The factor that would be considered a micro-effect is:
A stand of aspen trees dies and decays, leaving a clearing in the forest.
Micro-effects typically refer to more localized or smaller scale impacts on biodiversity, such as changes within a specific ecosystem or habitat. The death of a single stand of trees and the resulting clearing would have localized effects on the biodiversity in that particular area. In contrast, the other options involve larger scale impacts, such as invasive species introduction, deforestation for suburban development, or a hurricane that impacts larger areas.