Community Structure: Videos & Practice Problems

```html Community structure is defined by species richness, relative abundance, species diversity, and physical attributes. Species diversity, calculated using the Shannon Diversity Index, indicates biodiversity. Trophic structure describes feeding relationships, depicted as food chains or webs, with energy transfer efficiency limiting chain length. Foundation species, keystone species, and ecosystem engineers significantly impact communities. Population cycling is influenced by bottom-up (resource availability) and top-down (predator influence) controls. Higher species diversity enhances community stability and productivity. The Shannon Diversity Index is represented as $h$, with higher values indicating greater diversity. ```

Community Structure

Community structure is a fundamental concept in ecology that describes how a community is organized and its composition. It encompasses four key attributes that help us understand the dynamics within a community.

The first attribute is species richness, which refers to the total number of different species present in a community. This is a straightforward count that provides a basic understanding of biodiversity.

The second attribute is relative abundance, which measures the proportion of each species compared to the total number of individuals in the community. This helps to illustrate how evenly the species are distributed within the community.

Integrating both species richness and relative abundance gives us the concept of species diversity. This can be quantitatively assessed using the Shannon Diversity Index, denoted as H. A higher value of H indicates greater species diversity and a more biodiverse community. While calculating the Shannon Diversity Index is not always necessary, understanding its implications is crucial; a higher index value correlates with increased biodiversity.

The third attribute involves the interactions between organisms, which include competition, exploitation, mutualism, and commensalism. These interactions play a significant role in shaping community dynamics and species relationships.

The fourth attribute consists of physical attributes, which include both biotic and abiotic factors that influence species distribution and community structure. These factors can range from climate and soil type to the presence of other organisms.

To illustrate these concepts, consider two hypothetical communities, both with a species richness of four, including lavender, tulip, marigold, and shasta daisies. However, the relative abundance of these species differs between the two communities. Community 1 exhibits a more even distribution of species, leading to a higher Shannon Diversity Index (H = 1.39) compared to Community 2 (H = 0.98). This indicates that Community 1 is more diverse.

Higher species diversity is beneficial for communities as it often results in increased primary productivity, leading to more vegetation and biomass. Additionally, communities with greater species diversity tend to be more stable, showing resilience to disturbances and recovering more quickly after such events. Thus, fostering higher species diversity is generally advantageous for ecological health.

Community Structure Example 1

In evaluating the species diversity of two bird communities, it is essential to understand that species diversity is a comprehensive measure of biodiversity that combines species richness and relative abundance. Species richness refers to the total number of different species present in a community, while relative abundance indicates the proportion of each species compared to the total number of individuals.

To assess the two communities, we first note that both community 1 and community 2 have the same species richness, with three distinct bird species each. However, the key differentiator lies in their relative abundances. In community 1, the distribution of individuals is heavily skewed towards one species, with 9 bluebirds, 2 red birds, and 1 black bird, totaling 12 birds. This uneven distribution suggests a lower species diversity.

Conversely, community 2 exhibits a more balanced distribution among its species: 4 bluebirds, 5 red birds, and 3 black birds, also totaling 12 birds. This more equitable representation of species indicates a higher species diversity. The Shannon diversity index, while not explicitly calculated here, would reflect this balance, as it accounts for both the number of species and their relative abundances.

Thus, community 2 is determined to have greater species diversity due to its more equal distribution of individuals among the three species, making it a more stable and resilient ecosystem. This conclusion allows us to eliminate options that incorrectly attribute greater diversity to community 1 or misinterpret the role of species richness alone. The correct answer highlights that community 2's diversity stems from its balanced relative abundance among the species.

What is Trophic Structure?

Trophic structure is a fundamental concept in ecology that describes the feeding relationships among organisms within a community, essentially outlining the dynamics of who eats whom. This structure can be represented through two primary models: food chains and food webs. A food chain illustrates a linear sequence of organisms, where each organism is consumed by the next in a straightforward manner. While food chains are easy to understand, they often oversimplify the complex interactions present in an ecosystem.

In contrast, a food web provides a more intricate representation, showcasing a network of interconnected food chains that better reflects the diverse feeding relationships in a community. However, the complexity of food webs can also be a drawback, making them harder to interpret. Each organism's position within these structures is referred to as its trophic level, which ranges from primary producers, such as plants, to quaternary consumers, like apex predators.

For instance, in a simple food chain, aquatic plants (primary producers) are consumed by fish (primary consumers), which are then eaten by birds (secondary consumers). This linear model, while clear, does not capture the full range of interactions that occur in nature. Conversely, a food web illustrates multiple pathways of energy transfer, revealing the interconnectedness of various species.

