Community Ecology Practice Test

In a temperate pond, larval dragonflies consume mosquito larvae. When dragonflies are experimentally removed from several enclosures, mosquito larvae density increases and grazing on algae by mosquito larvae increases, causing algae biomass to decrease compared with control enclosures. No other species are added or removed, and abiotic conditions are similar across enclosures. Which interaction best explains the initial decrease in mosquito larvae in control enclosures?

In a kelp forest, sea urchins feed on kelp holdfasts, reducing kelp density. In areas where sea otters are common, urchin density is low and kelp density is high. In nearby areas without otters, urchin density is high and kelp density is low. Abiotic conditions are similar across areas. Which interaction best explains the difference in kelp density between areas?