Scientists investigated the recovery of coral reefs in a marine reserve after different periods of protection.
They recorded the number of coral species on several recovering reefs where major bleaching events had occurred between 1 and 40 years ago. Each recovering reef was compared with a nearby undisturbed, healthy 'control' reef.
Because different reef locations naturally support different total numbers of species, the scientists calculated the number of coral species on each recovering reef as a percentage of the number of species on its paired control reef.
Explain why the scientists presented their results as percentages of the control reefs rather than raw species counts.
Corals provide three-dimensional structures that act as habitats. Suggest two reasons why an increase in the diversity of coral species on a reef leads to an increase in the diversity of fish species.