There is growing interest in the ecological consequences of fear, as evidenced by the numerous studies on the nonconsumptive,
trait-mediated effects of predators. Parasitism, however, has yet to be fully integrated into research on the ecology of fear,
despite it having direct negative and often lethal effects on hosts and being the most common life history strategy on the
planet. This might at least be partly due to the traditional, but untested, assumption that anti-parasite responses are weak
relative to anti-predator responses. To test this hypothesis, we quantified the activity and location responses of
Bufo americanus tadpoles to one of six chemical cues: water; cercariae of
Echinostoma trivolvis, a trematode which infects and can kill amphibians; a snail releasing
E. trivolvis cercariae; an uninfected snail; food; or conspecific alarm chemicals signaling predation. There is also literature encouraging
research on the context dependency and pollution-induced disruption of fear responses. Consequently, before quantifying responses
to the chemical cues, half of the
B. americanus were exposed to the herbicide atrazine (201 μg/l for 4 days), a reported inhibitor of fear responses in fish. Tadpoles were
attracted to food, were indifferent to an uninfected snail, avoided alarm chemicals, and exhibited avoidance and elevated
activity in response to a snail shedding cercariae and cercariae alone. Atrazine had no detectable effects on
B. americanus’ responses to the tested cues despite the use of a higher concentration and longer exposure duration than has been repeatedly
shown to inhibit chemical cue detection in fish. The magnitude of anti-parasite and anti-predator responses were qualitatively
similar, suggesting that the fear of disease and its ecological consequences could be comparable to that of predation. Consequently,
we call for a greater integration of parasites into research on the ecology of fear and trait-mediated indirect effects.
Keywords Alarm chemical - Atrazine -
Bufo americanus
- Trait-mediated indirect effects - Trematode
Communicated by Anssi Laurila.