In the case of rodents, one component of the pruning mechanism may be an alteration in the community’s genetic composition. As rodent numbers spike during the surge, the pool of older, partially immune animals—the community firewall—is diluted by a rapidly expanding group of younger animals who have not yet acquired resistance to Y. pestis. This pool of unprotected young may constitute the biological equivalent of an oil slick; throw a match on it and it bursts into flame.
