[INTRO MUSIC]
So last time, we put a polar bear in the 
desert, and I still feel bad about that.
The good news is that in real life, a polar bear
probably won't just wander into the Sahara.
But not everything stays in the same habitat all the time.
A new species might come into a 
habitat, a species might die off.
Even the habitats themselves can change
as a result of things like floods and droughts.
The point is, habitats, and the food webs they support, can get out of whack and sometimes, it's not pretty.
Let's look at what happens when
an ecosystem gets out of balance.
Last week, we learned that a habitat is home
to a tangle of food chains called food webs.
The animals depend on each other for food,
but they don't just need each other;
they rely on the nonliving things in the habitat too.
This interaction of living and nonliving 
things in a habitat is called an ecosystem.
The things in an ecosystem are all connected,
just like when you touch one part of a
spiderweb and the whole thing vibrates.
When one link in the food web is threatened,
it can shake up the whole ecosystem.
Let's see what might happen, for example,
if an ecosystem loses a species.
Since we're talking about food webs, I
think we should look at spider monkeys.
They're called spider monkeys because they hang
upside down from their tails with their arms and legs
dangling - this is somehow completely adorable.
These primates live in a tropical rainforest habitat,
which is just bursting with some of the coolest
creatures out there; toucans, jaguars, sloths...
As we learned last time, these
animals need each other to survive.
And spider monkeys happen to play a particularly
important role in the rainforest food web:
they eat mostly fruit, which contains seeds, and we know that seeds are how plants make more plants.
When a spider monkey snacks on a 
berry, he gets to enjoy the tasty fruit
while also doing the plant a solid favor.
When the monkey moves on to another part of the forest
and, um, passes the fruit, he leaves the seeds behind.
Wait a while and then, voila! You have a new plant.
Imagine thousands of monkeys eating 
thousands of fruits every day.
More monkeys equals more plants and trees.
Those trees support lots of other animals:
insects and sloths eat those plants too.
And more spider monkeys, insects and
 sloths mean more food for carnivores.
Leopards dine on the sloths and spider
monkeys, while frogs eat the insects,
and of course our decomposers like fungi and
bacteria break down leftover plant and animal matter,
so we're talking around 50,000 plant and
animal species that rely on these plants.
Now imagine the spider monkey 
population starts to decline;
maybe they're hit with a strange new disease,
or maybe humans over-hunt them.
If the monkeys aren't around to eat the
fruit, then the seeds aren't scattered around,
and the forest stops growing,
leaving fewer fruits for fewer monkeys.
Not only that - fewer plants means less food for
other animals, like insects and our sloth friends.
That means the insect and
sloth numbers start to decline,
and that means less food for the animals that eat them.
All of a sudden, none of the animals
in our ecosystem have enough to eat,
all because of the loss of one species.
Do you see how this could get really bad?
Remove one piece of the food web, and 
you might knock down the whole thing.
That's bad news for us, too.
The good news is that
ecosystems WANT to be in balance.
After a natural disaster like a forest fire or
a flood, things might be wacky for a while,
but habitats can usually get back to normal.
But, if things get really bad, the
habitat might change forever:
the old species will leave, searching for a
better place to live, new species will come in.
Life will keep going, but it won't look the same.
In every ecosystem, the plants
and animals are connected;
you can't mess with one species
without affecting all the others.
Food webs are delicate, like spiderwebs. We don't want
to be all crazy pants and just go knocking them down.
[OUTRO MUSIC ]
