This video, we're going to talk about bottom-up and top-down effects on community structure. And so some population sizes actually cycle periodically in predictable ways due to interspecific community interactions or interactions between different species in the community. Now this population cycling can be explained from two different angles. It can be explained from the bottom-up or it can be explained from the top-down in terms of trophic levels. And so bottom-up control is when the population cycling is influenced by either resource availability or food availability at lower trophic levels, such as, for example, primary producers, which tend to be at the bottom of the trophic levels.
Now top-down control, on the other hand, is when the population cycling is influenced by organisms at higher trophic levels, such as, for example, predators. So the question is, which of these types of control is correct? Well, it turns out that it's actually situational. In some situations, the population cycling is almost exclusively determined from the bottom-up. But in other situations, it's almost exclusively determined from the top-down.
But in many cases, both of these types of control actually apply in a more even way. So let's take a look at our image down below where, on the left-hand side, we're showing you a food chain where at the very bottom, we've got resources, which could be the availability of water, nutrients, or sunlight, for example, which would be utilized by this primary producer here, the grass. And then the grass will be consumed by the hare, which is an herbivore. But the hare is also going to be the prey for this lynx up above, which is the predator. Now because the predator is at the very top of this food chain, it can only be influenced by trophic levels that are below it.
And so this lynx or predator can only be influenced from the bottom up. However, the hare or this prey here, because it falls in the middle of this food chain, it has a higher trophic level above it in the predator, the lynx, and a lower trophic level below it in the primary producer or the grass. And so the question is, well, is this hare's population cycling influenced by, from the top-down by predators or from the bottom-up by the grass? Now over here on the right, we're showing you a graph that has years on the x-axis and population size on the y-axis. And notice that we're showing you the population cycling of the hare and the lynx.
And what you'll notice is that these cycles are occurring about every 10 years or so. Now once again, we know that the lynx, because it's at the top, is influenced from the bottom up. But the question is, is the hare influenced from the bottom-up or the top-down? And so, in most cases, we tend to see population growth curves creating somewhat of a logistic or sigmoidal growth curve. So, we would expect to see this population somewhat plateau right around the carrying capacity.
But again, that's not what we see. We see this cycling. So the question here is really, what is happening to the population of hares right here where it starts to drastically decrease. And again, this is happening every 10 years or so. So, the question is, what is causing this drop in the hare population every 10 years?
Now, again, if it were the predators, consuming and killing and eating all of the hares and that's what's causing the primary drop in the hare population, then that means that the prey would be influenced from the top-down by the predator. But if the hare population is actually decreasing, not because of the predators above but because of the availability of primary producers below it in lower trophic levels, well, then that would mean that the hare is influenced from the bottom-up. Now it turns out that the case here is that the hare is influenced in a more even balance from the top-down and from the bottom-up. And another way that you can kind of visualize this is that the hare population is somewhat surpassing a carrying capacity, where it's going above the carrying capacity and then it's kind of forced back down below it. And it does that, you know, over time every couple of years.
You can think the same thing with the lynx, where maybe its carrying capacity is somewhere right here. And you can see, again, it goes above the carrying capacity, but only temporarily, and it's forced back below it. And so, that population cycling is something that we can see in some populations. So this here concludes our lesson on the bottom-up and top-down effects, and we'll be able to apply these concepts and problems moving forward. So see you all in our next video.
