Changes in the Age-Structure and Size of Populations of Wild Rabbits in South Australia, Following the Introduction of European Rabbit Fleas, Spilopsyllus Cuniculi (Dale), as Vectors of Myxomatosis.

Abstract

The age-structures of rabbit populations at 4 climatically different sites in South Australia were found to have altered significantly since the introduction of Spilopsyllus cuniculi (Dale) as a vector of myxomatosis. The ratio of young of the year to older rabbits in summer was greatly reduced. There was evidence of decreases in size of all observed populations and (for 1 population) in ability to increase rapidly in favourable years. The ratio of young of the year to older rabbits was a convenient and sensitive index of changes in population structure; for analytical purposes, the log transformation (ln [ratio] or ln [ratio + 1]) was biologically and statistically sound. Since the fleas became established, field-strain viruses may have become more virulent and morbidity rates may have increased, but the factors enabling rabbit fleas to enhance myxomatosis as a cause of mortality are unknown. The apparent virulence may have increased because of selection for viruses of high virulence, which are best suited for transmission by S. cuniculi, and possibly because the dose of virus transmitted by fleas is considerably greater than that by mosquitoes or other agents.