Resources and population growth | Interactions in ecosystems | Middle school biology | Khan Academy

So we have a picture here of these animals at a watering hole, and my question to you is: why don't we see more animals? There's clearly enough space for more animals, and we also know that if we focus on any one of these populations, say zebra, that every one zebra can have far more than one offspring in their lifetime.

Not only that, but those offspring can then have offspring, and so on and so forth. So it seems like over time these zebras should just be able to keep multiplying until they fill up all of this land. Similar for the buffalo, similar for the antelope. Why don't we see that? Pause this video and think about that.

So let's start by thinking about what any organism or a population of organisms, or a community of populations, need to survive. Most animals that live on the surface, like these animals, need air, especially oxygen. Plants need carbon dioxide from the air, they need water, they need a source of energy, which I will call food, or I could say it's energy here because it's not always in the form of food as we might recognize it.

And I guess we think really basically they need space in which to exist. Now, as I mentioned, there seems to be a lot of space here for the animals, so that doesn't seem to be the resource that is limiting their growth. So let's rule that out here.

Now it's also clear that it seems like there's a fairly large amount of air here, more than enough air for these populations to grow, so that doesn't seem to be a resource that's somehow putting a maximum on how many of these organisms there are. It does not look like a limited resource or a limiting resource.

Now what about water? Well, water does seem interesting here because this watering hole does not seem like it's very deep. It does not seem like there's a lot of water here, so this one could be what we would call a limited resource that is limiting population.

Maybe if they had more and more offspring, there just wouldn't be enough water for any member of a population, and then, of course, the populations would be competing with each other as well because they all need the same water to drink.

Now what about food? Well, you might recognize that zebra or antelope or buffalo tend to graze on grasses, and there are some grasses over here. But we also see that a lot of the grass is dead or dried out, and there's just a lot of areas with dirt here.

So it looks like the food and energy is also a limiting resource. We have to remind ourselves there's populations of different types of grasses, and why aren't they spreading more? Well, from the perspective of a plant, it looks like the air and space are for sure abundant, and even energy in the form of sunlight is abundant based on this picture.

So it's probably water that is the limited resource which is keeping us from having more plants and grasses in this picture. We can imagine a scenario in which it rains a lot; there's a lot of water. Water is abundant.

Then all of the different types of populations of grasses are able to expand, and then there's more than enough food and water for these animals to keep reproducing. In that case, space might become an issue.

But the important thing to realize here is that all organisms need resources in order to survive and in order to multiply and reproduce. Populations of many different species are often competing for those resources, like the water here, that will tend to put a limit on how much these populations can grow.

And the limit on one population can affect another. As we said, if you had more water, you could have more grass, which would allow for more food and energy for these other populations.