Community Structure - Online Tutor, Practice Problems & Exam Prep

Community structure encompasses species richness, relative abundance, and interactions among species, influencing biodiversity and ecosystem resilience. Trophic structure illustrates energy transfer through food chains and webs, highlighting bottom-up and top-down models. Key species roles include dominant, keystone, and ecosystem engineers, while disturbances can reshape communities, leading to primary and secondary succession. Biogeography affects species diversity through latitude and area, with larger areas supporting more species. Understanding these concepts is crucial for grasping ecological dynamics and conservation efforts.

Community Structure

So now that we've covered community interactions in our previous lesson videos, in this video, we're going to shift our focus over to community structure. Community structure refers to the makeup of a community, how that community is organized. It has 4 key attributes, which we have numbered down below. The first of these key attributes is species richness, which simply refers to the total number of different species in the community. Species richness is a simple count.

The next key attribute of community structure is relative abundance, which refers to the proportion of each species in relation to all of the individuals in the entire community. A weighted measurement of biodiversity in a community, which integrates both species richness and relative abundance, is species diversity. Species diversity can be mathematically calculated using the Shannon Diversity Index, commonly abbreviated with the letter h. In most cases, you don't need to worry about calculating the Shannon Diversity Index. However, you should know that the greater its value, the greater the species diversity, and the more biodiverse the community will be.

The third key attribute of community structure are all of the interactions between organisms, including the interactions we covered in our previous lesson videos, such as competition, exploitation, mutualism, and commensalism. The fourth key attribute of community structure are physical attributes, biotic and abiotic factors, including species distribution. Let's take a quick look at our image down below where it's asking us, which of these two communities is more diverse? Is it community number 1 or is it community number 2? Notice that both of these communities have a total of 4 different species of plants: lavender, tulip, marigold, and shasta daisies.

This simple count of the total number of different species is what we call the species richness. The species richness for both of these communities is 4. The relative abundance is the proportion of each species in relation to all individuals in the community. The relative abundance is indicated by these percentages that you can see below each of these communities. Most of us can correctly guess that it's community number 1 that is more diverse because it has a more even distribution of those 4 species.

However, it's not always so easy to tell which of the communities is more diverse. It's the Shannon Diversity Index that can mathematically determine which community has greater species diversity and is more biodiverse. For community number 1, the Shannon Diversity Index is indicated as 1.39, and for community number 2, its value is only 0.98. A greater Shannon Diversity Index indicates more species diversity in a more biodiverse community.

It is indeed the case that community number 1 is more diverse than community number 2. The last note that I'll leave you all with is that higher species diversity tends to lead to higher primary productivity, essentially more vegetation, which tends to lead to more biomass in the community. Higher species diversity also tends to lead to greater stability of communities. Communities that have higher species diversity tend to be more resistant to disturbances, and they tend to be more resilient and more quickly recover after a disturbance. This is why higher species diversity is often associated with being a good thing for a community.

This here concludes our brief lesson on community structure, and moving forward, we'll be able to learn a lot more and apply these concepts. So, I'll see you in our next video.

Community Structure Example 1

So here we have an example problem that says, based on the image below, which of the following communities of birds, either community number 1 or community number 2, has a greater species diversity and why? And we've got these 4 potential answer options down below. Now, of course, recall from our last lesson video that species diversity is a weighted measurement of biodiversity that integrates species richness or the total number of species in the community and relative abundance or the proportion of each species relative to all individuals in the community. Now, recall that species diversity can be mathematically calculated using the tool Shannon diversity index. However, we're not given Shannon diversity indexes here in this problem, so we need to use a different approach.

Now what you'll notice in comparing community 1 and community number 2 is that they both have a total of 3 species of birds. And so the species richness between community 1 and community 2 is both 3. Now, if the species richness is the same, then we need to turn to the relative abundance, the proportion of each species in the community. And so what you'll notice is in community number 1, there are 1, 2, 3, 4, 5, 6, 7, 8, 9 of these blue species of bird. Then we also have 2 species of this red bird, and we have one species of this black bird.

So this comes out to a total of 12 birds, and these are the abundances, between them. Now for community number 2, what you'll notice is that there are 1, 2, 3, 4 species of the bluebird. Then we have, 1, 2, 3, 4, 5 species of the red bird. And last but not least, we have 1, 2, 3 species of the black bird. And so what you'll notice is that in both communities, there are a total of 12 birds, but we have different relative abundances for each of the species.

