To begin with, while the passage states that the decline may be caused by insects, such as cedar bark beetles. The lecture argues that healthy cedars are not easily attacked by beetles. There are powerful chemicals in their leaves which are poisonous to the insects. Those which are killed by insects are sick when it is attacked. So, the decline is not caused by insects.
In addition, the author says that the brown bears may be responsible for the decrease, for they may eat the tree bark