Problem 7 - Campbell Biology

Question

Application/Analysis Which of the following could qualify as a top-down control on a grassland community? (A) limitation of plant biomass by rainfall amount (B) influence of temperature on competition among plants (C) influence of soil nutrients on the abundance of grasses versus wildflowers (D) effect of grazing intensity by bison on plant species diversity

Step-by-Step Solution

Answer

Option D

Step 1: Understand Top-Down Control

Top-down control refers to effects exerted by higher trophic levels (e.g., predators or grazers) on the abundance and diversity of lower trophic levels (e.g., plants or prey).

Step 2: Analyze Option A

Option A states the limitation of plant biomass by rainfall amount. This describes a bottom-up control by an abiotic factor, not a top-down control.

Step 3: Analyze Option B

Option B mentions the influence of temperature on competition among plants. This is another example of bottom-up control by an abiotic factor, not a top-down control.

Step 4: Analyze Option C

Option C discusses the influence of soil nutrients on the abundance of grasses versus wildflowers. This too is an example of bottom-up control by an abiotic factor, not top-down control.

Step 5: Analyze Option D

Option D describes the effect of grazing intensity by bison on plant species diversity. This is an example of top-down control where a higher trophic level (bison) influences the diversity of lower trophic levels (plants).

Step 6: Determine the Correct Answer

Based on the analysis, option D is the only one that qualifies as a top-down control in the grassland community.

Key Concepts

Trophic Levels

Trophic levels represent the different stages in a food chain. Each level consists of organisms that share similar roles in the flow of energy through an ecosystem. The primary levels are:

• Producers: Typically plants that produce energy through photosynthesis.
• Primary Consumers: Herbivores that eat producers.
• Secondary Consumers: Carnivores that eat herbivores.
• Tertiary Consumers: Top predators that eat secondary consumers.

Top-down control occurs when higher trophic levels (like carnivores or herbivores) regulate the population and community structure of lower trophic levels (like plants or herbivores). For instance, predation by a lion (tertiary consumer) reduces the number of herbivores (secondary consumers), which in turn affects plant biomass (producers).

Grassland Community

A grassland community is a unique ecosystem dominated by grasses, but also includes other plant species, herbivores, predators, and various microorganisms. Grasslands are typically found in regions with moderate rainfall and can be separated into different types, such as:

• Prairies: Found in North America, characterized by tall grasses.
• Steppes: Found in Europe and Asia, characterized by shorter grasses.
• Savannas: Found in tropical regions, characterized by a mix of grasses and scattered trees.

In grasslands, top-down control can significantly influence the community structure. For example, bison grazing can control the diversity and abundance of plant species, which indirectly affects the entire ecosystem.

Ecological Interactions

Ecological interactions are the relationships between different organisms within an ecosystem. These interactions can be classified into several types:

• Predation: One organism hunts and consumes another.
• Competition: Organisms compete for the same resources (e.g., food, habitat).
• Mutualism: Both organisms benefit from the interaction.
• Commensalism: One organism benefits, and the other is neither helped nor harmed.

Top-down control in ecological interactions often involves predators or grazers regulating the populations of their prey or the plants they eat. In our example, the grazing of bison reduces plant density and diversity, impacting other organisms that depend on those plants.

Grazing and Plant Diversity

Grazing by herbivores can significantly impact plant diversity in an ecosystem. Here are some ways grazing affects plant communities:

• Selective Grazing: Herbivores often prefer specific plant species, which can lead to those plants declining and less-preferred species becoming more abundant.
• Plant Resilience: Some plants may evolve defenses against grazing, such as thorns or toxic chemicals, increasing their survival chances.
• Seed Dispersal: Grazing can facilitate the spread of seeds through the digestive systems of herbivores.

In a grassland community, intense grazing by bison (as mentioned in Option D of the exercise) can lead to a reduction in plant species diversity. However, moderate grazing may actually promote diversity by preventing any single species from becoming too dominant, thus maintaining a balance.