Estimating the biodiversity of hay meadows in north-eastern Switzerland on the basis of vegetation structure

Transcription

1 Agricultur, Ecosysts and Environnt 93 (2002) Estiating th biodivrsity of hay adows in north-astrn Switzrland on th basis of vgtation structur Andra Schwab a, David Dubois a,, Padruot M. Frid a, Ptr J. Edwards b a Swiss Fdral Rsarch Station for Agrocology and Agricultur, Rcknholzstrt 191, CH 8046 Zürich, Switzrland b Swiss Institut for Tchnology, Gobotanical Institut, Zürichbrgstrt 38, CH 8044 Zürich, Switzrland Rcivd 17 Octobr 2000; rcivd in rvisd for 2 Novbr 2001; accptd 13 Novbr 2001 Abstract Biological divrsity is a ajor critrion in valuating th ffctivnss of asurs to nhanc th cological quality of rural aras. Assssnts of biodivrsity basd on slctd groups of indicator organiss, including both anial and plant groups, ar ti-consuing and rquir a high lvl of xprt knowldg: siplr thods ar thrfor ndd. Th biodivrsity of 18 hay adows in north-astrn Switzrland (Schaffhausr Randn, Canton Schaffhausn) was invstigatd using thr indicator groups (angiosprs, spidrs and tru bugs). Sipl structural paratrs dscribing th vgtation canopy wr invstigatd in trs of thir usfulnss as surrogats for biodivrsity asurs. Eightn sits varid widly in trs of spcis richnss, divrsity and abundanc for all indicator groups. Nubrs of angiospr spcis varid fro 21 to 57, spidrs 25 to 45, and tru bugs 12 to 37. Spcis nubrs of th thr indicator groups wr significantly corrlatd with ach othr (angiosprs vrsus spidrs: r = 0.53, P = 0.02; angiosprs vrsus tru bugs: r = 0.59, P = 0.01; tru bugs vrsus spidrs: r = 0.61, P = 0.01). Managnt had a strong influnc on spcis richnss, th sits undr xtnsiv anagnt showing th highst spcis nubr and divrsity. Th strongst corrlations wr thos btwn tru bugs and th two othr indicator groups. Howvr, th nubr of angiospr spcis pr 120 2, which was asy to assss, was also an accptabl prdictor of th othr two indicator groups. Svral paratrs of th vgtation canopy wr significantly rlatd to spcis nubrs and ultipl rgrssion odls basd on ths paratrs xplaind 25 60% of th varianc in spcis richnss and coposition. Canopy dnsity, plant bioass distribution and plant hight wr th ost iportant paratrs for all indicator groups. Th rsults indicat that paratrs dscribing th structur of th vgtation provid usful inforation about th rlativ spcis richnss of sits. In cobination with paratrs dscribing th surrounding land-us and an invntory of th angiosprs prsnt, thy offr a proising thod for assssing biodivrsity on a larg scal Elsvir Scinc B.V. All rights rsrvd. Kywords: Evaluation; Mthodology; Canopy paratrs; Grassland; Arana; Htroptra; Angiosprs; Switzrland 1. Introduction Biological divrsity has diinishd rapidly in rural aras of Switzrland ovr th last 70 yars as a Corrsponding author. Tl.: ; fax: E-ail addrss: david.dubois@fal.adin.ch. (D. Dubois). rsult of th intnsification of agricultur and associatd changs in faring thods (Islr-Hübschr, 1980; Mühlbrg and Slowik, 1997). Sinc 1992, thr hav bn ajor changs in Swiss agricultural policy aid at libralising th arkt and prooting sustainabl production. An iportant objctiv has bn to brak th traditional link btwn prics of agricultural products and farrs incos by /02/$ s front attr 2002 Elsvir Scinc B.V. All rights rsrvd. PII: S (01)

