It is a fundamental feature of evolution that natural selection acts on individuals to adapt to predation pressure via the development of anti‐predator mechanisms. As such mechanisms are costly in terms of energy and time, species living in habitats where predators are rare or absent were expected to show reduced predator responses. Such a reduction was expected for larvae of the native whitefish (Coregonus wartmanni) in Upper Lake Constance, as these pass their initial stage in a historically predator‐free pelagic habitat and only start to encounter predators in a later ontogenetic stage during which they shift to littoral habitat. However, a recent invader of the pelagic waters of the lake, the three‐spined stickleback (Gasterosteus aculeatus), is implicated in a severe decline in whitefish recruitment through larval predation. The predator‐naivety effect and its consequences on predation were experimentally tested by comparing predator defenses of different age classes of whitefish with those of roach (Rutilus rutilus) and perch (Perca fluviatilis), two species whose larvae grow up in a predator‐holding habitat. As predator, the three‐spined stickleback (Gasterosteus aculeatus) was selected. Sticklebacks actively hunted all prey species but predation was high on whitefish only. The addition of zooplankton (Daphnia) as alternative prey for sticklebacks in the experimental setup did not reduce predation on the whitefish larvae, indicating a clear preference for whitefish in this invasive predator. The escape responses of roach and perch were found to be clearly more complex and diverse to those of the predator‐naïve whitefish larvae. Particularly striking was that whitefish often did not show any apparent behavioral response to the approaching predator. Only large whitefish larvae (Table 1: length = 40.8 ± 0.6 mm) appeared capable of escaping predation, and this was correlated with a change in complexity of predator escape responses. Thus, differences in predation pressure for the different larvae species were highly related to the observed interspecific differences in predator defenses, and with the ontogenetic intraspecific change in whitefish from poor and inefficient predator escape responses to complex and more efficient predator escape responses. The evolutionary consequences of stickleback as an invasive predator on whitefish larvae behavior are discussed.
Predator-prey interactions play a key life history role, as animals cope with changing predation risk and opportunities to hunt prey. It has recently been shown that the hunting success of sticklebacks (Gasterosteus aculeatus) targeting fish larvae is dependent on both the size of the prey and the prior exposure of its species to stickleback predation. The purpose of the current study was to identify the behavioural predator-prey interactions explaining the success or failure of sticklebacks hunting larvae of three potential prey species [roach (Rutilus rutilus), perch (Perca fluviatilis) and whitefish (Coregonus wartmannii)] in a 3D environment. Trials were carried out for each prey species at four different size classes in a standardised laboratory setup and were recorded using a slow motion, stereo camera setup. 75 predator-prey interactions including both failed and successful hunts were subject to the analysis. 3D track analysis indicated that sticklebacks applied different strategies. Prey with less complex predator escape responses, i.e. whitefish larvae, were hunted using a direct but stealthy approach ending in a lunge, while the behaviourally more complex roach and perch larvae were hunted with a faster approach. A multivariate logistic regression identified that slow average speed and acceleration of the prey in the initial stages of the hunt increased the probability of stickleback success. Furthermore, predators adjusted their swimming direction more often when hunting larger whitefish compared to smaller whitefish. The results suggest that appropriate and adequately timed avoidance behaviours, which vary between prey species and ontogenetic stages, significantly increase the chances of outmanoeuvring and escaping stickleback predation. Small whitefish larvae can reach similar levels of swimming performance compared to older conspecifics, but display ineffective anti-predator behaviours, resulting in higher hunting success for sticklebacks. Thus, the development of appropriate anti-predator behaviours depending on size appears to be the crucial factor to escaping predation.
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