The energetic basis of population growth in animal kingdom

Population growth, and other population characteristics, have been computed and made available online for over 2000 animal species in the Add-my-Pet (AmP) collection, assuming constant food and temperature environments. The AmP collection – online database of Dynamic Energy Budget model parameters, implied properties and referenced underlying data – provides an unique opportunity to study how energetics of individuals relates to population growth. For the comparisons of traits, we assume that the background hazard rate is zero, but aging applies to all species and ‘thinning’ to species with high reproduction rates. The new concept ‘thinning’ is a state-dependent hazard rate such that the feeding rate of a cohort does not change in time: the increase of individual feeding rates due to growth is exactly compensated by a reduction in numbers. Thinning affects population growth rate, but the impact differs substantially between species. Some 11% of species do not survive thinning, even at abundant food. The population growth rate relates to the underlying energetics; we discuss and suggest explanations for how population growth rates fit into all known patterns in the co-variation of parameter values: body size-scaling, metabolic acceleration, waste-to-hurry, supply-demand spectrum and altricial-precocial spectrum. We show that, after reproduction, age at puberty dominates population growth. The specific population growth rate scales with maximum body weight in the same way as the weight-specific respiration scales with body weight. DEB theory, which explains both, shows, however, that no direct relationship exists between the population growth rate and respiration. We suggest that the similarity in scaling results from the equality between specific population growth and specific growth rate at maximum growth of structure, and might be an evolutionary relict from times that life consisted of dividing unicellulars; population and body growth are directly connected for unicellulars. We show that the specific growth rate at maximum growth equals 1.5 times the von Bertalanffy growth rate, in a DEB context, which is a new interpretation of the latter growth rate. We expected the population growth rate to co-vary with specific somatic maintenance rate, based on a previously discovered pattern, called the waste-to-hurry strategy, where growth and reproduction are increased by simultaneously increasing assimilation and somatic maintenance in species that live off temporarily abundant food supplies. We did find this effect in ecdysozoa and spiralia, which comprise roughly 95% of animal species, but hardly so in tetrapods. The reason might be that specific somatic maintenance also co-varies with specific maturity levels at puberty for tetrapods. The scaled functional response at which the population growth rate is zero is very close to that at which puberty can just be reached in absence of thinning, and somewhat higher in presence of thinning. The specific population growth rate at abundant food correlates negatively with the functional response for which population growth rate is zero. It also correlates negatively with the precociality index, i.e. the ratio of maturity levels at puberty and birth: the more precocial, the larger neonate size, the smaller reproduction rate, especially in restricted taxa such as mammals and cartilaginous fish. Like other traits, the population growth rate shows considerable segregation among taxa, where mammals have a relatively low rate, glires a relatively high rate among mammals, followed by marsupials; afrotherians have the lowest population growth rates.