How resource competition shapes individual life history for nonplastic growth: Ungulates in seasonal food environments

We analyze an age-, size- and sex-structured model to investigate how the interplay between individual-level energy budget dynamics and the feedback of population grazing on resources shapes the individual life history and the dynamics of ungulate populations, living in a predator-free, seasonal resource environment. We formulate a dynamic energy budget model for individual energetics, which accounts for energy requirements for maintenance and growth, and possibly pregnancy and lactation. Growth in structural mass is assumed prescribed. Dynamics of energy reserves are the resultant of energy acquisition through grazing and suckling of milk and the aforementioned energy-consuming processes. The dynamic energy budget model is used as the core for an individual-based population model, which captures general features of ungulate life history and population dynamics, although it is parameterized for a particular system. Model predictions reveal a characteristic dynamic pattern, in which years with low death tolls (<10% of the population dying) alternate with a single year of high death toll (up to 40% of the population dies). In these "collapse" years almost all individuals younger than 2 years die, creating holes in the population age distribution. The die-off of these age classes is shown to be caused by the energy requirements for growth that these individuals face. Individuals between 1 and 2 years of age are more at risk than foals, because they are burdened with the legacy of a poor body condition developed throughout their first winter. The characteristic dynamic pattern is more pronounced at high levels of resource productivity. In contrast, neither a period of snow cover, during which all foraging stops, nor a dependence of fecundity on female body condition change dynamics significantly.