Community Structure

I In this lesson, we'll be talking about community structure, which has four key components. The first two or species richness and relative abundance. Species richness is just the number of different species in a community. Are there five different types of organisms living here or other relative abundance looks at how many individuals of there are of a particular species relative to others in the community. So there's three types of species in the community, but one only has 10 individuals, and the others both have 100. That's the idea behind relative abundance. Now these two concepts air sometimes merged into this idea of species diversity, which is just a measure that takes into account both richness and relative abundance. And you can see an example of a chart that displays species diversity here, which is really just showing the change in species diversity in North America. Here we have the you know what it used to be, what it is now and what the difference between those two states is more or less now. The other key components of community structure are the interactions between all the species of the community and the physical aspect of the community like the size of the area, the altitude where where the community exists and the type of vegetation, uh, present. And you can see some other aspects in this image here behind my head now without going into too much detail. This idea of species diversity is really important because species diversity actually leads to greater biomass production and greater resistance to invasive species. So species diversity essentially helps bolster ecosystems. With that, let's go ahead and flip the page.

often when looking at a community, we want to know about its trophic structure, which is the transfer of energy through trophic levels. Now, Trophic levels are often expressed in a food chain, which is basically a network of organisms that shows who eats who. So let's take a look at this food chain right here. It's gonna start with this algae at the bottom and this algae is a producer. All the rest of these guys are consumers. So this algae is gonna be eaten by this organism and this organism is gonna eat that little shrimp thing and this fish is gonna eat that fish and so on and so forth. So each one of these levels is a trophic level. And basically what trophic structure is concerned with is how much energy gets transferred from one level to the next. Now, we're going to look at that in more detail later on Now, when looking at the food web, we see that it's linear. But in actuality in communities, these cysts, these networks are really more of a web than a chain. And you can see how this food chain we had over here on the left would actually sort of merge into a larger food web where there's more interconnections between the organisms. I mean, for example, what's to stop this bird from. I don't know, say eating this fish, That's why a food web is generally a better overall view of what's going on in the community and the main thing to realize about these trophic levels. And in the food web or in the food chain is that the energy that gets transferred between levels is transferred very inefficiently. So the length of the food chain is actually going to be limited by that in efficient energy transfer. And we're gonna again, we're gonna look at this in more detail, put some numbers to it, make it extra clear. Now one thing I want to point out while we're looking at these food webs and food chains is the idea of bottom up and top down models. Bottom up models look at food webs and look at the influence of lower trophic levels as determining what happens at higher levels. Top down models show community structures where usually a predator controls community structures by limiting herbivores. So the predator at the top of the food chain will control the elements of the food chain below it. So top down now with that, let's go ahead and flip the page.

some species play a special role in their community structure. Dominant species are those that dominate the community. Either in abundance is in the number of them or biomass. How much you know, organic matter they take up in the community now, unexamined of dominant species are gonna be these mangroves which tend to dominate, uh, communities that are in brackish swamp waters. Now, keystone species are gonna be species that have a significant effect on their community. But they're not the dominant species. And when you remove a keystone species, you're gonna dramatically changed that community structure. Great example of this or these starfish which play a super important role as predators in tide pools. And without them, sea urchin populations go just crazy. Now, lastly, I want to talk about ecosystem engineers, the most obvious example of which is the North American beaver. These organisms, these ecosystem engineers significantly modify or maintain their habitat, and beavers, as we know, create dams. They literally engineer their habitat, though some people don't like using the word engineer because they think it. It implies too much critical thinking in the process. Whereas, you know, a lot of these behaviors air gonna be mawr instinctual Now communities will change when they experience a disturbance disturbances, a temporary change in the environment, and it's going to result in a change in the ecosystem. This includes stuff like wild fires or storms, floods, even disease outbreaks. And they're gonna change communities by removing organisms or altering resource availability. And sometimes there's actually a recurring disturbance pattern that a community will experience, and this is known as a disturbance regime. Now the non equilibrium model says that communities are constantly changing after disturbances. That is, they don't just stay where they are. They develop and sort of in conjunction with that. The intermediate disturbance hypothesis says that moderate levels of disturbance actually lead to greater species diversity. So in a sense, moderate levels of disturbance are better for the community. Now, here we can see a great example of the aftermath of a disturbance. Notice how all of this is green and all of this is charred. This is the remains of a wildfire. This graph right here represents the intermediate disturbance hypothesis which shows that species diversity on the Y axis is gonna be highest at an intermediate level of disturbance, which you can't read because I'm blocking it. Here we go. Intermediate level of disturbance. See? Right in the middle. Too much disturbance. Species diversity plummets to little disturbance species. Diversity isn't as high as it could be. Now, with that, let's go ahead and flip the page.

