Data from: Density-dependent intraspecific aggression regulates survival in northern Yellowstone wolves (Canis lupus)

1. Understanding the population dynamics of top predators is essential to assess their impact on ecosystems and to guide their management. Key to this understanding is identifying the mechanisms regulating vital rates. 2. Determining the influence of density on survival is necessary to understand the extent to which human-caused mortality is compensatory or additive. In wolves (Canis lupus), empirical evidence for density-dependent survival is lacking. Dispersal is considered the principal way in which wolves adjust their numbers to prey supply or compensate for human exploitation. However, studies to date have primarily focused on exploited wolf populations, in which density-dependent mechanisms are likely weak due to artificially low wolf densities. 3. Using 13 years of data on 280 collared wolves in Yellowstone National Park, we assessed the effect of wolf density, prey abundance and population structure, as well as winter severity, on age-specific survival in two areas (prey-rich vs. prey-poor) of the national park. We further analysed cause-specific mortality and explored the factors driving intraspecific aggression in the prey-rich northern area of the park. 4. Overall, survival rates decreased during the study. In northern Yellowstone, density-dependence regulated adult survival through an increase in intraspecific aggression, independent of prey availability. In the interior of the park, adult survival was less variable and density-independent, despite reduced prey availability. There was no effect of prey population structure in northern Yellowstone, nor of winter severity in either area. Survival was similar among yearlings and adults, but lower for adults older than 6 years. 5. Our results indicate that density-dependent intraspecific aggression is a major driver of adult wolf survival in northern Yellowstone, suggesting intrinsic density-dependent mechanisms have the potential to regulate wolf populations at high ungulate densities. When low prey availability or high removal rates maintain wolves at lower densities, limited inter-pack interactions may prevent density-dependent survival, consistent with our findings in the interior of the park.

1. 探明顶级捕食者的种群动态，对评估其对生态系统的影响、指导其保护管理至关重要，而明确调控其生命率的机制则是实现这一认知的核心所在。
2. 明确种群密度对存活率的影响，有助于解析人类活动导致的死亡在多大程度上属于补偿性死亡或添加性死亡。针对灰狼（Canis lupus）的相关研究中，迄今仍缺乏密度依赖型存活率的实证证据。学界普遍认为扩散是灰狼根据猎物供给调整种群规模，或补偿人类猎杀压力的主要途径。但截至目前的相关研究多聚焦于受人为猎杀的灰狼种群——这类种群因人为因素导致种群密度偏低，其密度依赖调控机制往往较弱。
3. 本研究依托黄石国家公园（Yellowstone National Park）内280只佩戴项圈的灰狼的13年监测数据，针对公园内猎物丰富区与猎物匮乏区两个区域，分析了灰狼种群密度、猎物丰度、种群结构以及冬季严酷程度对灰狼年龄特异性存活率的影响。此外，我们还开展了死因特异性死亡分析，并探究了公园北部猎物丰富区域内驱动种内攻击行为的相关因素。
4. 整体而言，研究期间灰狼的存活率呈下降趋势。在黄石公园北部，密度依赖机制通过种内攻击行为的加剧调控成体存活率，且这一调控不受猎物可获得性的影响。而在公园内陆区域，尽管猎物可获得性有所下降，成体存活率的波动程度更低且不受密度依赖调控。黄石公园北部的猎物种群结构以及两个区域的冬季严酷程度，均未对灰狼存活率产生显著影响。亚成体与成体的存活率相近，但6龄以上成体的存活率显著更低。
5. 本研究结果表明，密度依赖型种内攻击行为是黄石公园北部灰狼成体存活率的主要调控因子，这暗示内在的密度依赖调控机制，有能力在有蹄类猎物密度较高的环境中调控灰狼种群规模。当猎物可获得性偏低，或人为猎杀率偏高导致灰狼种群维持在较低密度时，狼群间的互动有限，或可阻碍密度依赖型存活率调控的发生——这与我们在公园内陆区域观测到的结果一致。