Resources and population growth | Middle school biology | Khan Academy

0:00 [Narrator] For thousands of years,

0:01 people have written about huge swarms of locusts appearing out of nowhere,

0:06 eating entire fields of crops, and then just disappearing just as quickly.

0:13 What causes something like that to happen?

0:15 The answer is resources.

0:18 All living things need resources to survive and reproduce,

0:24 but what exactly is a resource?

0:27 Well, resource is anything an organism needs to live.

0:30 Some resources come from living things, like food, we call them biotic factors,

0:36 while others come from non-living parts of the environment,

0:39 like water, air, shelter, and space.

0:43 We call them abiotic factors.

0:46 When organisms have enough of these resources,

0:50 they're more likely to stay healthy and grow and produce offsprings.

0:57 And when many organisms have enough resources, their population can grow.

1:04 But there's an important catch: most resources are limited.

1:09 They're not available in an endless supply.

1:12 Think about it.

1:13 A field, for example, can only grow so much food,

1:17 a pond can only hold so much water,

1:20 and a forest can only have so many places to live.

1:25 So the question is what happens when a population gets bigger and bigger,

1:29 but the amount of resources stays the same?

1:33 To answer that question,

1:34 let's imagine a small group of rabbits living in a meadow.

1:39 At first, there's plenty of grass, water, and space for all of them.

1:45 The rabbits have enough to eat, so many survive and have babies,

1:49 and so the rabbit population starts to grow.

1:53 But as more and more rabbits live in the same meadow,

1:55 they all need to share the same grass and the same water.

2:01 Eventually, there isn't enough for everyone.

2:06 So what happens next?

2:08 Competition.

2:09 These rabbits begin competing for grass, for water, for space.

2:15 Some rabbits may not get enough food or space to survive,

2:19 and so fewer babies are born,

2:21 and as a result, the growth of the rabbit population slows down.

2:26 It may even shrink.

2:29 And this is not just the story of rabbits.

2:32 This is a common pattern we find everywhere in nature.

2:37 When there is plenty of resources, organisms thrive and the population grows,

2:43 but as populations grow larger and larger, resources start becoming limited.

2:50 As a result, competition increases and the population growth slows down,

2:55 or it can even stop.

2:57 And by the way, competition can also happen between different species.

3:00 For example, a bird and a squirrel might compete for the same seeds,

3:06 or different kinds of plants might compete for the same sunlight and water.

3:11 And this competition is not about animals being mean

3:15 to each other or being aggressive to each other.

3:18 No, no, no, it's simply what happens when

3:20 many organisms depend on the same limited resources.

3:25 As a result, this competition affects which organisms survive and reproduce.

3:33 Now, let's return to the locusts.

3:34 Locust plagues often begin after a period of heavy rainfall.

3:39 Why?

3:40 Because the rains help plants grow very quickly,

3:43 which creates lots of food for locusts.

3:46 With so much food available, locust populations grow very fast,

3:53 and locusts have a special ability that most

3:55 of the organisms don't really show in the same way.

3:58 When their numbers get very high and they become crowded together,

4:02 their bodies and behavior change.

4:05 They shift from living alone to actively seeking each other out,

4:10 gathering in huge groups and moving together as swarms.

4:15 These swarms devour crops and fields.

4:19 That's what turns ordinary population growth into massive

4:24 locust plagues people have feared for centuries.

4:29 But that growth doesn't last forever.

4:33 As the locust population gets larger, they begin to use up the available plants.

4:40 Eventually, there isn't enough food to support such a huge number.

4:44 Competition for resources increases and the population crashes back down,

4:50 making the swarms disappear.

4:54 This is why population growth and the resources are always connected.

5:01 Organisms need resources to live.

5:04 These resources, like food, water, oxygen,

5:07 shelter, space, sunlight, they're often limited.

5:12 When they appear plentiful, populations can grow, but as the populations grow,

5:18 individuals must share those limited resources, making the resources scarce.

5:23 And when the resources are scarce, population growth slows or even stops.

5:29 And so, look, the availability of resources helps

5:33 determine how big or small a population can become.