28. Pathways of Density Dependence and Natural Selection in Modern Humans

Abstract

Living things depend on a flow of energy and materials to grow, reproduce, and maintain their bodies. Populations are aggregations of individuals, so they too depend on resources. Humans use many fuels derived from the Earth’s photosynthetic energy, which in turn support a population that often occurs at unusually high densities for a mammal. Like most populations, growing human populations may experience negative feedbacks from population size unless the socio-economic system in which the population lives grows fast enough to maintain resource flows to individuals and to limit the downsides of high density. I map out a simple view of the pathways of density dependence through five main causes of negative feedback: poor nutrition, increased disease, increased toxins, altered life history strategies, and violent conflict. The pathways trace the different ways in which increasing population size can cause lower birth rates or higher death rates and set the stage for selection on contemporary human populations. Some of the pathways are not traditionally viewed as density-dependent, but since they all depend on a tension between population size and the ability of the socio-economic system to generate positive feedbacks, they are all a form of density-dependence. These pathways are also dependent on changes to the global environment, including warmer and more variable climates, and the way people respond to the feedbacks by altering socio-economic expectations or technology.