金 艷,劉 勇,袁興偉,凌建忠,程家驊
(中國(guó)水產(chǎn)科學(xué)研究院東海水產(chǎn)研究所,農(nóng)業(yè)部東海與遠(yuǎn)洋漁業(yè)資源開(kāi)發(fā)利用重點(diǎn)實(shí)驗(yàn)室,上海 200090)
一個(gè)典型的網(wǎng)絡(luò)是由許多節(jié)點(diǎn)(代表系統(tǒng)中個(gè)體)與連接兩個(gè)節(jié)點(diǎn)之間的邊(代表個(gè)體之間的關(guān)系)組成。不同于規(guī)則網(wǎng)絡(luò)(如二維方正晶格)和隨機(jī)網(wǎng)絡(luò)(節(jié)點(diǎn)之間的邊根據(jù)一個(gè)概率決定),一般認(rèn)為,復(fù)雜網(wǎng)絡(luò)(complex network)是真實(shí)復(fù)雜系統(tǒng)的拓?fù)涑橄?,具有其特有的統(tǒng)計(jì)網(wǎng)絡(luò)特征,能更真實(shí)地反映現(xiàn)實(shí)網(wǎng)絡(luò),并了解現(xiàn)實(shí)環(huán)境的復(fù)雜性[1]。近年來(lái),復(fù)雜網(wǎng)絡(luò)研究引起了國(guó)內(nèi)外不同領(lǐng)域的廣泛重視[2-3]。自從小世界網(wǎng)絡(luò)(small world network)[4]及無(wú)標(biāo)度網(wǎng)絡(luò)(scale free network)[5]的分布規(guī)律被發(fā)現(xiàn),復(fù)雜網(wǎng)絡(luò)理論開(kāi)始滲透到眾多不同的學(xué)科中。其中小世界網(wǎng)絡(luò)是指相對(duì)于節(jié)點(diǎn)數(shù)量,該網(wǎng)絡(luò)具有較短的平均路徑長(zhǎng)度,同時(shí)只需相對(duì)較少的長(zhǎng)程連接;無(wú)標(biāo)度網(wǎng)絡(luò)是指該網(wǎng)絡(luò)中度分布符合冪律分布的復(fù)雜網(wǎng)絡(luò)。
復(fù)雜網(wǎng)絡(luò)的理論已經(jīng)應(yīng)用到多個(gè)學(xué)科領(lǐng)域中,相比于社交網(wǎng)絡(luò)和計(jì)算機(jī)網(wǎng)絡(luò),生態(tài)網(wǎng)絡(luò)是復(fù)雜網(wǎng)絡(luò)在生態(tài)學(xué)上的一種應(yīng)用。物種與物種之間的密切聯(lián)系和制約(例如捕食-被捕食或者共生關(guān)系)形成了動(dòng)態(tài)平衡的生態(tài)網(wǎng)絡(luò)[6]。但復(fù)雜的生態(tài)網(wǎng)絡(luò)在一定程度上是脆弱的。網(wǎng)絡(luò)復(fù)雜性會(huì)影響一個(gè)種群密度的恢復(fù)力和可變性及群落組成的穩(wěn)定性和抗干擾能力,例如:物種入侵或物種滅絕[7-8]。了解生態(tài)網(wǎng)絡(luò)的復(fù)雜性及其對(duì)生態(tài)系統(tǒng)功能的影響有助于闡明影響生態(tài)系統(tǒng)穩(wěn)定性的機(jī)制,進(jìn)而預(yù)測(cè)環(huán)境擾動(dòng)的結(jié)果,最終降低如棲息地喪失、氣候變化和外來(lái)物種入侵等所帶來(lái)的危害。
