Introduction

Ecological succession is the process by which biological communities develop and change over time, especially after disturbances such as fires, logging, or the abandonment of agricultural land. One of the most important mechanisms driving this process is the role of nitrogen-fixing species, which can transform barren or nutrient-poor environments into fertile habitats for a wide range of organisms. This article explores the question: In an abandoned area, when nitrogen-fixing communities arrive first and are later replaced by non-nitrogen fixing species, which mechanism of succession does this scenario best illustrate? The options are facilitation, tolerance, inhibition, and adaptation. By examining each mechanism and reviewing real-world examples, we will clarify why this pattern is most consistent with the facilitation model of succession.

Understanding Ecological Succession

Ecological succession is the gradual process by which the species composition of a community changes over time. It can be primary (starting from bare rock or newly formed land) or secondary (starting after a disturbance where soil is already present). In both cases, the process involves a series of stages, each characterized by different types of plants and animals.

Succession is driven by a combination of biotic and abiotic factors, and several mechanisms have been proposed to explain how and why species replace one another over time. These mechanisms include facilitation, tolerance, inhibition, and adaptation.

The Arrival of Nitrogen-Fixing Communities

In many abandoned or disturbed areas, the first plants to colonize are often nitrogen-fixing species. These plants have a special ability: they form symbiotic relationships with bacteria that can convert atmospheric nitrogen (which is unavailable to most plants) into forms that plants can use, such as ammonia and nitrate. Examples of nitrogen-fixing plants include legumes (such as clover and beans) and certain trees and shrubs (such as alder and locust).

By fixing nitrogen, these early colonists enrich the soil, making it more fertile and hospitable for other plants. As the soil becomes richer in nitrogen and other nutrients, the conditions become more favorable for a wider range of species to establish themselves.

The Replacement by Non-Nitrogen Fixing Species

As succession progresses, the nitrogen-fixing communities are gradually replaced by non-nitrogen fixing species. These later species are often more competitive and better adapted to the improved soil conditions. They may outcompete the nitrogen-fixing pioneers for light, water, and other resources, leading to a shift in the community composition.

This pattern—where early colonizers modify the environment in a way that benefits later species, ultimately leading to their own replacement—is a classic example of facilitation.

The Four Mechanisms of Succession

Let’s examine each of the four mechanisms to determine which best explains the scenario described above.

1. Facilitation

Facilitation is the process by which early species modify the environment in ways that make it more suitable for later species to establish and grow. In the context of nitrogen-fixing communities, their presence increases soil fertility by adding nitrogen, which benefits non-nitrogen fixing species that arrive later. Over time, the improved conditions allow the later species to outcompete and replace the nitrogen-fixing pioneers. This is exactly what happens in the scenario described156.

2. Tolerance

Tolerance is a model of succession where later species are able to grow and persist under the conditions created by earlier species, regardless of whether those conditions are favorable or not. In the tolerance model, the presence of early species does not necessarily make the environment better for later species; instead, later species simply tolerate the existing environment. This is different from the scenario where early species actively improve conditions for later arrivals.

3. Inhibition

Inhibition occurs when early species make the environment less suitable for the establishment of later species. For example, some plants may release chemicals that inhibit the growth of potential competitors, or their dense growth may block light and nutrients. In the case of nitrogen-fixing communities, their effect is generally to improve soil conditions, not to inhibit later species.

4. Adaptation

Adaptation refers to the process by which species evolve traits that allow them to survive and reproduce in specific environments. While adaptation is important for the survival of all species, it is not a mechanism that explains the orderly replacement of one community by another during succession.

Why Facilitation Is the Correct Mechanism

The scenario in which nitrogen-fixing communities arrive first and are later replaced by non-nitrogen fixing species is a textbook example of facilitation. The early species modify the environment—in this case, by increasing soil nitrogen—which allows later species to establish and eventually outcompete the pioneers. This process is well-documented in both natural and human-altered landscapes, such as abandoned farmland, volcanic islands, and glacier forelands156.

Facilitation-driven succession is characterized by relatively ordered, predictable changes in community composition. The early species are not replaced because they are outcompeted in a negative sense, but because they have made the environment suitable for other, often more competitive, species.

Real-World Examples of Facilitation by Nitrogen-Fixing Plants

Red Alder in Pacific Northwest Forests

Red alder (Alnus rubra) is a common nitrogen-fixing tree in the Pacific Northwest of North America. After disturbances such as logging or fire, red alder is often one of the first trees to colonize the area. Its root nodules harbor bacteria that fix atmospheric nitrogen, enriching the soil. As the forest matures, conifers such as Douglas fir and western hemlock, which are not nitrogen fixers, become established and eventually outcompete the alder, leading to a shift in the dominant tree species3.

Coriaria arborea in New Zealand

On Mt. Tarawera in New Zealand, the nitrogen-fixing shrub Coriaria arborea plays a key role in primary succession. Its presence leads to significant increases in soil nitrogen and phosphorus, which in turn facilitate the growth of later successional trees such as Griselinia littoralis. The net effect of Coriaria on succession is facilitative, as it improves soil conditions for other species6.

Cyanobacteria in Glacier Forelands

In glacier forelands, cyanobacteria are among the first organisms to colonize newly exposed ground. They fix nitrogen and contribute to the development of soil organic matter, paving the way for the establishment of mosses, grasses, and eventually shrubs and trees7.

The Broader Importance of Facilitation in Succession

Facilitation is a fundamental process in ecological succession, not only for nitrogen-fixing plants but also for other early colonizers that modify light, temperature, moisture, or soil structure. By creating favorable conditions for later species, early pioneers drive the development of increasingly complex and diverse ecosystems.

Understanding the role of facilitation in succession is important for conservation, restoration, and land management. By recognizing which species act as facilitators, ecologists and land managers can design strategies to accelerate ecosystem recovery and promote biodiversity.

Common Misconceptions About Succession Mechanisms

Some people mistakenly believe that all species replacements during succession are due to competition or inhibition. While competition and inhibition do play roles in certain contexts, the scenario described in this article—where early species improve conditions for later species—is best explained by facilitation.

Conclusion

In an abandoned area, when nitrogen-fixing communities arrive first and are later replaced by non-nitrogen fixing species, this pattern is best explained by the facilitation mechanism of succession. Facilitation occurs when early species modify the environment in ways that benefit later species, allowing them to establish and eventually outcompete the pioneers. This process is essential for the development of complex, stable ecosystems and is well-documented in both natural and human-altered landscapes.

By understanding the role of facilitation and nitrogen-fixing plants in succession, we gain valuable insights into how ecosystems recover from disturbances and how we can support the restoration of healthy, diverse habitats.