Predators drive selection for adaptive plasticity in prey defense behavior

Plasticity to reduce activity is a common way prey evade predators. However, by reducing activity prey often experience lower individual growth rates because they encounter their own prey less often. To overcome this cost, natural selection should not simply favor individuals generating stronger plasticity to reduce activity rates, but also selection to resume activity once the threat of predation subsides. If such plasticity is adaptive, it should vary under environmental conditions that generate stronger selection for greater plasticity, such as predator density. Using a mesocosm experiment and observational study with a damselfly-prey/fish-predator system we show that fish predation exerts selection for greater plasticity in activity rates of damselflies. Such selection allows damselfly activity levels to initially decrease and then rebound when the threat of predation dissipates, potentially helping to ameliorate a hypothesized growth penalty from activity reductions. We also find that the extent of plasticity in activity to the threat of fish predation increases, albeit slightly (r2 = 0.04-0.063%), as fish densities increase across natural lakes, consistent with the idea that the magnitude of plasticity is shaped by environmental conditions underlying selection. Collectively, these results demonstrate how selection acts to drive adaptive plasticity in a common predator avoidance strategy.

Methods
This dataset was collected from a mesocosm study paired with a field study.

In the mesocosm study, we mixed damselfly larve collected from 3 lakes in Arkansas to ensure large phenotypic variation. We randomly assigned individuals to a caged-predator or free-ranging predator treatment. The caged-predator treatment represents behavioral responses of damselflies to predator cues without direct mortality by the predator, whereas the free-ranging predator treatment represents predator selection as they were free to swim about and consume damselflies. This experiment was conducted at an outdoor mesocosm facility at the University of Arkansas. Following the duration of the experiment, we collected the remaining damselflies in each treatment and performed behavioral assays to quantify predator-induced plasticity. Head positions of damselflies were marked on an underlying grid (1cm x 1cm) every 20 minutes for a total of 3 hours. Activity rate was then calcuted by measuring the linear distances between each head position of an individual to obtain the total minimum distance moved during the assays. We measured activity rates of each individual first without the presence of predator kairomones, then with predator kairomones, and once more after the removal of predator kairomones. We took photos of the damselflies and estimated head-widths (mm) using ImageJ software to control for body size effects on activity rates. Assays were performed at a greenhouse facility at the University of Arkansas under natural lighting conditions.

In the field study, we estimated fish predator densities from lakes in Arkansas by taking three standardized seine hauls (4 x 3m) through emergent vegetation. We recorded the number of centrarchid fish in each haul and samples were averaged per lake to estimate density. We also collected damselflies at these lakes and performed the same open-field behavioral assays as described above on 20 individuals from each lake. 

Data was recorded in excel and analyzed in R.