土壤動物粒徑譜研究進(jìn)展

徐國瑞,馬克明

1 中國科學(xué)院生態(tài)環(huán)境研究中心,城市與區(qū)域國家重點實驗室,北京 100085 2 中國科學(xué)院大學(xué),北京 100049

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土壤動物粒徑譜研究進(jìn)展

徐國瑞1,2,馬克明1,*

1 中國科學(xué)院生態(tài)環(huán)境研究中心,城市與區(qū)域國家重點實驗室,北京 100085 2 中國科學(xué)院大學(xué),北京 100049

群落結(jié)構(gòu)如何響應(yīng)環(huán)境變化是生態(tài)學(xué)研究長期關(guān)注的核心問題之一。粒徑譜由個體大小和多度構(gòu)建而來,與營養(yǎng)級轉(zhuǎn)換速率相關(guān)、反映生態(tài)系統(tǒng)過程動態(tài)以及表征生態(tài)系統(tǒng)穩(wěn)定性,可以將其視為一個綜合的功能多樣性指標(biāo)用于預(yù)測和表征群落的組成以及生態(tài)系統(tǒng)功能如何響應(yīng)環(huán)境壓力。粒徑譜研究最初始于水生生態(tài)系統(tǒng),近年來被引入到土壤動物群落生態(tài)學(xué)的研究中。簡要回顧粒徑譜的概念由來及理論基礎(chǔ),分析比較了當(dāng)前粒徑譜研究中的4種易混淆類型,介紹了常用的兩類土壤動物粒徑譜構(gòu)建方法及其生態(tài)學(xué)意義,梳理了土壤動物粒徑譜對環(huán)境梯度響應(yīng)與生態(tài)化學(xué)計量學(xué)相結(jié)合的研究進(jìn)展,并指出了應(yīng)用粒徑譜研究土壤動物群落的難點及限制條件。未來,在基礎(chǔ)理論研究方面,土壤動物粒徑譜應(yīng)關(guān)注個體大小與營養(yǎng)級位置及能量利用關(guān)系；在應(yīng)用方面,土壤動物粒徑譜可結(jié)合傳統(tǒng)的分類方法廣泛應(yīng)用于指示環(huán)境污染、生態(tài)恢復(fù)、保育生物以及土地利用變化等。

粒徑譜；土壤動物；生態(tài)化學(xué)計量；環(huán)境梯度

群落結(jié)構(gòu)如何響應(yīng)環(huán)境變化是生態(tài)學(xué)研究長期關(guān)注的經(jīng)典和前沿領(lǐng)域之一。Sutherland等[1]提出生態(tài)學(xué)仍需回答的100個根本問題中提到能否用一個綜合指標(biāo)的分布情況反映群落屬性對于環(huán)境變化的響應(yīng),比如個體大小？將群落生態(tài)位空間壓縮到單一維度,個體大小是直接涵蓋多重有意義生物學(xué)信息的重要指標(biāo)[2]。個體大小與新陳代謝密切相關(guān)[3],決定了一系列的物種特征,對食物網(wǎng)結(jié)構(gòu)和動態(tài)產(chǎn)生深刻影響,是生物體最為根本的一個屬性[2,4]。2011年Oikos上以“Body size and ecosystem dynamics”專題論述個體大小對于生態(tài)系統(tǒng)動態(tài)的影響[5],個體大小在群落研究中的重要性受到越來越多研究者的注意。種群多度變化除表征與其他物種和無機(jī)環(huán)境之間的相互關(guān)系外,更代表著種群在群落中的存在或滅絕[6]。個體大小的多度分布研究最初始于水生生態(tài)系統(tǒng)中,將生物(顆粒)按照大小分成不同的粒徑級別,計算不同的粒徑級別中的生物(顆粒)量,由此得出的生物(顆粒)量在不同徑級上的分布曲線稱之為粒徑(生物量)譜。粒徑譜結(jié)構(gòu)揭示了群落的大小分布與營養(yǎng)級結(jié)構(gòu)之間的相互作用[4],是環(huán)境變化背景下的群落結(jié)構(gòu)變化和功能響應(yīng)的綜合性指標(biāo)[7],可以與近年來備受關(guān)注功能多樣性[8-9]以及基于特征[10-11]的研究框架有益結(jié)合。粒徑譜有多種類型,包括豐度粒徑譜、顆粒粒徑譜、生物量譜[12]。粒徑譜方法在水生生物中得到了成功應(yīng)用[13],但陸域生態(tài)系統(tǒng)中的相關(guān)研究滯后,尤其是土壤動物的粒徑譜研究十分薄弱。

