* Physical Disturbance: This refers to changes in the physical environment that alter the structure and resources of an ecosystem. It can be caused by natural events like storms, fires, or falling trees, or by human activities like logging or construction.

In this case, the falling oak tree directly alters the physical environment:

* Kills ants: The tree's weight crushes the ants, a direct physical impact.

* Stops grass growth: The fallen tree blocks sunlight and potentially compacts the soil, preventing grass growth.

Let me know if you'd like to explore other types of disturbances or examples!