Energy flow through consumer-resource interactions is largely determined by
body size. Allometric relationships govern the dynamics of populations by
impacting rates of reproduction, as well as alternative sources of mortality,
which have differential impacts on smaller to larger organisms. Here we derive
and investigate the timescales associated with four alternative sources of
mortality for terrestrial mammals: mortality from starvation, mortality
associated with aging, mortality from consumption by predators, and mortality
introduced by anthropogenic subsidized harvest. The incorporation of these
allometric relationships into a minimal consumer-resource model illuminates
central constraints that may contribute to the structure of mammalian
communities. Our framework reveals that while starvation largely impacts
smaller-bodied species, the allometry of senescence is expected to be more
difficult to observe. In contrast, external predation and subsidized harvest
have greater impacts on the populations of larger-bodied species. Moreover, the
inclusion of predation mortality reveals mass thresholds for mammalian
herbivores, where dynamic instabilities may limit the feasibility of megafaunal
populations. We show how these thresholds vary with alternative predator-prey
mass relationships, which are not well understood within terrestrial systems.
Finally, we use our framework to predict the harvest pressure required to
induce mass-specific extinctions, which closely align with previous estimates
of anthropogenic megafaunal exploitation in both paleontological and historical
contexts. Together our results underscore the tenuous nature of megafaunal
populations, and how different sources of mortality may contribute to their
ephemeral nature over evolutionary time.Comment: 10 pages, 5 figures, 1 table, 4 appendices, 8 supplementary figure