土壤動物是生態(tài)系統(tǒng)中重要的生物組分,與生態(tài)系統(tǒng)關(guān)鍵功能過程緊密相關(guān)[14],在生物地球化學(xué)循環(huán)中起著重要作用[15]。不同大小的土壤動物發(fā)揮的生態(tài)系統(tǒng)功能與承載的生態(tài)系統(tǒng)服務(wù)價值不同[16]。作為分解者,中小型土壤動物(蜱螨類,線蟲等)顯著提高凋落物分解過程[17],促進(jìn)生態(tài)系統(tǒng)元素循環(huán)[18]。作為生態(tài)系統(tǒng)工程師,大型土壤動物(蚯蚓,螞蟻等)的自身活動改變土壤理化特征、調(diào)控其它生物的活動(如直接取食植物以及捕食微生物等),在營養(yǎng)級聯(lián)效應(yīng)中扮演重要角色[19]。越來越多的研究者認(rèn)同,土壤動物在驅(qū)動生態(tài)系統(tǒng)對環(huán)境變化響應(yīng)過程中具有重要作用[20]。但由于土壤動物的多樣性極高,個體小且存在大量幼蟲,分類鑒定到種的難度極高,土壤動物的研究及應(yīng)用發(fā)展緩慢[21]。近年來,研究者將水生生態(tài)系統(tǒng)中應(yīng)用成熟的粒徑譜概念引入陸域生態(tài)系統(tǒng)中[22-23],使得便捷地應(yīng)用土壤動物指示環(huán)境變化成為可能。

本文試圖在簡要回顧粒徑譜理論發(fā)展的基礎(chǔ)上,梳理土壤動物粒徑譜研究最新進(jìn)展,并探討整合粒徑譜理論與營養(yǎng)級理論及能量均值法則的可能性,嘗試提出未來土壤動物粒徑譜研究需要著重回答的問題與應(yīng)用的方向。

1 粒徑譜研究的概念由來、理論基礎(chǔ)與類型

1.1 粒徑譜的概念由來

粒徑譜的概念提出最早可以追溯到Elton所著的Animal Ecology一書[24],其理論模型研究主要經(jīng)歷了最初的Kerr營養(yǎng)級理論模型[25]-Sheldon捕食者/食物大小關(guān)系模型[26](K-S),改進(jìn)的Platt-Denman標(biāo)準(zhǔn)化生物量模型(P-D)[27-28]以及完全符合異速生長形式的Thiebaux-Dickie(T-D)穹頂模型[29-30]3個階段[12]。粒徑譜研究最初始于水生生態(tài)系統(tǒng)中,如浮游動植物[31]、底棲動物[32]、魚類[33]等,通過關(guān)注粒徑譜(對數(shù)化)斜率及截距的變化,監(jiān)測生態(tài)系統(tǒng)狀態(tài)[7,28]以及預(yù)測海洋資源產(chǎn)量[34]。集中在水域生態(tài)系統(tǒng)的粒徑譜研究經(jīng)過半個多世紀(jì)的發(fā)展已經(jīng)相對成熟,但在陸地生態(tài)系統(tǒng)中,尤其是土壤動物群落中,粒徑譜研究卻處于剛剛起步的階段。

1.2 粒徑譜的理論基礎(chǔ)

捕食者與被捕食者之間的大小關(guān)系是粒徑譜研究的重要理論基礎(chǔ)之一[26]。個體大小與營養(yǎng)級之間的正相關(guān)關(guān)系,即個體越大的生物其營養(yǎng)級位置往往也越高,在多種生態(tài)系統(tǒng)(尤其是水生生態(tài)系統(tǒng))中得到廣泛證實[35- 37],但由于陸域土壤生境的復(fù)雜性,土壤動物群落存在高度雜食性[38],其個體大小與營養(yǎng)級關(guān)系仍需進(jìn)一步研究。

