Finished Before You Even Started
Predators are known to affect prey while they are adults and juveniles, but what about when they haven’t even hatched yet? (Image Credit: Bernt Rostad, CC BY 2.0)
Predation risk affects egg mortality and carry over effects in the larval stages in damselflies (2018) Sniegula et al., Freshwater Biology, p. 1-9
In the natural world, one of the most dangerous things that a prey animal has to worry about is a predator. These organisms depend on the prey for their sustenance, and as such have become very good at finding ways to eat them. These are known as direct effects, as a predator eating prey is a direct interaction.
Another aspect of the predator-prey relationship is that of indirect effects, or effects that a predator has on prey that don’t involve it eating the prey animal. These can include predator-induced changes in the prey’s behavior, immune function, or even survival. These indirect effects are usually studied in prey species that are adults or juveniles, but the authors of today’s paper were interested in what indirect effects predators had on the eggs of prey species.
What They Did
The authors used eggs from females of three species of damselfly, to control for any possible species-specific variation in the response to predators. The authors took half of the eggs from each female and exposed them to water that had predator cues in it (fish, in this case), while the other half received water without any kind of cue.
The authors measured how long the eggs took to hatch, how many eggs from each clutch hatched, and how synchronous the hatching of the eggs was. This measure of synchrony was important, as synchronous hatching is predicted to be advantageous in the face of a predation threat, as many hatchlings at the same time would possibly overwhelm a predator and make it harder for them to catch and eat anything. The authors also measured the growth of the larvae that did hatch, to detect any carry-over effect from the egg-stage.
Did You Know: “Smelling” Underwater
This study is all about how predator cues in the water affect eggs, but how are eggs “sensing” anything? In aquatic ecosystems, chemical cues are able to travel through the water, much like an aroma from dinner would travel through the air from the kitchen to your nose. This allows the eggs to “know” when conditions may be best to hatch, and some studies have shown that some organisms can modify their hatching to take advantage of a good situation. In addition to predation, underwater organisms can also sense food, members of their same species, and disease vectors in the water.
What They Found
Fish cues caused significant egg mortality compared to the eggs from the no-cue water, with the eggs from one of the species of damselfly showing almost no hatching at all. The effect of predator cues on hatching date varied from species to species, with one species hatching later with predator cues, one species hatching earlier with predator cues, and one species showing no difference.
For the carry-over effects, eggs that hatched from water without fish cues grew much more than eggs in the fish-cue water, indicating that spending time as an egg and growing in the presence of a predator as an juvenile affected growth.
The key treatment in this study was the fish cues in the water, and although the authors made a point to replace the fish cue water often enough so that it never faded away and ceased to be a factor, they made no mention about whether or not the fish had fed on damselflies. An important part of the detection of predator cues in aquatic ecosystems is detecting not only if there are predators around, but if those predators recently ate animals like you (and how many they ate). If the authors didn’t actually feed the fish damselflies (of the same species) then the results could be the result of a general anti-predator response, instead of a species-specific phenomenon that may have been generated by fish that had fed on the same species.
The authors of this study have uncovered another layer in the complex mystery that is the natural world: indirect effects of predators on the egg stage. One theory in this system is that densities of insects in lakes with fish are lower than those without fish because the fish are effective predators and keep prey densities low, but these results indicate there may be more to it than that. Additionally, the findings from this study are in direct contrast to those found in similar studies on the indirect effects of predators on eggs in other animals, and highlight the complexity that is the ecology of natural systems.