And what you'll notice is that in community number 2, there's a more balance between, you know, the number of individuals in each species. Whereas in community number 1, it's pretty heavily leaning on this bluebird species where the vast majority of the birds are the blue ones. So because community number 2 has a more even balance with its relative abundance, this is really what makes it have a greater species diversity. So we can eliminate any answer option that suggests that community number 1 has a greater species diversity, such as eliminating answer option a and eliminating answer option b. And notice that option c says that community number 2 has a greater species diversity, but it says that it's because it has 3 different species.

And, really, that's not the correct answer because community number 1 also has 3 different species, so that's not correct. And this leaves answer option d, which is the correct answer, which says community number 2 has the greater species diversity because its 3 species have a more equal relative abundance than in community number 1. And so option d is the correct answer to this example problem. That concludes this example, and I'll see you on our next video.

What is Trophic Structure?

This video, we're going to answer the question, what is trophic structure? So trophic structure refers to the feeding relationships between organisms in a community. Essentially, who's eating who in the community. Now, trophic structure can be depicted either as a food chain or as a food web. Now a food chain is a linear sequence of organisms where each is eaten by the next.

Now food chains are quite simple to understand, but they're so simple that they don't always give the full picture of who's eating who in the community. Now on the other hand, a food web is a complex network of multiple interconnected food chains in a community. And so a food web gives a more complete picture of who's eating who in the community, but once again, they can get pretty complex, which can be a disadvantage in and of itself. Now trophic level refers to an organism's position in a food chain or a food web. For example, includes primary producers all the way up to quaternary consumers.

So let's take a quick look at our image down below where on the left-hand side we're showing you a simple food chain where we've got a primary producer in these aquatic plants on the bottom being consumed by the primary consumer, which is this fish. And this fish is being consumed by the secondary consumer, which is this bird. So pretty simple, pretty straightforward. But again, it's so simple that it doesn't give the full picture of who's eating who in the community. Now on the other hand, on the right-hand side, we're showing you a food web, which you'll notice is a network of multiple interconnected food chains, which gives a more complete picture of who's eating who in the community.

But as you can see here, can get pretty complex pretty quickly. Now, the last note that I'll leave you all off with here is that the length of food chains are limited. And so in nature, food chains don't get too long. They don't usually have more than just a small handful of organisms. And this is because of very inefficient energy transfers between trophic levels.

And so on average, only about 10% of the energy stored in one trophic level will actually get transferred up to the next trophic level. And so for example, if we had 1,000 kilograms worth of primary producer, well, that could only support 100 kilograms worth of primary consumer, since 100 is 10% of 1,000. And 100 kilograms of primary consumer could only support 10 kilograms worth of secondary consumer, since 10 is 10% of 100. And so this very inefficient energy transfer is what, again, limits food chains to being relatively short. Now, later in our course, we're going to revisit this idea of the very inefficient energy transfers.

But for now, this here concludes our lesson on what is trophic structure, and I'll see you all in our next video.

Community Structure Example 2

So here we have a pretty straightforward example problem that says fill in the blanks using the options below to complete the food chain, and it says that not all options will be used. So notice that here we have all of our options, and down below we have the food chain with the blanks throughout it. Now recall from our last lesson video that a food chain is a linear sequence of organisms where each is eaten by the next. Now notice that in this food chain, we have these two blue arrows already separating the organisms and both pointing to the right. So, of course, this means that we're going to need the arrow pointing to the right as well, separating our organisms.

So we can go ahead and put that arrow here. And we're not going to need this arrow pointing to the left. So we can scratch that off. So now we have to think, okay. Well, what do grasshoppers eat?

Do they eat mice or a mouse? Do they eat snakes, octopus, or grass? Well, of course, grasshoppers eat grass. So we can put grass here, and that's covered here. Then we have to think, okay, well, what eats a grasshopper?

Is it going to be an octopus, a snake, or a mouse? Well, in this food chain, it's going to be a mouse. We'll be next here. And so we can go ahead and highlight that. And then we have to think, well, what eats the mouse?

Well, in this case, is it going to be an octopus or a snake? And the answer is going to be a snake. Pretty easy, pretty straightforward. So we can highlight snake, and octopus is one that is not going to fit into this food chain, so we can cross it off. So that concludes this example problem, and I'll see you in our next video.

If a forest contains 47,000 kg of plant material, how many kg of tertiary consumers can it support?

• A. 4700 kg.
• B. 47 kg.
• C. 470 kg.
• D. 4.7 kg.

