Community Interactions - Video Tutorials & Practice Problems
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1
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Interspecific Interactions
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4m
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Hi. In this lesson, we'll be talking about community ecology and the interactions in communities. Now a a community is all the populations of different organisms that live in the same area, and these different species in a community are gonna 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. Now, this 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 because fitness is a strong driver of evolution, we're gonna see coevolution where species influence each other's, evolution due to the either harmful or beneficial interactions they have. So a common example is something like this bird, which can feed on these flowers because its bill has evolved to be, you know, just the right shape to get in there. And, you know, the bird is gonna get food, the flower is gonna get, to use the bird as a pollinator. Pretty common example. But the point I wanna make to you guys is that this goes beyond, you know, making 2 parts that fit together. Look at this interaction. Rabbits are fast. Right? Rabbits aren't fast just because it's fun to be fast or they're fans of the flash. They're fast 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 is 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. First one I want to look at is called commensalism, and this is an interaction where one organism benefits and the other is just unaffected. So here you can see an example of that. These fish hang around the shark basically waiting for some scraps to come their way. It's a good way to get a free meal. And, you know, the shark is basically they're just kind of freeloaders, but he's gonna get his meal anyways. Now it's worth noting that, you know, as with everything in biology, you know, these aren't super well defined boundaries, so to speak. I mean, we call this relationship between this shark and these fish commensalism, but it's not uncommon to see a shark eat one of the fish that's trailing it. You know, it happens, regularly. So, you know, commensalism, yes, but just know that there's always exceptions. So the next type of interaction I want to look at is competition. This is negative for both of the organisms involved. This is when individuals compete for the same resources, and it's not gonna work out well for either of them. And here you can see an example of that where 2 males are competing for mates, and, you know, this could result in a fatal injury, for example, or something like that. At worst, it's gonna mean one of them doesn't get to mate. I'm sorry. At at best, it's gonna mean one of them doesn't get to mate, at worst one of them could die. The next interaction I wanna talk about, or type of interaction I wanna talk about is positive for one organism and negative for the other. And this is like what we saw going on with the hair and the lynx. Right? That was an example of predation. But you can also have herbivory, which you see here, which is good for the cow, not so good for the plant it's eating. And parasitism, which we'll talk more about later. Now, lastly, you have mutualism, which is positive for both organisms. And you can see an example of that here between
2
concept
Ecological Niches and Competition
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3m
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The role that an organism or a population plays in its environment is called its ecological niche. Now, not every niche is gonna be glamorous, as you can see with this dung beetle right here. However, the point is more about finding a niche that isn't occupied by anyone else, so that you can best exploit the resources available there. Now, organisms are gonna have a range of, conditions that they can survive in, and we call this an organism's fundamental niche. This is just all the environmental conditions suitable to the organism, ignoring the interspecific interactions. Now the realized niche is gonna be the part of the fundamental niche that an organism will actually occupy due to the limiting factors imposed upon it. So this outer ring represents the fundamental niche, and this inner ring the organisms realized niche, or I should say inner area. Now, species in a community are going to compete. Both individuals within a species are gonna compete and individuals from different species are gonna compete with each other. Intraspecific competition is going to occur between individuals of the same species, and because these individuals are of the same species they're gonna have the same niche which means they're gonna compete for the same resources, which is why this is a density dependent competition. As density goes up so does competition. Now interspecific competition is when individuals of different species have overlapping niches, And this can result in either symmetric competition, where both species are negatively affected by the competition, or asymmetric competition where one is much more negatively affected than the other. Now it's common for one species to out compete another due to certain fitness trade offs. For example, a organism might have, you know, develop certain traits that make it really efficient at gathering some type of food, for example. However, there's a trade off for this, You know, by gaining that skill they're sacrificing an opportunity elsewhere. So it should note that species that excel at competing for certain resources will be lacking in other areas. Anyhow, let's flip the page.
3
concept
Competitive Exclusion and Niche Differentiation
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3m
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If species occupy the same niche or have overlapping niches, something's gonna have to change. Now if 2 species occupy the same niche and there's no disturbance in their environment, one will outcompete the other, says the competitive exclusion principle. You can see this represented in this chart here, where, you know, assuming that there's no disturbance, species 1 is going to way out compete species 2, and species 2 is gonna go extinct, or that population is gonna go extinct, I should say. Now, if competing species have overlapping niches, they can do what's, or they can experience what's called niche differentiation, where they'll actually use the environment differently to better coexist. And this can be achieved through resource partitioning, where similar species will differentiate their niches to better use the resources available. And, you know, mostly to use different resources to lower the competition between them. So you can see that happening here where we have these three species that are all overlapping in their niches, and they're going to have a niche differentiation and that's going to lower the competition between them. Now over time what you can see happening from this competition is character displacement. And a very famous example of that are the finches on the Galapagos Islands that Darwin characterized. Now this is when similar species, have distributions that overlap, and so they develop differences to reduce their competition. Now this example right here is super arbitrary, but I just wanna use it for illustrative purposes. I'm gonna jump out of the way here. So let's say that the yellow birds and the red birds overlap, they all want the bugs off this tree, But the red birds are going to, you know, they're bigger than the yellow birds, so they're gonna crowd them out. And they're gonna get the bugs from the middle of the tree, because, I don't know, that's where they want the bugs from, something like that. As I said, this is arbitrary. So if that's the case then the yellow birds could still get the bugs from the top of the tree or the bottom of the tree. So they're going to differentiate their niche to reduce competition, and over time, let's say that these birds evolved to specialize in, you know, eating the bugs at the middle of the tree versus the top of the tree. I don't know what trait that would be. But, you know, assuming that, you know, like their beaks, for example, started out kind of similar and then got more different based on the fact that these guys are eating off the top of the tree and these guys are eating off the middle of the tree, then we'd have character displacement. So essentially this is when species, develop differences to reduce competition, and you can see that again with the Galapagos finches, who all had, you know, these different differences developed so that they could take advantage of different types of food resources available. Alright. With that, let's flip the page.
