17. Which of the following species replacement sequence depicts tolerance model of community succession?
Introduction
Ecological succession is the process by which the species composition within a biological community changes over time, typically following a disturbance or the creation of new habitat167. This dynamic process is fundamental to ecosystem development and restoration. Among the various models that describe how species replace one another during succession, the tolerance model is a key framework. This article explores the question: Which of the following species replacement sequences depicts the tolerance model of community succession? By explaining the tolerance model and comparing it to other succession models, we clarify how to identify the correct sequence and deepen understanding of ecological succession.
Understanding Ecological Succession
Ecological succession can be divided into two main types: primary and secondary succession. Primary succession occurs in environments where no life previously existed, such as on newly formed volcanic islands or after glacier retreat. Secondary succession takes place in areas where a disturbance has removed most living organisms but soil and some biological material remain, such as after forest fires or agricultural abandonment67.
Succession progresses through stages, beginning with pioneer species and culminating in a stable, mature community known as the climax community. The mechanisms driving these changes are explained by several models, including facilitation, tolerance, and inhibition17.
The Three Main Models of Succession
1. Facilitation Model
In the facilitation model, early species modify the environment in ways that make it more suitable for the establishment of later species. For example, pioneer plants break down rock or enrich soil, enabling the arrival of more complex vegetation. Early species are eventually replaced as their own modifications make the environment less suitable for themselves and more favorable for others167.
2. Tolerance Model
The tolerance model is based on the idea that early species neither facilitate nor inhibit the growth and success of later species. Instead, species replacement is determined by differences in life history traits, such as growth rate, resource allocation, and tolerance to competition. All species have an equal chance to establish themselves early in succession, but as competition increases, those that are more tolerant of these conditions become dominant123.
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Key Feature:
Early species do not change the environment to favor or hinder later species. -
Species Replacement:
The sequence of species replacement is driven by their ability to tolerate increasing competition and environmental stress. -
Climax Community:
The climax community is composed of the most tolerant species, which can coexist in a densely populated area235.
3. Inhibition Model
In the inhibition model, early species make the environment less suitable for the establishment of later species by outcompeting them or releasing chemicals that inhibit growth. Replacement only occurs if the inhibiting species are removed by disturbance or death167.
Identifying the Tolerance Model in Species Replacement Sequences
Given a set of species (often labeled A, B, C, D), and arrows indicating replacement (e.g., A → B → C → D), the tolerance model is distinguished by the following:
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No Environmental Modification:
Early species do not modify the environment to benefit or hinder later species. -
Species Replacement Based on Tolerance:
Species replace one another based on their ability to tolerate increasing competition and environmental stress, not because of changes to the environment. -
Sequence Appearance:
The sequence may look similar to that of the facilitation model (A → B → C → D), but the underlying mechanism is different—replacement is due to tolerance, not facilitation235.
In standard diagrams, the tolerance model is often shown as a straightforward sequence where species replace each other over time, with no indication that early species are making the environment more or less suitable for later species.
Comparing the Models: Facilitation, Tolerance, and Inhibition
To clarify how to identify the tolerance model, let’s compare it to the other models:
| Model | Early Species’ Effect on Environment | Species Replacement Mechanism | Example Sequence | Key Feature |
|---|---|---|---|---|
| Facilitation | Makes environment more suitable | Early species facilitate later species | A → B → C → D | Early species change environment for others |
| Tolerance | No effect | Species replace based on tolerance | A → B → C → D | Replacement due to tolerance to competition |
| Inhibition | Makes environment less suitable | Early species inhibit later species | A inhibits B, C | Replacement only if early species are removed |
Note:
The sequence A → B → C → D can represent both facilitation and tolerance models in diagrams, but the underlying mechanism is different. In the tolerance model, replacement is due to species’ tolerance to competition, not environmental modification235.
How to Recognize the Tolerance Model in Practice
When presented with a species replacement sequence, look for the following indicators of the tolerance model:
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No mention of environmental modification:
Early species do not make the environment more or less suitable for later species. -
Replacement based on tolerance:
The sequence is explained by the ability of later species to tolerate increasing competition and environmental stress. -
Equal chance of establishment early on:
All species have an equal chance to establish themselves at the start, but as competition increases, only the most tolerant persist235.
Example: Tolerance Model in Forest Succession
A classic example of the tolerance model is seen in forest succession. After a disturbance, fast-growing pioneer species (such as certain grasses or shrubs) may initially dominate. As the forest develops and competition for light, water, and nutrients increases, slower-growing but more shade-tolerant tree species become dominant. The pioneer species are not eliminated because they made the environment more suitable for the trees, but because the trees are better able to tolerate the increasingly competitive conditions235.
Common Misconceptions
A common misconception is that all species replacement sequences are due to facilitation—early species making the environment better for later species. However, in the tolerance model, replacement is due to differences in species’ tolerance to competition, not environmental modification235.
Another misconception is that the inhibition model is the same as the tolerance model. In inhibition, early species actively make the environment less suitable for later species, whereas in tolerance, early species have no effect on the environment for later species167.
The Importance of Understanding the Tolerance Model
Understanding the tolerance model is essential for predicting how ecosystems will respond to disturbance, management, and environmental change. It helps ecologists and land managers design strategies for restoration and conservation, ensuring that the right species are introduced at the right time to promote healthy ecosystem development7.
Conclusion
The tolerance model of community succession is depicted by a species replacement sequence where early species do not modify the environment to favor or hinder later species. Instead, species replace one another based on their ability to tolerate increasing competition and environmental stress. In standard diagrams, this is often shown as a straightforward sequence (e.g., A → B → C → D), but the key feature is the absence of environmental modification by early species and replacement driven by tolerance to competition235.
Therefore, any species replacement sequence that reflects replacement based on tolerance to competition, without early species making the environment more or less suitable for later species, depicts the tolerance model of community succession.
