Box 2: Herbivore Spreaders?
Although our framework focuses primarily on predation of animals, it is worth noting that herbivores can spread disease between plant victims by similar mechanisms. Herbivory exhibits much resemblance to predation with the notable difference that it rarely results in plant death. Due to this trait, partial predation herbivore-spreading is better studied in plant-herbivore systems than in predator-prey systems. For example, plant parasites frequently pass through herbivores such as beetles and are deposited back on the same or a different plant in feces (Wielkopolan et al. 2021). Saprozoic nematodes frequently ingest pathogenic bacteria from dead plant tissue and spread bacteria to new plant hosts by defecating in soil (Chantanao & Jensen 1969; Nykyriet al. 2014). Although herbivores rarely kill whole plants, selective herbivory on uninfected individuals (Mauck et al.2015), or on the basis of traits that correlate with parasite load such as size/growth rate (Hoffland et al. 1996; Dietrich et al.2005; Cornelissen et al. 2008) are likely to decrease survival of uninfected plants, increasing disease in the population.
Although we rarely consider plants as having behaviors (but see (Karban 2015)), they do display plastic responses to herbivory which have been demonstrated to increase susceptibility to parasites. For example, herbivore wounding frequently increases jasmonic acid production in plants which in turn can downregulate the production of salicylic acid, a common compound in defense against parasites (Bostock 2005; Stoutet al. 2006; Smith et al. 2009; Vlot et al. 2009). Many of these herbivore-spreading systems, such as the partial predation of aphids or the physiological jasmonic acid/salicylic acid response are undoubtedly better characterized than any animal predator-spreading systems with the accompanying complications and nuances. Therefore, these systems may provide useful parallels for future predator-spreader study. To that end, we suggest that researchers studying predator-spreading and herbivore-parasite interactions acquaint themselves with the other body of literature.