Ecological interactions

0:00- [Lecturer] This is the African savanna. Here you see grazing zebras, wildebeests moving in herds, elephants pulling up grass, giraffes and lions, and so many other animals all at the same place and time. But how can so many different organisms, who all need food, water and space, how can they all live together in the same ecosystem?

0:24I mean, wouldn't they constantly like, I don't know, maybe get in each other's way or something? Well, to understand this, scientists study something called the ecological interactions. What is that? Let's find out. So this ecological interactions, this describes how organisms affect one another as they live in the same environment.

0:46What I mean is, it explains how an ecosystem functions as a whole. It tells us about how its population survives or grows, or, you know, even shrinks over time. And there are three major types of ecological interactions. We have competition, predation and herbivory, and we have symbiosis, which itself has a few other subtypes.

1:13So let's start with competition. You probably know that all organisms need resources to survive. They need food for energy, they need water, they need space to live, they need safe places to rest and reproduce, and so on. But in any ecosystem, these resources are limited.

1:32I mean, there's only so much grass or there's only so much water, right? So competition happens when organisms need the same limited resource. For example, on the savanna, the zebras and the wildebeests both feed on grasses. Now, if many animals are grazing in the same area, there may not be enough grass for all of them.

1:55That is competition. Notice that competition does not necessarily mean fighting. I mean, these zebras and wildebeests don't have to fight each other to compete. If one group ends up eating more grass, then less grass remains for the other group, right? That alone is competition.

2:12And by the way, competition can happen between different species, like zebras and the wildebeests, or it can happen between the same species, like between two zebras or two wildebeests feeding in the same area. And what do you think is gonna happen if this competition becomes very intense?

2:28Well, if resources become too limited, some organisms may not even get enough food, or water, or space, and that can reduce reproduction and lower population sizes all together. But here's something interesting. Sometimes organisms can reduce competition. How?

2:48By using resources differently. For example, giraffes feed high in acacia trees, while zebras or wildebeests, for example, mostly feed on the grass close to the ground. So because they're using different sources of food, they compete less directly, (laughing) right?

3:07So you can see how competition can help shape where organisms live and how many can survive in an ecosystem. All right, next up, let's talk about predation and herbivory. Let's start with predation. Some organisms survive by feeding on other organisms. So when one organism hunts and kills and eats another organism, that's what we call predation.

3:30In this example, the lion is the predator and the antelope over here is the prey. So the antelope is killed, and the lion gains the nutrition and energy. Okay, now, think about this. If the prey population increases in a certain area, what do you think is gonna happen to the predator population in that area?

3:51Well, now, since there's more food available, the predator population would also increase, right? In contrast, what if the prey population were to reduce in a particular area? Well, now less food is available, so there'll be more competition, and that would tend to reduce the predator population in the area.

4:11So notice how these interactions are connected, right? So predation helps shape the balance of populations in an ecosystem. But remember, not all feeding interactions involve animals eating other animals. What about animals eating plants? That's called herbivory.

4:32For example, giraffes eat acacia trees, elephants eat grass. These animals are herbivores. In this particular interaction, again, the animal gains energy and nutrients, and the plant is damaged. So again, similar question, what if we have an area with lots of plants?

4:51What would happen to the herbivore population in that area? Well, again, there's more food available, so the herbivore population in that area would increase. But again, if too many herbivores feed in one area, what would happen to the plants? The plant population may decrease, and that will now, again, affect the herbivore population, right?

5:15So you can see, just like with predation, even herbivory influences survival, reproduction, population sizes, all of that. This now brings us to the third major type of ecological interaction, symbiosis. What exactly is this? Well, take a look at this picture.

5:34It might look like these birds are troubling the buffalo, but they're not. These birds are called oxpecker birds, and they usually sit on large mammals like buffaloes. But why? Because they get to eat the ticks and other parasites from the mammal's skin. Why would they do that?

5:52Because the birds gain their food this way, and the mammals end up their parasites removed. So look, both organisms are benefiting. That is an example of symbiosis. It's a type of ecological interaction in which organisms form close, long-term relationships with each other.

6:13Now, this particular kind of symbiosis in which both the organisms are benefiting, we call it mutualism. And notice, the birds are not trying to sort of like help the buffalo. No, no, no, they're just trying to eat the food, and the buffalo ends up benefiting as a natural result of that behavior, right?

6:32But this is not the only kind of symbiosis. Take a look at this video. What do you notice? Hey, you see these tiny birds following the elephant, right? These birds are called cattle egrets. And they usually follow large grazing animals, like elephants or zebras.

6:54But why do they do that? Well, because as these large animals move through grass, insects are stirred up. So the birds catch and eat those insects. So who benefits this time? The birds do. But what about the elephants? They are not benefited at all, but they are also not harmed, right?

7:17This is called commensalism. It's also a type of symbiosis because, again, there is a close, long-term relationship between them, but it's a kind in which one organism benefits, but the other is mostly unaffected. And so now you can see where we are going with this.

7:36So you can kind of predict what the third one is going to be. For that, take an example of a tick feeding on some mammal's blood, like deer's blood, for example. What's happening over here? Well, notice the tick gains nutrients, but what happens to the mammal?

7:52It loses blood. It might become irritated or weakened. So this time, notice one organism benefits, but the other is harmed. This type of symbiosis is called parasitism. And so the tick becomes a parasite. And again, what's important is that this is not intentional harm.

8:15The tick is not trying to harm the mammal over here. It's simply a survival strategy. So coming back to our original question, how can so many organisms live together in an ecosystem? The answer lies in ecological interactions. All these organisms will compete for limited resources, they will feed on one another, and they will also form long-term relationships.

8:43All these interactions will influence who survives, who reproduces, and how many organisms can live in a particular area. So what's incredible is that ecosystems are not random collections of organisms. Hopefully, we can now appreciate how they are continuously shaped by various kinds of interactions.