根據(jù)物種類型及相互關(guān)系,生態(tài)網(wǎng)絡(luò)包含了多種類型,例如描述類似植物和授粉昆蟲(chóng)之間關(guān)系的共生網(wǎng)(mutualistic web)[9]、描述類似宿主與微生物之間關(guān)系的宿主 -寄生網(wǎng)(hostparasitoid web)[10]及描述獵物和捕食者之間關(guān)系的食物網(wǎng)(food web)等。其中,食物網(wǎng)描述了生態(tài)系統(tǒng)中的營(yíng)養(yǎng)關(guān)系,是研究生態(tài)系統(tǒng)功能的核心[11]。食物網(wǎng)的研究可以有效地將群落生態(tài)學(xué)研究和系統(tǒng)生態(tài)學(xué)的方法相結(jié)合,綜合生物多樣性和生態(tài)系統(tǒng)功能的研究[12]。本文將梳理復(fù)雜網(wǎng)絡(luò)理論在食物網(wǎng)中的應(yīng)用和研究進(jìn)展,并展望了其在水生生態(tài)系統(tǒng)中的應(yīng)用前景。
傳統(tǒng)的食物網(wǎng)研究一般分為兩個(gè)角度——種群和群落[13-14]及生態(tài)轉(zhuǎn)化效率[15-16]。種群和群落研究中,食物網(wǎng)的節(jié)點(diǎn)通常是物種種群,而連接它們的則是捕食-被捕食關(guān)系。這類網(wǎng)絡(luò)又分為兩種,一種是將一個(gè)特定范圍內(nèi)所有物種的相互攝食關(guān)系都表示出來(lái),另一種是從一個(gè)單一物種出發(fā)向上或者向下羅列攝食關(guān)系。相比于食物關(guān)系,生態(tài)轉(zhuǎn)化效率的研究主要強(qiáng)調(diào)能量、營(yíng)養(yǎng)物質(zhì)或生物量的集中與流動(dòng)。學(xué)界提出了一系列生態(tài)系統(tǒng)建模,用以計(jì)算生態(tài)效率參數(shù)[17-18], 如 Ecopath with Ecosim[19-20]、OSMOSE[21-22]、 Atlantis[23]及 Linear Inverse Model[24]。其 中 Ecopath with Ecosim 和 Atlantis在海洋生態(tài)系統(tǒng)中的應(yīng)用較多,但這些模型對(duì)基礎(chǔ)調(diào)查數(shù)據(jù)和信息需求量巨大,縮小了它們的應(yīng)用范圍。
反觀國(guó)內(nèi),海洋食物網(wǎng)研究起步較晚。張其永等[25]最早以閩南—臺(tái)灣淺灘漁場(chǎng)多種魚(yú)類食性分析為基礎(chǔ),觀察魚(yú)類食性類型與其消化器官,揭示了66種魚(yú)類組成的食物網(wǎng)及其營(yíng)養(yǎng)級(jí)。隨后,韋晟等[26]、張雅芝等[27]和鄧景耀等[28]分別從食性和營(yíng)養(yǎng)級(jí)角度研究了黃海魚(yú)類食物網(wǎng)、山東灣經(jīng)濟(jì)魚(yú)類食物網(wǎng)和渤海魚(yú)類食物網(wǎng)。20世紀(jì)90年代,唐啟升等[29]在渤海區(qū)進(jìn)行了關(guān)鍵種生態(tài)轉(zhuǎn)換效率實(shí)驗(yàn)、能值測(cè)定、營(yíng)養(yǎng)模型(ECOPATH)建立,以及群落結(jié)構(gòu)和生物多樣性等研究。萬(wàn)袆等[30]和蔡德陵等[31]利用穩(wěn)定同位素研究了渤海灣和東、黃海的食物網(wǎng),將同位素等新的分析手段引入了食物網(wǎng)的研究。近年來(lái),微量元素的分析和軟硬組織中的金屬積累量分析[32]及 DNA條形碼分子技術(shù)[33-34]也開(kāi)始被逐步引入食物關(guān)系的研究中,形成越來(lái)越完善的捕食群體描述。
