Diurnal and nocturnal functional response of juvenile Notonecta maculata considered as a consequence of shifting...

Presence or absence of light is considered to significantly affect predation within freshwater communities. In relation to light regime, the scope of the present study is to identify proximate factors accounting for different rates of predation in juvenile Notonecta maculata. It has been shown that foraging of juvenile N. maculata on Daphnia magna is reduced under dark conditions. These differences are accounted for by the presence/absence of light and are not regulated by any endogenous circadian rhythm. Direct observation of predation components revealed that in the dark, daphnid prey is detected at shorter distances, thus lowering the rate of encounter which finally results in a decreasing number of prey items eaten. Handling time was found to significantly increase during dark periods. Since the amount of food ingested per prey item increases to a certain extent with increasing handling time, it is suggested that the decreasing number of daphnids consumed is compensated by gaining a higher amount of food per item during a lengthier, more thorough, handling of the prey. A mechanistic model that describes the foraging process along a general predation cycle was parameterized based on the results of direct observations, instead of using classic functional response curves. This allows the comparison of model output to independent functional response data. A good correlation between observed and predicted data confirms the assumption that the reduction of the nocturnal predation rate is caused by shifting predation behaviour and indicates that the crucial light/dark differences in the foraging of N. maculata are considered in the approach.