Interactions between populations | Ecology and natural systems | High school biology | Khan Academy

In the introduction to ecology, we introduce the idea of a community, which is all about different populations that are in the same habitat, that share the same area, or that are in the same area. So, populations, and if we're thinking in terms of water or in the air, could be the shared share of similar volume. Even populations sharing a habitat, sharing a habitat.

In particular, in this video, we're going to focus on the interactions between those populations, the interactions between the different species. The technical term for that is interspecific interactions; I like to just say interactions between species. Now, the first one that is often thought about is the notion of competition. Competition is when different populations, different species, are competing for the same resources.

You can imagine a forest where you have different populations of plants that are competing for sunlight, that are competing for water, that are competing for nutrients in the soil. Even in this picture right over here, this is a picture of a community. All of these different populations of fish and other things, sea anemones and coral, they are sharing this same region, and many of them could be in competition with each other. They might be going after the same food, or they might be going after the same shelter someplace.

Oftentimes, when people are talking about these interspecies or interspecific interactions, you'll see something like this: a minus slash minus, or a negative sign slash a negative sign. That means that this type of interaction, when you have two species or two populations that are in competition with each other, the more that you have of one, it's going to have a negative effect on the other, and vice versa. If I'm a plant and if I'm in competition with another plant, and that one's taking my light, and if there's more of it taking my light, that's going to have a negative impact on me.

And vice versa, if I'm in competition with you and we eat the same thing, the more of me that there's around eating your food, that's going to have a negative impact on you and vice versa. So, the next form of interspecific interaction, or interaction between species, is predation. This is when one population likes to eat another population.

You might often associate predation with pictures like this that you see on television shows, on documentaries. You see a cheetah hunting; it looks like a cheetah hunting a gazelle or a deer of some kind. Actually, it says right here it's a young bushbuck, and this is predation. But this is not the only form of predation. This picture here of the goat eating grass, this is also predation. It's not quite as bloody and violent, but it is still predation because you have one species eating another species.

In this case, you have this animal, the goat, that is eating the grass, and this type of predation, the specific type of predation, is called herbivory. Herbivory is a type of predation, so we could say predation slash herbivory. Let me do a little slash here: slash herbivory, which is a special case of predation. You'll often see a plush slash minus: the more of, let's say, this species that you have, the species that is being eaten is going to benefit the predator, but the more of the predator that you have, it's going to have a negative effect on the actual prey.

Now, the next types of interactions are ones where you have long-term, fairly intimate interactions, where you have organisms that oftentimes live with each other or even on each other. This general term of organisms that have these long-term intimate interactions is symbiosis.

Now, in everyday language, when people talk about symbiosis, they're often talking about organisms that really benefit each other. But technically, symbiosis isn't just about benefiting each other; it could be that they're even hurting each other in some way, or that maybe one benefits while the other one really doesn't care.

So, there's several types of symbiosis. The first that we could talk about is parasitism. Para, para, and this looks a lot like predation, where the more the parasite benefits, the more of the host that there is, but the host is actually hurt by the parasite. There's all sorts of examples of parasitism. We have right over here a zoomed-in picture of a louse.

So, why is this parasitism? Well, if this louse, I should say, so this is parasitism. Parasitism, and we would call this louse here a parasite. Parasitism benefits by living in your hair because that's where it gets its food from. It can lay or live on your scalp; it gets its food by sucking your blood. It can also lay eggs in your hair. In some ways, you could view it as almost shelter from the rest of the environment, but it's negative for you. It will make you itchy; it is taking your blood; it is uncomfortable.

So, parasitism, once again, is good for the parasite, just like predation is good for the predator, but not so good for the host in the case of parasitism. Now, you have another situation where it is benefiting both sides, and that is called mutualism. Mutualism. Let me do that in a different color.

So, mutualism—this is where both sides benefit. Oftentimes, when people talk about symbiosis, they're really talking about mutualism, which is a specific type of symbiosis where both species, where both animals, organisms benefit. They don't have to just be animals.

This is an example of mutualism right here. Let me do that in a color you can see. So, this is mutualism, where you have a clownfish living within a sea anemone. The sea anemone is providing the clownfish shelter, while the clownfish is keeping away other fish that might eat that sea anemone. So, they are both benefiting from that interaction, and so that is mutualism.

Now, you have another category where one species is benefiting and the other one is maybe a little bit more indifferent. So, one species is benefiting, and then the other one, well, maybe it is a little bit indifferent, and we call that commensalism. Commensalism.

And once again, there's many examples of commensalism. This right over here is a picture of bacteria living on your skin. You do have bacteria living on your skin right now, except it's actually, well, oftentimes it's a good thing. Sometimes it's mutualism that it's providing protection from harmful bacteria, but sometimes the bacteria is surely benefiting. It's living off of nutrients on your skin; the skin is its habitat, but, well, oftentimes it doesn't really have an impact on you.

Now, commensalism. Let me write this down. Commensalism. Oftentimes the more that we study it and the more that we understand it, we realize that actually, maybe the host actually is benefiting, in which case it is mutualism, or maybe the host actually is getting hurt, in which case it is parasitism.

So, oftentimes commensalism isn't completely neutral for the host; it could go either way. And so these are all the different types of interactions. I encourage you to look around you, look at the world around you, and don't just limit yourself to animals. Think about bacteria, think about plants, and think about, within a habitat, what are all of the different interspecies interactions and how you might want to classify them.