4
concept
Predation
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2m
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Predation is when one organism consumes another organism. The organism that consumes is the predator and organism that gets consumed is the prey. Now often predators are gonna be carnivores, or meat eaters, animals that feed on flesh. A special case of predation is herbivory, which is predation when an animal consumes plant tissue. And often this is going to be done by herbivores or plant eaters, animals that feed on plant matter. Now predators can actually help play a role in keeping prey populations below carrying capacity, which can be very important for communities. Likewise, herbivore populations can be balanced by predation, also disease. And it should be noted that plants have evolved defenses against herbivores. Now we can see some examples of predation right here and some examples of meat eating. And I just gotta say really kudos to this heron, which speared the crap out of that fish. I mean, I couldn't believe it when I saw this image. Now, predators and prey, herbivores and plants, they're all part of this evolutionary arms race. Right? You know, it's like we were talking about with the rabbit, it runs fast to get away from a predator, not just because running fast is cool. So we're gonna take a look at some defenses that organisms have, and we're gonna kind of categorize them as constitutive defenses and inducible defenses. So constitutive defenses are always present regardless of predators being around or not. The stick bug is a great example. Always looks like a stick, always gonna be hard to see. Another example in the case of plants would be thorns. You know, thorns are grown on the plant, they're just there, that's sort of a constitutive defense for it. Now inducible defenses are physical, behavioral, or chemical traits that are induced in response to the presence of predators. And you can see here that this cat is about to get it from this skunk, which has an inducible defense that is chemical. It will spray it. Nasty stuff. Alright. With that, let's go ahead and flip the page.
5
concept
Mimicry
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3m
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Some organisms use their appearance as a defense. Aplasmatic coloration is warning coloration, usually in the form of bright colors that signal chemical defenses. So as we can see with this poisonous frog here, it is brightly colored. That bright bright red, it's just gonna pop. And it's there to signal to predators that, hey, I'm poisonous. I mean, you you can see me, if you want to eat me go for it, but not gonna be happy. Now cryptic coloration is the opposite. Instead of drawing attention to yourself you're trying to camouflage, and this is gonna make prey hard to see. Now believe it or not, there is a lizard on this tree right here. It is super well camouflaged. So I'm just gonna go ahead and kinda trace its outline, so you can see. Hopefully, you can, kind of, get the idea. But, yeah, there is a lizard hiding there, it is just incredibly well disguised. Now, speaking of disguises, mimicry is a really interesting evolved defense. And basically it's when one organism evolves to resemble another in some way, and this could be in appearance, in smell, in sound, and even in behavior. And we'll look at 2 examples of mimicry here. The first is Batesian mimicry, which is mimicry when a species sort of tries to imitate the warning signs of some other harmful species. So classic example is this, we have the coral snake up top, which is poisonous, and this mimic, the king snake right here, which is non poisonous. And the king snake is using Batesian mimicry to fool other organisms into thinking it's a coral snake, and therefore, poisonous, and therefore, stay away. Now, Mullerian mimicry is when 2 distinct species mimic each other's warning signs. So in this case we have 2 types of butterflies, we have a monarch up here, and what's called a viceroy butterfly. They look very very similar, and they actually both have the defense mechanism of being really bad tasting, they're very unpalatable. Now why would these 2 species mimic each other's warning signs? Well, it actually puzzled people for a long time. And then, basically someone created this mathematical model to show that by both species mimicking each other they actually increase their chances of survival because It's almost like this, if you're a predator and you learned that, you know, this one particular bug tastes bad and there's another bug that looks just like it, you're not really gonna tell the difference between them. So it actually increases the fitness of both organisms to sort of have this convergence in appearance. Very very strange interesting stuff. Now with that, let's go ahead and flip the
6
concept
Symbiosis
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2m
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Symbiosis is a close relationship between organisms of different species. In a symbiosis, the relationship can prove to be beneficial or harmful to the organisms involved. Now, I wanna take a look here at a great example of a beneficial symbiosis. What we're looking at are plant roots that have developed these nodes. These nodes contain bacteria that perform nitrogen fixation, which allows the plant to absorb nitrogen. Basically, these bacteria provide the plant with usable nitrogen. So their relationship is one of mutualism because both species benefit. The plant gets nitrogen, and it provides the bacteria with nutrition and a safe environment. Now, in this case the plant is actually acting as a host because it's harboring the symbiont, the bacteria inside of its roots. Now, it's worth noting that some relationships can be commensalism, where one organism benefits and the other is unaffected. It's also worth noting that some interactions can occur without a symbiotic relationship. For example, facilitation is an interaction where 1 or both species benefit. And getting back to, you know, nitrogen fixation by certain plants adding more nitrogen into the soil they can facilitate the growth of other species later on. So those 2 different types of plants wouldn't have a symbiotic relationship, but one would be benefiting the other. Now, let's talk about when these relationships sour a little. Things take a turn for the worst. Parasitism. This is a relationship where one species, the parasite, benefits at the expense of its host. Endoparasites are parasites that live in the body of their host, but some parasites live on the outside of their hosts, and they're called ectoparasites. Here is an example of a crazy fungal parasite, which in this case has completely taken over the body of a tarantula. This is called cordyceps fungus. That's all I have for this lesson. I'll see you guys next time.