Species Impact on Community Structure

This video, we're going to talk about how certain species have significant impacts on their communities. A foundation species is a species that has really strong community-wide effects. For example, providing habitat and or food to other species in the community due to its really large biomass in the community. An example of a foundation species is kelp in a kelp forest because it has relatively large biomass in that community and really strong community-wide effects as it provides habitat and food for other species in the community. Now, on the other hand, a keystone species is a species that has relatively low biomass, but it plays a key or important ecological role that's disproportionate to its relatively low abundance.

A classic example of a keystone species are bees. This is because bees have relatively low biomass and low abundance in their communities. However, bees play a very key and important ecological role in their communities as they serve as pollinators for many different plants. Lastly, we have ecosystem engineers, which are organisms that influence a community by significantly altering its physical environment. A classic example of an ecosystem engineer is beavers, because they physically alter their environment when they create these dams that can create new habitats for other species in the community when the dams slow down the flow of river water and create wetlands and lakes, for example.

So this here concludes this video, and I'll see you all in our next one.

Community Structure Example 3

So here we have an example problem that says, in 1995, 15 Alberta gray wolves were introduced to Yellowstone National Park. Now their population has expanded to over 100 and they have brought about significant changes. They primarily prey on elk, so the elk population has remained stable since the wolves' introduction. The leftover elk carcasses provide food for ravens, eagles, and bears, and the reduction in elk numbers has allowed more vegetation growth, which provided habitats for birds and beavers, allowing their respective populations to grow. How would you describe Alberta gray wolves in Yellowstone National Park and why?

And we've got these 4 potential answer options down below. Now, clearly, these Alberta gray wolves are having a huge impact on their community as they are initiating all these different changes described here in this problem. Now these Alberta gray wolves are not physically changing their environment, so they're not going to be ecosystem engineers. So we can cross that off. Now, what we can see here is that, if we can eliminate answer option c, we can also eliminate answer option d, which says all of the above.

So now we're between either foundation species or keystone species. Now a foundation and keystone species are both going to have huge impacts on their communities. The difference is that a foundation species is going to have large biomass in the community, whereas keystone species are going to have relatively low biomass in the community. And so what we need to realize here is that only 15 Alberta gray wolves were initially introduced. That's not many at all in this Yellowstone National Park.

And their population only grew to over a 100, which still is not very many wolves. And so these wolves, despite having low biomass and low abundance in their communities, are having a huge impact on their community. And this is what makes these gray wolves a keystone species. So, option b is the correct answer, which says keystone species: they're not abundant, but their presence has a large impact on the community. And so b here is the correct answer to this example problem.

That concludes this example, and I'll see you all in our next video.

Match each of the following species to their role in their respective communities:

Beaver: builds dams in rivers/lakes, significantly altering habitats in the area.

Ochre Sea Star: preys on mussels, preventing them from overpopulating intertidal zones & dominating the habitat.

Red Mangrove Tree: stabilizes shorelines by reducing erosion, provides nurseries for fish in coastal regions.

• A. Beaver: keystone species. Ochre Sea Star: ecosystem engineer. Red Mangrove Tree: foundation species.
• B. Beaver: ecosystem engineer. Ochre Sea Star: foundation species. Red Mangrove Tree: keystone species.
• C. Beaver: foundation species. Ochre Sea Star: ecosystem engineer. Red Mangrove Tree: keystone species.
• D. Beaver: ecosystem engineer. Ochre Sea Star: keystone species. Red Mangrove Tree: foundation species.

Consider an ocean community where corals are the most abundant species, where they "build" new habitats using calcium carbonate deposits, creating a coral reef. Which of the following terms can be applied to corals in this community?

• A. Foundation species only.
• B. Keystone species only.
• C. Ecosystem engineer only.
• D. Both a & b.
• E. Both a & c.
• F. Both b & c.

Bottom-Up & Top-Down Effects on Community Structure

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.

Community Structure Example 4

So here we have an example problem that says, imagine you're a farmer and a surge in the Colorado potato beetle population has taken its toll on your potato crops. Using the information in the food chain below, how could you go about reducing the Colorado potato beetle population while keeping your potato crops healthy? And so notice that again down below we've got this food chain that starts on the far left with the potatoes that you're trying to grow and keep healthy as a farmer. Now these potatoes are being eaten by Colorado potato beetles, which, again, is creating the problem for your potatoes. Now the Colorado potato beetle is eaten by ground beetles, and ground beetles are eaten by northern cardinals.

