Tue, Aug 16, 2022: 2:15 PM-2:30 PM
515C
Background/Question/MethodsUnderstanding the causes behind population cycles is a fundamental issue in ecology. One of the most prominent examples of cyclic species is the snowshoe hare (Lepus americanus), a prey species in the North American boreal forest that fluctuates in abundance every 9-11 years. According to the chronic stress hypothesis, demographic changes associated with the cycle are thought to be driven by direct and indirect predation, with predator induced stress being the major cause of decreased reproduction during the decline and low phases. However, food limitation has recently been shown to impact hare demography through behaviour and condition, but the impact of food on stress physiology remains unknown. We examined the effect of food on the stress physiology of hares using an individual based food-supplementation experiment over-winter across three cycle phases that naturally varied in predation risk. We assessed various measures of stress and condition in late winter in both treatments using hormone challenges and measured hematological indices of body condition. The goal was to 1) determine if access to abundant high-quality food may buffer against the effects of chronic stress, and 2) evaluate the physiological measures of stress and condition in hares during different cycle phases when predation risk varies.
Results/ConclusionsWe found that food supplementation increased hare stress response to a hormone challenge but simultaneously increased their ability to buffer against this heightened response, through increased corticosteroid binding capacity. We also found that food supplementation improved the overall body condition of hares as supplemented individuals had higher % packed red-blood cell volume (indicator of condition) and lower body mass loss over winter. Most notably, we found that indicators of stress in snowshoe hares did not track the yearly changes in predator density nor predator-prey ratio, as predicted by the chronic stress hypothesis. Overall, the indicators of chronic stress were more prominent during the increase phase of the cycle. In conclusion, our results showed no clear pattern between population level stress physiology and demography, and instead provided evidence that increased stress, in regards to the parameters measured here, is not required to cause population declines.
Results/ConclusionsWe found that food supplementation increased hare stress response to a hormone challenge but simultaneously increased their ability to buffer against this heightened response, through increased corticosteroid binding capacity. We also found that food supplementation improved the overall body condition of hares as supplemented individuals had higher % packed red-blood cell volume (indicator of condition) and lower body mass loss over winter. Most notably, we found that indicators of stress in snowshoe hares did not track the yearly changes in predator density nor predator-prey ratio, as predicted by the chronic stress hypothesis. Overall, the indicators of chronic stress were more prominent during the increase phase of the cycle. In conclusion, our results showed no clear pattern between population level stress physiology and demography, and instead provided evidence that increased stress, in regards to the parameters measured here, is not required to cause population declines.