基于個體大小推繹的異速生長關(guān)系構(gòu)成了粒徑譜研究的又一重要理論基礎(chǔ)[29-30]。個體大小影響細(xì)胞質(zhì)水平上的新陳代謝速率,對于生物個體、種群、群落乃至整個生態(tài)系統(tǒng)具有重要意義[3]。Huxley[39]最早提出了基于個體大小的異速生長冪次方程,Y=Y0Mb。式中Y可以是新陳代謝速率,發(fā)育時間,種群生長速率或是分子進(jìn)化速率；M指的是個體大小,通常以個體質(zhì)量作為表征；Y0是正態(tài)化常量,而b則是異速生長的冪次數(shù)。同年,Kleiber[40]提出了有機(jī)體新陳代謝速率的尺度推繹方程,I=I0M3/4,式中I0是與個體大小無關(guān)的正態(tài)化常量,可以是基礎(chǔ)/野外/最大呼吸速率。個體大小與多種系統(tǒng)特征也存在異速生長推繹關(guān)系,如家域范圍(b=1)[41],消化速率與次級生產(chǎn)(b=3/4)[3,42],營養(yǎng)循環(huán)速率(b=-1/4)[3,43-44]。

生物異速生長推繹(Allometric scaling)冪次數(shù)曾存在基于歐式幾何推繹的2/3[45- 49]與基于分形理論推繹的3/4[3,50- 53]之間的長期爭論,至今仍無定論[54]。也有學(xué)者認(rèn)為在存在資源限制的真實局地群落中,冪次數(shù)并不是一個固定值,而是涵蓋了眾多生物與環(huán)境條件的綜合值,因此建議將其看做是兼具生物和生態(tài)學(xué)功能的有用通量[7,55- 57]。

1.3 粒徑譜類型

2 土壤動物粒徑譜的構(gòu)建方法與生態(tài)學(xué)意義

不同大小土壤動物發(fā)揮的生態(tài)系統(tǒng)功能[74]和承載的生態(tài)系統(tǒng)服務(wù)[16]不同,如表1中土壤動物各大小類群在營養(yǎng)循環(huán)和土壤團(tuán)聚體形成的作用各異。2005年Mulder[22]首次運用粒徑譜方法在土壤動物群落,引起了土壤學(xué)者的普遍關(guān)注,成為近年來土壤動物群落研究的一個新的研究方法。在介紹構(gòu)建方法前,需要明確個體大小的定義。盡管個體長度[75]和生物體積[67,69]都可以表征個體大小,但多數(shù)研究者認(rèn)為個體質(zhì)量是更為本質(zhì)或更為準(zhǔn)確的一個指標(biāo),因為個體質(zhì)量在個體水平和種群水平與功能過程更為相關(guān)[3]。因此,建議構(gòu)建土壤動物粒徑譜時,應(yīng)盡量選擇個體質(zhì)量(干重優(yōu)于鮮重)作為個體大小的具體表征。粒徑譜研究中通?？紤]豐度(多度)粒徑譜和生物量譜兩種類型,二者可以相互轉(zhuǎn)化[76],如下：

log(N)=b×log(M)+a

(1)

而log(B)=log(MN)=log(M)+log(N)

(2)

由(1)和(2)可得：

log(B)=log(M)+log(N)=log(M)+b×log(M)+a=(1+b)×log(M)+a

(3)

式中,B為生物量,M為個體大小,N為豐度(多度)。

下面主要以豐度粒徑譜構(gòu)建為例,介紹當(dāng)前主要運用的2類土壤動物粒徑譜的構(gòu)建方法。

2.1 土壤動物L(fēng)SDR粒徑譜構(gòu)建方法

LSDR粒徑譜的構(gòu)建的第一步是獲取不同類群土壤動物的質(zhì)量,大型土壤動物可以通過直接稱重法獲取個體質(zhì)量,而對于中小型土壤動物可以通過測量體長,根據(jù)體長-質(zhì)量方程轉(zhuǎn)換為個體質(zhì)量。常見土壤動物類群的質(zhì)量-體長轉(zhuǎn)變方程見表2,干重-鮮重轉(zhuǎn)換方程見表3。

