Quantifying 25 years of disease‐caused declines in Tasmanian devil populations: host density drives spatial pathogen spread

Abstract

Infectious diseases are strong drivers of wildlife population dynamics, however, empirical analyses from the early stages of pathogen emergence are rare. Tasmanian devil facial tumour disease (DFTD), discovered in 1996, provides the opportunity to study an epizootic from its inception. We use a pattern‐oriented diffusion simulation to model the spatial spread of DFTD across the species' range and quantify population effects by jointly modelling multiple streams of data spanning 35 years. We estimate the wild devil population peaked at 53 000 in 1996, less than half of previous estimates. DFTD spread rapidly through high‐density areas, with spread velocity slowing in areas of low host densities. By 2020, DFTD occupied >90% of the species' range, causing 82% declines in local densities and reducing the total population to 16 900. Encouragingly, our model forecasts the population decline should level‐off within the next decade, supporting conservation management focused on facilitating evolution of resistance and tolerance.

Disruption of metapopulation structure effectively reduces Tasmanian devil facial tumour disease spread at the expense of abundance and genetic diversity

“…This procedure removed any patches where a local devil population was unlikely to persist due to restricted habitat availability, while maintaining a connected metapopulation network. Predicted devil density of each local population was extracted from recently published density estimates (Cunningham et al 2021). We overlaid the TASVEG2 vegetation map over Cunningham et al's density estimates map for the year 1986 (10 years prior the estimated appearance of DFTD in Tasmania, and the starting point of our simulations) and extracted the mean devil density over the area of each habitat patch.…”

“…Devil facial tumour disease (DFTD) is a transmissible cancer that has decimated populations of Tasmanian devils (Sarcophilus harrisii) across nearly the entire geographical range of the species (Hawkins et al 2006;Cunningham et al 2021). The disease is a clonal cancerous cell line originated from a mutated Schwan cell in a female devil in north-eastern Tasmania in the 1990's (Pearse and Swift 2006;Murchison et al 2010).…”

“…Landscape-scale spread of the disease occurred relatively fast, with the disease now found on over 90% of the landmass of Tasmania, and the south-west of the island being the only area where the disease has not been detected (Cunningham et al 2021).…”

“…At the start of each simulation, local population size (i.e. number of individuals) was initialised independently for each population using population density estimates for the year 1986(Cunningham et al 2021), habitat patch area, and the same density multiplier used to calculate carrying capacity (see Metapopulation structure; Table…”

Dominant carnivore loss benefits native avian and invasive mammalian scavengers

“…Devils have experienced severe population declines due to a transmissible cancer, devil facial tumour disease (DFTD) [24]. The disease has progressively spread across Tasmania over 25 years, causing average population declines of 83% across ∼90% of Tasmania [25, 26]. The progressive spread of DFTD has established a natural experiment by creating regions of Tasmania with different disease histories and consequently, widely variable densities of top carnivores.…”