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.