An oak tree falls in a forest. it kills some ants and stops grass from growing below it. what type of disturbance is this?

A Small-scale biological disturbance.

Explanation:

In ecology a disturbance is an event that causes a change in the environment. There are small-scale disturbances and high-magnitude disturbances. Small-scale disturbances are typically driven by biological processes -for example the fall of a tree- while high magnitude disturbances are typically driven by physical processes -for example, storms-. Small-scale disturbances as single-tree blowdowns are very important  to create and maintain diversity and heterogeneity in the forest.