When anthropogenic-related disturbances overwhelm demographic persistence mechanisms

1. Population decline is associated with increased vulnerability to extinction, but also with possible density-, frequency-, or distance-related ‘rarity advantages’ that increase recruitment success as individuals become isolated from their congeners. Distinguishing between these alternatives (risk versus recovery of rare populations via demographic processes) has become critical, given how anthropogenic disturbances are causing population declines globally. 2. Here, we demonstrate how distance-related rarity advantages are evident in spatially isolated recruits of a canopy-dominant but regionally rare species of oak that appears to be suffering recruitment collapse. As distance from parent trees increased, seedlings had significantly more leaves and experienced reduced insect browsing and intraspecific competition. Long-term field-based experimental treatments revealed these advantages to be associated with rapid rates of juvenile maturation and survival that are unobserved in natural settings. 3. The discrepancy between the experimental and natural settings was explained by trophic collapse and habitat loss - two changes ubiquitous to many terrestrial ecosystems – that combine to concentrate vertebrate herbivores in habitat remnants and cause 100% juvenile mortality via the browsing of taller juveniles. Exotic grass cover, long associated with oak recruitment failure, significantly suppressed seedling height and leaf production, but appeared to delay mortality by hiding shorter seedlings from vertebrate herbivores. 4. Synthesis. Our work demonstrates how rarity advantages have the potential to positively influence the population performance of a declining species, but are short-circuited by intense herbivory associated with human-based environmental change. Regionally, there appear to be few existing conditions on the contemporary landscape that favor juvenile survival, suggesting ongoing recruitment difficulties without intervention. Our work clarifies how extinction risk can in some cases be best defined by how anthropogenic disturbances affect, and are offset by, demographic-based persistence mechanisms, than simply by present-day abundance or distribution.

1. 种群下降既会提升物种的灭绝易感性，同时也可能带来与密度、频率或距离相关的稀有优势（rarity advantages）——当个体与其同种个体相互隔离时，这类优势可提升种群补充成功率。鉴于全球范围内人为干扰（anthropogenic disturbances）正不断引发种群下降，区分这两种对立情形（即稀有种群通过种群动态过程（demographic processes）面临灭绝风险，或是实现种群恢复）已成为至关重要的议题。
2. 本研究证实，在一种冠层优势但区域稀有的栎属物种的空间隔离补充个体中，与距离相关的稀有优势表现显著——该物种正面临种群补充崩溃的困境。随着与母树的距离增加，幼苗的叶片数量显著增多，同时遭受的昆虫啃食与种内竞争压力均有所降低。长期野外受控实验结果显示，这类优势与幼体快速成熟及存活速率相关，而这一现象在自然环境中并未被观测到。
3. 实验环境与自然环境间的差异可通过营养级崩溃（trophic collapse）与栖息地丧失得到解释——这两种变化在众多陆地生态系统中均普遍存在，它们共同促使脊椎动物草食动物聚集在栖息地残存片段中，并通过啃食较高的幼体导致100%的幼体死亡。长期以来被认为与栎树种群补充失败相关的外来草本覆盖，显著抑制了幼苗的株高增长与叶片生成，但似乎通过将矮小幼苗隐藏起来，使其免受脊椎动物草食动物的啃食，从而延缓了幼苗的死亡。
4. 综合分析与结论：本研究表明，稀有优势具备对种群下降的物种种群表现产生积极影响的潜力，但这类优势会因人为驱动的环境变化引发的高强度草食作用而被阻断。从区域尺度来看，当前景观中几乎不存在有利于幼体存活的条件，这意味着若无人为干预，种群补充困难的问题将持续存在。本研究阐明，在某些情况下，灭绝风险的最佳界定方式并非仅依据当前的物种丰度或分布范围，而是取决于人为干扰如何影响以及基于种群动态的存续机制（demographic-based persistence mechanisms）如何抵消这类影响。