Data from: Using experimentation to understand the 10-year snowshoe hare cycle in the boreal forest of North America

1. Population cycles have long fascinated ecologists from the time of Charles Elton in the 1920s. The discovery of large population fluctuations in undisturbed ecosystems challenged the idea that pristine nature was in a state of balance. The 10-year cycle of snowshoe hares (Lepus americanus Erxleben) across the boreal forests of Canada and Alaska is a classic cycle, recognized by fur traders for more than 300 years. 2. Since the 1930s ecologists have investigated the mechanisms that might cause these cycles. Proposed causal mechanisms have varied from sunspots to food supplies, parasites, diseases, predation, and social behaviour. Both the birth rate and the death rate change dramatically over the cycle. Social behaviour was eliminated as a possible cause because snowshoe hares are not territorial and do not commit infanticide. 3. Since the 1960s large-scale manipulative experiments have been used to discover the major limiting factors. Food supply and predation quickly became recognized as potential key factors causing the cycle. Experiments adding food and restricting predator access to field populations have been decisive in pinpointing predation as the key mechanism causing these fluctuations. 4. The immediate cause of death of most snowshoe hares is predation by a variety of predators, including the Canada lynx (Lynx canadensis Kerr). The collapse in the reproductive rate is not due to food shortage as was originally thought, but is a result of chronic stress from predator chases. 5. Five major issues remain unresolved. First, what is the nature of the predator-induced memory that results in the prolonged low phase of the cycle? Second, why do hare cycles form a travelling wave, starting in the centre of the boreal forest in Saskatchewan and travelling across western Canada and Alaska? Third, why does the amplitude of the cycle vary greatly from one cycle to the next in the same area? Fourth, do the same mechanisms of population limitation apply to snowshoe hares in eastern North American or in similar ecosystems across Siberia? Finally, what effect will climatic warming have on all the above issues? The answers to these questions remain for future generations of biologists to determine.1. 种群周期（population cycles）自20世纪20年代查尔斯·埃尔顿（Charles Elton）的时代起，便长期吸引着生态学家的关注。在未受干扰的生态系统中发现的大规模种群波动，对“原生自然处于平衡状态”这一传统观点提出了挑战。加拿大与阿拉斯加寒带森林中的美洲兔（snowshoe hare，*Lepus americanus* Erxleben）的10年周期是经典的种群周期案例，早在300多年前便被毛皮商人所认知。 2. 自20世纪30年代起，生态学家便着手探究引发此类周期现象的潜在机制。所提出的因果机制涵盖范围极广，从太阳黑子活动、食物供给，到寄生虫感染、疾病传播、捕食行为与社会行为。在该周期进程中，种群的出生率与死亡率均会发生剧烈变化。由于美洲兔不具有领地意识，且不存在杀婴行为，社会行为这一假说已被排除在可能的成因之外。 3. 自20世纪60年代起，大规模野外操控实验被用于探寻种群动态的主要限制因子。食物供给与捕食行为很快被认定为引发该周期的潜在关键因素。其中，通过向野外种群补充食物以及限制捕食者接触的实验，最终明确了捕食行为才是导致种群波动的核心机制。 4. 绝大多数美洲兔的直接死因来自各类捕食者的猎杀，其中包括加拿大猞猁（Canada lynx，*Lynx canadensis* Kerr）。种群繁殖率的下降并非如最初所想那般由食物短缺所致，而是捕食者持续追逐引发的慢性应激所导致的结果。 5. 目前仍有五大核心问题尚未解决。其一，捕食者诱导产生的记忆究竟具有何种特性，使得种群周期得以维持在长期的低谷阶段？其二，为何美洲兔的种群周期会形成传播行波：从加拿大萨斯喀彻温省的寒带森林中心起始，随后横跨加拿大西部与阿拉斯加？其三，为何同一区域内的不同种群周期，其波动幅度会存在巨大差异？其四，北美东部或西伯利亚类似生态系统中的美洲兔种群，是否适用相同的种群限制机制？最后，气候变暖将对上述所有问题产生何种影响？这些问题的答案，仍有待未来一代生物学家去揭晓。