農田與地景因子對水稻田節肢動物多樣性的影響: 從物種到群聚

Abstract

農業地景為蘊育生物多樣性的動態鑲嵌體，其環境條件會隨空間尺度而變化。為了保護農業地景的生物多樣性及所提供的生態系統服務，我們需要了解環境因子在各空間尺度下與生物多樣性的相關性。此外，我們需要了解個別物種層級的反應如何匯集成群聚層級的反應，因為生物多樣性涵蓋了個別物種的個體數量(族群密度)和物種的共存關係(物種豐富度)。本研究採集第一季稻作的節肢動物(物種及功能性皆豐富的類群)，並探討農田因子(有機耕作、水深、作物高度)和地景因子(森林覆蓋率、樣區相對位置)如何影響節肢動物的族群密度和分佈。我們利用可分析時空因子的聯合物種分佈模型(JSDM)探討：1)各物種對當地(農田)和地景因子是否有不同的反應? 2)是否能用營養功能群(trophic guild)的概念解釋各物種對以上環境因子之不同反應? 3)個別物種層級的反應如何匯集並影響節肢動物群聚的生物多樣性(物種豐富度)? 我們的結果顯示，整體環境因子的平均重要性不高(平均Pseudo-R2 = 0.21)，表示本農業系統中物種的族群密度主要是受隨機性因素的影響。不過，環境因子的重要性在各個物種間差異頗大(最高Pseudo-R2 = 0.87，最低Pseudo-R2 = 0.01)，此外，營養功能群僅解釋了約15％的物種反應變異量，顯示物種反應具有很高的差異性。根據我們建構的群聚層級物種豐富度統計模型，我們發現有機耕作沒有顯著地影響物種豐富度，但是作物高度和水深與物種豐富度有負相關，而森林覆蓋率會增加了物種豐富度。此外，物種反應的差異性反映了各個環境因子的重要性。最後，我們發現此農業系統中物種族群密度的建構過程並不會受到物種擴散能力的限制。根據以上結果，本研究討論相關的應用性，例如農民所關注物種之經營管理(如害蟲和天敵)。綜上所述，本研究顯示稻田節肢動物族群的動態具有高度隨機性，即使在同一營養功能群中，各物種對環境因子的反應也具有高度的多樣性，而本研究亦提出相關機制的假說。此外，本研究的結果協助釐清動態鑲嵌農業地景中生物多樣性的建構過程，彰顯從物種層級了解生物多樣性的重要性。

Species in agricultural landscapes live and move among dynamic mosaics where environmental conditions vary across spatial scales. To safeguard agro-biodiversity and associated ecosystem services, it is important to understand the spatial scales of environmental variables relevant to biodiversity. Furthermore, we need to examine how species-specific responses to environmental variables give rise to community-level patterns. This is because biodiversity is an emergent property that arises from the abundance and co-occurrence of individual species. However, few biodiversity studies have accounted for the heterogeneity in species responses, impeding our understanding of ecological processes that structure biodiversity. In this study, we sampled paddy-field inhabiting arthropods, a speciose and functionally rich taxa, across the first growing season and examined how local (organic farming, water depth, and crop height) and landscape factors (forest cover and geographic distance) affect their abundance and distribution. We built a spatio-temporally explicit joint species distribution model (JSDM) to answer three main questions: 1) How do species differ in their response to local and landscapes factors? 2) Can trophic guild explain the variation in species responses to environmental covariates? 3) How do species-specific responses scale-up to structure arthropod community richness? Our results show that the mean importance of environmental covariates, measured as the percentage of explained variance (Pseudo-R2), was low overall (mean Pseudo-R2 = 0.21), suggesting the prevalence of stochasticity in structuring species abundances in our system. However, environmental importance varied substantially across species (highest Pseudo-R2 = 0.87 and lowest Pseudo-R2 = 0.01), and trophic guild explained only approximately 15% of species responses, which indicates high idiosyncrasy in species responses both within and among guilds. Using our trained model to predict community-level species richness, we found that organic farming had no effect, crop height and water depth decreased, while forest cover increased species richness. Moreover, the importance of each environmental covariate was reflected by the variability of species responses. Lastly, we found no evidence of dispersal limitation (i.e. geographic distance) in structuring species abundances. We discuss the implications of these findings, specifically with regards to the management of specific species that may be of interest to farmers such as insect pests and predators. Taken together, our study suggests that many paddy field arthropods exhibit transient dynamics and that environmental effects are highly species-specific, even within the same trophic guild. We provide some hypotheses on why this is so. Most importantly, our study highlights the need and value of understanding biodiversity from the species level to understand the processes that structure them in dynamic mosaic agricultural landscapes.