Data from: The fluctuating world of a tundra predator guild: bottom-up constraints overrule top-down species interactions in winter

Global warming is predicted to change ecosystem functioning and structure in Arctic ecosystems by strengthening top-down species interactions, i.e. predation pressure on small herbivores and interference between predators. Yet, previous research is biased towards the summer season. Due to greater abiotic constraints, Arctic ecosystem characteristics might be more pronounced in winter. Here we test the hypothesis that top-down species interactions prevail over bottom-up effects in Scandinavian mountain tundra (Northern Sweden) where effects of climate warming have been observed and top-down interactions are expected to strengthen. But we test this a-priori hypothesis in winter and throughout the 3-4 year rodent cycle, which imposes additional pulsed resource constraints. We used snowtracking data recorded in 12 winters (2004-2015) to analyse the spatial patterns of a tundra predator guild (arctic fox Vulpes lagopus, red fox Vulpes vulpes, wolverine Gulo gulo) and small prey (ptarmigan, Lagopus spp). The a-priori top-down hypothesis was then tested through structural equation modelling, for each phase of the rodent cycle. There was weak support for this hypothesis, with top-down effects only discerned on arctic fox (weakly, by wolverine) and ptarmigan (by arctic fox) at intermediate and high rodent availability respectively. Overall, bottom-up constraints appeared more influential on the winter community structure. Cold specialist predators (arctic fox and wolverine) showed variable landscape associations, while the boreal predator (red fox) appeared strongly dependent on productive habitats and ptarmigan abundance. Thus, we suggest that the unpredictability of food resources determines the winter ecology of the cold specialist predators, while the boreal predator relies on resource rich habitats. The constraints imposed by winters and temporary resource lows should therefore counteract productivity-driven ecosystem change and have a stabilizing effect on community structure. Hence, the interplay between summer and winter conditions should determine the rate of Arctic ecosystem change in the context of global warming.据预测，全球变暖将通过强化下行（top-down）物种互作——即对小型植食动物的捕食压力与捕食者间的种间干扰——改变北极生态系统的功能与结构。然而，既往研究多偏向夏季季节。由于非生物胁迫更为严苛，北极生态系统的特征在冬季可能表现得更为显著。我们在此检验一项假说：在斯堪的纳维亚山地苔原（瑞典北部）这一已观测到气候变暖影响、且下行互作预计会增强的区域，下行物种互作的效应强于上行（bottom-up）效应。但我们是在冬季以及完整的3-4年啮齿动物周期内检验这一先验（a-priori）假说，该周期会带来额外的脉冲式资源限制。我们使用2004-2015年间12个冬季的雪地追踪数据，分析了苔原捕食者功能群[北极狐（Vulpes lagopus）、赤狐（Vulpes vulpes）、貂熊（Gulo gulo）]与小型猎物[岩雷鸟（Lagopus spp.）]的空间分布格局。随后，我们针对啮齿动物周期的各个阶段，通过结构方程模型检验这一先验下行假说。该假说仅得到微弱支持：下行效应仅分别在啮齿动物资源处于中等和高水平时，对北极狐（受貂熊的微弱下行调控）与岩雷鸟（受北极狐捕食）显著可见。总体而言，上行资源限制对冬季群落结构的影响更为显著。寒带特化捕食者（北极狐与貂熊）的景观关联特征存在变异性，而北方域捕食者赤狐则显著依赖高生产力生境与岩雷鸟种群丰度。据此我们认为，食物资源的不可预测性决定了寒带特化捕食者的冬季生态策略，而北方域捕食者则依赖资源富集的生境。因此，冬季环境限制与临时性资源匮乏应会抵消由生产力驱动的生态系统变化，并对群落结构起到稳定作用。综上，在全球变暖的背景下，夏季与冬季环境条件的相互作用将决定北极生态系统的变化速率。