Question

Observations of algal species showed that their diversity was higher in pools where there were grazing snails compared to pools without snails. Which one of the following statements best explains this result?

• A. Snails feed preferentially on the more abundant algal species.
• B. Snails avoid feeding on algal species.
• C. Snails feed only on the less abundant algal species.
• D. Snails feed equally on all the algal species irrespective of algal abundance.

Hint:

In ecology, grazers and predators often {increase diversity} by suppressing dominant competitors. Look for options that describe {preferential removal of abundant species}.

Answer 1

The Correct Option is A

The given question is about the diversity of algal species in pools with and without grazing snails. Let's analyze the options to understand why the correct answer is: "Snails feed preferentially on the more abundant algal species." 

1. The observation is that algal diversity is higher in pools with snails. This implies that snails somehow contribute to or maintain higher diversity.
2. We are provided with four options:
   • Snails feed preferentially on the more abundant algal species.
   • Snails avoid feeding on algal species.
   • Snails feed only on the less abundant algal species.
   • Snails feed equally on all the algal species irrespective of algal abundance.
3. Analysis of Each Option:
4. 1. Snails feed preferentially on the more abundant algal species. 
   This feeding pattern could control the dominance of the most abundant species, allowing less abundant species to flourish and leading to higher diversity.
5. 2. Snails avoid feeding on algal species. 
   If snails do not feed on any algae, their presence would not be expected to impact algal diversity.
6. 3. Snails feed only on the less abundant algal species. 
   If snails feed only on less abundant species, those species may become even less common or potentially go extinct, reducing diversity.
7. 4. Snails feed equally on all the algal species irrespective of algal abundance. 
   This would not necessarily lead to higher diversity, as the grazers would not preferentially control dominant species.
8. Thus, the first option is the most suitable explanation. Snails feeding on the more abundant algae prevent a few species from dominating the pool, which helps maintain or increase overall diversity. This aligns with the intermediate disturbance hypothesis, where moderate levels of predation can enhance diversity.

Hence, the correct answer is: Snails feed preferentially on the more abundant algal species.

Answer 2

Step 1: Identify the ecological pattern.
Algal diversity is higher in pools with grazing snails than in pools without snails.
This suggests that grazing is playing a role in maintaining or enhancing species diversity.
Step 2: Recall the concept of top-down control.
Grazers can act as top-down regulators by reducing the dominance of competitively superior species.
When dominant species are suppressed, less competitive species can persist, increasing overall diversity.
Step 3: Analyze option (A).
If snails feed preferentially on the more abundant (dominant) algal species,
they prevent competitive exclusion and allow rarer species to coexist.
This mechanism is well-known to increase species diversity.
Hence, (A) is correct.
Step 4: Analyze other options.
(B) Snails avoiding algae contradicts the premise of grazing effects. Incorrect.
(C) Feeding only on rare species would further reduce diversity. Incorrect.
(D) Equal feeding on all species would not specifically reduce dominance and is less effective at maintaining high diversity. Incorrect.
Step 5: Conclusion.
Higher algal diversity in the presence of snails is best explained by selective grazing on dominant species: \[ \boxed{(A)} \]