Hunter: Hunters alone are blamed for the decline in Greenrock National Forest's deer population over the past ten years. Yet clearly, black bears have also played an important role in this decline. In the past ten years, the forest's protected black bear population has risen sharply, and examination of black bears found dead in the forest during the deer hunting season showed that a number of them had recently fed on deer.
In the hunter's argument, the boldface portion plays which of the following roles?