Demographic variability and density‐dependent dynamics of a free‐ranging rhesus macaque population

Abstract

Density-dependence is hypothesized as the major mechanism of population regulation. However, the lack of long-term demographic data has hampered the use of density-dependent models in nonhuman primates. In this study, we make use of the long-term demographic data from Cayo Santiago’s rhesus macaques to parameterize and analyze both a density-independent and a density-dependent population matrix model, and compare their projections with the observed population changes. We also employ a retrospective analysis to determine how variance in vital rates, and covariance among them, contributed to the observed variation in long-term fitness across different levels of population density. The population exhibited negative density-dependence in fertility and the model incorporating this relationship accounted for 98% of the observed population dynamics. Variation in survival and fertility of sexually active individuals contributed the most to the variation in long-term fitness, while vital rates displaying high temporal variability exhibited lower sensitivities. Our findings are novel in describing density-dependent dynamics in a provisioned primate population, and in suggesting that selection is acting to lower the variance in the population growth rate by minimizing the variation in adult survival at high density. Because density-dependent mechanisms may become stronger in wild primate populations due to increasing habitat loss and food scarcity, our study demonstrates it is important to incorporate variation in population size, as well as demographic variability into population viability analyses for a better understanding of the mechanisms regulating the growth of primate populations.

Managing the Cayo Santiago rhesus macaque population: The role of density

“…Maintaining long-term viability of this population allows for the continuation of studies of primate populations which are difficult or impossible to address in the wild [Southwick, 1989]. These include studies on post-partum mating control [Johnson et al, 1993], age-related differences in vital rates [Bercovitch & Berard, 1993; Hoffman et al, 2010; Hoffman & Maestripieri, 2012], sex-related differences in vital rates [Manson, 1994; Hoffman et al, 2008], social ranking-related demographics [Blomquist et al, 2011], and density-dependent dynamics [Hernández-Pacheco et al, 2013]. Maintaining the long-term viability of the Cayo Santiago rhesus macaque population requires development of modeling tools that will allow the analysis of the effects of management practices on the size, growth rate and structure of the population.…”

“…Vital rates at Cayo Santiago are characteristically higher than those of wild rhesus populations, resulting in remarkably high mean annual population growth rates (~12%) [Koford, 1966; Rawlins et al, 1984; Rawlins & Kessler, 1986b; Hernández-Pacheco et al, 2013]. Such demographics forced management efforts towards live capture and removal of individuals (removals from now on) to maintain what was estimated as the optimum size for ongoing research (300 individuals), a size determined principally by cost of colony maintenance (e.g., food supply, staff) and visual census taking efforts [Carpenter, 1972; Rawlins, 1989].…”

“…Such demographics forced management efforts towards live capture and removal of individuals (removals from now on) to maintain what was estimated as the optimum size for ongoing research (300 individuals), a size determined principally by cost of colony maintenance (e.g., food supply, staff) and visual census taking efforts [Carpenter, 1972; Rawlins, 1989]. Although different removal schemes have been implemented since then, population dynamics at Cayo Santiago has never been stable [Hernández-Pacheco et al, 2013]. In fact, the colony has been characterized by periods of exponential growth following removal events, raising questions regarding the effects of animal removal on the dynamics of the population [Carpenter, 1972; Rawlins, 1989].…”

“…), and (3) by age as immature individuals require less trapping effort and are better candidates for the Caribbean Primate Research Center’s NIH-supported specific-pathogen-free (SPF) breeding colonies at its Sabana Seca Field Station (SSFS). The observed dynamics from 1973 to 2000 indicate that the colony has exhibited several periods of exponential growth that have been interrupted only by removal events [Hernández-Pacheco et al, 2013] (see also Fig. 1).…”

“…Although such density-independent models have proven to be accurate in describing Cayo Santiago’s dynamics during decadal periods of low density [Rawlins & Kessler, 1986b], the population has recently been found to be regulated by negative density-dependence in female fertility [Hernández-Pacheco et al, 2013]. This finding needs to be incorporated into colony management as the population’s vital rates are differentially affected by removal events.…”