Ecologists at the University of Toronto have found that the mere presence of predators is enough to increase mortality in dragonflies, even when there is no imminent threat of attack, suggesting that the stress related to the knowledge that a potential predator is nearby increases susceptibility to other factors that cause death.
Previous studies have shown that "nonlethal" predators – i.e. predators that are in the vicinity but cannot actually gain access to their prey – affect mortality in species with complex life cycles, such as dragonflies. But they have focused only on effects during the larval stages, which immediately precede adulthood. The findings published in the November issue of Ecology, focused instead on the effect on adult dragonflies, as well as metamorphosis, the high-stress stage between larval and adult stages.
Leucorrhinia intacta, the species of dragonfly used in the study, has already been proven to show plasticity in the presence of predatory fish. Plasticity is the behavioral, physiological and morphological changes prey undergo that can reduce predator risk, but alternatively increase risk of death from other sources.
The first part of their experiment corroborated the earlier work done on larvae. Using dragonfly larvae from two ponds, one with a fish and one with an invertebrate as the top predator, they investigated how "nonlethal" predators affected the rate of mortality in larvae.
As expected, survival rates were 2.5-4.3 times greater among larvae raised in the absence of predators than those raised in the presence of "nonlethal" predators. However, there was no compensatory growth among surviving larvae. Previously, it has been shown that the death of some individuals results in an increase in the growth of those that survive, because of a decrease in competition for food and resources. However, this was not the case, which researchers attributed to the high levels of the food source, zooplankton, given to the larvae.
In the second part, they utilized dragonfly larvae from a pond with only fish as the top predator. They then allowed them to grow, pass through metamorphosis and into adulthood. The adults were then measured for difference in body size using head width, throat length and forewing length.
Their results were significantly different between dragonflies exposed to "nonlethal" predators, and those that were not exposed to any predator fish. Larval mortality, the same data measured in the first part, increased 1.2 times for exposed dragonflies, a significant decrease than previously noted. This can be explained by two reasons.
In the first part, the aquarium used was markedly smaller which resulted in an increase in predator density. Additionally, the predators in the first experiment were fed other Leucorrhinia intacta larvae. This affected the potential risk perceived by the predators' prey.
Furthermore, the metamorphosis failure for exposed dragonflies was 11%, compared to just a 2% failure rate in non-exposed fish. These results suggested, in their words, "a stress response above and beyond the normal stresses associated with metamorphosis." They theorized that the increased mortality was due to the dragonflies becoming more vulnerable to other mortality factors such as reduced energy gains, and exposure to pathogens.
This hypothesis has previously been asserted in studies focusing on the effect of stress hormones. Some behavioral changes associated with the stress of avoiding predators, can result in trade-offs that affect survival. One example is the response that occurs in the brain. This involves the release of neuro-hormones that can have a potential to generate a cascade of negative physiological responses.
These fatal responses to "nonlethal" predators can have complex consequences for predator-prey relationships and food chains and webs. For example, the preemptive death of prey due to the stress associated with "nonlethal" predators, can actually work against their predators, as this increases the likelihood that the prey will be devoured by scavengers and decomposers rather than the predators themselves.
