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. 早自20世纪20年代查尔斯·埃尔顿（Charles Elton）的开创性研究以来，种群周期便一直是生态学界备受关注的核心课题之一。在未受人为干扰的生态系统中发现大规模种群波动的现象，直接挑战了“原始自然处于绝对平衡状态”的传统认知。分布于加拿大与阿拉斯加寒温带针叶林内的雪兔（snowshoe hare, Lepus americanus Erxleben）10年周期，是经典的种群周期案例，这一现象早在300余年前便被毛皮贸易从业者所记录。
2. 自20世纪30年代起，生态学家们便持续探究驱动此类种群周期的潜在机制。先后被提出的成因假说涵盖太阳黑子活动、食物资源、寄生虫感染、疾病、捕食作用以及社会行为等诸多方向。在完整的种群周期中，出生率与死亡率均会发生剧烈波动。由于雪兔不具备领地习性且不存在杀婴行为，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，大规模操控性野外实验被用于甄别种群的核心限制因子。食物供应与捕食作用很快便被认定为引发该种群周期的潜在关键因素。其中，向野外种群补充食物以及限制捕食者接触的实验，最终精准确定了捕食作用是引发此类种群波动的核心机制。
4. 绝大多数雪兔的直接致死原因是各类捕食者的捕食行为，其中包括加拿大猞猁（Lynx canadensis Kerr）。种群繁殖率的骤降并非如早期研究所推测那般由食物短缺导致，而是捕食者持续追逐引发的慢性应激反应的结果。
5. 目前仍有五大核心问题尚未得到解决。其一，引发种群周期持续处于低谷阶段的捕食者诱导记忆，其本质究竟为何？其二，雪兔的种群周期为何会形成传播波：从萨斯喀彻温省的寒温带针叶林中心区域起始，逐步蔓延至加拿大西部与阿拉斯加全境？其三，在同一区域内，为何不同周期的种群波动幅度会存在显著差异？其四，北美东部的雪兔种群或是西伯利亚地区类似生态系统中的雪兔种群，是否也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？上述问题的答案，仍有待后世生物学家们逐一揭晓。