| 摘要: | 生態元素比理論(ecological stoichiometry)結合元素流動量、能量傳遞效率、獵物選擇，提供食物網多面向並整合的研究架構。生物元素比的中心理論在於探討資源元素比 （主要考量碳氮磷比例）與消費者元素比的差異造成消費者生長效率降低。例如，若浮游植物碳氮(C:N)與碳磷(C:P)比過高會降低浮游動物合成生物質量與成長速率，因此浮游動物傾向選擇碳氮磷比接近自己的獵物。除了生物元素比，體型大小亦會影響浮游動物的代謝率以及掠食者-獵物體型比 (predator-prey mass ratio, PPMR)，進而影響水域環境的食物鏈長度。過去研究從實驗室控制的培養實驗中發現生物元素比以及體型大小確實會改變生物能量傳遞效率與獵物選擇偏好，但野外自然環境的證據仍十分缺乏。另外生物元素比與體型大小的關聯也少有研究。我的論文嘗試藉野外觀察實驗與理論生態模擬探討(1) 獵物元素比如何影響浮游動物生產率，(2) 體型與生物元素比的關聯及體型決定生物元素比的機制，(3) 營養鹽供給與獵物元素比如何影響掠食者-獵物體型比。我在亞熱帶區海洋野外的實驗結果顯示，浮游動物生產率與獵物（浮游植物）C:N及C:P比例呈負相關，顯示氮與磷元素對浮游動物生產力的重要性。亞熱帶海洋以及翡翠水庫的浮游生物元素比研究則發現，C:N比例是體型大小的函數：浮游植物C:N比例隨體型上昇，而異營為主的生物C:N比例隨體型下降，而C:N比例的最大值出現在50μm。我以資源獲取率異速增長(allometry)的浮游生物食物網模型模擬來解釋其機制：浮游植物碳累積速度隨體型大小增加，而呼吸碳消耗降低異營為主生物的C:N比例。異營生物C:N比例隨體型上昇而降低的結果意涵著浮游動物傾向捕食體型較大、C:N比例較小且接近自身元素比需求的獵物。我測量亞熱帶海洋系統的浮游生物穩定同位素資訊，發現當營養鹽濃度高，足以支持較多大型獵物時，掠食者-獵物體型比較小，表示浮游動物在營養鹽充足的環境中會捕捉較多大型獵物。本研究結合野外實驗觀察與理論模擬指出生物元素比與體型大小共同改變次級生產率與獵物選擇，顯示二者交互作用影響食階能量傳遞的重要性。;Ecological stoichiometry integrates elemental fluxes, energy transfer efficiency, and prey selectivity into a comprehensive food web research framework. The central idea of ecological stoichiometry is that the mismatch between resource stoichiometry (elemental carbon:nitrogen:phosphorus ratio; C:N:P) and stoichiometric requirement of consumers hinders the carbon assimilation efficiency of consumers. Because high phytoplankton C:N and C:P ratios are disadvantageous to zooplankton growth, zooplankton would consume prey with preferred C:N:P ratio. In addition to stoichiometry, body size determines metabolic rates, and predator-prey mass ratio (PPMR, body size ratio of predator and its prey) determines food chain length in aquatic systems. Through lab-manipulated incubation experiments, stoichiometry and body size are found potential traits that influence trophic transfer efficiency and prey selection; however, evidence from in situ observations in natural aquatic food webs remains elusive. Furthermore, the interactions between body size and stoichiometry are rarely studied. Here, I examined (1) how prey stoichiometry (C:N:P ratio) influences zooplankton production, (2) how body size determines plankton stoichiometry, and (3) how nutrient and prey stoichiometry alter community PPMR in plankton grazing food webs, by in situ observations and theoretical modeling. I found that prey (phytoplankton) C:N and C:P ratios are indeed negatively correlated with zooplankton production in subtropical marine systems, indicating that N and P are essential to zooplankton. In both subtropical marine and freshwater grazing food webs, I found a unimodal C:N ratio pattern with respect to plankton body size: C:N ratio increases and reaches the maximum at 50 μm autotrophs, and then decreases with body size in heterotrophs. This pattern is explained by the accumulation of C through allometric resource affinity and respiratory C loss in my size-based food web model. The decreasing of C:N ratio with body size and heterotrophy suggests that predators prefer large prey and thus decrease PPMR when large size prey are sufficient; my field observations based on stable isotope analysis reveal that PPMR is smaller when nutrient supply is higher and supports more large prey. My research combining in situ observations and theoretical modeling point out that prey stoichiometry and size affect zooplankton production and modifies predation, indicating the important role of stoichiometry and allometry in trophic transfer efficiency in nature. |
