Hurley Mark A, Hebblewhite Mark, Gaillard Jean-Michel
Idaho Department of Fish and Game Boise ID USA.
Wildlife Biology Program Department of Ecosystem Sciences and Conservation W.A. Franke College of Forestry and Conservation University of Montana Missoula MT USA.
Ecol Evol. 2020 Jun 9;10(12):5464-5475. doi: 10.1002/ece3.6289. eCollection 2020 Jun.
Density-dependent competition for food reduces vital rates, with juvenile survival often the first to decline. A clear prediction of food-based, density-dependent competition for large herbivores is decreasing juvenile survival with increasing density. However, competition for enemy-free space could also be a significant mechanism for density dependence in territorial species. How juvenile survival is predicted to change across density depends critically on the nature of predator-prey dynamics and spatial overlap among predator and prey, especially in multiple-predator systems. Here, we used a management experiment that reduced densities of a generalist predator, coyotes, and specialist predator, mountain lions, over a 5-year period to test for spatial density dependence mediated by predation on juvenile mule deer in Idaho, USA. We tested the spatial density-dependence hypothesis by tracking the fate of 251 juvenile mule deer, estimating cause-specific mortality, and testing responses to changes in deer density and predator abundance. Overall juvenile mortality did not increase with deer density, but generalist coyote-caused mortality did, but not when coyote density was reduced experimentally. Mountain lion-caused mortality did not change with deer density in the reference area in contradiction of the food-based competition hypothesis, but declined in the treatment area, opposite to the pattern of coyotes. These observations clearly reject the food-based density-dependence hypothesis for juvenile mule deer. Instead, our results provide support for the spatial density-dependence hypothesis that competition for enemy-free space increases predation by generalist predators on juvenile large herbivores.
食物的密度依赖性竞争会降低生命率，其中幼崽存活率通常是最先下降的。对于大型食草动物基于食物的密度依赖性竞争，一个明确的预测是随着密度增加幼崽存活率会下降。然而，对无天敌空间的竞争也可能是领域性物种密度依赖性的一个重要机制。幼崽存活率如何随密度变化预测，关键取决于捕食者 - 猎物动态的性质以及捕食者和猎物之间的空间重叠，特别是在多捕食者系统中。在这里，我们进行了一项管理实验，在5年时间里降低了广食性捕食者郊狼和专食性捕食者美洲狮的密度，以测试在美国爱达荷州对幼年骡鹿捕食所介导的空间密度依赖性。我们通过追踪251只幼年骡鹿的命运、估计特定原因死亡率以及测试对鹿密度和捕食者数量变化的反应，来检验空间密度依赖性假说。总体而言，幼崽死亡率并未随鹿密度增加而上升，但广食性郊狼导致的死亡率上升了，但在实验性降低郊狼密度时并非如此。与基于食物竞争假说相反，在参考区域美洲狮导致的死亡率并未随鹿密度变化，但在处理区域下降了，与郊狼的模式相反。这些观察结果明确否定了幼年骡鹿基于食物的密度依赖性假说。相反，我们的结果为空间密度依赖性假说提供了支持，即对无天敌空间的竞争增加了广食性捕食者对幼年大型食草动物的捕食。
