Nonconsumptive Predator-Driven Mortality Causes Natural Selection on Prey

Abstract

Predators frequently exert natural selection through differential consumption of their prey. However, predators may also cause prey mortality through nonconsumptive effects, which could cause selection if different prey phenotypes are differentially susceptible to this nonconsumptive mortality. Here we present an experimental test of this hypothesis, which reveals that nonconsumptive mortality imposed by predatory dragonflies causes selection on their damselfly prey favoring increased activity levels. These results are consistent with other studies of predator-driven selection, however, they reveal that consumption alone is not the only mechanism by which predators can exert selection on prey. Uncovering this mechanism also suggests that prey defensive traits may represent adaptations to not only avoid being consumed, but also for dealing with other sources of mortality caused by predators. Demonstrating selection through both consumptive and nonconsumptive predator mortality provides us with insight into the diverse effects of predators as an evolutionary force.

Predator cues have contrasting effects on lifespan ofPardosa milvina(Araneae: Lycosidae)

“…Prey typically must make trade-offs between anti-predator behavior and foraging, so most responses (e.g., reduced body size, slower development) are attributed to decreased energy intake (Lima 1998;Persons et al 2002;Benard 2004;Relyea 2007;Hawlena & Schmitz 2010b; but see Davenport et al 2014). However, in our study and the odonate studies described above, prey had ad libitum access to food (Stoks 2001;McCauley et al 2011;Siepielski et al 2014) or were provided a standardized amount of food without the presence of competitors, thus making reduced energy intake an unlikely explanation for decreased longevity. Although increased survival has been observed in response to predator cues, this effect is due to thinning (sensu Relyea 2007), a phenomenon unrelated to our results due to the lack of interactions among spiders.…”

“…Similarly, larval odonates exposed to predator cues have been found to have increased mortality rates compared to unexposed individuals (Stoks 2001;McCauley et al 2011;Siepielski et al 2014). In one study, reduced damselfly lifespan was accompanied by slower growth and development rate, and smaller, more asymmetric wings after metamorphosis (Stoks 2001).…”

“…Interestingly, stress induced by predator cues can decrease prey development and growth and increase prey mortality independent of changes in foraging (Stoks 2001;McCauley et al 2011;Siepielski et al 2014). If predator presence leads to a decrease in prey population sizes without directly consuming or changing the foraging success of their prey, then our conceptual models of predator-prey dynamics may be incomplete due to incorrectly attributing prey mortality to consumptive effects (Peckarsky et al 2008;McCauley et al 2011).…”

“…In one study, reduced damselfly lifespan was accompanied by slower growth and development rate, and smaller, more asymmetric wings after metamorphosis (Stoks 2001). However, these detrimental effects on prey beyond decreased longevity are not always present: prey exposed to predator cues had lower survival but did not differ from the control treatment in terms of body size as larvae or adults (McCauley et al 2011;Siepielski et al 2014). Interestingly, we did not find evidence for reduced foraging to explain how predator cues may reduce prey lifespan; odonates were also found to have unaltered energy intake in the presence of predator cues (Stoks 2001;McCauley et al 2011;Siepielski et al 2014).…”

“…However, these detrimental effects on prey beyond decreased longevity are not always present: prey exposed to predator cues had lower survival but did not differ from the control treatment in terms of body size as larvae or adults (McCauley et al 2011;Siepielski et al 2014). Interestingly, we did not find evidence for reduced foraging to explain how predator cues may reduce prey lifespan; odonates were also found to have unaltered energy intake in the presence of predator cues (Stoks 2001;McCauley et al 2011;Siepielski et al 2014). Prey typically must make trade-offs between anti-predator behavior and foraging, so most responses (e.g., reduced body size, slower development) are attributed to decreased energy intake (Lima 1998;Persons et al 2002;Benard 2004;Relyea 2007;Hawlena & Schmitz 2010b; but see Davenport et al 2014).…”