Where to draw the line?

As I mentioned before, ecosystems, not species, are the ideal scale at which environmental concern should be focused. However, this is not as easy as it sounds. While the idea of ecological preservation is to preserve all species in a given ecosystem, is there ever a point where this is impossible, or when a species should be allowed to go extinct for the greater good?
Consider the case of the Rocky Mountain Locust. The Rocky Mountain locust was once one of the most abundant insects in the world. They descended upon the crops of the pioneers in the American West in swarms of billions, devouring all vegetation in their path. Laura Ingalls Wilder, of Little House on The Prairie fame, described such an infestation in her book On The Banks Of Plum Creek. Yet by the beginning of the 20th century, the locusts had all but vanished. They were not intentionally exterminated--pesticides had not been invented yet--but rather their breeding grounds were destroyed when farmers plowed under them.
The loss of the Rocky Mountain Locust was not lamented. No one mourned for it, not the way they mourned for the herds of bison that once inhabited the Great Plains. Except for insect experts--of which there were few at the time-- no one remembered it as anything other than a pest.
Yet the demise of the locust had far-reaching consequences. The Eskimo Curlew, a migratory shorebird, depended on the locusts for food during its northward migrations. Deprived of their main prey and mercilessly pursued by hunters, the curlews declined heavily, and by 1963 they, too, were extinct.
The story of the Rocky Mountain Locust shows that no species is exempt from the attention of conservation. The extinction of anything will affect the ecosystem around it.