Ecologists investigated the recovery of epiphytic plant communities (mosses and lichens) on host oak trees in a replanted woodland following different periods of regeneration.
They recorded the number of epiphytic species on several oak trees in the replanted woodland where intensive logging had ceased between 5 and 60 years ago. Each regenerating oak tree was compared with a nearby healthy, undisturbed oak tree in an ancient woodland, which served as a paired 'control'.
Because oak trees at different locations naturally support different baseline numbers of epiphytic species due to variations in microclimate and bark texture, the ecologists calculated the number of epiphytic species on each recovering tree as a percentage of the species count on its paired control tree.
Explain why the ecologists presented their results as percentages of the control trees rather than using raw species counts.
Epiphytic plants provide complex physical structures on tree bark that act as microhabitats. Suggest two reasons why an increase in the diversity of epiphytic species on a tree trunk leads to an increase in the diversity of invertebrate species.