Trophic cascades following disease-induced decline of an apex predator: Tasmanian devils, environmental variability and top-down vs bottom-up control

Background/Question/Methods

Top predators are ecosystem architects, exerting strong top down interactive control on smaller predators and prey. Large predators are under threat worldwide, and their decline can have cascading effects on ecosystems, sometimes tipping systems in alternative states and often with dramatic loss of biodiversity. The largest extant marsupial carnivore, the Tasmanian devil (Sarcophilus harrisi) is threatened with extinction from an unusual transmissible cancer, devil facial tumour disease (DFTD). Since it emerged 15 years ago, DFTD has spread to most of the devil’s range causing an overall 83%  population decline, with local declines up to 95% where the disease has been longest. We analyzed a long-term Tasmania-wide spotlight survey dataset, pre-dating DFTD emergence by more than a decade, to assess the effects of the outbreak of DFTD and subsequent devil decline on other mammalian fauna; mesopredators and herbivorous and omnivorous prey. We divided the island state of Tasmania into four regions based on four year timeframes since disease outbreak: early (1996-99), mid (2000-03), late (2004-07) and as yet disease free.

Results/Conclusions

Devil decline was immediate and extensive following disease arrival with no indication of population recovery. Feral cats (Felis catus), a destructive introduced placental predator, increased following DFTD arrival in long-term diseased regions, and showed a strong and significant negative relationship with devils. The eastern quoll (Dasyurus viverrinus), the smallest mesopredator, showed dramatic declines corresponding to DFTD arrival, although rainfall deficiency was also a significant predictor of these declines. The strength of top down regulation on mesopredators was influenced by environmental variables. In landscapes with lower rainfall and more farmland, bottom up forces of prey availability appear to be the regulatory mechanism controlling mesopredator populations, in contrast to other areas of the state where top down control was the most important process. Herbivore prey species show some evidence of population differences before and after DFTD arrival, but other environmental influences were more important. This dataset provides a unique opportunity to assess population changes at a large temporal and spatial scale following top-predator decline. It is imperative such studies are conducted in order to develop management priorities to maintain ecosystem function in the absence of apex predators.