Question

In western North American grasslands, bison (Bison bison ) and prairie dogs (Cynomys ludovicianus) are considered keystone herbivores. What changes would you predict in the plant community of these grasslands if you set up an experiment in which bison were excluded from some plots, prairie dogs from other plots, and both species from a third set of plots. Fahnestock and Detling (2002) did this experiment for three years and got no vegetation changes in any of the plots. Is this sufficient information to reject the idea that these herbivores are keystone species?

Short answer

No, three years is not enough to definitively reject the keystone species idea; more comprehensive studies are needed.

Step-by-step solution

Understand Keystone Species

Keystone species are organisms that have a disproportionately large effect on their ecosystem relative to their abundance. The presence or absence of keystone species can significantly shape the ecological community and influence the composition and diversity of the plant and animal life therein. In this context, bison and prairie dogs are believed to have significant impacts on grassland plant communities.

Analyze the Experiment Parameters

In the experiment by Fahnestock and Detling (2002), bison and prairie dogs were excluded from various plots on the North American grasslands to observe changes in vegetation. The scientific principle here is to compare plots with and without these herbivores to identify their ecological role as potential keystone species.

Evaluate Experiment Outcomes

After three years, the experiment showed no significant changes in the vegetation of any plots, whether bison, prairie dogs, or both were excluded. This is an important observation as it suggests that their exclusion did not alter the plant community as expected.

Draw Conclusions About Keystone Status

The lack of vegetation changes suggests one of several possibilities: the herbivores may not be keystone species, or the time frame of the experiment was insufficient to detect changes, or other ecological factors could be maintaining plant community stability. Since keystone species typically drive substantial ecological changes, the findings alone might imply a lack of evidence supporting their keystone status.

Assess the Sufficiency of the Information

While the experiment did not detect changes in plant communities, determining keystone status requires comprehensive long-term studies, multiple ecological factors consideration, and replication across different environments. A single short-term study is not sufficient to categorically reject their keystone status.

Key concepts

Ecological Experiments

Ecological experiments are essential tools that help scientists better understand complex relationships within ecosystems. In these experiments, researchers manipulate one or more environmental factors to observe the resulting effects on the system.

In the context of the exercise, the experiment conducted by Fahnestock and Detling involved excluding bison and prairie dogs from specific plots within North American grasslands. This experimental design is known as a manipulation experiment, and it aimed to discern the role of these herbivores as keystone species within their habitat.

Ecological experiments often need to be:

• Long-term: Some ecological processes take time to manifest, and short-term studies might miss crucial changes.
• Replicated: Repeating the experiment in different areas ensures that the results are not unique to one location.
• Comprehensive: Considering multiple factors helps account for other influences that might affect the results.

These components are key to adequately understanding ecological dynamics.

Grassland Ecosystems

Grassland ecosystems are rich natural habitats dominated by grasses and other herbaceous plants. They cover a significant portion of the Earth's landmass and support a diverse range of plant and animal species.

One characteristic of grasslands, like those in North America, is their reliance on certain animal species to shape the environment. Bison and prairie dogs are examples of native species that have historically influenced these ecosystems through grazing and burrowing, respectively.

In these environments:

• Grazing animals, such as bison, impact the plant community by selectively eating certain species, which can promote plant diversity by preventing any single species from dominating.
• Prairie dogs, through their burrowing activities, aerate the soil and create microhabitats that benefit certain plants and other organisms.

Understanding the role of these animals is crucial for managing and conserving grassland ecosystems.

Herbivore Impact

Herbivores play a critical role in ecosystems through their feeding habits and other activities. Their impact on plant communities can be significant, and they often help to shape the landscapes they inhabit.

In the experiment discussed, bison and prairie dogs were expected to influence the grassland vegetation in noticeable ways.

The potential impacts of herbivores include:

• Regulating plant population: By feeding on dominant plant species, herbivores prevent any single species from monopolizing resources, thus encouraging a more diverse plant community.
• Soil and nutrient cycling: Herbivores contribute to nutrient cycling through their waste products, which enrich soil fertility.
• Physical landscape change: The activities of herbivores like prairie dogs can alter the physical landscape, creating habitats for other organisms and influencing plant growth.

Their role can be dynamic and complex, with outcomes varying based on ecological conditions.

Plant Community Dynamics

Plant community dynamics refer to the ever-changing composition and structure of plant populations within an ecosystem. Factors influencing these dynamics include herbivory, competition, climate, and disturbance.

In grassland ecosystems where bison and prairie dogs are present, these dynamics can be heavily influenced by their activities. For instance, grazing might reduce plant height but promote growth in plant species resistant to heavy grazing.

Key aspects of plant community dynamics include:

• Species diversity and abundance: The variety and number of species present can fluctuate due to various environmental pressures.
• Successional stages: Natural disturbances or human activities can initiate succession, where plant communities evolve over time towards stability.
• Interactions with animals: Herbivores and other fauna interact with plants in ways that can alter community structure and function.

Evaluating these dynamics requires examining both biotic factors like the presence of keystone species and abiotic influences such as weather patterns.