Sabrina: 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 out, even the habitats themselves might change as a result of floods and droughts.

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 non-living things in the habitat, too. This interaction of living and non-living 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 somehow completely adorable. 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 pretty 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 food 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 them monkeys aren't there 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, but fewer plants means less food for other animals like insects and our sloth friends. That means that the insect and sloth numbers start to decline, and that means less food for the animals that eat them. All of the 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 spider webs. We don't want to be all crazy-pants and just go knocking them down.

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