Quantifying predator–prey body size relationships is key to understanding food webs. Food web models often assume that all individuals of predator species prefer the same relative body size of prey, using a single constant called preferred predator–prey mass ratio (preferred PPMR). In contrast, empirical studies have shown that relative prey body size in diet varies with individual predator size, challenging the food web models based on size-invariant preferred PPMR and their predictions. We point out that this apparent inconsistency arises because empirical PPMR in those previous studies has been measured only through dietary data (i.e. realized PPMR rather than preferred PPMR) without considering the effects of environmental prey availability, suggesting the possibility that preferred PPMR may be in fact independent of individual predator size. Here, we present a new approach to revisit the assumption of size-invariant preferred PPMR in food web models. The approach compares two measures of PPMR calculated from prey compositions in predator diet and environmental prey composition, respectively (i.e. realized PPMR vs. environmental PPMR). The deviations between realized and environmental PPMRs are considered as a proxy of individual variations in relative prey size preference (i.e. preferred PPMR). We apply this idea to long-term dietary data of an omnivorous predatory fish species collected from a lake ecosystem over four decades. Our results showed that the preferred PPMR is independent of individual predator size when the foraging mode (i.e. the major prey type) of the predator is considered while the realized PPMR is size-dependent regardless of the foraging mode, especially when analysed analogously to previous empirical studies. We suggest that the apparent inconsistency between theoretical assumption and empirical observation of PPMR is due to the conceptual and methodological confusion and could be resolved by distinguishing between preferred and realized PPMRs. Further, in contrast to the previous arguments based on realized PPMR, we provide the first empirical support for size-invariant preferred PPMR. Future studies are encouraged to apply our ideas to other species/systems to test the robustness of size-invariant preferred PPMR and to better describe food web models. A lay summary is available for this article.
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