Question

An example of the needs of an ecosystem being at odds with those of humans is _____.

Answer 1

The answer would be your third choice 
"the extermination of the prairie dog pest"  

Answer 2

The correct answer is - limiting livestock to prevent overgrazing.

In the past few centuries, the humans have been founding it very hard to maintain a balance between their own interest, and the well being of the ecosystems, the nature.

There has been some examples though, that the two can and should coexist, and some of those nice examples is the limitation of the livestock in accordance to the size of the area, as well as with the food resources for it. By limiting the number of the livestock, the ecosystems have the opportunity to be in healthy condition, as there will be no danger of overgrazing, thus destruction of the basis of it.