Effects of temperature and browning on the functional response of a freshwater top predator.

Freshwater lakes are becoming warmer and browner, with poorly known ecosystem consequences. A major unresolved issue is how these changes will affect the feeding rates of predators that regulate top-down trophic cascades. We explored the effects of temperature and browning on the functional response and feeding rates of a keystone predator, the Northern pike Esox lucius. We first derived a simple mechanistic model on the effect of temperature and browning on predator feeding rates. To test predictions from the model, we performed two laboratory experiments where we estimated pike functional responses in brown and clear water at three temperatures and quantified feeding rates along a gradient from completely clear to extremely brown. We find strikingly weak effects of temperature and browning on pike feeding rates, even under extreme levels of browning. Pike showed an asymptotic Type II functional response under most conditions but switched to a dome-shaped Type IV functional response in cold clear water, possibly due to seasonal changes in the schooling behaviour of prey. Our results suggest that temperature and browning may have interactive effects on predator functional responses mediated via changes in prey behaviour and support the view that browning affects piscivorous fish mainly through bottom-up effects rather than changes in foraging efficiency.