Seasonal Effects of Habitat on Sources and Rates of Snowshoe Hare Predation in Alaskan Boreal Forests

Survival and predation of snowshoe hares (Lepus americanus) has been widely studied, yet there has been little quantification of the changes in vulnerability of hares to specific predators that may result from seasonal changes in vegetation and cover. We investigated survival and causes of mortalities of snowshoe hares during the late increase, peak, and decline of a population in interior Alaska. From June 2008 to May 2012, we radio-tagged 288 adult and older juvenile hares in early successional and black spruce (Picea mariana) forests and, using known-fate methods in program MARK, evaluated 85 survival models that included variables for sex, age, and body condition of hares, as well as trapping site, month, season, year, snowfall, snow depth, and air temperature. We compared the models using Akaike’s information criterion with correction for small sample size. Model results indicated that month, capture site, and body condition were the most important variables in explaining survival rates. Survival was highest in July, and more generally during summer, when alternative prey was available to predators of hares. Low survival rates coincided with molting periods, breeding activity in the spring, and the introduction of juveniles to the sample population in the fall. We identified predation as the cause of mortality in 86% of hare deaths. When the source of predation could be determined, hares were killed more often by goshawks (Accipiter gentilis) than other predators in early successional forest (30%), and more often by lynx (Lynx canadensis) than other predators in black spruce forest (31%). Great horned owls (Bubo virginianus) and coyotes (Canis latrans) represented smaller proportions of hare predation, and non-predatory causes were a minor source (3%) of mortality. Because hares rely on vegetative cover for concealment from predators, we measured cover in predation sites and habitats that the hares occupied and concluded that habitat type had a greater influence on the sources of predation than the amount of cover in any given location within a habitat. Our observations illustrate the vulnerability of hares to predators in even the densest coniferous habitat available in the boreal forest, and indicate strong seasonal changes in the rates and sources of predation.

北美雪兔（Lepus americanus）的生存与被捕食动态已得到广泛研究，但针对因植被与遮蔽物的季节变化所引发的雪兔对特定捕食者的易感性变化，相关量化研究仍较为匮乏。本研究于阿拉斯加内陆地区，在雪兔种群的后期增长、峰值及衰减三个阶段，对其生存状况与死亡原因展开调查。2008年6月至2012年5月期间，研究人员在早期演替林与黑云杉（Picea mariana）林生境中，共对288只成年及老龄幼兔进行了无线电标记；随后借助MARK软件的已知命运模型，评估了85个生存模型，模型纳入的变量包括雪兔的性别、年龄、身体状况，以及捕获位点、月份、季节、年份、降雪量、雪深与气温。研究采用校正后的小样本赤池信息准则（Akaike’s information criterion）对模型进行比较。模型结果显示，月份、捕获位点与身体状况是解释存活率的核心变量。夏季（尤其7月）的存活率最高，彼时捕食者拥有替代猎物可供选择。低存活率时段则对应换毛期、春季繁殖活动，以及秋季幼兔加入种群样本的时期。研究确认86%的雪兔死亡原因为捕食。当捕食来源可被判定时，早期演替林中苍鹰（Accipiter gentilis）为主要捕食者（占比30%）；而在黑云杉林中，加拿大猞猁（Lynx canadensis）的捕食占比最高（31%）。大雕鸮（Bubo virginianus）与郊狼（Canis latrans）的捕食占比相对较低，非捕食性死亡仅占总死亡的3%。鉴于雪兔依赖植被遮蔽以躲避捕食者，研究人员对捕食位点与雪兔栖息生境的遮蔽物覆盖度进行了测量，结果表明，相较于某一生境内特定位置的遮蔽物总量，生境类型对捕食来源的影响更为显著。本研究结果揭示，即便在寒温带针叶林可获取的最茂密针叶生境中，雪兔仍面临捕食者的威胁，同时证明捕食率与捕食来源存在显著的季节动态变化。