Forest Fragmentation and Selective Logging Have Inconsistent Effects on Multiple Animal-Mediated Ecosystem Processes in a Tropical Forest

Forest fragmentation and selective logging are two main drivers of global environmental change and modify biodiversity and environmental conditions in many tropical forests. The consequences of these changes for the functioning of tropical forest ecosystems have rarely been explored in a comprehensive approach. In a Kenyan rainforest, we studied six animal-mediated ecosystem processes and recorded species richness and community composition of all animal taxa involved in these processes. We used linear models and a formal meta-analysis to test whether forest fragmentation and selective logging affected ecosystem processes and biodiversity and used structural equation models to disentangle direct from biodiversity-related indirect effects of human disturbance on multiple ecosystem processes. Fragmentation increased decomposition and reduced antbird predation, while selective logging consistently increased pollination, seed dispersal and army-ant raiding. Fragmentation modified species richness or community composition of five taxa, whereas selective logging did not affect any component of biodiversity. Changes in the abundance of functionally important species were related to lower predation by antbirds and higher decomposition rates in small forest fragments. The positive effects of selective logging on bee pollination, bird seed dispersal and army-ant raiding were direct, i.e. not related to changes in biodiversity, and were probably due to behavioural changes of these highly mobile animal taxa. We conclude that animal-mediated ecosystem processes respond in distinct ways to different types of human disturbance in Kakamega Forest. Our findings suggest that forest fragmentation affects ecosystem processes indirectly by changes in biodiversity, whereas selective logging influences processes directly by modifying local environmental conditions and resource distributions. The positive to neutral effects of selective logging on ecosystem processes show that the functionality of tropical forests can be maintained in moderately disturbed forest fragments. Conservation concepts for tropical forests should thus include not only remaining pristine forests but also functionally viable forest remnants.

森林片段化（forest fragmentation）与选择性采伐（selective logging）是驱动全球环境变化的两大核心因子，可对诸多热带森林的生物多样性与环境条件造成改变。目前学界尚未有研究采用全面系统的方法，探讨此类人为干扰对热带森林生态系统功能的影响。本研究以肯尼亚一处热带雨林为研究对象，针对6项动物介导的生态系统过程（animal-mediated ecosystem processes）开展了调查，并记录了参与上述过程的所有动物类群（taxa）的物种丰富度（species richness）与群落组成（community composition）。我们采用线性模型与规范元分析（meta-analysis），检验森林片段化与选择性采伐是否会对生态系统过程及生物多样性产生影响；同时借助结构方程模型（structural equation models），厘清人为干扰对多重生态系统过程的直接效应，以及与生物多样性相关的间接效应。研究结果显示，森林片段化会提升分解作用（decomposition），降低蚁鸟捕食（antbird predation）强度；而选择性采伐则持续提升传粉（pollination）、种子扩散（seed dispersal）与军蚁劫掠（army-ant raiding）行为的发生水平。森林片段化会改变5个动物类群的物种丰富度或群落组成，而选择性采伐未对生物多样性的任一组分产生显著影响。小型森林片段中，功能重要物种的丰度变化与蚁鸟捕食率降低及分解速率提升存在显著关联。选择性采伐对蜜蜂传粉、鸟类种子扩散与军蚁劫掠行为的正向效应为直接效应，即与生物多样性变化无关，其成因大概率为这些移动性极强的动物类群的行为改变。我们据此得出结论：在卡克梅加森林（Kakamega Forest）中，动物介导的生态系统过程对不同类型的人为干扰呈现出差异化响应。本研究结果表明，森林片段化通过改变生物多样性间接影响生态系统过程，而选择性采伐则通过改变局域环境条件与资源分布直接对生态系统过程施加影响。选择性采伐对生态系统过程呈现正向至中性的效应，这说明热带森林的生态功能可在中度干扰的森林片段中得以维持。因此，热带森林的保护策略不仅应涵盖现存的原始森林，还应纳入具备功能完整性的森林残余斑块。