摘要:人類干擾與氣候變遷對生態系的協同效應日益受到關注,對應這些協同效應,我們急需一套有效的監測與調查方法。生態系的食階結構與其隨環境變遷而產生的變異代表著能量在生態系各部位傳遞的狀況,正好可提供用於生態系監測的一個方法,這同時也是生態學家長久以來意欲探討的生態系屬性之一,對生態系的管理與保育至關重要。
在本計畫中我們提出一個有別於以往的新方法來研究水生生態系統食階結構與其隨環境變動而產生的變異。過去的食階結構研究主要著眼於掠食者/獵物間的特定關係,這類在物種層級上探討食性關係的傳統方法,在實際操作上有相當的難度。首先,鑑種工作曠日廢時;再者,要量化物種之間的食性關係更是困難。在此,我們發展出避免這些難題的方法,改以測量水域生態系中的生物個體體型大小,並依各體型大小分組,探討各體型組間的食性關係。之所以發展出依據體型大小來探討食階結構的構想,是因為到目前已有許多研究都證實在水域系統中多半是大型生物捕食小型生物。
本研究的特點是以個體體型大小分佈與體型食階關係來取代傳統的物種組成與食性分析(誰吃誰)。浮游生物體型大小分佈的資料可由採集瓶與浮游生物網取得,並以新建立的體型自動分類影像系統:流式影像儀(FlowCam)與浮游動物掃描器(ZooScan)來測量。將各體型組的樣本,進行穩定同位素(δN15 與 δC13)分析,估算出掠食者與獵物之體型大小關係。最後整合上述資料,推估食階結構。
此外,我們亦提出一新方法,由體型大小頻譜與體型食階關係來估計食階能量傳遞效率(這是食階研究中最難測量的性質)。根據生物代謝理論我們推導出:log(N)=(log(TE)/log(PPMR)-0.75)*log(M),N是一特定體型組生物量的豐度,M是該組的體型大小,體型大小頻譜定義為log(N) vs log(M),TE則是食階能量傳遞效率,而PPMR為掠食者獵物之體型比。PPMR可由各體型組的穩定同位素分析得到,而藉由流式影像儀、浮游動物掃描器與顯微鏡的測量可得到體型大小頻譜資料。如此便能估算出此理論中的食階能量傳遞效率。再結合初級生產力測量,便能估算次級生產力。接下來再以獨立測量之次級生產力來測試這整套方法。
我們的研究目標是要由下列幾點生態系的特性來探討水域系統中浮游生物的食階結構與變異:1.體型大小分佈;2. 依體型大小分類之食階與掠食者與獵物體型大小比例;3. 食階能量傳遞效率。我們將會探討這三項特性在人為干擾與氣候變遷下的變動。
我們預計將每週在翡翠水庫進行採樣,以測試我們的構想。選擇以翡翠為採樣地點是因為這裡是重要的水源地,而且交通方便。此外,中研院的研究人員持續在這裡收集各項環境變因,可供日後分析環境變動使用。
Abstract: Synergistic effects of anthropogenic disturbance and global climate changes on ecosystems are pressing concerns. The ability to effectively and efficiently monitor and investigate these effects is needed! One of the most desirable ecosystem properties that ecologists eager to understand is trophic structure and its variation responding to environmental changes. This understanding is essential to ecosystem management and conservation, because the variation of trophic structure represents energy transfers among components of the ecosystem.
Here we propose a novel size-based approach to study trophic structure and its variation of aquatic ecosystems responding environmental changes. This is contrast to the traditional approach that is based on species-specific predator-prey relationships. One practical caveat of the traditional species-based approach is that collecting species-specific data is time consuming and costly and that examining quantitatively who eat whom is difficult. The size-based approach developed here can overcome those difficulties, because size data and trophic interactions among size groups are relatively easy to measure, particularly in aquatic systems. Importantly, this size-based approach is based on well-documented evidence that, often in aquatic systems, large organisms eats small organisms.
The merits of our approach lie in using organism size distribution to replace traditional species composition and using size-specific trophic interactions to replace diet content analyses (who eating whom). We focus on the planktonic component, because it represents the basic foundation of aquatic systems and is sensitive to environmental changes. The size distribution data of plankton can be collected efficiently with combination of bottle samples and plankton net samples and measured using our newly established automatic sizing instruments: FlowCam and ZooScan. The size-based predator-prey interactions can be measured using size-fractionated stable isotope (δN15 and δC13). We can then integrate these measurements to obtain the trophic structure.
Furthermore, we propose a novel approach to estimate trophic transfer efficiency (one of the most difficult properties to measure in trophic studies) based on estimating the size distribution (size-spectrum) and size-specific trophic levels. Based on metabolic theory, we derive this equation: log(N)=(log(TE)/log(PPMR)-0.75)*log(M), where N is abundance of a size class, M is size class, and size-spectrum is define as the slope of log(N) vs log(M), TE is trophic transfer efficiency, and PPMR is predator-prey biomass ratio. PPMR can be obtained from size-fractionated stable isotope analysis and the size spectrum can be measured from FlowCAM and ZooScan. As such, trophic transfer efficiency can be estimated from this theory. Moreover, together with primary production measurement, secondary production can be estimated. We shall then use independent measurements of secondary production to test our approach.
Our research goals are to investigate trophic structure and its variation in the planktonic component of aquatic systems based on three elements: 1. size distribution, 2. size-specific trophic levels and predator-prey mass ratio, and 3. trophic transfer efficiency. We will investigate how these elements respond to anthropogenic disturbance and climate change.
We will test our ideas by conducting weekly sampling and measurements in the Fei-Tsui water reservoir. The location is chosen because of its importance as source of drinking water and its easy accessibility. In addition, environmental variables have been continuously collected by colleagues from Academia Sinica. When available, we will also carry out sampling in various marine environments. That is, we can use the spatial variability as surrogate of temporal variability in testing our approaches.