A micro-effect on biodiversity typically refers to a localized change that impacts a specific ecosystem or population rather than broader, systemic changes.
In this case, the factor that would be considered a micro-effect is: A stand of aspen trees dies and decays, leaving a clearing in the forest.
This event would primarily affect the local ecosystem where the aspen trees were, rather than causing widespread changes to biodiversity at a larger scale, such as an invasive species introduction or a hurricane event.