So you just watched the video How Wolves Change Rivers. You may have seen this before but I hope you watched it again. I don't get tired of it. If only I had the production value that they have. But it's really a wonderful video and it gives us a sense of the complexity of the Yellowstone ecosystem even though it's much more complex than they were able to represent.
It still gives us more more broader sense of the ecosystems than I'm able to do in these short videos. And of course now what I want to do is that happen in the Yellowstone ecosystem as an example. But one thing I want to mention before I go into that is the idea that while you saw the Yellowstone example of a trophic cascade and in how wolves change rivers, what you also saw was that that trophic cascade extended beyond just impacts on the biotic community and also extended to the abiotic community, right? And that's the whole piece about how wolves change rivers. And so that's especially what I love about using that video for this course.
All right, so let's look simply, though, at the trophic cascade that we observed in the video. Now, remember that when I show you these arrows right here, these yellow arrows, I'm using those to represent the flow of energy. So we see that wolves eat elk, right? And so elk, energy flows from elk to wolves.
Elk are herbivores of plants. Willows and aspen are a couple of the native plants that are preferred food sources. And so we have energy flowing from those plants to the elk.
Okay, so that's what the yellow arrows are, energy moving through the trophic chain. But then what we saw was that wolves originally declined. right, in the Yellowstone ecosystem.
They were adult increased, their population skyrocketed, and then as a result of that the willows and the aspen were browsed down quite a bit, and so those particular plants ended up declining as well. So we have this typical you know ABA pattern decline, increased decline pattern that we see, that we know as a trophic cascade, right. But in the video you also heard again, about this complexity.
And there's one particular piece of that complexity that I want to focus on so that we can learn another concept in community ecology. Alright, so what I want to focus on is the fact that it wasn't just, and if you look down here back at our little diagram, it wasn't just that elk, the elk population increased and then the plant population declined, the native plants declined. It was also that the elk population changed their behavior.
They changed the way that they acted, the way that they moved around the space of Yellowstone in the absence of wolves. And similarly, we didn't just see a decline in plants. We saw a shift in the plant community. So I'm using these delta symbols to represent change. So it wasn't just this straight decline, increased decline.
We also saw change, change in behavior, change in community structure of the plants. Why did we see that change? That's what I want to look at. Yes, part of it was just due to the absence of wolves, but part of it was due to something else.
And this is where coyotes come in. So here we have elk are also sometimes eaten by coyotes. So coyotes will take the young, the elk calves, and maybe, maybe occasionally, if working together, could take down. a very sick or old adult elk, but that doesn't happen very often. So all of a sudden the wolves are gone, right?
These top predators are gone. The major predators of elk are gone. And what's left that ends up increasing in population size is coyotes.
And because the coyotes are not putting the same kind of... pressure, predation pressure on the elk, the elk change their behavior, right? So usually the wolves through what we call a landscape of fear kind of urge the elk to move around, right? So we have elk tend to be a little bit nomadic, for lack of a better word, so they move from place to place.
And the wolves are essentially instigating that moving just by their presence. And so So, uh, A consequence of that, an indirect effect from the wolves, is that the plant community doesn't really get over-browsed in any one area, right? Because the elk are moving from place to place. But when the coyotes are really the kind of the predators that are present, the elk aren't as intimidated by them, and they don't have to move around.
So the elk tend to stay close to the riparian areas next to the rivers where they prefer to be. because they're not being forced out by wolves, right? And so they overbrowse the native plants that exist there.
And so this change of behavior and this change of plant community is somewhat mediated by the kind of presence of coyotes as one of the top predators, if you will, in the system in the absence of wolves. And so why is this happening? And what do we call this? This blue square here I'm using to represent the relationship between the wolves and the coyotes and kind of the phenomenon that happens when wolves are gone and coyotes kind of increase in number. Top predators, they're large predators, right?
Coyotes are medium-sized predators and they're kind of in the middle, right? They're not the top trophic level. They're one of the middle levels of consumers.
And so we call them mesopredators. Other mesopredators are foxes and badgers and... skunks and things like that. So these are medium-sized predators.
And so while we are seeing a trophic cascade here, right, the wolf-elk plant trophic cascade, we're also seeing the influence of what we call mesopredator release. And essentially, just to give you a definition of mesopredator release, mesopredator release is the increase... in mesopredator population size, so the increase in numbers of mesopredators, and in this case, we're talking about coyotes, when they are released from competition with the top predators, right? And why are they released from competition?
They're released from competition because of the major decline or extirpation or loss of the top predator. So a top predator is lost or just significantly declines in population size. The mesopredator that normally is being out-competed by that top predator, in this case for den sites and some prey items and just space, territory.
So they're normally being out-competed. Now they're released from that competition. Their numbers increase and we see reverberating effects throughout the ecosystem. So you don't have to, if I'm ever asking you about a system like this, or if you're thinking about what are the phenomena, what are the ecosystem phenomena that are at play in a system like this, you don't have to choose between trophic cascade and mesopredator release, right? They're not mutually exclusive.
