The council of Durton, which wants to avoid overly depleting the surrounding forest through exports of lumber, is considering subsidizing the import of certain kinds of wood to offset partly the deforestation. Such a step could actually put the forest in more severe danger, since insects often get into wood and can be introduced as a highly destructive force into habitats where they are non-native. Moreover, in the vast majority of cases in which a lumber-exporting city has become a lumber-importing city, the forest surrounding that city has been wiped out a few years after the switch.
The answer to which of the following would be most useful for evaluating the argument above?