Inter-annual dynamics and persistence of small mammal communities in a selectively logged tropical forest in Borneo

Understanding temporal change and long-term persistence of species and communities is vital if we are to accurately assess the relative values of human-modified habitats for biodiversity. Despite a large literature and emerging consensus demonstrating a high conservation value of selectively logged tropical rainforests, few studies have taken a long-term perspective. We resampled small mammals (≤1kg) in a heavily logged landscape in Sabah, Borneo between 2011 and 2016 to investigate temporal patterns of species-level changes in population density. We found that small mammal population density in heavily logged forest was highly variable among years, consistent with patterns previously observed in unlogged forest, and uncovered evidence suggesting that one species is potentially declining towards local extinction. Across nine species, population densities varied almost sevenfold during our six-year study period, highlighting the extremely dynamic nature of small mammal communities in this ecosystem. Strictly terrestrial murid species tended to exhibit strong temporal dynamics, whereas semi-arboreal foraging species such as treeshrews had more stable dynamics. We found no relationships between population density and fruit/seed mass, and therefore no evidence that our patterns represent responses to inter-annual mast fruiting of the dominant canopy dipterocarp trees. This may be due to the removal of most of the canopy during logging, and hence the dipterocarp seed resource, although it possibly also reflects spatiotemporal limitations of our data. Our results underline the importance of understanding long-term variability in animal communities before developing conservation and management recommendations for human-altered ecosystems.

若要精准评估受人类活动改造的生境对生物多样性的相对保护价值，厘清物种与群落的时间动态及长期存续机制至关重要。尽管已有大量研究成果，并逐渐形成共识：选择性采伐 (selectively logged) 热带雨林具有较高的保护价值，但相关研究极少采用长期视角。本研究于2011至2016年间，在婆罗洲沙巴州的重度采伐区域内，对体重≤1千克的小型哺乳类 (small mammals) 开展重采样工作，以探究种群密度在物种层面的时间变化模式。研究发现，重度采伐森林内的小型哺乳类种群密度在各年间波动显著，这与此前在未采伐森林中观测到的模式一致；同时还发现证据表明，某一物种可能正朝着局部灭绝 (local extinction) 方向衰退。在为期6年的研究周期内，9个物种种群的密度波动幅度接近7倍，凸显出该生态系统中小哺乳类群落极强的动态性。严格陆生的鼠科 (Muridae) 物种往往表现出显著的时间动态变化，而以半树栖方式觅食的物种（如树鼩 (treeshrews)）的种群动态则更为稳定。我们未发现种群密度与果实/种子生物量之间存在显著关联，因此并无证据表明观测到的变化模式是对优势冠层龙脑香科 (dipterocarp) 树木年际结实大年 (mast fruiting) 的响应。这一结果可能源于采伐过程中大部分冠层被移除，进而导致龙脑香科植物的种子资源大幅减少；不过这也有可能反映了本研究数据在时空尺度上的局限性。本研究结果强调，在为受人类活动改造的生态系统制定保护与管理方案前，厘清动物群落的长期变异性极为关键。