The diagram shows part of the food web in Yellowstone National Park. Humans
 began removing gray wolves from the park in the early 1900s. In 1995, gray wolves
 were reintroduced to the park.
 Beaver
 Coyote
 Aspen
 Gray wolf
 Willow
 Elk
 Grasses.
 Which two populations are most likely to decrease after gray wolves are added to this food web?