5. What species took over the desert ecosystem when the hummingbirds were
removed?

Hummingbirds pollinate native cactus. In areas of the Sonoran Desert with few hummingbirds, (5)invasive species such as buffelgrass have taken over the ecosystem. The theory that the balance of ecosystems can rely on one keystone species was first established in 1969 by American zoology professor Robert T. Paine.

D. Any or all of the above could be causing the problem.

Explanation:

Pollution has been said to have adverse effect on living organisms. In this question, a decrease in number of intertidal organisms has been observed. Since a major freeway runs along a stretch of the ocean shore, the most likely cause of the death of the intertidal organisms is the release of any form of toxic waste into the water body.

Oil pollutants causes death of living organisms by hindering the entrance of oxygen into the water body. It makes it difficult for oxygen, which is a requirement for life, to penetrate the water body. Hence, living organisms in the water body will suffocate and die.

Toxic wastes also causes the death of organisms when ingested in excess. Waste water causes deterioration in the oxygen content of the water body. Hence, any or all of the above options can likely cause the reduction in number of the intertidal organisms.