然而,自 CAMERANO[35]繪制食物網(wǎng)后長(zhǎng)達(dá)一個(gè)世紀(jì)的研究中,傳統(tǒng)食物網(wǎng)一直是由一個(gè)二元網(wǎng)絡(luò)表示,網(wǎng)絡(luò)中的點(diǎn)和線體現(xiàn)了攝食關(guān)系的存在與否。這類傳統(tǒng)的食物網(wǎng)雖然包含了眾多物種,但也存在一些描述上的劣勢(shì),例如:存在營(yíng)養(yǎng)級(jí)模糊、許多物種間都存在著看似同等的營(yíng)養(yǎng)關(guān)系、物種在網(wǎng)絡(luò)中的地位與重要性較模糊等問(wèn)題。
有別于傳統(tǒng)食物網(wǎng)對(duì)物種及攝食關(guān)系的靜態(tài)描述,復(fù)雜網(wǎng)絡(luò)理論更有助于了解動(dòng)態(tài)網(wǎng)絡(luò)的穩(wěn)定性。在網(wǎng)絡(luò)穩(wěn)定性研究中,最先受到關(guān)注的是連接分布(connectivity distributions)。研究發(fā)現(xiàn)自然界中的食物網(wǎng)并不符合“富者更富(rich get richer)”的馬太效應(yīng)(Matthew effect)[36-37]。舉個(gè)例子,當(dāng)多個(gè)捕食者都將一個(gè)物種納入食譜時(shí),競(jìng)爭(zhēng)會(huì)加大,此時(shí)捕食者在保留該物種在食譜里的同時(shí),會(huì)傾向于捕食其它物種。此外,越來(lái)越多的研究認(rèn)為自然界中的連接分布并非是隨機(jī)的,譬如在共生網(wǎng)中很多傳粉昆蟲(chóng)有特定的攝食對(duì)象。
復(fù)雜食物網(wǎng)中的連接分布存在兩種結(jié)構(gòu):簇狀結(jié)構(gòu)(clustered)和嵌套結(jié)構(gòu)(nested)。簇狀結(jié)構(gòu)是指多個(gè)具有共性的節(jié)點(diǎn)組成“團(tuán)”,具有同簇節(jié)點(diǎn)相互連接密集、異簇節(jié)點(diǎn)相互連接稀疏的特點(diǎn)[38]。嵌套結(jié)構(gòu)指的是某一物種的連接對(duì)象都是相對(duì)其更為泛化的物種的連接對(duì)象的一個(gè)子集[39]。該結(jié)構(gòu)是食物網(wǎng)中較為常見(jiàn)的類型。嵌套結(jié)構(gòu)對(duì)生態(tài)網(wǎng)絡(luò)穩(wěn)定性具有重要意義,如果某一物種消失,其它更泛化的物種也能夠接替該物種的生態(tài)作用。通過(guò)與零假設(shè)的隨機(jī)模型(null models)進(jìn)行比較,研究發(fā)現(xiàn)嵌套型結(jié)構(gòu)中數(shù)量較少的物種與出現(xiàn)頻率高的物種會(huì)產(chǎn)生特定的相互作用[40]?,F(xiàn)有的研究還發(fā)現(xiàn)在水生動(dòng)物的食物網(wǎng)中,嵌套結(jié)構(gòu)似乎與個(gè)體大小關(guān)系較緊密,因?yàn)樾€(gè)體捕食者的食譜一般是大個(gè)體捕食者食譜的子集[41-42],也有學(xué)者認(rèn)為嵌套結(jié)構(gòu)還與系統(tǒng)發(fā)生學(xué)關(guān)系相關(guān),進(jìn)化史相似的物種往往容易形成嵌套結(jié)構(gòu)[43]。但到目前為止,嵌套結(jié)構(gòu)的形成機(jī)制仍舊不清楚,是數(shù)量驅(qū)動(dòng)還是地理劃分驅(qū)動(dòng)也不能下定論,是普通的連接還是特殊的連接調(diào)節(jié)著這種結(jié)構(gòu)的穩(wěn)定性也尚不明了。
