National parks can be a double-edged sword because

The given blank can be filled with that the crowds of people, which are attracted can disrupt the ecosystem.

Explanation:

The national parks can be regarded as a double-edged sword as the individuals, which visits the park can also harm the park's ecosystem. The main motto of the establishment of the national parks is to provide protection to the species and the ecosystem from the destruction done by human beings.  

Therefore, certain measures are needed to be taken in order to minimize the destruction, like limiting the daily limits of the crowd, or by enhancing the price of the tickets. Safety measures like security should be increased in order to keep an eye on the crowds. Ultimately reminding the people about the importance and privilege attained by the national parks.