表1 不同大小土壤動物在營養(yǎng)循環(huán)和土壤結(jié)構(gòu)與團(tuán)聚體形成中的作用[16,23,77,78]

需要特別指出的是極微型土壤動物,如原生動物等,因其個體過小,未檢索到其對應(yīng)的質(zhì)量-體長轉(zhuǎn)換方程。另一點是土壤微生物作為土壤動物重要的食物來源,將其與土壤動物一并整合到土壤生物粒徑譜研究中,對于整體上理解土壤食物網(wǎng)的結(jié)構(gòu)具有重要意義。然而由于微生物個體變異大,如少數(shù)真菌個體可以占據(jù)965hm2的面積[94],相關(guān)研究極少。Mulder等人[76]通過熒光染色與自動成像技術(shù)觀測微生物的細(xì)胞體積,進(jìn)而通過細(xì)胞體積與碳容量的轉(zhuǎn)換比(3.2×10-13g C/μm3)以及0.5的干鮮重比得到了微生物的個體質(zhì)量。

表2 質(zhì)量-體長轉(zhuǎn)換方程

M:個體質(zhì)量,L:體長,W:體寬

表3 干重-鮮重轉(zhuǎn)換方程

獲取了各類群土壤動物質(zhì)量后,分類群計算土壤動物的平均質(zhì)量,而后以各類群的平均質(zhì)量與多度(或多度+1)的對數(shù)值作圖,即構(gòu)建出土壤動物的LSDR粒徑譜。

2.2 土壤動物ISD粒徑譜構(gòu)建方法

在獲取各類群的個體質(zhì)量后(步驟同LSDR),略去分類學(xué)信息,完全按照各類群的個體質(zhì)量大小排序,分成不同徑級。劃分徑級方法主要包括兩種,第一種是按照倍增數(shù)列劃分[95],如1—2μg,2—4μg,4—8μg…等,第二種是按照具體研究群落的個體大小范圍進(jìn)行等分[76],等分?jǐn)?shù)由研究者自行確定,將各個徑級的個體數(shù)加和作為Y值,取各個徑級的中間值作為X值,進(jìn)行雙對數(shù)回歸,即為ISD粒徑譜。

2.3 土壤動物粒徑譜的生態(tài)學(xué)意義

兩種構(gòu)建方法得到的粒徑譜均具有2個重要的參數(shù),粒徑譜的斜率(Slope,b)與截距(Y-intercept,a)。土壤動物粒徑譜斜率可以反映群落中不同大小的土壤動物的分布情況[96],而截距則代表著在固有資源供應(yīng)限制情況下的生態(tài)系統(tǒng)容量[97]。Mulder[23]將異速生長視角引入土壤無脊椎動物對于生態(tài)系統(tǒng)功能響應(yīng)的研究中,指出粒徑譜可以作為衡量人類活動對于景觀結(jié)構(gòu)以及食物網(wǎng)結(jié)構(gòu)的綜合測度指標(biāo),可以作為土地利用對恢復(fù)生態(tài)學(xué)和保護(hù)生物學(xué)影響模型構(gòu)建的替代參數(shù),量化土壤群落組成及與其相關(guān)的生態(tài)系統(tǒng)服務(wù),評估生態(tài)系統(tǒng)風(fēng)險。

3 土壤動物粒徑譜對于環(huán)境梯度的響應(yīng)

土壤動物異速生長推繹關(guān)系并不是一個固定不變的值,而是局地群落的一個功能屬性,可以將其作為一種囊括豐富信息的新的多樣性測度指標(biāo)[55-56]。粒徑譜能夠響應(yīng)環(huán)境梯度變化(氣候因素,生產(chǎn)力以及擾動),反映生態(tài)系統(tǒng)功能[7]。

3.1 土壤動物粒徑譜對于環(huán)境梯度的響應(yīng)