2 198 A. Schwab t al. / Agricultur, Ecosysts and Environnt 93 (2002) aking dirct paynts to farrs rlatd to nvironntal critria. Fars which rciv financial support ar obligd to allocat at last 7% of thir land for so-calld cological copnsation aras. Financial incntivs ar also availabl for othr anagnt practics which proot consrvation. Although, ths schs hav bn widly adoptd, thir ffctivnss in prooting biological divrsity is difficult to assss du to th lack of suitabl valuation thods. A straightforward, ti-saving thodology is ndd to asur th fficincy of such anagnt practics. Usual biodivrsity assssnt procdurs, which involv aking invntoris of slctd indicator groups, ar uch too xpnsiv for larg-scal survys, spcially whn anial taxa ar involvd. Hänggi (1989) calculatd th labour costs for an valuation basd on th spidr fauna of 10 sits (thr traps pr sit with svn captur priods fro May to August) as follows: 2.5 days fild work, 7 days sorting traps, 10 days idntification, 1.5 days analysis of th rsults. Thus, th total labour rquird for a odst survy of 10 sits was 21 days. Prvious attpts to dvlop siplr thods of valuating biodivrsity on farland hav bn basd only on th assssnt of angiospr spcis, and hav provd inadquat in prdicting invrtbrat divrsity (Zwölfr t al., 1984; Gloor, 1996). In this study, th spcis richnss of 18 grassland sits was invstigatd using thr indicator groups. Th taxa chosn wr th angiosprs bing th group ost coonly rcordd in biodivrsity studis and th tru bugs (Htroptra) and spidrs (Arana), both of which hav provd to b usful indicators of biodivrsity in agricultural aras (Wachann, 1989; Kichl and Trautnr, 1992; Dulli and Obrist, 1998). Th study also aid at tsting whthr th structur of th vgtation can b usd as an altrnativ to aking invntoris of indicator spcis. Svral coponnts of th vgtation structur influnc invrtbrat abundanc (Lawton, 1983; Brown, 1991). Variation in canopy coponnts and intractions btwn th ay produc diffrnt habitat possibilitis for invrtbrats (Dnno and Rodrick, 1991). Various structural paratrs of th vgtation wr asurd with a viw to dtct thos which ost closly rflctd diffrncs in grassland biodivrsity. 2. Matrial and thods 2.1. Study ara and study sits Th invstigation was carrid out in 1998 in th Mrishausr Randn (Swiss coordinats: 683,000/ 291, ,500/293,000, world-wid coordinats 8 36 E/46 44 N), which lis at th north-astrn nd of th Swiss Jura. Avrag yarly rainfall in th narby town of Schaffhausn (437 a.s.l.) is 866, with th highst valus in sur and th lowst in lat wintr. Th an annual tpratur is 7.8 C, with a axiu an onthly tpratur in July (23.2 C) and a iniu in January ( 3.9 C) (SMA, 1999). Th rgion is ainly forstd but with so agricultural land including a larg nubr of traditional hay adows btwn 780 and 830 a.s.l. (total ara of agricultural land: 243 ha). Th soils ovrli a nutrint-poor liston and ar shallow and frly draining (sklton-fr top layr of c; orthic rndzina according to th FAO classification). Thr typs of hay adows wr distinguishd. Extnsiv (E) sits (n = 6), not frtilisd, cut onc or twic pr yar. Low (L) intnsiv sits (n = 6), lightly frtilisd with anur, cut twic pr yar. Modrat (M) intnsiv sits (n = 6), rgularly frtilisd with slurry, cut two to thr tis pr yar. Eightn adows, six pr anagnt intnsity, wr slctd within a 1.5 k-radius of ach othr. In ach of th 18 adows, a plot was usd to sapl th spcis in th thr indicator groups and to asur th vgtation paratrs Assssnt of indicator groups and vgtation paratrs Th choic of th tru bugs (Htroptra) as a group for study was basd on Dulli and Obrist, 1998, conclusion that thy ar th bst indicators of invrtbrat biodivrsity. This is bcaus thy includ phytophagous, saprophagous and prdatory spcis (Dolling, 1991), larva and adults liv in th sa habitat and rspond strongly to nvironntal changs (Morris and Lakhani, 1979; Otto, 1996, tru bug spcis nonclatur follows Günthr and Schustr, 1990).

