According to the intermediate disturbance hypothesis, species richness may be relatively low in a community where disturbances occur very infrequently. The low species richness of undisturbed communities occurs because of
A. a loss of resources from the community
B. competitive exclusion of early-successional species
C. a loss of species that are unable to tolerate disturbances
D. increased predation rates
The Correct Answer and Explanation is:
The correct answer is B. competitive exclusion of early-successional species.
The intermediate disturbance hypothesis suggests that species richness is highest in ecosystems where disturbances occur at moderate frequencies and intensities. This hypothesis contrasts with both ecosystems with very frequent disturbances (which often support few species) and those with infrequent disturbances (which tend to have low species richness).
In communities where disturbances are infrequent, such as in mature or climax communities, species that are well-adapted to stable conditions typically dominate. These species are often competitively superior to other species, especially early-successional species that rely on disturbances to create space for colonization. The result is competitive exclusion, where the dominant species outcompete other species for resources, preventing their establishment and reducing the overall species richness.
Why the other options are incorrect:
- A. A loss of resources from the community: While disturbances might temporarily deplete certain resources, in the context of very infrequent disturbances, resources are typically stable, and the lack of disturbance doesn’t lead to a resource depletion. Instead, resource competition increases as fewer new opportunities for growth arise.
- C. A loss of species that are unable to tolerate disturbances: This option is more relevant in communities with frequent disturbances. Species that are unable to tolerate disturbances tend to be excluded from these environments. In contrast, in undisturbed communities, it’s the dominance of a few competitive species that reduces species richness, not the inability to tolerate disturbances.
- D. Increased predation rates: Predation rates may vary depending on the ecosystem, but they are generally not the main factor driving low species richness in undisturbed communities. Competitive exclusion by dominant species is the more critical factor.
In summary, in ecosystems with infrequent disturbances, competitive exclusion by dominant species reduces opportunities for early-successional species to establish, leading to a lower species richness. This aligns with the intermediate disturbance hypothesis, which posits that moderate disturbance levels allow both competitive and early-successional species to coexist, maximizing species diversity.