氣候變化(溫度升高,降雨增加等)和環(huán)境擾動(土地利用變化,森林管理,農(nóng)田施肥等)可能會通過影響土壤動物生物量的分布,進(jìn)而影響土壤生物提供的生態(tài)系統(tǒng)功能[98-99]。由于在眾多生物學(xué)過程中的重要作用,溫度對于群落中生物量的分布以及食物網(wǎng)的作用受到眾多研究者的關(guān)注,2012年英國皇家科學(xué)院Philosophical Transactions B 組織了“Climate change in size-structured ecosystems”?？?專題論述具有大小結(jié)構(gòu)的生態(tài)系統(tǒng)如何響應(yīng)氣候變化[100]。作為變溫動物,土壤無脊椎動物受環(huán)境溫度變化的影響可能更甚[101]。研究表明,暖溫帶森林生態(tài)系統(tǒng)凋落物層土壤動物的粒徑譜關(guān)系無論在生長季還是休眠季均可以響應(yīng)海拔變化,溫度是驅(qū)動粒徑譜斜率在海拔上變化的主要因素[102]。溫度也可通過影響資源的可獲得性對于土壤動物的生物量分布產(chǎn)生影響；在生長季粒徑譜的斜率主要受凋落物層質(zhì)量影響,而休眠季主要受凋落物數(shù)量影響。也有研究[103]發(fā)現(xiàn)土壤微節(jié)肢動物的粒徑譜的斜率在不同溫濕度環(huán)境下并未產(chǎn)生顯著差異,但粒徑譜的截距在不同濕度下有差異:更濕潤的環(huán)境,截距更高,說明濕度可提高生態(tài)系統(tǒng)最大容納的個體多度。該研究中溫度的作用不明確可能和其關(guān)注的土壤動物類群大小范圍變化較窄以及溫度跨度較小有關(guān)。

由于地上-地下生態(tài)系統(tǒng)之間緊密鏈接[15],植物生產(chǎn)力的變化會對土壤動物群落大小結(jié)構(gòu)產(chǎn)生影響。土壤動物粒徑譜可以很好地反映其大小分布情況如何響應(yīng)植物生產(chǎn)力變化。凋落物作為土壤動物的基礎(chǔ)食源,以凋落物質(zhì)量表征植物生產(chǎn)力,Comor[96]等發(fā)現(xiàn)植物生產(chǎn)力對土壤動物的粒徑譜斜率有影響,但這種影響并不均等,相對小的土壤動物從更高的生產(chǎn)力中獲利更多。

土壤動物對于擾動環(huán)境的響應(yīng)主要集中在土壤線蟲的研究。George[104]等人將傳統(tǒng)的成熟度指數(shù)與粒徑譜結(jié)合起來應(yīng)用于土壤線蟲群落研究中,發(fā)現(xiàn)在添加木灰的處理中相對小的個體受損比其他處理(皆伐與未受擾動)更多。施肥導(dǎo)致群落水平上大個體土壤線蟲增加的比小個體土壤線蟲的幅度大。但也有研究證實施肥并未改變粒徑譜斜率但顯著增加其截距[105],施肥使得土壤線蟲大個體與小個體獲利情況相對均等。群落水平上土壤線蟲大小與土壤有機(jī)碳,總氮,可利用性P和K正相關(guān)[106]。土壤線蟲群落粒徑譜也可用于指示土地利用變化,牧場和農(nóng)田生態(tài)系統(tǒng)的粒徑譜斜率比草地和次生林的更為陡峭[107],這與干擾狀態(tài)下相對大的土壤動物受損更多的假設(shè)相符。Zhao[107]指出基于個體大小構(gòu)建的粒徑譜(ISD)比基于分類信息構(gòu)建的土壤線蟲粒徑譜(LSDR)有更高的解釋率,可能更為實用。

