Question

Use the discussion of secondary succession in an abandoned agricultural field presented in Section 19.3 to differentiate among the models of facilitation, competition, and tolerance as they apply to the process of succession.

Short answer

In the context of secondary succession in an abandoned agricultural field, explain the differences between facilitation, competition, and tolerance models. The facilitation model emphasizes the role of pioneer species in creating a more suitable environment for subsequent species to grow, such as weeds stabilizing the soil and providing shade. In contrast, the competition model focuses on the struggle for resources, like sunlight, soil nutrients, and water, as plant life becomes more abundant, with only the more competitive species persisting. The tolerance model suggests that a species' ability to tolerate various environmental conditions determines its place in the succession process, with hardier species colonizing the disturbed area earlier and more sensitive species appearing later as conditions improve.

Step-by-step solution

Understand Secondary Succession

Secondary succession is the process of ecological change that occurs in an ecosystem that has been previously established but was disturbed. In the case of an abandoned agricultural field, the initial disturbance was caused by human activity, and now nature is trying to reclaim the area.

Explain Facilitation Model

Facilitation is the model of succession in which early colonizing species (pioneer species) have a positive effect on the establishment and growth of later species. Pioneer species modify the environment, making it more suitable for successive species. In the context of an abandoned agricultural field, pioneer species like weeds can stabilize the soil and provide shade, which helps retain soil moisture and supports the growth of larger plants.

Explain Competition Model

The competition model of succession is based on the idea that as resources become scarce, species must compete with one another for these resources. As succession progresses, the environment becomes more crowded, and only the species that can outcompete others will persist. In the abandoned agricultural field scenario, as plants grow and become more abundant, they compete for resources like sunlight, soil nutrients, and water. Over time, more competitive species displace less competitive species, leading to a change in the ecosystem's overall composition.

Explain Tolerance Model

The tolerance model suggests that the ability of a species to tolerate different environmental conditions determines its place in the succession process. Species that can tolerate a wide range of conditions can colonize a disturbed area earlier, while species with more specific requirements will arrive later when conditions are more suitable for their needs. For example, in an abandoned agricultural field, hardy species that can tolerate poor soil quality might be early colonizers while more sensitive species might come later.

Compare the Models in the Context of Secondary Succession

In the abandoned agricultural field, the facilitation model would emphasize how pioneer species make the environment more suitable for the growth of subsequent species. The competition model would focus on the struggle for resources as the available space and resources diminish with the increase in plant life. Finally, the tolerance model would consider how varying abilities to withstand environmental conditions influence which species can successfully colonize the field at different points in the succession process. By differentiating among the facilitation, competition, and tolerance models, we can better understand the complex process of secondary succession in abandoned agricultural fields and other ecosystems. Each model provides valuable insight into the factors that influence species as they vie for dominance in a changing environment.

Key concepts

Facilitation Model

In the realms of secondary succession, the facilitation model portrays a nurturing starting phase where certain species, known as pioneer species, act as ecosystem engineers. They roll out the welcome mat for other species by altering the environment in positive ways.

Think of these early arrivals like the hardy weeds in an abandoned agricultural field. They take control of the barren soil, preventing erosion with their roots and providing shade that keeps the ground moist. As a result, these conditions encourage new species to settle in the area, as they find the modified environment more habitable. It's like setting the table before a feast; the early species lay the groundwork for a diverse ecological party to follow.

Competition Model

As we move through the stages of secondary succession, the competition model comes into play. This model is akin to a crowded marketplace where everyone is vying for the best produce. In ecological terms, the marketplace is the ecosystem, and the coveted goods are resources: light, water, and nutrients.

Species within the environment must compete for these scarce resources to survive. Over time, in places such as our textbook's abandoned agricultural field, this competition can become intense. Some species emerge as better competitors, able to secure more resources, while others fall behind and eventually fade away. The end result is a survival-of-the-fittest scenario, shaping the community composition based on who can outcompete whom.

Tolerance Model

The tolerance model shifts our focus from the competitive arena to individual species' capabilities. This model suggests that not all species need to be tough competitors; rather, they must simply tolerate the existing conditions. Think of it as a game of endurance.

In the example of an abandoned agricultural field, some pioneers are the rugged types that can endure poor soil. They don't need much to thrive. Later on, as the soil enriches and the environment stabilizes thanks to these tough species, others with more delicate constitutions start to appear. The tolerance model tells us that succession isn't just a battle for resources; it's also about waiting for the right moment when conditions are ripe for success.

Ecological Succession

Underpinning all these models is the broader concept of ecological succession, the gradual and natural process through which ecosystems change and develop over time. Secondary succession, in particular, kicks off in areas where a disturbance has occurred, but life hasn't been wiped out completely.

It's like rebuilding a city after a natural disaster: the foundation is still there, but the structures need to be rebuilt. The abandoned agricultural field is a classic example where new growth stages upon the old, each wave of species modifying the landscape, setting the stage for the next. This constant ebb and flow of life stages creates a dynamic tapestry of biodiversity.

Pioneer Species

The trailblazers of secondary succession are the pioneer species. These hardy organisms don't just survive in the tough conditions of a disturbed environment; they thrive. In our agricultural field, these would be the first weeds and grasses that spring up.

These biological pioneers are ecological innovators. Their presence alone begins to revitalize the soil, adding organic matter as they live and die. Gradually, they create conditions that make it possible for other, more sensitive plants and animals to join in. The contributions of pioneer species are invaluable, serving as the ecological sparks that ignite the fire of succession, leading to a flourishing ecosystem once more.