Well, since there is little genetic variation, all the animals in the population are fairly similar, from a purely physical standpoint. Therefore, if there was some new form of whale lice that kills northern right whales, then presumably at least nearly 1/3 of the population would be vulnerable to this new form of whale lice. This is over simplified, but in essence a huge portion of the population is much more vulnerable to a single threat due to common disease resistances, or the lack thereof.