除了連接分布,連接強(qiáng)度(link strength)同樣也是影響網(wǎng)絡(luò)穩(wěn)定的重要因素。20世紀(jì)90年代的許多研究均發(fā)現(xiàn)了自然界中普遍存在的較弱的相互作用(interaction strength)[44-47],這意味著食物網(wǎng)中大量弱相互作用與少量強(qiáng)相互作用共同參與整體網(wǎng)絡(luò)穩(wěn)定性的維護(hù)[46,48]。強(qiáng)弱作用組合分布的重要性也逐漸受到關(guān)注。BASCOMPTE等[49]運(yùn)用該理論評(píng)估了過(guò)度捕撈對(duì)海洋食物網(wǎng)穩(wěn)定性的影響。另外,NEUTEL等[50]在研究網(wǎng)絡(luò)穩(wěn)定性影響因素的時(shí)候發(fā)現(xiàn)“回路長(zhǎng)度”是穩(wěn)定性的一個(gè)重要決定因素,即從一個(gè)物種出發(fā)沿著連接在不重復(fù)經(jīng)過(guò)其它物種的情況下回到該物種的通路長(zhǎng)度。雖然網(wǎng)絡(luò)內(nèi)部的相互作用逐漸被量化,但受到不同強(qiáng)度定義和估算方法的影響,導(dǎo)致同一現(xiàn)象的描述可能出現(xiàn)相反的結(jié)果[51-52]。
最初復(fù)雜網(wǎng)絡(luò)理論的應(yīng)用是利用二元網(wǎng)絡(luò),對(duì)網(wǎng)絡(luò)中物種數(shù)量(S)、連接數(shù)量(L)、連接密度(L/S)、度的分布等基本參數(shù)加以計(jì)算,從而高效得出一些以往必須通過(guò)生態(tài)實(shí)驗(yàn)才能獲知的結(jié)論,例如以此方法發(fā)現(xiàn)了食物網(wǎng)中大多數(shù)能量流動(dòng)都會(huì)集中經(jīng)過(guò)少數(shù)幾個(gè)物種[53],而該結(jié)論與以往生物多樣性及生態(tài)系統(tǒng)功能(biodiversityecosystem function,BEF)研究中物種移除實(shí)驗(yàn)的結(jié)果相一致[54]。但因二元網(wǎng)絡(luò)缺乏定量數(shù)據(jù),所以無(wú)法體現(xiàn)各個(gè)物種或者某一種關(guān)系在一個(gè)網(wǎng)絡(luò)中的重要程度。于是,學(xué)者們開(kāi)始嘗試將生物個(gè)體特征測(cè)定和群體數(shù)量豐度相結(jié)合[42,55],例如有研究者提出用三變量網(wǎng)絡(luò)(tri-variate-webs)將豐度、個(gè)體大小及攝食關(guān)系結(jié)合在一起[56]。以個(gè)體平均大小和種群數(shù)量豐度做散點(diǎn)圖,再用攝食關(guān)系將其連接起來(lái)[57-58]。從而將個(gè)體特征及行為的重要性引入了食物網(wǎng)的研究。
物種之間的關(guān)系實(shí)則是在個(gè)體層面上發(fā)生的。個(gè)體在行為和生理上的差異最終也可以反映到生態(tài)系統(tǒng)進(jìn)程上[59],一些適應(yīng)性行為更是會(huì)影響食物網(wǎng)的結(jié)構(gòu)[60]。BROWN等[61]很早就指出理想的食物網(wǎng)研究是綜合了個(gè)體、群體到整個(gè)系統(tǒng)。但當(dāng)下大多數(shù)對(duì)食物網(wǎng)描述的研究仍是建立在群體豐度和大小均值的基礎(chǔ)上[62]。這種方法在宏觀生態(tài)學(xué)的研究中十分有效,但是容易造成假陽(yáng)性的結(jié)果[63-64]。