3.2 土壤動物粒徑譜與生態(tài)化學(xué)計量

生態(tài)化學(xué)計量學(xué)主要研究能量與多種元素(以C、N、P為主)在生物體與環(huán)境中的平衡關(guān)系,其動態(tài)平衡決定了細(xì)胞質(zhì)過程、個體消費者-資源互作、種群密度以及群落和生態(tài)系統(tǒng)格局[108- 111]。生態(tài)化學(xué)計量學(xué)的一個重要理論基礎(chǔ)是生長速率假說,即生物體必須通過調(diào)節(jié)自身的C、N、P比值適應(yīng)不同的生長速率,認(rèn)為P對于生物生長和繁殖密切相關(guān),異養(yǎng)生物高生長速率需要對應(yīng)低C：P比與N：P比[110,112]。異速生長與生態(tài)化學(xué)計量學(xué)之間的聯(lián)系在水生生態(tài)系統(tǒng)中得到廣泛證實,直到2009年,Mulder[76]將粒徑譜(生物量譜)與生態(tài)化學(xué)計量聯(lián)系起來,在涵蓋了廣泛類群(真菌、細(xì)菌以及土壤動物)的歐洲草原土壤食物網(wǎng)中證實了生長速率假說,即由于P對于核糖體RNA以及蛋白質(zhì)產(chǎn)量的重要影響,P的可獲得性越高,土壤生物的相對生長速率越快,其粒徑譜斜率也越陡峭。這項研究首次揭示了P的可獲得性和土壤食物網(wǎng)異速生長的耦合關(guān)系。

此后,Ott等[113]在森林生態(tài)系統(tǒng)的凋落物層無脊椎動物類群中驗證了生態(tài)系統(tǒng)Na短缺假說(凋落物中的Na含量低,而其消費者依賴Na保持膜透性,對Na要求高,即分解者系統(tǒng)中存在Na短缺),發(fā)現(xiàn)Na元素與土壤無脊椎動物的粒徑譜斜率高度相關(guān)。結(jié)構(gòu)物質(zhì)假說認(rèn)為N元素對于核酸產(chǎn)生與結(jié)構(gòu)物質(zhì)(蛋白質(zhì),微纖絲,幾丁質(zhì))具有普遍重要性,這一假說在草甸與森林生態(tài)系統(tǒng)中都得到了證實[96,113]。最新的研究發(fā)現(xiàn)凋落物層的S元素與粒徑譜斜率負(fù)相關(guān),S元素對于土壤動物的生長可能具有潛在抑制作用[102]。S元素對于土壤無脊椎群落大小分布的影響的機(jī)理需進(jìn)一步研究。未來生態(tài)化學(xué)計量與粒徑譜的整合研究中應(yīng)納入更多的化學(xué)元素并將動物自身的化學(xué)元素計量特征考慮在內(nèi),比較生物體與環(huán)境之間的元素計量關(guān)系,找出限制土壤動物生長的化學(xué)計量元素,更直接探索生物生長自身與環(huán)境資源供給的動態(tài)平衡關(guān)系。

3.3 土壤動物粒徑譜與其他功能多樣性指標(biāo)的結(jié)合研究

土壤動物粒徑譜的應(yīng)用還可以與傳統(tǒng)的多樣性指標(biāo)、功能多樣性以及基于特征的研究方法廣泛結(jié)合。如與豐富度、多度以及群落組成指標(biāo)相結(jié)合,從不同側(cè)面揭示全球變化致使土壤微節(jié)肢動物群落縮小(Community downsizing)的可能機(jī)制[114]或與營養(yǎng)級研究相結(jié)合,揭示環(huán)境壓力下的碎屑食物網(wǎng)營養(yǎng)級緊實(Trophic contraction)以及土壤動物群落拆解(Community disassemble)機(jī)制[103]。有研究將土壤動物分為不同功能群進(jìn)行粒徑譜構(gòu)建,進(jìn)而更為詳盡地回答土地利用變化如何影響土壤動物群落的功能變化[107]。另外,基于取食功能團(tuán)權(quán)重計算的個體大小可以有效補(bǔ)充基于分類信息與在評估土壤食物網(wǎng)狀態(tài)時的不足[115]。在土壤線蟲研究中,可與基于譜系、取食以及個體大小特征集合而成熟度指數(shù)相結(jié)合,共同反應(yīng)土壤線蟲群落狀態(tài),有效指示土壤質(zhì)量。也有研究指出,土壤線蟲個體大小的群落加權(quán)均值(Community-weighted mean)是指示土壤生態(tài)系統(tǒng)對于干擾響應(yīng)的有效指標(biāo)[106]。

