Ecological effects of a plague epizootic on the activities of rodents inhabiting caves at Lava Beds National Monument, California.

Abstract

In 1972–1973 an epizootic of sylvatic plague, caused by Yersinia pestis , occurred in rodents inhabiting lava caves at Lava Beds National Monument, California. The epizootic involved Neotoma cinerea and Peromyscus maniculatus . Comprehensive epizootiological surveys during and after the epizootic elucidated the natural history of these rodents in the cave habitat. This information was used to develop criteria for understanding plague ecology in lava caves. Neotoma cinerea , the Bushy-tailed Woodrat, successfully occupies the dark zone of caves through its ability to establish scent trails with material from mid-ventral sebaceous glands. Information is presented on use, location, kind, and composition of the den, composition of the nest, composition of the food caches, location of urine and fecal stations, and behavior of woodrats in the cave habitat. Vast accumulations of material at certain den sites indicate occupation of these dens by many generations of woodrats; these dens are preferred over smaller, more recently built dens. Peromyscus maniculatus , the Deer Mouse, inhabits both the twilight zone and dark zone in caves, whereas P. crinitus , the Canyon Mouse, occupies only the twilight zone. It is suggested that P. maniculatus establishes scent trails with material from mid-ventral sebaceous glands, enabling this species to dwell in the dark zone. The absence of these glands in P. crinitus apparently prevents this species from penetrating the dark zone. Peromyscus maniculatus was found modifying and using abandoned woodrat dens as home sites. Preference of N. cinerea for old, established dens and predilection of P. maniculatus for use of abandoned dens would enhance the spread of a plague epizootic through contact with infected fleas living in nests. The role of these rodents in the trophic economy of lava caves is discussed. Neotoma cinerea makes the greatest contribution to this economy, with the woodrat and materials that comprise its den supporting directly and indirectly most of the assemblage of invertebrates found in these lava caves. Mortality from a plague epizootic, therefore, drastically alters the inflow of materials into the dens, resulting in changes in both the materials and the invertebrate assemblage of the den. These changes were found to be useful in surveillance to determine the extent and the relative timing of the plague epizootic in the cave habitat.