Conclusions
The potential for predators to protect their prey populations from the harmful effects of parasitism is intuitive and has promising conservation and public health implications (Packer et al. 2003; Ostfeld & Holt 2004). In agricultural pest systems, management sometimes has the opposite goal: using predators to facilitate pathogen spread to better limit pest abundances (Roy et al. 2001; Zhanget al. 2015; Lin et al. 2019). However, there is surprisingly mixed evidence for the effect of predators on parasitism in their prey, with roughly as many studies finding that predators increase disease as finding a decrease (Richards et al. 2022). This variability raises the question of what factors lead to predators reducing disease vs. spreading it – a question that we must be able to answer with confidence if we wish to manipulate predation as a management strategy. Attempts to manage infectious diseases in wildlife based on incomplete understandings of natural systems can have catastrophic outcomes, as when culling of badgers in the United Kingdom repeatedly led to an increase in bovine tuberculosis and not the hoped for decline (Donnelly et al. 2003, 2006, 2007). Unfortunately, at present, we are far from being able to confidently predict who will be a predator spreader. However, the mechanisms that we identify in this perspective provide a framework for rigorously studying predator spreading; we also provide initial hypotheses regarding factors that should promote predator spreading via these different mechanisms, and guidance for how to carry out studies on this topic in the future. These studies – and consistent, comprehensive results reporting – are essential if we are to better understand this phenomenon that is of basic interest and applied importance. We often learn the most when predictions break down and unexpected outcomes occur. While we expect many of our predictions here will prove reasonably sound, we anticipate that the most exciting research questions in the next generation of predator-prey-parasite ecology will grow out of discovering the places where these predictions fall apart.