Trophic Cascades Following the Disease‐Induced Decline of an Apex Predator, the Tasmanian Devil

Abstract

As apex predators disappear worldwide, there is escalating evidence of their importance in maintaining the integrity and diversity of the ecosystems they inhabit. The largest extant marsupial carnivore, the Tasmanian devil (Sarcophilus harrisii) is threatened with extinction from a transmissible cancer, devil facial tumor disease (DFTD). The disease, first observed in 1996, has led to apparent population declines in excess of 95% in some areas and has spread to more than 80% of their range. We analyzed a long-term Tasmania-wide data set derived from wildlife spotlighting surveys to assess the effects of DFTD-induced devil decline on populations of other mammals and to examine the relative strength of top-down and bottom-up control of mesopredators between 2 regions with different environmental conditions. Collection of the data began >10 years before DFTD was first observed. A decrease in devil populations was immediate across diseased regions following DFTD arrival, and there has been no indication of population recovery. Feral cats (Felis catus) increased in areas where the disease was present the longest, and feral cat occurrence was significantly and negatively associated with devils. The smallest mesopredator, the eastern quoll (Dasyurus viverrinus), declined rapidly following DFTD arrival. This result suggests the species was indirectly protected by devils through the suppression of larger predators. Rainfall deficiency was also a significant predictor of their decline. Environmental variables determined the relative importance of top-down control in the population regulation of mesopredators. In landscapes of low rainfall and relatively higher proportions of agriculture and human settlement, top-down forces were dampened and bottom-up forces had the most effect on mesopredators. For herbivore prey species, there was evidence of population differences after DFTD arrival, but undetected environmental factors had greater effects. The unique opportunity to assess population changes over extensive temporal and spatial scales following apex predator loss further demonstrated their role in structuring ecosystems and of productivity in determining the strength of top-down control.

Transmissible cancer in Tasmanian devils: localized lineage replacement and host population response

“…Future research should include tumour lineage dynamics within an evolutionary epidemiological analytical framework, as well as assessing variation in host tolerance to different DFTD genetic variants under different ecological or immunological conditions. Transmissible clonal cancers in wildlife have only recently been recognized as a threat to biodiversity conservation [12,43], including both direct effects on species decline and the trophic cascades that result from the loss of keystone species and ecological functions, such as is occurring in Tasmania with the severe decline of its top mammalian predator [44]. The Tasmanian devil's facial tumour disease is a new transmissible cancer that we have studied in the wild almost since its emergence.…”

Predators drive community structure in coral reef fish assemblages

“…For example, in California declines in coyote populations led to increases in native (skunks, racoons, foxes) and exotic (feral cats, opossums) meso-predators causing local extinctions of scrub-feeding birds (Crooks and Soule 1999). Similar interactions have occurred for Tasmanian devils and quolls in Australia, following the near extinction of Tasmanian devils due to a disease outbreak (Hollings et al 2014). In this study, we found evidence to suggest that predator/mesopredator interactions occurred at the Palm Islands between large fisheries-targeted predators such as coral trout and snappers, and smaller non-target meso-predators such as small groupers and cods.…”

Relaxation of risk-sensitive behaviour of prey following disease-induced decline of an apex predator, the Tasmanian devil

“…B 282: 20150124 similar (0.4-0.6/10 km) in each of three regions we studied [19]. Numbers declined dramatically following disease arrival, but no such decline was evident in the disease-free northwest [19]. The higher levels of terrestrial activity of possums in areas with lower predation risk, evident from the hair-trap surveys and the earlier detection of food patches, may represent an increase in ground-foraging behaviour (microhabitat use), changes in vegetation use (macrohabitat use), greater population abundance or combinations of all three.…”