Drivers of a temporal change in the adult sex ratio of a Roosevelt elk (Cervus canadensis roosevelti) population

Ecological processes driving female-skewed adult sex ratios (ASRs; males:females) in populations with polygynous mating systems have been addressed theoretically, but empirical support is scarce. The theoretical framework of the female substitution hypothesis (FSH) asserts that a female-skewed ASR at carrying capacity reflects an overall fitness benefit for females and for males competitive in acquiring access to reproductive females. The FSH predicts that as population abundance increases females should acquire forage more efficiently than males, thereby leading to passive displacement of males. The result is declining ASR associated with differential habitat use by both sexes as food resources are depleted by female scramble competition. We characterized the temporal variation in ASR in a Roosevelt elk population inhabiting the Redwood National and State Parks, California, across 24 years, and determined which of two possible ecological mechanisms was the driver of a declining ASR. The first mechanism explored was that increasing female abundance associated with declining forage in the study area led to the passive displacement of males into the study periphery over time. The second mechanism explored was that a declining ASR was precipitated by a lack of males within the study area and the study periphery. Systematic population surveys from a vehicle were done to estimate abundance and ASR as well as assess male abundance in the study periphery. Forage biomass was estimated in quarter-m2 plots randomly placed in meadows inhabited by female elk. Our multiple regression analysis revealed an inverse relationship between abundance and ASR indicating density dependence. We found numerous males in the study periphery when females were abundant. Our least squares models indicated declining food resources across years when female abundance increased. Our results showed that the first, and not the second, ecological mechanism examined was responsible for a female-skewed ASR. Our findings provide empirical support for the theoretical framework of the FSH in a nonmigratory population protected from hunting.

Context-dependent group size: effects of population density, habitat, and season

Group size can vary in relation to population density, habitat, and season. Habitat and season may also interact with population density and affect group size through varying foraging benefits of social aggregation in different ecological contexts. We tested the hypothesis that group size varies across ecological contexts, including population density, habitat type, and season, for woodland caribou (Rangifer tarandus) in ten herds over 25 years in Newfoundland, Canada. We predicted that group size would increase as a function of population density. Based on the foraging benefits of social aggregation, we predicted larger groups as habitat openness increased because open areas tend to have higher quality foraging resources. We predicted larger groups during winter when foraging resources are covered in snow because caribou and other social animals exploit social information about the location of foraging resources. In contrast to our prediction, group size decreased as a function of population density. In support of our prediction, group size was larger in winter than calving and summer, and we found that group size increased with habitat openness in some, but not all, cases. Patterns of animal grouping are context-dependent and the additive effect of different ecological contexts on variation in group size informs our understanding of the implicit trade-offs between competition, predation risk, and profitability of forage.