Who is eating whom? Morphology and feeding type determine the size relation between planktonic predators and their ideal prey


Body size determines the position of organisms in plankton food-webs. The mass or diameter ratio between predators and their optimal prey is therefore a central element of size-based models in their attempt to link consumer groups across trophic levels. Despite a renaissance of size-based approaches in plankton ecology, however, this relation still lacks a generic and also mechanistically sound formulation. An empirically derived constant value of this ratio can not describe the wide scatter in optimal prey diameter for specific predator size classes, especially in the mesozooplankton range. In this study, I propose that a given morpho-metric ratio between feeding related apparatus and total body volume decreases when predator size increases. This ratio decrease is due to the additional need of structural components in larger organisms for maintaining intra-body transport. Non-isometric scaling results in a non-linear dependency of optimal prey size on predator diameter. This dependency defines an average relation which enables the quantitative definition of feeding mode. This new trait variable explains a component of the variability in optimal prey diameter that is independent from predator diameter. Feeding mode as a trait can be interpreted as activity during grazing, mostly in terms of speed regulation in swimming or in feeding current generation. Feeding mode, in concert with the classical trait predator size, accurately determines optimal prey size. This is extensively tested using literature data for the entire plankton domain. The theory predicts an increasing feeding activity in larger consumer species. It elucidates how successional shifts in the composition of zooplankton communities are linked to weakly coupled changes in mean body size and feeding mode.
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