Community Dynamics - Online Tutor, Practice Problems & Exam Prep

Disturbances significantly impact community structure and function, influencing species diversity. The intermediate disturbance hypothesis suggests that moderate disturbances foster the highest species diversity by allowing both competitive and less competitive species to coexist. Ecological succession, a gradual process of community change, can be primary (starting from bare rock) or secondary (recovering from disturbances with soil intact). Pioneer species initiate primary succession, while secondary succession occurs more rapidly due to existing soil. Early arriving species can facilitate, tolerate, or inhibit later species, shaping community dynamics.

Disturbances Make Communities Dynamic

This video, we're going to talk about disturbances and how disturbances make communities dynamic. And so first, recall from our previous lesson videos that a disturbance can be defined as a relatively short-lived event that significantly changes and or disrupts the structure and function of a community. And recall that a disturbance can change the availability of resources in the environment, which can create new opportunities for different species in the community. Now the impact level of a disturbance actually depends on the type of disturbance that it is, the frequency of the disturbance, and the intensity of the disturbance. And so we're going to be focusing mainly on the frequency and intensity.

Now the intermediate disturbance hypothesis tells us that it's actually moderate or intermediate levels of disturbances that have just the right frequency and just the right intensity, that actually fosters the most species diversity. And so we can see this down below in the image. So notice that we've got this graph here that has the disturbance impact level on the x-axis and the species diversity on the y-axis. And the data is creating this curve that you can see right here, where with really low disturbance impact levels, meaning that there are very few disturbances and or the disturbances are very mild, this creates a community where the best competitors are going to dominate the community, and weaker competitors are going to be excluded from these communities via the competitive exclusion principle. And so these low disturbance impact levels are not going to foster the greatest species diversity.

They foster very low species diversity, as we can see here with this corresponding image down below, which has only one species of this dominating tree here. Now on the opposite end, with very high disturbance impact levels, meaning that the disturbances are quite frequent and or quite severe, This actually prevents many different species from establishing and surviving in these communities. And really only the best adapted species to these disturbances are able to survive and thrive in these communities, and those tend to be our selected species. And so once again, this high disturbance impact level is not going to foster the greatest species diversity. It fosters relatively low species diversity.

As you can see once again with this corresponding image here, we're only showing you one species of these shrubs that are r-selected and dominating this community. Now right here in the middle with moderate or intermediate levels of disturbances with just the right frequency and just the right intensity, this actually fosters the highest species diversity. And this is because moderate, disturbance impact levels allows for both competitive and less competitive species to coexist. And so notice that here in the corresponding image that we have multiple different species of trees and shrubs and bunny rabbits and deer and birds, indicating that these moderate disturbance impact levels foster the greatest species diversity. So this here concludes our lesson, and we'll be able to apply these concepts and problems moving forward.

So see you all in our next video.

What is the reason that moderate levels of disturbance foster the highest diversity in a community?

• A. It opens up more habitat for less competitive species.
• B. It prevents r-selected organisms from continually thriving, as they would with frequent/severe disturbances.
• C. It prevents the best competitors from dominating, as they would with few/mild disturbances.
• D. All of the above.

Which of the following should lead to an increase in a community's diversity?

• A. Fairly frequent, very severe disturbances.
• B. Very frequent, moderate disturbances.
• C. Fairly frequent, moderate disturbances.
• D. Very infrequent, minor disturbances.

Ecological Succession

In this video, we're going to begin our lesson on ecological succession. Ecological succession is a very gradual and slow process by which a community structure changes over time. It's important to recall from our previous lesson videos that the community structure includes a variety of different aspects and attributes, including species richness, relative abundance, species diversity, all of the interactions in the community, and physical attributes as well. Changes to any one of these aspects would alter the community structure over time. Ecological succession occurs because each existing community actually alters its environment, and the change in the environment creates new opportunities that can allow new species to thrive in the community.

The thriving of new species changes the community structure over time. Theoretically, ecological succession is occurring at all times in all communities. However, it's often a very gradual and slow process where significant changes can take hundreds or thousands of years in some cases. Disturbances can somewhat reset the process of ecological succession, which also accelerates the process. It's important to note that the final and most stable stage of ecological succession is what we call the climax community.

