Divergent effects of forest edges on host distribution and seed disperser activity influence mistletoe distribution and recruitment

1. Species interactions define functional diversity and community stability across ecosystems, and depend on the spatial distribution, the habitat requirements, and the sensitivity to disturbances of all interacting partners. Hence, assessing the effects of such anthropogenic disturbances on multi-species interactions may be essential to improve adaptation and mitigation measures for biodiversity conservation. 2. We determined the importance of edge effects on the interaction and distribution of three keystone species in South American temperate rainforests: the hemiparasitic mistletoe Tristerix corymbosus, its main host (the liana Campsidium valdivianum) and its only seed disperser (the marsupial Dromiciops gliroides). 3. The discordant impacts of forest edges on host (positive) and seed disperser (negative) affected mistletoe distribution at large spatial scales, owing to the combined effects of increased dispersal limitation and decreased host availability. More importantly, marsupial abundance had contrasting effects on mistletoe abundance at small and large spatial scales – suggesting a potential trade-off between local and long-distance dispersal. 4. We found the number of adult mistletoes per host increased with host size, which likely indicates that mistletoe colonization accumulated over the host's life-span. However, the number of juveniles found per host peaked at medium-sized hosts, increased with marsupial abundance and host availability and showed a negligible response to edges. 5. Synthesis: The lack of spatial congruence between host and seed disperser probably explains the scarcity of mistletoes in the study area, although the discordant drivers of juvenile and adult distributions suggest that there is a trade-off between recruitment patterns but also potential dispersal limitation at small scales. In essence, the interdependence among species linked by (mutualistic and antagonistic) interactions makes them more sensitive to anthropogenic disturbance, exacerbating its impact on the diversity and functioning of forest ecosystems.

1. 物种相互作用塑造了各类生态系统的功能多样性与群落稳定性，其依赖于所有互作伙伴的空间分布、栖息地需求以及对干扰的敏感性。因此，评估此类人为干扰对多物种相互作用的影响，或是优化生物多样性保护的适应性管理与缓解措施的核心环节。
2. 本研究阐明了边缘效应（edge effects）对南美温带雨林中3种关键物种（keystone species）的相互作用与分布的重要意义：半寄生槲寄生（hemiparasitic mistletoe）*Tristerix corymbosus*、其主要宿主藤本植物（liana）*Campsidium valdivianum*，以及其唯一的种子传播者有袋类动物（marsupial）*Dromiciops gliroides*。
3. 森林边缘对宿主产生正向影响、对种子传播者产生负向影响的相悖效应，在大空间尺度上影响了槲寄生的分布，这源于扩散限制加剧与宿主可利用性降低的共同作用。更为关键的是，有袋类动物的丰度在小尺度与大空间尺度上对槲寄生丰度产生了截然相反的影响——这暗示本地扩散与长距离扩散之间可能存在权衡关系。
4. 研究发现，单宿主的成年槲寄生数量随宿主体型增大而增加，这大概率表明槲寄生的定殖随宿主寿命累积。然而，单宿主的幼株数量在中等体型宿主中达到峰值，且随有袋类动物丰度与宿主可利用性的提升而增加，对边缘效应的响应则可忽略不计。
5. 综合分析：宿主与种子传播者之间缺乏空间一致性（spatial congruence），或可解释研究区域内槲寄生的稀缺性；尽管幼株与成株分布的驱动因素存在差异，这表明种群补充模式之间存在权衡，同时小尺度上也存在潜在的扩散限制。本质上，由（互利与拮抗）相互作用联结的物种间的相互依存性，使得它们对人为干扰更为敏感，进而加剧了人为干扰对森林生态系统多样性与功能的负面影响。