Community Dynamics: Videos & Practice Problems

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

Disturbances play a crucial role in shaping ecological communities, acting as short-lived events that significantly alter the structure and function of these systems. A disturbance can change the availability of resources, creating new opportunities for various species to thrive. The impact of a disturbance is influenced by its type, frequency, and intensity, with a particular focus on the latter two factors.

The intermediate disturbance hypothesis posits that moderate levels of disturbance, characterized by optimal frequency and intensity, promote the highest species diversity within a community. This relationship can be visualized in a graph where the x-axis represents disturbance impact level and the y-axis indicates species diversity. The resulting curve illustrates that low disturbance levels lead to dominance by the best competitors, resulting in low species diversity due to the competitive exclusion principle. In such scenarios, only a few species, often the most competitive, thrive, as seen in communities with minimal disturbances.

Conversely, high disturbance levels, marked by frequent or severe events, hinder the establishment and survival of many species. In these environments, only the most adapted species can endure, leading to low species diversity as well. This is evident in communities where only a single species dominates, reflecting the challenges posed by extreme disturbances.

In contrast, moderate disturbance levels create a balanced environment where both competitive and less competitive species can coexist. This coexistence fosters a rich diversity of life, as illustrated by communities that support various species of trees, shrubs, and wildlife. Thus, understanding the dynamics of disturbances is essential for appreciating how they contribute to biodiversity and ecological resilience.

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.

Answer: (D)

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.

Answer: (C)

Ecological Succession

Ecological succession is a gradual process through which the structure of a community changes over time. This community structure encompasses various elements such as species richness, relative abundance, species diversity, and the interactions among species, as well as physical attributes of the environment. Changes in any of these factors can lead to alterations in the community structure.

The process of ecological succession is driven by the fact that existing communities modify their environment, creating new opportunities for different species to thrive. As new species establish themselves, they further influence the community structure, leading to ongoing changes. Although ecological succession is a continuous process occurring in all communities, it typically unfolds slowly, often taking hundreds or thousands of years to manifest significant changes. However, disturbances—such as natural disasters or human activities—can reset and accelerate this process.

The climax community represents the final and most stable stage of ecological succession. It is characterized by a relatively stable structure, but it is essential to understand that even in this stage, ecological succession continues, albeit at a much slower rate. For example, a community that starts with grasses may evolve into a forest over time, with the forest potentially serving as the climax community. Despite its stability, the forest will still experience gradual changes.

In the study of ecological succession, two primary types are recognized: primary ecological succession and secondary ecological succession. Each type has distinct characteristics and processes that contribute to the overall understanding of how ecosystems develop and change over time.

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.

Answer: (B)

Primary Ecological Succession

Primary ecological succession, often referred to simply as primary succession, occurs when organisms colonize an area that is devoid of soil, such as regions left barren by volcanic eruptions or glacial retreats. This process is distinct from secondary succession, which takes place in areas where soil is already present. The absence of soil is a critical factor that influences the duration and nature of primary succession, making it a much slower process compared to secondary succession.

At the onset of primary succession, the first organisms to inhabit the barren landscape are known as pioneer species. These species, which typically include mosses and lichens, are specially adapted to survive in harsh, abiotic conditions without the presence of soil. They play a vital role in the gradual formation of soil, which is essential for the establishment of more complex plant communities.

The process of soil formation is slow and can take hundreds to thousands of years. Initially, the landscape consists of bare rock, as illustrated in time scale diagrams where soil is represented by darker colors. Over time, the weathering of rock and the biological activity of pioneer species contribute to the development of soil, allowing for the colonization of small annual plants. As these plants thrive, they create conditions that support the growth of more diverse species, leading to the establishment of an intermediate community characterized by grasses and perennials.

As succession progresses, the community evolves further, eventually supporting shrubs and shade-intolerant trees, such as pines. The final stage of this ecological process is known as the climax community, which is a stable ecosystem that can include shade-tolerant trees like oaks and hickories. Although the climax community continues to undergo ecological changes, these occur at a significantly reduced rate, marking the culmination of primary succession.

In summary, primary ecological succession is a lengthy process that begins with pioneer species colonizing barren landscapes, leading to soil formation and the gradual establishment of diverse plant communities, ultimately resulting in a stable climax community.

Secondary Ecological Succession

Secondary ecological succession refers to the process by which a community recovers after a disturbance while retaining its soil. This is a crucial distinction from primary succession, which begins in lifeless areas without soil. The intact soil in secondary succession allows for a much faster recovery, typically taking only a few decades to a few centuries to reach a climax community, compared to the hundreds or even thousands of years required in primary succession.

In secondary succession, the presence of soil enables a wider variety of species to act as pioneer species. These can include small annual plants, grasses, and perennials, which are the first to colonize the disturbed area. As succession progresses, these pioneer species give way to an intermediate community characterized by shrubs and shade-intolerant trees, such as pines. Ultimately, the process culminates in a climax community, which is a stable ecosystem where ecological changes continue at a much-reduced rate. This climax community may include shade-tolerant trees like oaks and hickories.