3 A. Schwab t al. / Agricultur, Ecosysts and Environnt 93 (2002) Spidrs ar also rgardd as a usful indicator group for biodivrsity (Marc t al., 1999). Many authors (Duffy, 1966; Conrady, 1987; Schidlr, 1990; Utz, 1991; Barthl, 1997; Dnnis t al., 1998) hav docuntd associations btwn spidr and plant counitis, and rportd how th architctur of vgtation influncs th spcis richnss of th spidr counity. Pozzi t al. (1998) dvlopd an valuation thod using spidrs to quantify th consrvation valu of particular sits in Switzrland (spidr spcis nonclatur follows Maurr and Hänggi, 1990). Spidrs and tru bugs wr sapld using a standardisd swp-nt thod (Ran, 1958; Otto, 1996). Th swp-nt had a diatr of 40 c and was fittd with a havy cloth. Sapls wr collctd vry 4 wks fro May to Sptbr (5 saplings pr plot) by aking 50 swps ovr 50. Sapling was prford btwn 11 a.. and 4 p.., on days whn th sun was shining and th tpratur was at last 17 C. Barbr pitfall traps (Adis, 1979; Curtis, 1980) wr usd to sapl surfac-activ arthropods. This thod is blivd to provid a good stiat of th nubr of spidr spcis in a counity (Luff, 1975; Thoas and Marshall, 1999). Th pitfall traps consistd of 200 l plastic cups insrtd into th ground and fittd a funnl st flush with th substrat. Each trap containd a 100 l solution of 75% alcohol. Traps wr protctd fro rain by transparnt covrs (17 c 17 c) supportd by a tal arch 10 c abov th ground. At ach sit, thr pitfall traps wr placd 2.5 apart in a row. Sapling was carrid out fro 8 May to 12 Jun and fro 3 July to 7 August (i.. 10 wks, cf. Dulli t al., 1999). Traps wr ptid onc a wk, th spidrs and tru bugs bing prsrvd in 70% alcohol in polythyln tubs. All collctions wr cobind to on sapl pr plot (n = 18) for ach indicator group. Shortly bfor th first cut, a spcis list of angiosprs was ad for an ara of at ach sit; th prcntag covr of spcis was stiatd for an ara of 16 2 (nonclatur following Laubr and Wagnr, 1996). In July, various paratrs dscribing th structur of th vgtation wr rcordd (Tabl 1) Data analysis Statistical analyss wr prford using th Statistica statistical packag (Vrsion 6.0, 98th Edition) and Canoco (Vrsion 4, 98th Edition). Data wr valuatd for auto-corrlation, colinarity, norality and hoognity of variancs. Whn ncssary, th transforation ln(x + 1) was usd to noralis th data (norality of th distribution was tstd with Shapiro Wilks W-Tst). Unlss othrwis statd, th lvl accptd for statistical significanc was Spcis nubr and Sipson s divrsity indx wr usd as paratrs dscribing th divrsity of a sit. Man valus wr usd for statistical analysis throughout th study. Rankings wr calculatd to rflct thr diffrnt typs of critria for valuating th sits. Th first critrion was spcis nubr, sits bing rankd sparatly for ach indicator group (thr rankings). Th scond critrion was Sipson s indx, also for ach indicator group sparatly. Th third critrion xprssd consrvation valu. Th nubr of angiospr spcis charactristic of nutrint-poor grassland was usd as an indx of th consrvation valu of th plant counity (rfrrd to as val. plant indx). A scond indx of consrvation valu, rfrrd to as val. spidr indx, was basd on an valuation thod dvlopd by Hänggi (1987) and Pozzi t al. (1998). Ths authors assignd all spidr spcis in Switzrland a valu fro 1 to 36 to rflct thir consrvation iportanc in trs of rarity and spcificity (habitat fidlity). Th an valu of th scors for th spidrs prsnt at any sit providd an indx of th consrvation status of th sit. Corrlations btwn spcis richnss of th thr indicator groups wr calculatd to dtrin whthr a singl group could b usd as a surrogat for th total biodivrsity. Furthr analysis of th data was carrid out by stpwis ultipl linar rgrssion. In intrprting th rsults, attntion was paid to th liitation of such odls whn any xplanatory variabls ar corrlatd (ulti-colinarity). To invstigat which canopy paratrs had a significant influnc on spcis coposition, a partitioning of th varianc using canonical corrspondnc analysis was carrid out. This tchniqu idntifis th linar cobination of xplanatory variabls which axiiss th disprsion of th spcis scors (for furthr xplanation to CCA s Tr Braak, 1986). Th statistical significanc of th variabls was dtrind by a Mont Carlo prutation tst.