例如:在分析捕食 -被捕食者的體重比率時(shí),WOODWARD等[48]發(fā)現(xiàn)基于個(gè)體重量比基于種群重量的均值更接近真實(shí)情況。種群大小均值雖然便于計(jì)算[63,65-66],但是在依賴大小均值作為營(yíng)養(yǎng)級(jí)決定因素的生態(tài)位模型(niche modl)中,則可能出現(xiàn)被捕食者個(gè)體均值大于捕食者的結(jié)論[67]。不得不提的是許多物種在其生活史中存在食性轉(zhuǎn)變,隨著個(gè)體成長(zhǎng),可能從獵物變成捕食者,進(jìn)而改變其營(yíng)養(yǎng)地位。所以在世代重疊的情況下,群體均值并不能有效衡量食性轉(zhuǎn)變上的個(gè)體差異[68]。
食物網(wǎng)的研究為個(gè)體生理、群落能量和穩(wěn)定性的結(jié)合提供了途徑,但如何將個(gè)體特征的數(shù)據(jù)納入食物網(wǎng)研究依然是一大挑戰(zhàn)。研究者已經(jīng)開(kāi)始嘗試將個(gè)體基因、表型、行為或代謝率等個(gè)體特征融入食物網(wǎng)模型中。MELIAN等[69]結(jié)合基因和微進(jìn)化對(duì)食物網(wǎng)的研究發(fā)現(xiàn)種內(nèi)捕食關(guān)系差異對(duì)食物網(wǎng)的特征有顯著影響。除了個(gè)體特征數(shù)據(jù)的重要性外,動(dòng)物行為也在生態(tài)網(wǎng)絡(luò)中起到至關(guān)重要的作用[70-72]。例如,被捕食者發(fā)現(xiàn)捕食者時(shí)會(huì)表現(xiàn)出躲避、減少活動(dòng)頻率或減少攝食等行為[73-75],又如捕食者會(huì)通過(guò)微環(huán)境的變化進(jìn)行獵食[76]。這些行為都會(huì)改變網(wǎng)絡(luò)中的相互作用強(qiáng)度[77-78]。關(guān)注生態(tài)網(wǎng)絡(luò)中的個(gè)體行為為我們提供了一個(gè)探索網(wǎng)絡(luò)結(jié)構(gòu)的調(diào)控方式。BECKERMAN等[77]基于覓食理論率先發(fā)現(xiàn):個(gè)體食譜寬度模型(individual diet breadth models)可用于預(yù)測(cè)真實(shí)食物網(wǎng)中的關(guān)聯(lián)性及其與物種豐度的關(guān)系。隨后在BECKERMAN等[77]的工作基礎(chǔ)上,PETCHEY等[79]引入個(gè)體大小作為新的參數(shù),進(jìn)而生成了一種確定性模型(deterministic model),該模型可以成功預(yù)測(cè)真實(shí)食物網(wǎng)中65%的營(yíng)養(yǎng)關(guān)系。VALDOVINOS等[80]則評(píng)估了適應(yīng)性覓食行為(adaptive foraging behavior)與網(wǎng)絡(luò)結(jié)構(gòu)的關(guān)系。他們發(fā)現(xiàn)該行為對(duì)嵌套型網(wǎng)絡(luò)結(jié)構(gòu)有負(fù)面影響,但有利于提高連接穩(wěn)定性。通過(guò)拓展這些模型,我們可以更好地解釋個(gè)體行為在整個(gè)生態(tài)系統(tǒng)中的作用,尤其是一些特殊行為在該網(wǎng)絡(luò)中的驅(qū)動(dòng)作用。