The climax community can be thought of as the end goal or the end result of ecological succession. However, it's important to note that we have "final" in quotes because even the climax community will still undergo ecological succession, but at a significantly reduced rate. It is the most stable stage of ecological succession, but ecological succession still occurs even in the climax community, just at a slower rate. Notice in this image on the left-hand side, we have a community that just consists of these grasses. But over time, ecological succession occurs and the community structure changes, creating this forest that you can see here.

If this forest were the climax community, it would be considered the final and most stable stage of ecological succession. But once again, ecological succession can still be occurring even in the climax community. Moving forward in our course, we're going to talk about two different types of ecological succession: primary ecological succession and secondary ecological succession. I’ll see you in our next video to talk more about that.

Which of the following statements most accurately describes ecological succession?

• A. The population of predators and prey increase & decrease in cycles.
• B. The species that make up the community gradually change over time as the environment changes.
• C. The diversity of a community increases over time.
• D. A stable, unchanging community that changes due to a severe disturbance.

Primary Ecological Succession

This video, we're going to talk about primary ecological succession or just primary succession for short, which is when organisms colonize a currently uninhabited area with no soil, such as, for example, when an area is left completely barren after volcanic activity or after the retreating of glaciers and organisms come and colonize those areas. That would be primary succession. Now the absence of soil is something very important to take note of because this is what truly differentiates primary succession from secondary succession, which we'll get to talk more about in our next video. And also, soil formation is a very slow and gradual process that takes a really long time, and many plants and vegetation require soil in order to come and colonize those areas. And so this is what makes primary succession a much longer process in comparison to secondary succession.

Now, pioneer species is the term that we give the very first species that arrive on the scene and colonize the area during primary succession. And these pioneer species need to be able to withstand very harsh and abiotic conditions since, again, those areas are uninhabited areas. And they do not require any soil as, again, there's no soil there. And these pioneer species can actually help in the soil formation process, which again is very slow and gradual. And so let's take a look at our image down below where you can see, this is a time scale going from left to right.

And so at the very beginning of this primary ecological succession process, notice that the area is just bare rock and has absolutely no soil. And so notice the soil in this image is represented by the dark brown color that you can see over here on the right side of the image. And again, we don't see that soil over here on the left, and so that's something very important to take note of. Now the very first species to arrive on the scene are called pioneer species, and they tend to be mosses and lichens, which can arrive on the scene via wind-blown spores. And over time, the weather, along with the activity of these pioneer species, can create conditions that invite other species to colonize the area, such as small annual plants.

And eventually, the community is referred to as an intermediate community and can acquire grasses and perennials, and eventually, it will get shrubs and shade-intolerant trees, such as pines. And then the final and most stable stage of ecological succession is reached, which, recall from our previous lesson videos, is the climax community. And so the climax community is still going to undergo ecological succession, but at a significantly reduced rate. And so it's considered the final stage of ecological succession, and it can have shade-tolerant trees such as oaks and hickory, for example. Now once again, because this soil formation process here takes such a long time, this is what makes primary ecological succession take several hundred or a thousand of years in some cases, making it a much longer process than secondary ecological succession.

So this here concludes our lesson on primary ecological succession. And in our next lesson video, we'll get to talk about secondary ecological succession. So I'll see you all there.

Secondary Ecological Succession

In this video, we're going to talk about secondary ecological succession or just secondary succession for short, which is when a community recovers after a disturbance that leaves the soil intact, which is critically important to distinguish it from primary succession, which recall has no soil initially. The soil remaining intact here in secondary succession eliminates the very slow, gradual, and time-consuming process of soil formation, which allows secondary succession to occur on a much shorter timescale, where it only takes a few decades up to just a few centuries to reach the climax community in secondary succession. Whereas recall with primary succession, it would take several hundred and even up to a thousand years in some cases to reach the climax community. And also, the soil remaining intact in secondary succession broadens the amount of species that can serve as pioneer species in secondary succession. So let's take a look at our image down below where notice on the far left, we're showing you how primary succession ends with a climax community.

And then notice that we're showing you a disturbance occurring here, a forest fire that wipes out a significant portion of the community. However, the soil is going to remain intact here with secondary succession. And again, the soil in this image is represented as all of this dark brown color that you can see I'm highlighting here. And so again, because the soil remains intact, this broadens the amount of species that can serve as pioneer species. And so they could be, for example, small annual plants or grasses and perennials.