ecological succession is a natural process that changes the species structure of a community over time. Eventually, it'll lead to a steady state known as a climax community. Now you could think of the steady state almost Aziz, the community that succession was building towards. Now succession can come due to natural processes or unnatural ones. You can see here the remains of forest after a fire, and after some succession, that area will start looking like this. As you know, the species structure changes, and, for example, we get this ground cover back now. Primary succession is a type of succession that occurs when new substrate is formed or exposed, so that usually this will happen when glaciers retreat and exposed rocky earth underneath or volcanoes erupt, making a fresh lava flow that will harden. This could also be achieved by floods and landslides. Landslides. Now primary succession is going to occur when this new substrate is exposed. It's devoid of life, and then life and organisms colonize it. So it's going to start out like Rocky. Nothing. And you're going to need stuff like I don't know, like in Thio, help break down some material and create some soil and eventually the soil will build up and you'll be able to support mawr and more organisms until you'll eventually reach some type of climax community. No secondary succession is when a previously inhabited area so not like the start of a primary succession, which is a completely barren area previously inhabited area, is colonized by new organisms after a disturbance. So secondary succession requires some type of disturbance, like that image at the top of the page we saw. Now this disturbance can be something like a fire. It can also be man made these me human disturbances, for example, deforestation can lead to secondary successions and basically what will happen during six secondary successions. What we see charted out here first this top row is just showing the destruction which leads Thio essentially the starting point for the succession. And from there organisms will colonize and eventually you know and you sort of climax community will regrow. So with that, let's go ahead and flip the page

interactions between organisms in a community can affect that community structure. One way this can happen is is with facilitation, where the presence of some earlier species makes conditions more favorable for a later species. In fact, we'll see this with what are called pioneering species, which are the first hardy species to arrive and colonize during a primary succession. And these were going to help develop soil for later organisms. And they'll be, you know, stuff like like in moss shrubs, various weeds, all common common pioneer species. Now, some less positive interactions are tolerance, where existing species just don't affect the subsequent species that arrives and inhibition, where the presence of one species will actually prevent another species from forming. And here I just want to show some pioneering species moving into this hardened lava flow. In an example of primary succession, and behind me, we have an EP If it this organism actually is living on the surface of this tree. So in a sense, this tree was providing facilitation, it essentially made the conditions favorable for this later species. This EPA fight to arrive. Now I want to briefly take a look. At a great example of primary succession which occurred in Glacier Bay in Alaska. Uhh! You can see sort of, ah, aerial view of this location behind me. And basically what happened is a rapid glacial retreat over the past 200 years exposed this area and led to primary succession. Now, the recently uncovered areas are just gonna have pioneer species. And you can see an example of that here. Now, over time, small shrubs called dry as they're going to inhabit the more developed areas. You could see that happening here, and then they will be replaced by alder. She can see here, then spruce trees will move in, and eventually these areas that air developed with spruce trees will allow for hemlocks to arrive forming these spruce hemlock forests. Now, actually, some of these areas in Glacier Bay are not going to stay. Hemlock, spruce forests there actually do thio the acidity of the see magnum moss that is found there. They're actually going to turn into ponds and Boggs not everywhere, just some of the areas. So with this, hopefully you can see an example of primary succession and how it moves through these various stages. With that, let's go ahead and flip the page

bio geography has large scale effects on communities and species diversity, especially in regards toe latitude in area. First, let's take a look at how latitude is going to affect things. Differences in latitude will lead to differences in climate, and differences in latitude will also result in differences in sunlight and precipitation. For example, we know the tropics relieve or rather receive ample sunlight and ample precipitation. This leads to lots of EV apa transpiration, which is the evaporation of water from the soil and plants moving into the atmosphere. And this helps make this a very moist, humid, warm climate, which allows for very rich species diversity. Now the relationship between species and area is modeled here in the species area curve, but you can see in this graphic. Basically, the trend is that larger areas have larger numbers of species, so as you increase the area, you're also increasing the number of species. The last model I want to look at is what's known as the island Equilibrium model, and this is basically looking at the balance between species immigration and extinction rates that helps maintain stable species counts. Now, on our X axis here basically have a number of species. And on this side we have our extinction rate. And here we have our immigration rate. And what's important to know is as you increase uh, well, hopefully what you can see is that the balance between the immigration rate and the extinction rate is what's going to determine the species number on the island. So if you have a lot of immigration and a small amount of extinction, you're going to have ah lot of species on your island. So, for example, here, you know where it says large island nearby. We're gonna have a good amount of immigration and a low amount of extinction. And that's going to give us a pretty high number of species, not something to obsess over. Just something to sort of understand how community structures work. All right, that's all I have for this lesson. I'll see you guys next time