousands of females spawn successfully.

♪♪ [ Waves crashing ] Then... it's over.

Just two hours after the first scouts hit the beach, the only evidence of what happened is the glint of city lights off a few eggs on the sand.

These eggs, exposed to the elements, won't last long... but millions of others are now safe below the surface.

♪♪ Mission -- accomplished.

♪♪ Far from the coast, the grebes are starting to think about nesting, too.

[ Birds chirping ] Months after their first date, the pair still find time to dance... ♪♪ And he still brings her gifts.

♪♪ He certainly seems like a keeper.

She's ready to commit.

[ Bird warbles ] It's time to build a nest.

♪♪ ♪♪ Before long, a floating platform emerges from the surface... ♪♪ ...giving the female a chance to observe -- or maybe "supervise" -- the male.

♪♪ He's a hard worker, but he takes plenty of breaks to... well, let's just say "reinforce their bond."

The trouble is, his legs are built for swimming... ♪♪ ...and not much else.

♪♪ His flare on the mount is impressive... ♪♪ ...but balancing... ♪♪ ...that's tricky.

♪♪ Nice save!

♪♪ But if he really wants to be a father, he'll have to try a little harder.

♪♪ Just the dismount now... ♪♪ ...and... ♪♪ Nailed it!

♪♪ The next generation is on its way.

♪♪ And these two will be ready.

♪♪ [ Birds chirping ] ♪♪ Back in the garden, the hummingbird nest is further along.

♪♪ For the young chicks, sugar water isn't enough.

[ Birds chirping ] They need protein and fat to grow.

♪♪ But the feeders are still important, fueling Mom up to find insects for the little ones.

♪♪ In just 3 weeks, the chicks are as big as their mother... ♪♪ ...and they're ready to join the melee at the neighborhood feeders... ♪♪ ...where the constant dogfights have attracted some attention.

♪♪ A roadrunner.

♪♪ Like the cartoon, the real roadrunner is clever.

♪♪ It's also...a hunter.

♪♪ Predators often ignore hummingbirds... ♪♪ ...they're so small and so fast, they're hardly worth the trouble.

♪♪ But so many drawn to one place... it's too much to resist.

♪♪ The more experienced hummingbirds can sense that something's not right.

But the youngsters don't seem to notice.

♪♪ A near miss.

♪♪ But they'll be back.

♪♪ ♪♪ It's over quickly for the young hummingbird.

♪♪ New resources for hummingbirds have made new opportunities for their enemies, too.

♪♪ [ Waves crashing, birds calling ] Feeders aren't the only things in San Diego that attract a crowd.

On a warm spring day, the beaches are packed.

[ Footfalls echo ] Beneath the bare feet and sandcastles... ♪♪ ...the grunion eggs have transformed.

♪♪ A single cell has grown into a tiny fish... [ Heart beating ] ♪♪ ...complete with eyes and a beating heart.

♪♪ ♪♪ Microscopic nematode worms, normally bacteria eaters, won't pass up a fish dinner.

♪♪ But as long as the eggs' tough outer membranes are intact, the embryos inside are safe.

♪♪ [ Waves crashing ] Lately, the grunion's ancient life cycle has run into a very modern problem.

♪♪ Humans don't just love beaches.

♪♪ We love them clean.

♪♪ For grunion eggs... ♪♪ ...these machines are a death sentence.

♪♪ But San Diego recently banned spring beach cleaning below the high-tide mark.

♪♪ So, these eggs will live to see another day... ...and with it, the tide that will reunite them with the sea.

[ Crashing waves echo ] The first touch of saltwater... [ Crashing waves echo ] [ Crashing waves echo ] The time is finally here.

♪♪ ♪♪ ♪♪ ♪♪ ♪♪ In the churn of the surf and the lulls between the waves... [ Choir vocalizing ] ♪♪ ...millions of tiny fry wriggle from their eggs.

♪♪ ♪♪ Swept down the beach, a new generation of grunion leaves the sand and worms behind... ♪♪ ...and begins lives in the vast Pacific.

♪♪ The grebes are preparing for new arrivals, too.

[ Water lapping, birds chirping ] The eggs -- safe in their floating nest -- are almost ready to hatch.

♪♪ But this isn't a natural lake.

♪♪ We decide when the water's gotten too high.

Grebes don't get a vote.

So, to quench San Diego's thirst and protect an aging dam, the city releases billions of gallons of water.

♪♪ ♪♪ ♪♪ ♪♪ In a few days, the nests are too high for the parents to reach.

♪♪ Baking under the sun, they soon become unrecognizable.

♪♪ On this reservoir, not a single egg survives.

[ Wind rushing ] Water from the dam flows through a landscape utterly transformed to meet our needs.

Suburbs where chaparral once stood... ...rivers without curves... [ Birds calling ] ...estuaries re-engineered as harbors.

But in this coastal park... life goes on... ...and this female ground squirrel is making some changes of her own.

She's hard at work, collecting grass trimmings.

♪♪ Below the surface, she's built a cozy nest... and inside are three helpless kits.