But eventually, it will lead to what we call an intermediate community that can have shrubs and eventually shade intolerant trees such as pines. But then we know that all ecological succession will end in a climax community, which we know is a very stable community where ecological succession is still occurring, but at a significantly reduced rate. And so it can have shade-tolerant trees, such as oaks and hickory, for example. And so this here concludes our lesson on secondary ecological succession, and moving forward, we'll be able to apply these concepts in problems. So I'll see you all there.

Community Dynamics Example 1

So here we have an example problem that says to identify whether each of the following scenarios are examples of primary succession, which we'll abbreviate with the letters "PS", secondary succession, which we'll abbreviate with the letters "SS", a climax community, which we'll abbreviate with the letter "CC", or none of the above, which we'll abbreviate with the letter "X".

Now the first scenario says that after a severe landslide washes away all the soil in a forest, new species colonize the area. Since all of the soil is being washed away and initially there's no soil for the new species to colonize the area, this is what makes this primary succession. So we can put "PS" here for that.

Now the next one says an apex predator population in a river community declines due to a new disease. This one doesn't provide enough information to distinguish if soil is being impacted, and so we can't really determine whether primary or secondary succession is occurring here. Since this disease seems to be only impacting a single species within the community, which we would expect to have multiple different species, and there's no reason to assume that this predator couldn't recover back to its original state after the few individuals die from the disease. With not enough information to conclusively determine that this predator is part of a climax community that has reached its final stage and is in its most stable state, the best option is to classify it as none of the above, with an "X" here.

So we'll go ahead and do that. Now the next one says an ocean community has been stable for many years but still experiences occasional disturbances. The stability of this community indicates that this is a climax community. So we'll put "CC" here for that. Recall that the climax community is somewhat of the final and most stable stage of ecological succession where ecological succession is still occurring but at a significantly reduced rate.

Now the next one here says a volcanic eruption underwater creates a new oceanic island, which takes years for plants to colonize. Because a brand new oceanic island is being created and initially there will be no soil, this makes this primary succession. So we can put "PS" here for that.

Last but not least, the final scenario says a big wildfire kills virtually an entire community, but the soil is left intact allowing for new plant species to grow. Since the soil is left intact here, this makes this secondary succession. So we can put "SS" here for that.

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

Listed below are 4 stages of ecological succession in Glacier Bay, Alaska. However, they are in the wrong order. Select the answer option that puts these events in the correct chronological order.

I. The area is invaded by alder trees, which grow up to 9 meters tall.

II. Small flowers (dryas) begin to dominate the plant community.

III. Over hundreds of years, large trees such as spruce & hemlock become the dominant vegetation.

IV. Glacier ice melts, exposing bare rock with no living organisms. Mosses & liverworts colonize this area.

• A. IV, I, III, II.
• B. II, I, III, IV.
• C. III, I, IV, II.
• D. IV, II, I, III.

Effects of Early-Arriving Species on Late-Arriving Species

This video, we're going to very briefly talk about the effects of early arriving species on late arriving species. In ecological succession, there are only three ways that early arriving species can affect late arriving species, and this is through either facilitation, tolerance, or inhibition.

Now, facilitation, as its name implies, is when early arriving species make conditions more favorable for certain later species, somewhat promoting or facilitating the arrival of these later species into the community. Notice down below in our image for facilitation, this species right here is like, "Let me help," and is promoting and facilitating these other species here to arrive into the community.

Tolerance is when the existing species do not affect the arrival of certain later species. They neither promote nor inhibit the arrival of these other species. So notice that down below in our image for tolerance, this species here is just like "I don't care" and is neither promoting nor inhibiting these other species' arrival into the community.

Last but not least, inhibition, as its name implies, is when the presence of certain species inhibits the establishment or regrowth of another species. In our image down below, notice that this species is like "back off" and has a "not welcome" sign here.

So really, that concludes this video, and we'll be able to apply these concepts and problems moving forward. So see you all in the next one.

Match the following terms to each of the scenarios below:

◆ Facilitation ◆ Tolerance ◆ Inhibition

1. Large trees create large, shaded areas, making it difficult for photosynthetic organisms to thrive.

2. Presence of alder plants increases nitrogen content in soil, allowing willow & poplar seedlings to thrive.

3. The early growth of algae in a community has no significant impact on the future presence of birds.

• A. 1 – facilitation, 2 – tolerance, 3 – inhibition.
• B. 1 – tolerance, 2 – facilitation, 3 – inhibition.
• C. 1 – inhibition, 2 – tolerance, 3 – facilitation.
• D. 1 – inhibition, 2 – facilitation, 3 – tolerance.