隨著科學(xué)技術(shù)手段的更新,近年來(lái)復(fù)雜食物網(wǎng)的研究有新的突破。從關(guān)注量級(jí)變化(如物種數(shù)量和關(guān)系)發(fā)展為關(guān)注網(wǎng)絡(luò)結(jié)構(gòu)(如聚集情況和回路長(zhǎng)度),從現(xiàn)象歸納描述逐漸向機(jī)理探索。融合不同學(xué)科的理論和方法,研究人員已經(jīng)確定了復(fù)雜系統(tǒng)結(jié)構(gòu)、物種配置、連接強(qiáng)度及其分布等網(wǎng)絡(luò)特征在維持食物網(wǎng)穩(wěn)定性中的重要性。新興的量化食物網(wǎng)研究方法為從描述物種均值向探索個(gè)體特征重要性發(fā)展提供了有利幫助。
食物網(wǎng)研究可用于評(píng)估生態(tài)系統(tǒng)的抗干擾特性。研究發(fā)現(xiàn)食物網(wǎng)具有小世界網(wǎng)絡(luò)的特性,即食物網(wǎng)中物種之間的路徑長(zhǎng)度較短。在包括較多物種的食物網(wǎng)中,物種間的最短路徑常常不超過(guò)3個(gè)節(jié)點(diǎn),即頂層捕食者和底層的植物或腐生生物之間的路徑距離很短[6,44,81]。這意味著在復(fù)雜生態(tài)網(wǎng)絡(luò)中一個(gè)物種變動(dòng)引起的干擾,其傳播速度遠(yuǎn)遠(yuǎn)超過(guò)了人們之前的預(yù)期[81-82]。
食物網(wǎng)研究揭示了環(huán)境變化對(duì)群落或生態(tài)系統(tǒng)的影響。例如,LEDGER等[83]模擬河流內(nèi)的食物網(wǎng)發(fā)現(xiàn)水文環(huán)境的干擾顯著減少了物種豐度,降低了網(wǎng)絡(luò)中連接數(shù)量,因而改變了網(wǎng)絡(luò)結(jié)構(gòu)。另有研究發(fā)現(xiàn)淡水酸化會(huì)顯著影響網(wǎng)絡(luò)的大小和結(jié)構(gòu),但因受制于冗余的原因,這些網(wǎng)絡(luò)系統(tǒng)仍能繼續(xù)維持至少部分的功能[82,84]。
網(wǎng)絡(luò)結(jié)構(gòu)的研究還被用于預(yù)測(cè)諸如物種滅絕等生物多樣性損失造成的影響,動(dòng)態(tài)模型被用于探索原發(fā)的和次級(jí)滅絕物種[85-86],物種在網(wǎng)絡(luò)中的位置也可被用于評(píng)估其重要性,度量其滅絕隱患大?。?7]。
此外,復(fù)雜網(wǎng)絡(luò)的理論也被用于評(píng)估篩選生態(tài)食物網(wǎng)中維持網(wǎng)絡(luò)穩(wěn)定的關(guān)鍵物種[81]。VASAS等[88]和楊濤等[89]分別通過(guò)網(wǎng)絡(luò)分析的方法確定了切薩皮克灣及萊州灣內(nèi)的關(guān)鍵種,為生物多樣性保護(hù)提供了基礎(chǔ)。
復(fù)雜食物網(wǎng)研究還能為生態(tài)保護(hù)及漁業(yè)管理提供新的視角,從單物種保護(hù)向整體保護(hù)轉(zhuǎn)變,更多的關(guān)注物種和物種之間的關(guān)系。近年來(lái)海洋食物網(wǎng)退化,食物網(wǎng)結(jié)構(gòu)簡(jiǎn)化(食物鏈縮短、營(yíng)養(yǎng)回路減少、連接度降低),都會(huì)影響食物網(wǎng)的穩(wěn)定性[90]。雖然當(dāng)下食物網(wǎng)還不是退化指標(biāo),但是在未來(lái)的研究中,尤其是海洋環(huán)境中,食物網(wǎng)亦可用于此類評(píng)估[91]。
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