4 土壤動物粒徑譜研究的應(yīng)用難點以及限制條件

土壤動物分類精度越高對于專業(yè)知識的要求及時間投入也就越多。盡管已有研究表明適度降低分類精度(如在屬[76]、形態(tài)種[57]或科[102]一級水平上構(gòu)建土壤動物L(fēng)SDR粒徑譜),土壤動物的粒徑譜斜率依然可以指示環(huán)境變化,但也有研究指出不同的分類精度會影響土壤食物網(wǎng)中的異速生長推繹關(guān)系[116]。因此,土壤動物分類精度對于粒徑譜的影響還需進(jìn)一步探討,未來有必要針對不同研究關(guān)注的具體科學(xué)問題和主要土壤動物類群,進(jìn)行預(yù)實驗比較不同分類精度下粒徑譜響應(yīng)環(huán)境的敏感度與有效性。

土壤動物的個體發(fā)育階段也會對粒徑譜的應(yīng)用產(chǎn)生重要影響,土壤動物某些類群(如線蟲、彈尾類、鞘翅目、雙翅目、鱗翅目等)的個體大小在發(fā)育階段會有幾個數(shù)量級的變化,這種變化往往伴隨著食性的改變[117-118]。LSDR粒徑譜構(gòu)建過程中,缺失發(fā)育階段信息,會增大粒徑譜響應(yīng)環(huán)境變化的噪音,對于重點關(guān)注土壤中大量幼蟲或是發(fā)育階段有多個齡期在土壤中度過的類群時,需謹(jǐn)慎應(yīng)用或解釋。

對于關(guān)注土壤動物群落變化,但缺少系統(tǒng)地土壤動物分類知識專業(yè)訓(xùn)練的研究者而言,ISD的相對簡單構(gòu)建方法以及圖像掃描軟件的發(fā)展,使土壤動物粒徑譜研究變得簡單易行[57]。取樣充分與具有代表性是應(yīng)用ISD粒徑譜方法研究土壤動物群落的先決條件,徑級劃分的寬度與邊界直接影響群落的粒徑譜結(jié)構(gòu)[119]。由于完全忽略了分類學(xué)信息,應(yīng)用ISD方法構(gòu)建土壤動物粒徑譜時,土壤動物的標(biāo)準(zhǔn)化獲取以及統(tǒng)計分析時如何劃分徑級是兩大難點。未來的研究中,將ISD與LSDR兩種粒徑譜構(gòu)建方法結(jié)合起來,對于綜合反應(yīng)土壤動物群落如何響應(yīng)環(huán)境變化具有重要意義。

5 未來方向

營養(yǎng)級理論是粒徑譜推繹關(guān)系(斜率,截距等)變化鏈接生態(tài)系統(tǒng)功能重要理論基礎(chǔ)。粒徑譜與營養(yǎng)級的相關(guān)關(guān)系主要在水生生態(tài)系統(tǒng)中的ISD粒徑譜得到證實,而適合描述土壤動物群落的LSDR和ISD粒徑譜與營養(yǎng)級的關(guān)系仍需進(jìn)一步研究。水生態(tài)系統(tǒng)中,粒徑譜斜率越為平緩,則代表該群落的營養(yǎng)級轉(zhuǎn)化速率越高,越陡峭其營養(yǎng)級轉(zhuǎn)化速率越小。土壤生境比水體生境更為復(fù)雜,影響土壤生物生長與活動的無機(jī)環(huán)境因素很多,另外土壤動物高度的雜食性可能會導(dǎo)致土壤動物粒徑譜與營養(yǎng)級關(guān)系變得復(fù)雜。Mulder[99]利用酶活性信息定義土壤動物取食功能群間接證實了土壤食物網(wǎng)與ISD粒徑譜之間存在關(guān)系,但這并沒有直接回答土壤動物個體大小與營養(yǎng)級之間或是土壤動物粒徑譜斜率與營養(yǎng)級轉(zhuǎn)化效率之間的關(guān)系。近年來,隨著穩(wěn)定性同位素技術(shù)在土壤中的應(yīng)用發(fā)展,使得直接測量不同種類的大中型土壤動物的營養(yǎng)級位置成為可能,但對于土壤中小型動物營養(yǎng)級位置的研究仍然較為困難。對于具有明確食物關(guān)系的土壤生物,研究捕食者與食物二者之間個體大小比有助于解釋粒徑譜與營養(yǎng)級的關(guān)系。另外,個體大小與營養(yǎng)級的關(guān)系在不同尺度上的研究結(jié)果可能不一致,如水生生物的個體大小與營養(yǎng)級的關(guān)系薄弱,但在群落水平上呈現(xiàn)出了顯著正相關(guān)[120],因此有必要從不同尺度上(如種類,功能群,群落等)驗證個體大小與營養(yǎng)級的關(guān)系。

