Question

During the process of succession in a community, species that are good colonisers are gradually replaced by species that are good competitors. Which one or more of the following statements is/are consistent with this pattern?

• A. Initially, there is great resource limitation.
• B. Keystone species must establish first to facilitate the later establishment of higher trophic level species.
• C. Trees are the climax stage of terrestrial communities and generally have low competitive ability, but high dispersal ability.
• D. For many taxa, there is a tradeoff between dispersal ability and local competitive ability.

Hint:

In succession-related GATE questions, always look for {life-history tradeoffs}, especially between {dispersal} and {competitive ability}. These tradeoffs drive predictable changes in community composition over time.

Answer 1

The Correct Option is D

The question revolves around the ecological concept of succession, a process where different communities colonize an environment over time. In the initial stages, species that are good colonizers due to high dispersal ability establish themselves. Over time, these species are replaced by competitors that are better adapted to the stable environment created after initial colonization. To determine which statement is consistent with this pattern, let's analyze each option: 

1. Initially, there is great resource limitation.
   This statement refers to the starting conditions of succession. While resource limitation can be a factor initially, it does not directly explain the shift from colonizers to competitors.
2. Keystone species must establish first to facilitate the later establishment of higher trophic level species.
   This statement is not consistent with the basic succession pattern described. Keystone species are crucial in an ecosystem, but this pattern focuses more on colonizers and competitors rather than on the trophic structure.
3. Trees are the climax stage of terrestrial communities and generally have low competitive ability, but high dispersal ability.
   This statement is incorrect. By the climax stage, species typically have high competitive ability rather than high dispersal ability, as they must compete for limited resources in a stable environment.
4. For many taxa, there is a tradeoff between dispersal ability and local competitive ability.
   This statement captures the essence of the succession pattern. Early colonizers typically have high dispersal abilities that allow them to quickly occupy new areas. As succession progresses, species with greater competitive abilities establish themselves as the environment stabilizes and resources become more limited.

The correct answer is that for many taxa, there is a tradeoff between dispersal ability and local competitive ability. This aligns with the ecological understanding of succession where initial colonizers are gradually outcompeted by species that thrive in resource-limited, stable environments.

 

Answer 2

Step 1: Understand the successional pattern.
Ecological succession often begins with early colonisers that are excellent at dispersal and rapid establishment.
Over time, these species are replaced by late-successional species that are superior competitors for local resources.
Step 2: Analyze option (A).
In the early stages of succession, resources such as space, light, and nutrients are usually abundant, not limiting.
Resource limitation typically increases later as biomass accumulates.
Hence, (A) is incorrect.
Step 3: Analyze option (B).
This statement relates to keystone species and trophic facilitation, not directly to the coloniser–competitor replacement pattern.
Hence, (B) is incorrect.
Step 4: Analyze option (C).
Trees, which often dominate climax terrestrial communities, generally have high competitive ability but relatively low dispersal ability.
The statement reverses these traits.
Hence, (C) is incorrect.
Step 5: Analyze option (D).
A well-established ecological principle is the tradeoff between dispersal ability and competitive ability.
Good colonisers disperse widely but are weak competitors, while late-successional species disperse poorly but compete effectively.
This tradeoff directly explains the observed successional pattern.
Hence, (D) is correct.
Step 6: Conclusion.
The replacement of good colonisers by good competitors during succession is best explained by: \[ \boxed{(D)} \]