A rainforest can lose a species and recover. A reef can unravel from losing coral.

Lose one species in the Amazon and the forest will likely carry on. Lose coral on a reef and the whole neighborhood can fall apart.

The difference comes down to how the living system is built. In the Amazon, one of the most diverse places on Earth, support is widely shared. Woody climbing vines wind around huge trees and thicken into stems that help brace them. Those trees feed plant eaters such as the tapir, a hoofed forest mammal, and the agouti, a small forest rodent, which roam and drop seeds so new growth can sprout elsewhere. Afterward, vast numbers of insects break down the scraps, returning nutrients that keep the soil fertile.

That combination of many species, varied genes within species, and many small systems nested inside one large forest weaves a dense safety net. If one strand breaks, others still carry the load, letting the forest absorb the gap and recover.

A coral reef works differently. Coral is a loom that much of the reef's life is woven around. It provides shelter, hiding places, and nursery areas for thousands of fish, crabs and other crustaceans, and mollusks, and it trades support with fungi and bacteria.

That heavy reliance makes coral what biologists call a keystone organism, meaning much of the reef community relies on it to live. When destructive fishing, pollution, and more acidic seawater damage coral or kill it, its dependents have nowhere else to turn, and the harm spreads through the reef.