Density independent decline from an environmentally transmitted parasite

Abstract

Invasive environmentally transmitted parasites have the potential to cause declines in host populations independent of host density, but this is rarely characterized in naturally occurring populations. We investigated (1) epidemiological features of a declining bare-nosed wombat (Vombatus ursinus) population in central Tasmania owing to a sarcoptic mange (agent Sarcoptes scabiei) outbreak, and (2) reviewed all longitudinal wombat-mange studies to improve our understanding of when host population declines may occur. Over a 7-year period, the wombat population declined 80% (95% CI 77-86%) and experienced a 55% range contraction. The average apparent prevalence of mange was high 27% (95% CI 21-34), increased slightly over our study period, and the population decline continued unabated, independent of declining host abundance. Combined with other longitudinal studies, our research indicated wombat populations may be at risk of decline when apparent prevalence exceeds 25%. This empirical study supports the capacity of environmentally transmitted parasites to cause density independent host population declines and suggests prevalence limits may be an indicator of impending decline-causing epizootics in bare-nosed wombats. This research is the first to test effects of density in mange epizootics where transmission is environmental and may provide a guide for when apparent prevalence indicates a local conservation threat.

Keywords: Sarcoptes scabiei; Vombatus ursinus; bare-nosed wombat; density dependence; environmental transmission; frequency dependent transmission.

Figure 1.

Declining wombats per survey day over time (a) and time period (b), increasing apparent prevalence of sarcoptic mange over time (c) and time period (d), and density independent relationship between wombats per survey day and the apparent prevalence of sarcoptic mange (e). Position of points jittered in (b) and (d) for visual clarity.

Figure 2.

Reduction in the distribution of wombats from a site in central Tasmania between time periods 2015–2018 and 2020–2022, and mange disease status of wombats.