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
- Certain species of ibex and weevils depend on the same food source. A study finds that the removal of ibex results in an increase in the number of weevils, but the removal of weevils does not affect the ibex. Which one of the following options best describes this interaction?
- Fungus S infects elm trees and fungus T infects chestnut trees, and both kill their hosts within a year of infection. A forest with equal density of elm and chestnut trees is colonized by both pathogenic fungi. A 100 years later, the elm tree population has declined and the chestnut trees have become extinct. Which one of the following mechanisms would NOT contribute to this outcome?
- 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?
