Tilman's theory predicts the outcome of competition between two consumers sharing two resources on the basis of the shape of zero net-growth isoclines (ZNGIs). In his theory, intra-specific differences in resource use are not accounted for. Here we extend this theory to include situations where organisms undergo ontogenetic diet shifts, as these characterize the life histories of many species. In a situation that without diet shifts would lead to neutral coexistence of consumer species, we investigate whether ontogenetic diet shifts lead to niche partitioning. We analyze a model describing competition for two resources between two competitors with distinctive diets over ontogeny, using copepods (showing ontogenetic diet shifts) and daphnids (not showing ontogenetic diet shifts) as appropriate representatives. We show that an ontogenetic diet shift affects the shape of the ZNGI, changing it from reflecting perfectly substitutable resources to reflecting essential resources. Furthermore, we show that resource supply determines population stage structure and stage-dependent resource consumption in copepods and influences the competitive outcome with daphnids. In particular, we show that in itself, an ontogenetic diet shift can provide a competitive advantage if the supply of the adult resource is lower than the supply of the juvenile resource but that it always causes a disadvantage if the supply of the adult resource exceeds that of the juvenile resource.
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