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.