The bacteria that cause plague, Yersinia pestis, maintain their existence in what type of cycle?

Yersinia pestis, the causative agent of plague, primarily exists in a cycle that involves rodents and fleas. This relationship is key to its transmission and persistence in the environment. Rodents, particularly rats, serve as the primary reservoir for the bacteria. The fleas that infest these rodents act as vectors, transmitting Yersinia pestis between hosts. When fleas bite infected rodents, they acquire the bacteria and can then spread it to other rodents or, in some cases, to humans. This rodent-flea cycle is fundamental for the survival of the bacterium in nature.

Moreover, the other options do not accurately represent the ecological relationships involved with Yersinia pestis. The plant cycle and human-host cycle do not reflect the natural history and transmission dynamics of the plague bacterium, and a predator-prey cycle does not pertain directly to its life cycle, as the primary interactions are between rodents and fleas rather than predators capturing prey. Thus, the cycle involving rodents and fleas is vital for understanding how plague persists and is transmitted in various environments.