Abstract
Although most large-herbivore populations are regulated by a combination of top-down and bottom-up forces, relationships between herbivores and their habitat are of fundamental importance to wildlife managers. Variation in nutritional resources, and how herbivores respond to that variation, influences rates of nutrient intake, which directly affects nutritional condition, pregnancy rates, timing of parturition, offspring birth mass, and survival. Accordingly, nutritional approaches hold great potential for uncovering the mechanisms that govern population performance of large herbivores, and for assessing the nature and magnitude of bottom-up limitation. We quantified relationships between the foodscape (i.e., spatiotemporal variation in forage quality and abundance) in southwestern Oregon and black-tailed deer (Odocoileus hemionus columbianus) behavior and demography, with a focus on the influence of nutrition on fawn survival. We hypothesized (1) that performance of black-tailed deer is limited by the availability of high-quality forage, and (2) that foodscape use by maternal females during spring and summer directly influences reproductive success (i.e., fawn birth mass and survival). From 2016–2023 we collared adult female black-tailed deer and their offspring to track movement and survival. We also conducted intensive vegetation sampling and used generalized additive modeling to map the foodscape available to deer. Suitable forage biomass (i.e., biomass of forage that exceeded quality thresholds for supporting one fawn) was highly variable across space and time, and our top foodscape model explained 70% of that variation (adjusted R2 = 0.70). We observed a strong, positive relationship between foodscape use by maternal females prior to parturition and fawn birth mass. Although maternal foodscape use after parturition did not influence the probability of fawn survival, survival increased with birth mass. These results suggest that the effects of nutrition on fawn survival in our study system are indirectly mediated by dam behavior (i.e., foodscape use) and the corresponding effects on fawn birth mass. Our study adds to a growing body of literature supporting a fundamental link between foodscape use and population performance of large herbivores. Wildlife managers can use the dynamic models we developed to assess habitat quality and to make quantitative predictions about how different management actions (e.g., forest thinning) are likely to influence habitat quality and performance of black-tailed deer.