21. How can the removal of a top predator lead to a trophic cascade? A) By increasing the biodiversity of the ecosystem (B) By causing an increase in the population of herbivores, which then overconsume vegetation C) By decreasing the amount of sunlight available to plants D) By reducing the amount of oxygen in the water

The answer is (B).<br /><br />Here's why:<br /><br />A **trophic cascade** occurs when changes at one trophic level (e.g., predators) cascade down to impact other levels. Removing a top predator means its prey (often herbivores) will face less predation pressure. This allows the herbivore population to grow. If the herbivore population grows significantly, they can overgraze and significantly reduce or eliminate plant populations, impacting the entire ecosystem.<br /><br />Why the other answers are incorrect:<br /><br />* **(A):** While trophic cascades *can* sometimes influence biodiversity, the primary effect is the described cascading impact on populations, not a direct increase in biodiversity. In fact, overgrazing by unchecked herbivores can *decrease* biodiversity.<br />* **(C):** Predator removal doesn't directly affect sunlight availability.<br />* **(D):** Predator removal doesn't directly affect oxygen levels in aquatic systems. While decomposition of excess dead plant matter (due to overgrazing) *could* lead to localized oxygen depletion in water, this is a secondary, indirect effect, not the primary mechanism of a trophic cascade.<br />