Dangerous prey is associated with a type 4 functional response in spiders

Prey can defend themselves against predators in many different ways. Social insects, such as ants, possess particularly effective defensive systems. Some predators are better adapted to prey defence than others. We compared the capture and defence efficiency in three spider species that differ in their level of myrmecophagy. We used three ant species differing in body size and aggression in a functional response experimental set-up that measured capture frequency at different prey densities. We found a type 4, dome-shaped functional response, and we propose a new mechanistic model to describe this type. Estimated parameters (searching efficiency, handling time, inhibition by prey) were then compared among spider and ant species to quantify density-dependent defensive effects on the predator's capture efficiency. We also compared survival of spiders during experiments. We found that myrmecophagous Zodarion spiders hunted ants with the highest capture efficiency and had the highest survival, suggesting that these spiders are adapted to living with high densities of ants. Polyphagous Xysticus spiders captured ants with lower efficiency and had the lowest survival, indicating that these spiders are adapted to the capture of solitary ant workers. Polyphagous nonanteating Pardosa spiders did not capture ants but had high survival, and are apparently adapted to living with high densities of ants. The new proposed model of the type 4 functional response can be applied to other predator-prey systems in which the prey is dangerous and a decrease in predator hunting has a similar dependence on prey density.