Sloppy Predators
Perhaps the most iconic predator spreader in the ecological literature is the “sloppy predator”. The sloppy predator contributes to increased parasitism by consuming its prey in a way that increases the transmission or dissemination of parasites as compared to other sources of prey mortality (Cáceres et al. 2009; Duffy et al.2019). The classic example of this phenomenon is the larvalChaoborus predator which shreds and regurgitates infectedDaphnia dentifera prey (Cáceres et al. 2009; Strausset al. 2016). This predation behavior spreads fungal parasite spores in the water column where they are ingested by foraging D. dentifera . In contrast, D. dentifera that die from parasite virulence rapidly settle out of the water column, limiting onward transmission of fungal spores. In this system, both prey biology and environmental stratification contribute to limit transmission after prey death from virulent effects of the parasite, while prey and predator behavior increase transmission after predation by the sloppy predator.
In general, predator behaviors that spread prey blood, tissue, or viscera are likely to contribute to transmission as long as prey behaviors expose them to these new sources of infection. These prey behaviors include indiscriminate grazing or filter feeding, as we see in the D. dentifera example, but also scavenging behaviors which may lead prey to consume conspecifics post-predation when they would not otherwise. (Because our focus in this review is on scenarios where the predator does not become infected, we are not considering cases of cannibalism.) Similarly, prey biology and the interaction between environment and prey space-use impact whether prey are more likely to encounter the parasite after predation or after death from virulent mortality. If prey shed few parasites during their life and/or if parasite shedding from live prey is concentrated in areas avoided by healthy prey, then there is a clear opportunity for sloppy predation to increase transmission. In fact, questions of spatio-temporal patterns of predator spreading are likely of broad relevance to this topic (Box 1).
Efforts to discover and experiment on a wider array of sloppy predator-spreader systems should begin by identifying predatory behaviors that spread blood, tissue, or viscera. In these systems, field manipulation of predator presence/absence should be supported and contextualized with lab or field mesocosm simulation of predator sloppy eating byproducts to determine whether sloppiness mechanistically results in transmission.