Grazing by large herbivores can increase plant diversity by which of the following mechanisms? (i) Reducing abundance of dominant plants and favouring rare species
(ii) Accelerating rates of nutrient cycling in the ecosystem
(iii) Promoting photo-respiration by increasing ambient $CO_2$ concentration
(iv) Decreasing stomatal conductance which promotes biomass production
Grazing Mechanisms and Plant Diversity
Large herbivores significantly impact plant communities. Grazing influences plant diversity through several ecological mechanisms. We need to evaluate the given statements to understand how grazing increases plant diversity.
Analysis of Grazing Impact Statements
- Statement (i): Reducing abundance of dominant plants and favouring rare species. This is a key effect of grazing. By consuming biomass, herbivores prevent overly competitive, dominant plant species from monopolizing resources (like light and nutrients). This selective pressure allows less competitive, often rarer, species to establish and grow, thus increasing overall plant diversity.
- Statement (ii): Accelerating rates of nutrient cycling in the ecosystem. Grazing animals contribute to faster nutrient cycling. They consume plant tissues, and their waste products (urine and feces) return essential nutrients to the soil rapidly. This enhanced nutrient availability can support a wider range of plant species with different nutrient requirements, contributing to diversity.
- Statement (iii): Promoting photo-respiration by increasing ambient $CO_2$ concentration. Grazing does not directly increase ambient $CO_2$ levels in a way that significantly promotes photo-respiration. Photo-respiration is an internal plant process influenced by factors like temperature and gas concentrations within the leaf, not primarily driven by grazing intensity on external $CO_2$. This statement is incorrect.
- Statement (iv): Decreasing stomatal conductance which promotes biomass production. Grazing involves the removal of plant biomass, which generally reduces total biomass, not promotes it. While plants might respond physiologically to grazing, the primary effect is biomass reduction. Decreased stomatal conductance under stress can conserve water but doesn't typically lead to increased overall biomass production in the grazed context and isn't a mechanism for increasing diversity via grazing. This statement is incorrect.
Conclusion on Grazing Mechanisms
Based on the analysis, statements (i) and (ii) accurately describe mechanisms by which grazing by large herbivores can increase plant diversity. Statements (iii) and (iv) describe incorrect or unrelated effects.
Therefore, the correct mechanisms are (i) and (ii).
Important Questions from Community interactions
- A native species of butterfly at one site in North America used to feed on plant species P. In the 1960s, an exotic plant species Q was introduced at this site. The butterfly switched to feeding on Q, so much so that it was no longer found feeding on P. When the plant species Q was removed from the site in the 1990s, the butterfly species went locally extinct. Later, a population of the same butterfly species from elsewhere, that still fed on plant P, came and occupied the same site. Which one or more of the processes listed below has/have occurred here?
- Which one of the following is a result of antagonistic coevolution?
- 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?
- Which one of the following ecological processes best explains the observation thatseedling establishment increases with distance from the parent tree in a forest?
- Depending on soil nutrient availability, which one or more of the following interaction(s) can occur between soil mycorrhizal fungi and plants?