Understanding secondary succession is essential for applying ecological concepts to real-world scenarios, particularly in the context of ecosystem recovery and management following disturbances such as forest fires.

Community Dynamics Example 1

In ecological studies, understanding the different types of succession is crucial for grasping how ecosystems develop and change over time. Succession can be categorized into primary succession, secondary succession, climax communities, and scenarios that do not fit these categories.

Primary succession occurs in environments where no soil exists, such as after a severe landslide that washes away all soil in a forest. In this case, new species colonize the area, indicating the beginning of a new ecological community. This process is characterized by the gradual establishment of life in a barren landscape.

Secondary succession, on the other hand, takes place in areas where soil remains intact after a disturbance, such as a wildfire that destroys a community but leaves the soil unharmed. This allows for the rapid regrowth of plant species, as the existing soil provides a foundation for new life.

A climax community represents the final and most stable stage of ecological succession. It is characterized by a stable ecosystem that can withstand occasional disturbances while maintaining its structure and function. For example, an ocean community that has been stable for many years, despite experiencing occasional disturbances, exemplifies a climax community.

In some scenarios, there may not be enough information to classify the type of succession. For instance, if an apex predator population declines due to a disease, it may not be clear whether this situation indicates primary or secondary succession, or if it relates to a climax community. In such cases, the scenario may be classified as none of the above.

Overall, recognizing these distinctions helps in understanding the dynamics of ecosystems and the processes that lead to their development and stability.

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.

Answer: (D)

Effects of Early-Arriving Species on Late-Arriving Species

In ecological succession, the interactions between early arriving species and late arriving species can significantly influence community dynamics. These interactions can be categorized into three main types: facilitation, tolerance, and inhibition.

Facilitation occurs when early species create conditions that are more favorable for the establishment of later species. This process promotes the arrival of these subsequent species into the community. For example, certain plants may improve soil quality or provide shade, making it easier for other species to thrive.

Tolerance describes a scenario where existing species neither promote nor inhibit the arrival of later species. In this case, the presence of early species does not affect the establishment of new species, allowing them to coexist without direct interaction. This can lead to a diverse community where various species can thrive independently.

Inhibition is the opposite of facilitation, where the presence of certain species hinders the establishment or regrowth of others. This can occur through competition for resources, allelopathy (the release of chemicals that inhibit other species), or physical barriers. In this scenario, early species may dominate the area, preventing later species from establishing themselves.

Understanding these interactions is crucial for studying ecological succession and the development of ecosystems, as they highlight the complex relationships that shape biodiversity and community structure.

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.

Answer: (D)

Here’s what students ask on this topic:

What is the intermediate disturbance hypothesis in community dynamics?

The intermediate disturbance hypothesis suggests that moderate levels of disturbance foster the highest species diversity within a community. This is because intermediate disturbances create conditions that allow both competitive and less competitive species to coexist. Low disturbance levels lead to dominance by the best competitors, reducing diversity, while high disturbance levels prevent many species from establishing, also reducing diversity. Therefore, moderate disturbances strike a balance, promoting a diverse range of species.

How does primary ecological succession differ from secondary ecological succession?

Primary ecological succession occurs in an area with no initial soil, such as after volcanic activity or glacier retreat. It begins with pioneer species like mosses and lichens that can survive harsh conditions and contribute to soil formation. This process is slow, taking hundreds to thousands of years. In contrast, secondary ecological succession happens in areas where soil remains intact after a disturbance, such as a forest fire. This allows for a quicker recovery, taking only decades to a few centuries, as the existing soil supports a broader range of pioneer species.

What role do pioneer species play in primary ecological succession?

Pioneer species are the first organisms to colonize a barren area during primary ecological succession. They can withstand harsh, abiotic conditions and do not require soil to grow. Examples include mosses and lichens. These species contribute to soil formation by breaking down rock and accumulating organic material, creating conditions that allow other species to establish. Over time, this leads to a more complex community structure, eventually reaching a climax community.

What are the effects of early arriving species on late arriving species in ecological succession?

Early arriving species can affect late arriving species in three ways: facilitation, tolerance, and inhibition. Facilitation occurs when early species make conditions more favorable for later species, promoting their arrival. Tolerance means early species neither help nor hinder the arrival of later species. Inhibition happens when early species prevent the establishment or growth of later species. These interactions shape the community dynamics and influence the progression of ecological succession.

What is a climax community in ecological succession?

A climax community is the final and most stable stage of ecological succession. It represents a relatively stable ecosystem where species composition remains relatively constant over time. Although ecological succession continues to occur, it does so at a significantly reduced rate. Climax communities are characterized by mature, shade-tolerant species, such as oaks and hickories in forests. They provide a stable environment that supports a diverse range of species interactions and ecological processes.