Density-dependent effects of prey defences

In this study, we show that the protective advantage of a defence depends o n prey density. For our investigations, we used the predator-prey model sys tem Chaoborus-Daphnia pulex. The prey, D. pulex, forms neckteeth as an indu cible defence against chaoborid predators. This morphological response effe ctively reduces predator attack efficiency, i.e. number of successful attac ks divided by total number of attacks. We found that neckteeth-defended pre y suffered a distinctly lower predation rate (prey uptake per unit time) at low prey densities. The advantage of this defence decreased with increasin g prey density. We expect this pattern to be general when a defence reduces predator success rate, i.e. when a defence reduces encounter rate, probabi lity of detection, probability of attack, or efficiency of attack. In addit ion, we experimentally simulated the effects of defences which increase pre dator digestion time by using different sizes of Daphnia with equal vulnera bilities. This type of defence had opposite density-dependent effects: here , the relative advantage of defended prey increased with prey density. We e xpect this pattern to be general for defences which increase predator handl ing time, i.e. defences which increase attacking time, eating time, or dige stion time. Many defences will have effects on both predator success rate a nd handling time. For these defences, the predator's functional response sh ould be decreased over the whole range of prey densities.