The conclusion is that the pillar corals are heavily dependent on the sunlight in order to survive and flourish.
In warm waters, other organisms compete with them, and the pillar corals are out-competed, so the sunlight is blocked, partially, or totally, so they are not able to function.
Because they are not able to function, the pillar corals are dying out, thus living space for other organisms to take their niche in the ecosystem.
