Hi. In this lesson, we'll be talking about community ecology and the interactions in communities. A community is all the populations of different organisms that live in the same area, and these different species in a community interact with one another. We call this interspecific interactions. And these interactions can be beneficial or harmful to the survival and reproduction of the organisms involved. The idea of being beneficial for survival and reproduction gets at the idea of fitness, which is the ability of an organism to survive and reproduce compared to the average member of their population. Because these interactions affect fitness, and since fitness is a strong driver of evolution, we're going to see coevolution where species influence each other's evolution due to the either harmful or beneficial interactions they have. A common example is a bird that can feed on flowers because its bill has evolved to be just the right shape to get in there. The bird gets food, and the flower uses the bird as a pollinator—a pretty common example.
But the point I want to make to you guys is that this goes beyond making two parts that fit together. Look at this interaction: Rabbits are fast, not because it's fun to be fast or because they are fans of the Flash, but because there are predators hunting them. They need to be fast to get away. So, I want you to think about the evolution that takes place due to these interactions, almost like an evolutionary arms race between these organisms in the battle for survival. Let's actually take a look at the different types of interactions. The first one I want to look at is called commensalism, which is an interaction where one organism benefits and the other is just unaffected. Here, you can see an example of that with fish hanging around a shark, basically waiting for some scraps to come their way. It's a good way to get a free meal. And, although they're kind of freeloaders, the shark is going to get his meal anyway. It's worth noting that these boundaries are not super well-defined in biology. We call this relationship between the shark and these fish commensalism, but it's not uncommon to see a shark eat one of the fish that's trailing it—it happens regularly. So, commensalism, yes, but just know that there are always exceptions.
The next type of interaction I want to look at is competition, which is negative for both of the organisms involved. This occurs when individuals compete for the same resources, and it's not going to work out well for either of them. Here, you can see an example of that where two males are competing for mates; this could result in a fatal injury, for example. At best, it's going to mean one of them doesn't get to mate; at worst, one of them could die. The next type of interaction I want to discuss is positive for one organism and negative for the other, as seen with the hare and the lynx—an example of predation. But you can also have herbivory, which is good for the cow but not so good for the plant it's eating. We'll talk more about parasitism later. Lastly, there's mutualism, which is positive for both organisms, as you can see in an example here between...