Linking species to ecosystems: predation-mediated nutrient flows

生态学理论和应用研究的一个基本问题是物种如何影响生态系统功能。越来越多的证据表明，大型食草动物对植物群落内的养分循环有很大的影响。捕食者是否能直接通过减少食草动物数量或间接通过改变食草动物行为来改变营养动态尚不清楚。我的研究解决了食草动物如何在“恐惧的环境”中改变它们的行为以避开捕食者，然后评估了营养流动的后果。利用阿尔伯塔省中东部斜坡的野外研究数据，我检查了狼和麋鹿分布在时间和空间上的相关性，以测试麋鹿是通过集中在狼没有使用的地区（即避难所），还是通过离开狼经常出没的地区（例如，空壳游戏），或者避开狼首选猎物使用的栖息地（例如，隔离）来避开狼。然后，通过直接观察麋鹿在开阔草原上扫描捕食者（即警惕），我确定了警惕对摄食的影响。根据咬伤率和植物生物量之间的关系，我预测动物有多少“空闲时间”可以用来扫描捕食者，并将其与观察到的分配给警惕的时间进行比较。接下来，我确定咬伤率是否会随着动物花更多的时间保持警惕而下降，以及这种情况如何随着同种觅食者的数量而变化。最后，我通过比较从低到高狼捕食风险梯度地区的净碳和氮回报（粪便和尿液中减去饲料）来评估麋鹿分布和摄食行为的变化是否会改变养分流动。我的工作提供了一种机制的理解，即捕食的影响如何通过营养流动向下级联到较低的营养水平，这在目前大型哺乳动物系统中是不可用的。

A fundamental issue of theoretical and applied research in ecology is how species affect ecosystem functioning. Evidence has accumulated that large-bodied herbivores exert a strong influence on nutrient cycling within plant communities. Whether predators can modify nutrient dynamics either directly by reducing herbivore numbers or indirectly by altering herbivore behaviour remains unclear. My research addresses how herbivores alter their behaviours to avoid predators in a "landscape of fear", and then evaluates consequences for nutrient flow. Using data from field studies in the central east slopes of Alberta, I examine correlations between the distributions of wolves and elk in both time and space to test whether elk avoid wolves by concentrating in areas not used by wolves (i.e., refuges), by moving out of areas when wolves frequent them (e.g., shell game), or by avoiding habitats used by the wolves' preferred prey (e.g., segregation). Then, using direct observations of elk scanning for predators (i.e., vigilance) in open grasslands, I determine the effect of vigilance on intake. From a relationship between bite rate and plant biomass I predict the amount of "spare time" animals have available to scan for predators, and compare this to the observed time allocated to vigilance. Next, I determine whether bite rate declines as animals spend more time vigilant and how this changes with the number of conspecific foragers. Finally, I assess whether changes in elk distribution and feeding behavior alter nutrient flows by comparing net carbon and nitrogen return (in feces and urine minus forage removed) across a gradient of areas from low to high wolf predation risk. My work provides a mechanistic understanding of how the influence of predation can cascade down to lower trophic levels through nutrient flows that is not currently available for large mammal systems.