On a trip to the beach, Jacob noticed many signs that read "Keep off dunes." His mother told him that walking or digging on the dunes makes them vulnerable to erosion. What other human activity would also contribute to the erosion of the sand dunes?

Almost any human interaction. One main cause: Construction.

Explanation:

To build anything, the construction area must first be cleared. In the case of a pier being built, the dunes would have to be disrupted in order to burrow down to bedrock to provide adequate support for the pier.