Here is an example of where we're seeing both happen. So we are seeing trophic cascade with the wolf-elk plant situation, but we're also seeing that the coyotes are being released from competition, mesopredator release, and that that increase in coyotes because of that is also having impacts on everything that exists below it. And so both of these, both the trophic cascade and the implications of the mesopredator release, they're trading down towards the lower trophic levels. And what we call that is top-down effects.
Because it starts at the top because of some change at the top and reverberates down the trophic levels. Later on in a future module, in a future case study, we're going to talk about what we call bottom-up effects. So effects that start maybe at the primary producer level and reverberate up the ecosystem. That does occur, but we don't call it trophic cascades.
Trophic cascades cascade down like a waterfall. cascading down like a waterfall. Waterfalls don't flow up, so trophic cascades go from the top to the bottom. All right, so hopefully you here see the difference between trophic cascade and mesopredator release.
Now I want to tell you one more example of mesopredator release. and trophic cascade incidentally. So this is an example from the marine ecosystem.
And here the three kind of levels, the three trophic levels we're looking at are the great sharks, the large bodied sharks like hammerheads as you see here. And then the next trophic level down is small sharks, the smaller bodied sharks, and rays like this. You can see a ray right here. And then the next trophic level down that we're going to look at is scallops, right, is occupied by scallops. And what we see here is that when the great sharks started declining, when their population size started to decline, because again, largely because of over hunting by humans or because of entanglements in fishing nets and things like that, So we have a decline in the large-bodied great sharks.
That led to an increase in the smaller sharks and rays. And that led to a decline in scallop populations. So before we get to implications for humans, I want to say that this is clearly a trophic cascade. But there's also, I also have that blue block that's representing mesopredator release. And this one is a slightly different story.
So in the... The wolf-coyote example, wolves are largely competitors of coyotes. They will aggressively defend territory against coyotes, but they don't often eat coyotes. They're not like, coyotes aren't their preferred prey.
Whereas great sharks, there are many species of great sharks that will actually prey on smaller sharks and rays. So they prey on smaller sharks and rays, and that's why it's a but they also will compete with smaller sharks and rays. for food and for territory and things like that.
And so we also have a mesopredator release. So there are two kind of mechanisms of this top-down effect when we're looking at the marine example here. One is the release from predation and that's where the trophic cascade occurs. And the other is the release from competition and that's where we have mesopredator release. Now the result happens to be the same here, that we have a decrease in the great sharks, an increase in the small sharks and rays, and a decrease in scallops.
But I just wanted to use this as another opportunity to explain trophic cascade and mesopredator release. Again, head to the discussion board if you have any questions or need clarification on that. So now implication for humans, and this is one of the reasons that I like to use this example in addition to the wolf example. So, Because the Yellowstone example is this kind of national park, separate from us seeming example that we get to just watch as bystanders.
But this example really impacted people, scallop fishermen in particular, because the scallop industry in the southeastern coast of the United States really took a major hit when this... trophic cascade when the effects started to manifest at the lower trophic levels. So when the scallops started to be over predated by the smaller bodied sharks and the rays, now we're competing with huge numbers of rays and smaller sharks for scallops, which are a really important industry. for many communities in the southeastern United States. So there are sometimes not just these kind of aesthetic or ecosystem serve ecological type values that are lost, but actual economic values can be lost because of the top-down effects, because of the reverberating effects of a trophic cascade like this.
All right. And so... We just, oh, you know what?
We just looked at two examples of trophic cascades in the last couple of case studies. So we looked at the sea otter, urchin, kelp forest example where we had a trophic cascade. And then we looked at the wolf yellowstone ecosystem trophic cascade.
And then finally the the great shark, smaller sharks and rays, scallop, trophic cascade. We also learned about keystone species, right? The sea otter was a keystone species. The wolf incidentally is considered a keystone species.
And then we also learned about this concept of mesopredator release. which is where you have a top predator that declines or disappears from an ecosystem and that releases the smaller body, the medium-sized predators from competition and that has implications for everything in the lower trophic levels of that biological community. I have foundation species written up here and I haven't talked about that yet, but I will talk about that in the future.
in a future video. So stay tuned for Foundation Species. Okay, one last thing I wanted to mention. So I wanted to leave you with this thought about the roles of humans in trophic cascades.
So I mentioned the implications for humans of this great shark trophic cascade and the mesopredator release that we saw there. But what I mean by the roles of humans is not just how we're affected economically or how we're affected because of the loss of wolves and the symbol of wilderness from Yellowstone, but also the roles that humans have in creating trophic cascades, right? That's really big.
So we talked about humans being one of the major reasons that, well, humans were essentially the reason that sea otters declined, that wolves were extirpated from Yellowstone in the first place. Right. and that great sharks declined. So we really were responsible for these top predators in these systems declining or disappearing, triggering trophic cascades.
So it was our human impacts that triggered these trophic cascades. Now, it's also our, we can also have positive impacts in that we help the sea otters rebound by placing them in moratorium and we reintroduced wolves into Yellowstone. So we can have positive impacts that turn the trophic cascade around. But I just want us to keep in mind that these systems aren't separate from us, that we actually enter these systems in lots of different ways. And we're also impacted by them in lots of different ways.
So that's what I'll leave you with.