Certain types of finished wood are coated in toxic termite repellent. Some types of termite, however, have developed a resistance to the toxin, and they retain the toxin in their bodies after ingesting the wood. When certain birds, such as sparrows, ingest the toxified termites, they die. Thus, by using toxic termite repellent, a homeowner puts the entire neighborhood sparrow population at risk.
Which of the following would be most useful to determine in order to evaluate the argument?