♪♪ Just a few days old, the kits are growing fast... [ Squirrels chittering ] ...and as soon as their eyes begin to open... ♪♪ ...they start exploring.

♪♪ Before long, the bright outside beckons.

♪♪ ♪♪ Baby steps.

♪♪ But the kits get bolder... and soon start to enjoy everything the park has to offer.

♪♪ With a whole new generation out and about, the colony is bustling.

And it's drawn some attention.

[ Birds calling ] American Crows love the park, too.

It's an easy place to find food.

[ Birds squawking ] But today, they seem to have more than leftovers on their minds.

♪♪ The kits don't seem to grasp the danger.

♪♪ ♪♪ [ Bird squawks ] ♪♪ The crows bide their time... ...the kits can't stay underground forever.

♪♪ ♪♪ [ Bird squawking ] [ Squawk echoes ] ♪♪ ♪♪ ♪♪ ♪♪ [ Squawk echoes ] ♪♪ ♪♪ One fewer squirrel in this colony.

♪♪ Its siblings have learned an important lesson.

♪♪ People haven't eliminated predators in the parks after all.

We've just made space for more opportunistic hunters.

[ Birds squawking ] ♪♪ Until recently, there were no crows in San Diego.

But starting in the 1980s, their numbers exploded.

♪♪ No one really knows why crows didn't invade the city earlier.

♪♪ They're smart, adaptable, and most importantly, they thrive around people.

♪♪ With bellies full, the crows settle down for the night.

♪♪ As the sun sinks low in the west, other lights come awake.

[ Fireworks exploding ] And the nightly pyrotechnics of San Diego's theme parks add an extra splash of color.

These bright nights distract us from a ritual older than our entire species.

[ Wind rushing ] Just offshore, single-celled algae called dinoflagellates are starting to reproduce.

♪♪ If disturbed, a chemical reaction inside each cell creates a tiny spark of light.

♪♪ When the conditions are just right, the algae multiply to uncountable numbers, and the coast becomes electric.

♪♪ ♪♪ ♪♪ ♪♪ ♪♪ ♪♪ ♪♪ ♪♪ But most San Diegans never notice... ...and in a few hours... ...the show is over.

♪♪ [ Wind rushing ] As summer approaches, the same water that gave life to trillions of tiny glowing plankton... evaporates into the air above.

♪♪ West winds push this cool, oceanic air inland, creating a marine layer enshrouding the coast.

♪♪ Known to many as the "May Gray" and "June Gloom," this fog delivers the last moisture to the hills before the unrelenting summer.

♪♪ Some comes to rest here... on a rock that the locals know well.

♪♪ ♪♪ It's a place to drink... ♪♪ ...and to bathe... ♪♪ ...and to socialize.

♪♪ A reliable puddle like this one can even summon one of San Diego's most elusive animals.

♪♪ These hills support a thriving population of bobcats.

♪♪ ♪♪ ♪♪ ♪♪ But when the June Gloom evaporates... so, too, does their lifeline.

♪♪ A century ago, many animals would leave the hills when their seasonal water sources dried up.

♪♪ Today, people live here.

♪♪ And for us, water isn't just for drinking.

♪♪ It's ornamental, too.

♪♪ As the hills wither, our decorative water features become irresistible.

♪♪ ♪♪ Animals shift their schedules, coming at night to avoid crossing paths with humans... [ Raccoons chittering ] ♪♪ Even the shy bobcats have found a way to live alongside us... ♪♪ ...where water now flows all year long.

♪♪ [ Birds squawking, chirping ] ♪♪ [ Birds chirping, cawing ] The way we manage water shapes the lives of creatures all over San Diego.

♪♪ On this reservoir... the city kept the year's rain behind the dam.

♪♪ The grebe nests here survived... ♪♪ ...and left these new parents with a different challenge.

♪♪ Actually... two.

♪♪ An hour after hatching, the grebe chicks can swim... ♪♪ ...climb... ♪♪ ...and eat.

♪♪ And sometimes, their eyes are bigger than their beaks.

♪♪ Every meal is a competition... ♪♪ ...and an exercise in patience for the parents.

♪♪ ♪♪ Before long, the piggyback rides start to get impractical.

♪♪ But no one is ready for this chapter to end quite yet.

♪♪ These reservoirs are strange, unpredictable homes... ♪♪ ...but they're the only ones these grebes know... ♪♪ ...and despite all odds, they've made it here.

[ Wind rushing ] And as the season draws to a close, nature's cycle begins again.

♪♪ ♪♪ Across the city, animals are finding new ways to thrive in a world that humans are changing a little more with each passing season.

♪♪ It was San Diego's abundance... diversity... ♪♪ ...and natural beauty that drew people here to create America's Finest City in the first place.

[ Bobcat sniffing ] ♪♪ Now, it's up to us to make sure San Diego remains America's wildest city, too.

♪♪ ♪♪ ♪♪ ♪♪ ♪♪ To learn more about what you've seen on this "Nature" program, visit pbs.org.

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