臺灣柳杉人工林中山蘇內的節肢動物多樣性與群聚結構

Abstract

包含蟎 (Acari) 和跳蟲 (Collembola) 在內的小型節肢動物是土壤生態系的主要動物類群。然而，由於土壤物理結構的複雜性和隱蔽性，我們非常缺乏土壤小型節肢動物的群聚知識。大部分土壤小型節肢動物群聚的研究主要注重於群聚的空間變異及環境變異對群聚的影響，而時間上的變化相對未知。山蘇為研究土壤小型節肢動物的演替提供獨特的系統。山蘇是熱帶與亞熱帶森林常見的附生植物，碗狀與海綿構造的根球能累積枯落物並保持水分，這提供合適的微棲地以容納豐富且多樣的土壤小型節肢動物。山蘇的大小能代表節肢動物群聚的相對年齡，因此能在小區域內收集不同年齡的土壤小型節肢動物群聚。我透過比較大小山蘇中土壤小型節肢動物群聚各自的β多樣性，研究土壤小型節肢動物的時間變化與群聚組成過程。當群聚組成主要由確定性過程主導，生物或非生物因子將變得更加重要，從而導致群聚趨同。而當隨機性過程主導時，群聚則不會趨同，甚至趨異。我們在臺灣北部的柳杉 (Cryptomeria japonica) 人工林，採樣大小山蘇各九顆。總共鑑定一百八十九種型態種與20,157隻小型節肢動物個體。我使用假設模型與基於取樣完整度標準化的β多樣性去除α和γ多樣性對β多樣性的影響。最終發現，大山蘇的β多樣性顯著小於小山蘇。結果代表土壤小型節肢動物的群聚組成受確定性過程主導。我進一步發現，pH值、碳氮比及含水率的變異，在大小山蘇之間都沒有顯著差異，也和群聚相異度沒有相關。這代表在土壤小型節肢動物的確定性的群聚組成過程中，生物因子可能扮演較重要的角色。另一個有趣的發現是，β多樣性的下降只出現在豐度的群聚資料，發生率資料則沒有。我推測原因是山蘇類似土壤的結構降低物種交互作用的強度，導致不完全的競爭排除。本研究記錄了物種交互作用對群聚組成重要性的珍貴實際證據，並且更深入了解土壤小型節肢動物群聚如何隨著時間改變。

Microarthropods, primarily mites (Acari) and springtails (Collembola), are the major groups of animals living in the soil ecosystem. However, due to the complexity and low visibility of the soil physical structure, our knowledge on the community assembly rules of soil microarthropods is scarce. Most studies of soil microarthropod communities focus on the spatial variation of the community or the effect of environmental variation on the community. How soil microarthropod community change over time is relatively unknown. When community assembly is primarily affected by deterministic processes, biotic and/or abiotic filtering will become important over time and then cause communities convergence. Alternatively, when stochastic processes are dominant, communities will not converge or will even diverge. Bird’s nest ferns provide a unique system for studying the succession of soil microarthropods. They are common epiphytes in tropical and subtropical forests. Their bowl-shaped and sponge-like bowl-shaped bases accumulate litter and hold water, providing habitats for thousands of individuals of soil microarthropods. The various sizes of the ferns growing in a forest can serve as a proxy for the ages of the microarthropod communities that are isolated from each other within a relatively small and homogenous area. I studied the temporal change and community assembly processes of soil microarthropods by comparing the β-diversity of microarthropod communities in nine large and nine small bird’s nest ferns from a subtropical Cryptomeria japonica plantation in northern Taiwan. From the 18 communities, 20,157 individuals belonging to 189 morphospecies of microarthropods were identified. Null models and coverage-based standardized β-diversity showed that the β-diversity of large ferns was significantly smaller than small ferns even after taking into account the dependence of β-diversity on both α and γ-diversity, suggesting that microarthropod communities in bird’s nest fern were primarily affected by deterministic processes. Furthermore, the variances of pH value, C:N ratio, and water content were not significantly different between large and small ferns and did not correlate with community dissimilarity. Accordingly, biotic filtering may play a relatively important role in soil microarthropod community assembly. Interestingly, the decreasing pattern of β-diversity was only observed in abundance-based data and not in incidence-based data, implying that the soil-like structure in bird’s nest ferns might reduce the strength of interspecific competition and prevent exclusion. This study documents rare empirical evidence that supports the importance of species interaction on community assembling in soil microarthropods and provides new insights into how soil microarthropod community change over time.