Top carnivore decline has cascading effects on scavengers and carrion persistence

Abstract

Top carnivores have suffered widespread global declines, with welldocumented effects on mesopredators and herbivores. We know less about how carnivores affect ecosystems through scavenging. Tasmania's top carnivore, the Tasmanian devil (Sarcophilus harrisii), has suffered severe disease-induced population declines, providing a natural experiment on the role of scavenging in structuring communities. Using remote cameras and experimentally placed carcasses, we show that mesopredators consume more carrion in areas where devils have declined. Carcass consumption by the two native mesopredators was best predicted by competition for carrion, whereas consumption by the invasive mesopredator, the feral cat (Felis catus), was better predicted by the landscape-level abundance of devils, suggesting a relaxed landscape of fear where devils are suppressed. Reduced discovery of carcasses by devils was balanced by the increased discovery by mesopredators. Nonetheless, carcasses persisted approximately 2.6-fold longer where devils have declined, highlighting their importance for rapid carrion removal. The major beneficiary of increased carrion availability was the forest raven (Corvus tasmanicus). Population trends of ravens increased 2.2-fold from 1998 to 2017, the period of devil decline, but this increase occurred Tasmania-wide, making the cause unclear. This case study provides a little-studied potential mechanism for mesopredator release, with broad relevance to the vast areas of the world that have suffered carnivore declines.

The mesoscavenger release hypothesis and implications for ecosystem and human well‐being

“…In Tasmania, Australia, areas where Tasmanian devils ( Sarcophilus harrisii ) have declined due to facial tumour disease have resulted in an increased abundance of feral cats ( Felis catus ) and forest ravens ( Corvus tasmanicus ) (Cunningham et al . ). The authors show that forest ravens have increased across all of Tasmania during the period of Tasmanian devil decline.…”

“…Similarly, carrion persisted 2.6 times longer in areas where Tasmanian devils declined due to a facial tumour disease (Cunningham et al . ). Carcasses were scavenged three times slower in the absence of vultures in the Laikipia District of central Kenya (Ogada et al .…”

Trophic rewilding establishes a landscape of fear: Tasmanian devil introduction increases risk‐sensitive foraging in a key prey species

“…In addition to predation pressure, energetic state influences an animal's GUD (Brown 1992, Bedoya-Perez et al 2013. Other research shows that devils modify the behaviour of mesopredators and prey; for example, spotted-tailed quolls Dasyurus maculatus temporally partition activity to avoid devils at high density (Cunningham et al 2019a), and feral cats Felis catus willingly feed on carcasses in areas where devils are rare, but less so in areas where devils are abundant, possibly a response to increased risk of encountering a devil at a carcass (Cunningham et al 2018). Maria Island and the control region were both exposed to similarly low rainfall preceding the 'after' period, yet the control region saw a significantly larger drop in GUDs, and the two regions had significantly different slopes.…”

Temporal partitioning of activity: rising and falling top‐predator abundance triggers community‐wide shifts in diel activity

“…Quolls shifted their activity peak from dawn, when devils are abundant, to dusk, when devils were rare, strongly suggesting competitive release. Carrion is a resource that is largely unavailable to quolls where devils are abundant, but is used by them where devils have declined (Cunningham et al 2018). Interaction plots from GAMs predicting the temporal activity of a species based on changes in the devil population.…”

“…Interference competition in carnivores, involving aggressive exclusion from a resource (Linnell and Strand 2000), is probably a stronger driver of temporal partitioning than exploitation competition (Carothers and Jaksić 1984). Devils aggressively protect carcasses and exclude other species from feeding (Cunningham et al 2018), but there is only anecdotal information about devils killing or actively persecuting mesopredators (Jones 2003), as some other top predators do (Palomares and Caro 1999). Devils additionally pose a strong risk of kleptoparasitism for quolls, but less so for cats due to their smaller prey size.…”