It was a deliberate attempt to deprive Native Americans of their historical means of support. You can't besiege a Native American city because they had already been pushed off their historical lands. But if you can eliminate a major source of food for them, it's possibly even more effective.

In general people under-estimate the capability of humanity to eradicate other forms of life. We've managed to eliminates organisms as plentiful as things like rinderpest for example. Large animals are actually easier. Were it not for a last minute change, almost all forms of whales would have been extinct in the 20th century. We've been the most advanced life form on Earth for a long time, possibly even before modern humans arrived. But in the past 1000 years or so we really became the most dominant life form. Our population has exploded and we can pretty much push out any other form of life we choose.