能量均等法則(EER)認(rèn)為生物的能量利用不受生物個體大小影響,即不同大小的生物類群其能量利用均等。粒徑譜理論結(jié)合新陳代謝理論可以推繹出群落中各類群生物的能量利用情況[121, 122]。關(guān)于土壤動物的能量利用研究十分薄弱,且已有的少數(shù)研究結(jié)果并不一致。能量均等法則在森林土壤節(jié)肢動物的某些類群中得到證實[122, 123],但最近Enthes[124]利用涵蓋多營養(yǎng)級的土壤動物類群更為翔實的數(shù)據(jù)證明森林中的土壤動物群落并不存在能量均等法則。生物群落的能量分布情況對于理解群落結(jié)構(gòu)及其對于環(huán)境變化的響應(yīng)至關(guān)重要,未來的基礎(chǔ)理論研究應(yīng)著重關(guān)注土壤動物個體大小與營養(yǎng)級位置及能量利用關(guān)系。

土壤生物的高度多樣性以及分類局限性使得大范圍應(yīng)用傳統(tǒng)方法評估土壤質(zhì)量十分困難,而土壤動物粒徑譜具有以下3個優(yōu)點：(1)構(gòu)建方法簡單,便于應(yīng)用；(2)與土壤功能與環(huán)境響應(yīng)高度相關(guān)；(3)對于分類知識要求不高。因此土壤動物粒徑譜可以與傳統(tǒng)的分類學(xué)知識以及食物網(wǎng)理論互相補(bǔ)益,成為預(yù)測土壤質(zhì)量的標(biāo)準(zhǔn)化評估工具[57]。未來可以將土壤動物的ISD粒徑譜與LSDR粒徑譜相結(jié)合廣泛應(yīng)用于指示環(huán)境污染,生態(tài)恢復(fù),保育生物以及土地利用變化中。

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Advances in the body size spectra study of soil fauna

XU Guorui1,2, MA Keming1,*

1StateKeyLaboratoryofUrbanandRegionalEcology,ResearchCenterforEco-EnvironmentalSciences,ChineseAcademyofSciences,Beijing100085,China2UniversityofChineseAcademyofSciences,Beijing100049,China

Community structure and their responses to environmental changes have long been a major concern in ecology. Constructed by body size and abundance, size spectra correlate with trophic transfer efficiency, respond process rate dynamics, and characterize ecological stability. Thus, body size spectra (BSS) can be an indicator of functional diversity to predict and characterize how community composition and ecosystem function respond to environmental pressures. The initial focus of BSS was in aquatic ecosystems until recently, when it was used in the community ecology of soil invertebrates. The present study briefly reviewed the concept, background, and theoretical basis of BSS. Four different but often confused BSS patterns were also distinguished. Then we showed how to construct the two frequently used types of BSS in soil fauna and their ecological significances. We introduced research progress on how the BSS of soil organisms respond to environmental gradients and relate to ecological stoichiometry. Limitations and constraints of the BSS application to soil fauna were also noted. In basic theoretical research, we should focus on the correlation of body size with trophic level and energy use in soil animals. Combined with traditional classification method, BSS of soil fauna can be widely used to indicate environmental pollution, ecological restoration, conservation biology, and land use changes.

body size spectra; soil fauna; ecological stoichiometry; environmental gradients

國家自然科學(xué)基金面上項目(31470481)

2016- 01- 2; 網(wǎng)絡(luò)出版日期:2016- 10- 29

10.5846/stxb201601260185

*通訊作者Corresponding author.E-mail: mkm@rcees.ac.cn

徐國瑞,馬克明.土壤動物粒徑譜研究進(jìn)展.生態(tài)學(xué)報,2017,37(8):2506- 2519.

Xu G R, Ma K M.Advances in the body size spectra study of soil fauna.Acta Ecologica Sinica,2017,37(8):2506- 2519.