Introduction

Ecological succession is the process by which the composition of species in a biological community changes over time, leading to increasingly complex and stable ecosystems. Several models explain how succession unfolds, including the facilitation, inhibition, and tolerance models. The tolerance model, in particular, emphasizes the role of species’ life history traits and their ability to coexist or outcompete others as environments change. This article addresses the question: In the tolerance model of succession, how is the environment modified by early occupants, and how does this affect the recruitment of early and late successional species? By analyzing the options and delving into ecological theory, we clarify the correct answer and provide a comprehensive overview of the tolerance model’s impact on species recruitment.

What Is the Tolerance Model of Succession?

The tolerance model is one of the three classic models of ecological succession, alongside facilitation and inhibition. According to the tolerance model, early successional species neither facilitate nor inhibit the establishment of later species. Instead, the sequence of species replacement is determined by differences in life history traits, such as growth rate, resource allocation, and tolerance to competition or environmental stress123.

In this model, all species have an equal chance to establish themselves in the early stages of succession. As the community develops and competition for resources increases, species that are more tolerant of these conditions—typically K-selected, slow-growing, and long-lived—become dominant. Early successional species, which are often r-selected and fast-growing, are eventually outcompeted and their abundance declines235.

Key Features of the Tolerance Model

• No Environmental Modification by Early Species:
  Early occupants do not significantly alter the environment in ways that either help or hinder the establishment of other species. The environment remains largely unchanged by their presence237.

• Recruitment Based on Life History Traits:
  The ability of a species to establish and persist depends on its tolerance to competition and environmental conditions, not on changes brought about by previous occupants123.

• Species Coexistence and Replacement:
  Over time, species that are better adapted to the increasingly competitive environment become dominant, while less tolerant species are outcompeted and decline in abundance235.

Analyzing the Options

Let’s examine each of the four options to determine which best describes the effect of early occupants on the environment and species recruitment in the tolerance model.

1. Less Suitable for Subsequent Recruitment of Early Successional Species (ESS), but Has Little or No Effect on the Recruitment of Late Successional Species (LSS)

This option suggests that early occupants make the environment less suitable for their own kind but do not affect late successional species. However, in the tolerance model, early occupants do not modify the environment in a way that explicitly favors or disfavors any group. The decline of early successional species is due to increased competition and their lower tolerance to these conditions, not to environmental modification123.

2. Less Suitable for Subsequent Recruitment of ESS and More Suitable for the Recruitment of LSS

This option implies that early occupants make the environment less suitable for early successional species and more suitable for late successional species. This description is more aligned with the facilitation model, where early species modify the environment to benefit later species. In the tolerance model, the environment is not modified to favor any particular group; recruitment is determined by species’ tolerance to competition and environmental stress123.

3. More Suitable for Subsequent Recruitment of ESS and Less Suitable for the Recruitment of LSS

This option suggests that early occupants make the environment more suitable for early successional species and less suitable for late successional species. This is not consistent with any of the classic succession models and does not reflect the dynamics of the tolerance model123.

4. Less Suitable for the Subsequent Recruitment of Both ESS and LSS

This option is also not consistent with the tolerance model. Early occupants do not make the environment less suitable for both early and late successional species. In fact, the tolerance model states that the environment is not significantly modified by early occupants, and recruitment is determined by species’ life history traits and tolerance to competition123.

Clarifying the Correct Interpretation

The tolerance model is unique in that early occupants do not alter the environment in ways that affect the recruitment of other species. Instead, the sequence of species replacement is driven by differences in tolerance to competition and environmental stress. As the community develops, competition increases, and species that are more tolerant of these conditions—typically late successional species—become dominant. Early successional species are outcompeted, but this is due to their lower tolerance to competition, not to environmental modification by previous occupants123.

Therefore, none of the provided options perfectly describe the tolerance model as it is classically defined. However, if forced to choose the closest option based on the premise that early occupants do not modify the environment to favor or disfavor any group, the most accurate interpretation is that the environment is not significantly altered by early occupants, and recruitment is determined by species’ tolerance to competition and environmental stress.

However, if the question is interpreted as asking about the outcome of succession (not the mechanism), the environment becomes less suitable for early successional species due to increased competition, while late successional species persist due to their higher tolerance. But this is a result of the process, not a direct effect of environmental modification by early occupants.

Given the phrasing of the options and the classic definition of the tolerance model, the most accurate answer (if forced to choose) is that early occupants have little or no effect on the environment for later species, but the environment becomes less suitable for early successional species due to increased competition—not due to environmental modification. However, this nuance is not fully captured by any of the options.

If the question is strictly about the tolerance model’s mechanism (not the outcome), then none of the options are fully correct, but option (1) is the closest, as it suggests that the environment becomes less suitable for early successional species (due to competition, not environmental modification) and has little or no effect on late successional species (because the environment is not modified).

However, in most ecological texts, the tolerance model specifically states that early species do not modify the environment in ways that affect later species’ recruitment123.

Summary Table

The Broader Context of Succession Models

Understanding the differences between the facilitation, tolerance, and inhibition models is essential for predicting how ecosystems will respond to disturbance and change. The tolerance model highlights the importance of species’ life history traits and their ability to tolerate competition, rather than environmental modification by early occupants123.

Common Misconceptions

A common misconception is that early occupants always modify the environment to favor later species (facilitation) or to hinder them (inhibition). In the tolerance model, early occupants do not significantly alter the environment, and species replacement is driven by differences in tolerance to competition and environmental stress123.

Conclusion

In the tolerance model of succession, early occupants do not modify the environment in ways that affect the recruitment of other species. The sequence of species replacement is determined by differences in life history traits and tolerance to competition. As the community develops, competition increases, and species that are more tolerant of these conditions—typically late successional species—become dominant. Early successional species are outcompeted, but this is due to their lower tolerance to competition, not to environmental modification by previous occupants123.

Among the options provided, none perfectly capture the tolerance model’s mechanism. However, if forced to choose, option (1) is the closest, as it suggests that the environment becomes less suitable for early successional species (due to competition, not environmental modification) and has little or no effect on late successional species (because the environment is not modified).