AN EXPERIMENTAL FIELD-STUDY TO EXAMINE WHETHER CAPILLARIA-HEPATICA (NEMATODA) CAN LIMIT HOUSE MOUSE-POPULATIONS IN EASTERN AUSTRALIA

Host-parasite studies involving host populations that outbreak offer a n appropriate model for examining whether micro- and macro-parasites c an limit or regulate mammal populations. This paper details a replicat ed experimental field investigation to examine the effect of the nemat ode parasite Capillaria hepatica on populations of house mice (Mus dom esticus). A 2-year study was conducted at seven sites with matching fa rming practices, soil types, topography and habitat heterogeneity on t he Darling Downs in south-eastern Queensland, Australia, where mice ca use substantial economic, social and environmental problems. A 4-km(2) sampling zone was designated on each site and sites were assigned ran domly to one of three untreated and four treated groups. The parasite was released successfully on three occasions at three markedly differe nt stages of mouse population dynamics. The first release was in winte r 1992 into a low-density, non-breeding population. Mice on treated si tes had significantly lower survival for six months after the release than mice on untreated sites. The parasite had a relatively high impac t on survival of young mice (<72 mm long) two months after its release . The greatest impact on old mice (>76 mm) occurred a month later. The most pronounced effects of C. hepatica on mouse abundance occurred du ring the four months after its release (June-September). Mice on the u ntreated sites, however, had poor survival in September, so by October their population abundance was at a level similar to that of the trea ted populations. Once breeding began in mid-October C. hepatica had no noticeable effect on mouse population dynamics. This was because the parasite (i) had no effect on breeding of mice, (ii) had minimal trans mission and (iii) had a diminishing effect on survival after October. The apparent lack of transmission of C. hepatica was probably due to a combination of low population density, the transient nature of the mo use population and predominantly dry weather for six months after the release. A second release was made in February 1993 into a breeding, m edium-density host population that was rapidly increasing in abundance . Less than 2% of the population was affected during the release so in terest focused on transmission rather than the effect of the parasite on the host's demographic machinery. Transmission did occur at a low r ate and the parasite persisted for 4.5 months (to June) when it was de cided to boost the proportion of mice infected in order to follow its effect on the overwintering population and the demographic effects dur ing the next breeding season. This last release was compromised by syn chronous, widespread and rapid decline in mouse densities. Densities f ell from greater than 500 ha(-1) to less than 1 ha(-1) in less than si x weeks. Two messages emerge from these studies. First, C. hepatica wi ll not limit mouse populations if it is released into a low-density po pulation during a long dry period on the Darling Downs. Second, we nee d to know more about the factors that influence the survival and trans mission of the parasite under field conditions.