The ecophysiology of food chains

The properties of species that are connected in a food chain depend on each other. This paper shows that supply stress determines the trophic position of species, being low at the bottom, and high at the top of the chain. Supply species eat what is available, while demand species eat what they need, almost independently of what is available. Supply stress involves maturity maintenance, which is proportional to maturity and somatic maintenance, which is proportional to structure. Maturity and structure are specific for Dynamic Energy Budget (DEB) theory, which is the natural context for this paper. Species at the bottom of the food chain allocate a large fraction of their assimilation to reproduction, while species at the top do the opposite. The structure of DEB theory implies that species need to have a low supply stress to allocate a substantial fraction of their assimilation to reproduction. The AmP collection of data on energetics and DEB parameters and traits shows that the overall reproduction efficiency decreases with increasing trophic position, at least for the most extreme demand taxa: birds and mammals. This amplifies the difference of investing into reproduction at the bottom of the chain to efficient demand-driven predators at the top of the chain. The low overall production efficiency, i.e. the life-time cumulated energy in neonates over the life-time cumulated energy in consumed food, explains why the maximum length of food chain is 4 to 5; the hexapods have a relatively high efficiency which possibly links to why they are the most speciose animal taxon.