One critical aspect of trophic structure is the limitation on the length of food chains. Typically, food chains do not extend beyond a few organisms due to the inefficiency of energy transfer between trophic levels. On average, only about 10% of the energy from one trophic level is passed on to the next. For example, if there are 1,000 kilograms of primary producers, they can support only 100 kilograms of primary consumers, and those 100 kilograms can sustain just 10 kilograms of secondary consumers. This significant energy loss at each level restricts the length of food chains and highlights the importance of energy dynamics in ecological systems.

Community Structure Example 2

In ecology, a food chain represents a linear sequence of organisms, illustrating how energy and nutrients flow through an ecosystem. Each organism in the chain serves as a food source for the next, creating a pathway of energy transfer. For instance, in a simple food chain, grass serves as the primary producer, which is consumed by herbivores such as grasshoppers. This relationship can be depicted with an arrow pointing from grass to grasshoppers, indicating that grasshoppers eat grass.

Next in the chain, grasshoppers are preyed upon by secondary consumers, such as mice. This relationship is also represented by an arrow pointing from grasshoppers to mice, showing that mice eat grasshoppers. Finally, mice are consumed by tertiary consumers, such as snakes, completing the food chain with an arrow pointing from mice to snakes.

In this example, the octopus does not fit into the food chain, as it is not part of the sequence involving grass, grasshoppers, mice, and snakes. Understanding these relationships is crucial for grasping the dynamics of ecosystems and the roles different organisms play within them.

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.

Answer: (B)

Species Impact on Community Structure

In ecological communities, certain species play crucial roles that significantly influence their environment and the organisms within it. A foundation species is one that has a substantial impact on its community due to its large biomass, providing essential habitat and food for other species. An exemplary foundation species is kelp in kelp forests, which supports a diverse array of marine life through its extensive growth.

In contrast, a keystone species is characterized by its relatively low biomass but holds a critical ecological role that is disproportionate to its abundance. A prime example of a keystone species is bees, which, despite their small numbers, are vital for pollinating many plants, thereby maintaining the health and diversity of their ecosystems.

Additionally, ecosystem engineers are organisms that significantly modify their physical environment, creating new habitats for other species. Beavers serve as a classic example of ecosystem engineers; their dam-building activities alter water flow, leading to the formation of wetlands and lakes that support various forms of wildlife.

Understanding these roles—foundation species, keystone species, and ecosystem engineers—highlights the interconnectedness of species within ecosystems and the importance of biodiversity in maintaining ecological balance.

Community Structure Example 3

In the context of ecological dynamics, the introduction of Alberta gray wolves to Yellowstone National Park serves as a compelling example of a keystone species. Initially, 15 wolves were introduced in 1995, and their population has since expanded to over 100. Despite this increase, the overall biomass of these wolves remains relatively low compared to other species in the ecosystem.

Keystone species are defined by their significant impact on the ecological community, despite their low abundance. In this case, the gray wolves primarily prey on elk, which has led to a stable elk population. The reduction in elk numbers has facilitated the growth of vegetation, creating new habitats for various species, including birds and beavers. This cascading effect illustrates the wolves' role in maintaining the balance of the ecosystem.

In contrast, foundation species typically have a large biomass and play a crucial role in shaping the community structure through their abundance. Since the gray wolves do not fit this description, they cannot be classified as foundation species. Instead, their ability to influence the ecosystem significantly, despite their relatively small population size, confirms their status as a keystone species.

Thus, the correct classification for Alberta gray wolves in Yellowstone National Park is that they are a keystone species, as their presence has a profound impact on the ecological community, demonstrating the intricate interdependencies within ecosystems.

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.

Answer: (D)

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.

Answer: (E)

Bottom-Up & Top-Down Effects on Community Structure

Understanding community structure involves examining how population sizes fluctuate due to interactions between different species, which can be explained through bottom-up and top-down effects. Bottom-up control refers to population dynamics being influenced by resource availability at lower trophic levels, such as primary producers like plants. In contrast, top-down control occurs when higher trophic levels, such as predators, impact population sizes.

Both types of control can be situational; in some ecosystems, population cycling is predominantly bottom-up, while in others, it is top-down. Often, both influences are present, creating a more complex interaction. For instance, consider a food chain where grass (a primary producer) is consumed by hares (herbivores), which in turn are preyed upon by lynxes (predators). The lynx, being at the top of the food chain, is influenced by the availability of grass, demonstrating bottom-up control. However, the hare's population is affected by both the grass below and the lynx above, making it subject to both top-down and bottom-up influences.

Population cycles can be observed in graphs plotting years against population size, revealing patterns such as those seen in hare and lynx populations, which cycle approximately every ten years. Typically, one might expect a logistic growth curve, where populations plateau at carrying capacity. However, the observed cycling suggests that hare populations can exceed their carrying capacity, leading to a subsequent decline. This decline can be attributed to either predation by lynxes or resource limitations from the grass. Research indicates that hare populations are influenced by a balance of both top-down predation and bottom-up resource availability.