4 200 A. Schwab t al. / Agricultur, Ecosysts and Environnt 93 (2002) Tabl 1 Paratrs usd for dscribing th 18 hay adows (plot siz: 16 2 ) in Mrishausr Randn, Switzrland Catgoris Critria a or b Sit location Exposition and inclination Hight of sit abov sa lvl Adjacnt surrounding (six catgoris) Qualitativ dscription of th canopy Hight of canopy (fiv catgoris) Dnsity of canopy (fiv catgoris) Hoognity of canopy (fiv catgoris) Quantitativ dscription of th canopy Maxiu hight of canopy (c) Avrag hight of canopy (c) Maxiu hight of flowring hrbs (c) Avrag hight of flowring hrbs (c) Nubr of angiospr spcis on 16 2 and Maxiu hight of flowring grass spcis (c) Distribution of plant covr and bioass Plant covr abov 60 c (%) Plant covr btwn c (%) Plant covr btwn 0 20 c (%) Plant bioass abov 60 c (%) Plant bioass btwn c (%) Plant bioass btwn 0 20 c (%) Total plant covr (%) Covr and bioass of functional groups Hrb covr (%) Grass covr (%) Lgu covr (%) Hrb bioass (%) Grass bioass (%) Lgu bioass (%) Covr of furthr soil covring parts Gap covr (%) Moss covr (%) Littr covr (%) Othr covrs (%),.g. stons, woody spcis Classification basd on plant hight Hight of canopy coponnt 1 n (c) Bioass of canopy coponnt 1 n (%) Plant covr of canopy coponnt 1 n (%) Light absorption by th canopy PAR 0 c PAR 20 c PAR 30 c PAR 60 c PAR 80 c Htrognity of th canopy S.D. PAR 0 c (intra-plot standard dviation) S.D. PAR 20 c S.D. PAR 30 c S.D. PAR 60 c S.D. PAR 80 c a PAR = photosynthtic activ radiation; S.D. = standard dviation. b = asurd; = stiatd. 3. Rsults Eightn adows rprsntd a rang of grassland typs fro spcis-rich plant counitis doinatd by Brous rctus to spcis-poor counitis with Arrhnathru latius. A total of 100 angiospr spcis, 112 spidr spcis (fro 21,658 adult spcins) and 94 tru bug spcis (fro 5,666 adult spcins) wr rcordd. Angiospr spcis richnss and coposition diffrd considrably according to anagnt. Th counitis of th xtnsiv sits wr doinatd

5 A. Schwab t al. / Agricultur, Ecosysts and Environnt 93 (2002) by B. rctus, which had a an covr of 25%, followd by Salvia pratnsis and Trifoliu capstr, both with about 5% covr. Th an nubr of angiospr spcis pr was 58. Th ost abundant spcis in th low intnsiv sits wr Trifoliu pratnss.l. (15% covr), A. latius (8%) and S. pratnsis (7%), and thr was a an of 40 spcis pr Madows undr odratly intnsiv anagnt had approxiatly qual aounts (ca. 10%) of Dactylis glorata, Tristu flavscns and A. latius, and a an of 32 angiospr spcis pr sapl sit. For ach group, thr wr significant diffrncs in spcis nubr (ANOVA; Fig. 1) btwn th xtnsiv and odratly intnsiv sits, but no significant diffrncs in spcis divrsity as asurd by Sipson s indx. Angiospr spcis nubr also diffrd significantly btwn xtnsiv and low intnsiv anagd adows. Nithr Sipson s divrsity indx nor spcis richnss diffrd btwn low and Fig. 1. Rsponss of indicator spcis to anagnt intnsity (dian ± in/ax). Bars with th sa lttr wr not significantly diffrnt. Mor angiospr (a) (ANOVA:P <0.001), spidr (b) (ANOVA:P <0.06), and tru bug spcis (c) (ANOVA:P <0.05) found in xtnsiv than in odrat intnsiv adows.