Now keeping this in mind, option a says remove all the potato crops to exert bottom up control on the Colorado potato beetle population. But as a potato farmer, you don't want to remove all of your potato crops. You're trying to keep your potato crops healthy so that you can sell your potatoes. So removing all of the potato crops is not going to be the best option here. So let's eliminate answer option a.

Now, moving on, option b says, introduce Northern Cardinals to the farm to exert top down control. Now, notice the Northern Cardinals are here. Now, if we introduce northern cardinals, that means its population size will increase, which we can indicate with an up arrow. That means there will be more northern cardinals to eat the ground beetles. That means that the ground beetles will go down.

And if the ground beetles go down, that means that there's less ground beetles to eat the Colorado potato beetle. And that means the Colorado potato beetles will go up and they will continue to eat more and more of your potatoes and it will be a disaster. So answer option b is not going to be the correct answer either. Now notice answer option c says, introduce ground beetles to the farm to exert top down control. Now if we introduce ground beetles, its population size will go up.

Then it will eat more of the Colorado potato beetle, so its population size will go down, and there will be less Colorado potato beetles to eat your potatoes, and so you can grow healthy potato crops. So this means that answer option c is the correct answer to this example problem. So we can go ahead and indicate that's the case. Now just in case, notice option d says, remove ground beetles from the farm to exert top down control on the Colorado potato beetles. Now again, if you remove ground beetles, its population will go down.

There will be less of them to eat the Colorado potato beetle, so its population goes up, and then there will be more Colorado potato beetles to eat your potatoes. And, again, that's not what you want, so option d is not correct. So again, option c is the correct answer to this example, and I'll see you all in our next video.

Which of the following could be characterized as top-down control in a community?

• A. Otters keeping the sea urchin population down by consuming them.
• B. Sea urchin population getting smaller means less food for otters, causing the otter population to shrink.
• C. Sea urchin population expanding because of an abundance of kelp, their primary food source.
• D. Ideal climatic conditions for kelp growth causes their population to increase.

Here’s what students ask on this topic:

What are the key components of community structure in ecology?

The key components of community structure in ecology include species richness, relative abundance, species interactions, and physical aspects of the community. Species richness refers to the number of different species present in a community. Relative abundance measures how many individuals of each species exist relative to others. Species interactions encompass various relationships like predation, competition, and mutualism. Physical aspects include factors such as the size of the area, altitude, and type of vegetation. These components collectively influence ecosystem resilience, biomass production, and overall biodiversity.

How does trophic structure affect energy transfer in an ecosystem?

Trophic structure describes the transfer of energy through different trophic levels in an ecosystem, typically illustrated by food chains and food webs. Energy transfer between trophic levels is highly inefficient, with only about 10% of the energy being passed on to the next level. This inefficiency limits the length of food chains. Trophic levels include producers (e.g., algae), primary consumers (e.g., herbivores), secondary consumers (e.g., small fish), and tertiary consumers (e.g., larger fish or birds). Understanding trophic structure helps in studying energy flow and ecosystem dynamics.

What roles do dominant and keystone species play in community structure?

Dominant species are those that are most abundant or have the highest biomass in a community, significantly influencing its structure and function. For example, mangroves dominate brackish swamp waters. Keystone species, although not necessarily abundant, have a disproportionate impact on their community. Removing a keystone species, like starfish in tide pools, can dramatically alter the community structure. Keystone species often play critical roles in maintaining the balance and diversity of their ecosystems.

What is the difference between primary and secondary succession?

Primary succession occurs in lifeless areas where new substrate is exposed, such as after a volcanic eruption or glacial retreat. It begins with pioneer species like lichen and moss that help create soil, eventually leading to a climax community. Secondary succession happens in areas where a disturbance has occurred but soil and some organisms remain, such as after a fire or deforestation. It involves the re-colonization of the area by new species, leading to a new climax community. Both processes contribute to ecosystem recovery and biodiversity.

How does biogeography influence species diversity in communities?

Biogeography, the study of the distribution of species and ecosystems in geographic space and through geological time, significantly influences species diversity. Latitude affects climate, sunlight, and precipitation, with tropical regions receiving ample sunlight and precipitation, leading to high species diversity. The species-area curve shows that larger areas tend to have more species. The island equilibrium model explains how the balance between species immigration and extinction rates determines species diversity on islands. Larger, closer islands have higher immigration rates and lower extinction rates, resulting in greater species diversity.