In summary, the dynamics of community structure are shaped by the interplay of bottom-up and top-down effects, with population cycling reflecting the complex relationships between species at different trophic levels. Understanding these interactions is crucial for studying ecological systems and predicting changes in population dynamics.

Community Structure Example 4

In this scenario, a farmer faces a challenge with the Colorado potato beetle, which is adversely affecting potato crops. Understanding the food chain is crucial for finding a solution. The food chain consists of potatoes, which are consumed by Colorado potato beetles. These beetles are preyed upon by ground beetles, and in turn, ground beetles are eaten by northern cardinals.

To address the beetle problem, several options are considered. The first option suggests removing all potato crops to exert bottom-up control on the Colorado potato beetle population. However, this is not viable for a farmer who aims to maintain healthy crops for sale, so this option is eliminated.

The second option proposes introducing northern cardinals to exert top-down control. While this might seem beneficial, increasing the cardinal population would lead to a decrease in ground beetles, which would then allow the Colorado potato beetle population to rise, exacerbating the problem. Thus, this option is also dismissed.

The third option involves introducing ground beetles to the farm. By increasing the ground beetle population, they would consume more Colorado potato beetles, leading to a decrease in their numbers. This would ultimately protect the potato crops, making this option the most effective solution.

The final option suggests removing ground beetles from the farm. This would result in fewer ground beetles to control the Colorado potato beetle population, allowing it to increase and further threaten the potato crops. Therefore, this option is not suitable either.

In conclusion, the best approach for the farmer is to introduce ground beetles, as this top-down control method effectively reduces the Colorado potato beetle population, ensuring the health of the potato crops.

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.

Answer: (A)

Here’s what students ask on this topic:

What is species richness and how does it contribute to community structure?

Species richness refers to the total number of different species present in a community. It is a fundamental component of community structure, as it provides a simple count of species diversity. High species richness often indicates a more complex and stable ecosystem, as a greater variety of species can fulfill different ecological roles, leading to more robust interactions and resilience against disturbances. However, species richness alone does not account for the relative abundance of each species, which is also crucial for understanding community dynamics. Together with relative abundance, species richness contributes to the overall species diversity, which can be quantified using indices like the Shannon Diversity Index. This index helps ecologists assess the biodiversity and health of ecosystems, guiding conservation efforts and ecological research.

How does the Shannon Diversity Index measure species diversity?

The Shannon Diversity Index (H) is a mathematical tool used to quantify species diversity within a community. It considers both species richness (the number of different species) and relative abundance (the proportion of each species). The formula for the Shannon Diversity Index is:

$H=\; -\sum p_{i}ln{p}_i$

where pi is the proportion of individuals belonging to the i-th species. A higher H value indicates greater species diversity and a more even distribution of species. This index is valuable for ecologists to compare biodiversity across different communities and assess the impact of environmental changes or conservation efforts on ecosystem health.

What roles do keystone species play in community structure?

Keystone species play a critical role in maintaining the structure and stability of their ecosystems, despite having relatively low biomass or abundance. Their ecological impact is disproportionate to their numbers, as they influence the diversity and population dynamics of other species. For example, bees are keystone species due to their role as pollinators, supporting plant reproduction and, consequently, the entire food web. The removal of a keystone species can lead to significant changes in community structure, often resulting in reduced biodiversity and ecosystem collapse. Understanding the role of keystone species is essential for conservation efforts, as protecting them can help preserve the overall health and resilience of ecosystems.

What is the difference between a food chain and a food web?

A food chain is a linear sequence of organisms where each organism is eaten by the next one in the chain. It represents a straightforward path of energy flow and nutrient cycling within an ecosystem. However, food chains are often oversimplified and do not capture the complexity of real ecosystems. In contrast, a food web is a more comprehensive representation, consisting of multiple interconnected food chains. It illustrates the complex network of feeding relationships among organisms in a community, providing a more accurate depiction of energy flow and ecological interactions. While food webs offer a detailed view of ecosystem dynamics, they can be complex and challenging to analyze.

How do bottom-up and top-down controls affect community structure?

Bottom-up and top-down controls are mechanisms that influence community structure through trophic interactions. Bottom-up control occurs when the availability of resources, such as nutrients or primary producers, determines the population sizes and dynamics of higher trophic levels. In contrast, top-down control involves predators regulating the populations of their prey, which can cascade down the food chain, affecting lower trophic levels. Both controls can operate simultaneously, with their relative influence varying depending on the ecosystem and specific interactions. Understanding these controls helps ecologists predict changes in community structure and manage ecosystems effectively, especially in response to environmental changes or human interventions.