The Abert's squirrel relies on the Ponderosa for survival. (National Park Service)

The Ponderosa pine serves as a host species for the Abert’s squirrel. This squirrel is ecologically dependent on the Ponderosa, eating its inner bark for food and using the tree as its shelter. As a result, the Ponderosa experiences a decrease in the ability to reproduce, lower seed quality, and less overall growth. 

It is suggested that these squirrels play a large role in the evolution of Ponderosa pines. They may in fact be important agents of natural selection in Ponderosa pine populations. This is because they only attack trees with specific genetic traits. As a result, those trees suffer great reductions in total fitness. In other words, these squirrels choose to attack trees that have weaker, less beneficial genetic traits and prevent them from reproducing. This ensures that the trees with stronger, beneficial genetic traits will survive and reproduce, passing on their traits. In this way, natural selection is choosing the trees with the better genetic traits to survive, pushing the evolution of those specific traits and their development.