6 202 A. Schwab t al. / Agricultur, Ecosysts and Environnt 93 (2002) Fig. 1. (Continud). odratly intnsiv anagnt. Thr wr significant corrlations btwn th total spcis nubrs of vry pair of taxonoic groups, th strongst corrlations involving th nubr of tru bug spcis (tru bugs vrsus spidrs: r = 0.61, P = 0.001; angiosprs vrsus tru bugs: r = 0.59, P = 0.01; angiosprs vrsus spidrs: r = 0.53, P = 0.02). Th sits wr rankd using thr diffrnt critria valuating thir biological divrsity (Tabl 2). Th su of th ight rankings providd an ovrall indx of biodivrsity. Extnsivly usd adows had scors of 33 60, whil low and odratly intnsiv sits achivd scors of and , rspctivly. Tabl 3 shows th Sparan rank corrlation cofficints btwn all pairs of rankings. Th paratrs involvd in th largst nubr of corrlations (four out of a possibl svn) wr nubr of tru bug spcis, val. plant and val. spidr indx. Corrspondnc analyss wr calculatd for ach indicator group sparatly (Tabl 4). Sits clustrd according to anagnt typ into thr groups, sit E7 bing an outlir in all analyss. Th first axis scors of all thr corrspondnc analyss wr significantly corrlatd (angiosprs vrsus spidrs: r = 0.95, angiosprs vrsus tru bugs: r = 0.92, and tru bugs vrsus spidrs: r = 0.89; all P<0.00). Th scond axs wr also corrlatd significantly for angiosprs vrsus tru bugs and tru bugs vrsus spidrs (r = 0.47, P = 0.05; r = 0.77, P<0.00, rspctivly). To idntify which asur of vgtation structur bst rflctd spcis divrsity, th canopy paratrs listd in Tabl 1 wr tstd for thir corrlation with spcis nubrs in th thr indicator groups. Canopy paratrs with significant corrlations (Tabl 5) wr usd in a ultipl linar rgrssion odl. Th paratrs which significantly influncd th spcis richnss of th thr groups ar listd in Tabl 6. Half of th variability in angiospr spcis richnss was xplaind by th structural paratrs, (photosynthtic activ radiation) PAR 20 c bing th ost iportant variabl. Clos to 50% of th variation in th total nubr of spidrs was xplaind by th aount of angiospr bioass c in cobination with th hight of canopy coponnt 3. Narly 70% of th variation in th tru bug data was xplaind by th axiu hight of flowring hrbs and th standard dviation of th PAR 0 c. Paratrs not usd for th rgrssion ithr did not corrlat with th nubr of spcis or wr corrlatd with paratrs alrady includd in th rgrssion quation. Th rlationships btwn obsrvd and xpctd valus showd that th cobination of variabls usd in th odl achivd a good fit to th data, with no obvious signs of non-linaritis (Fig. 2). Managnt intnsity influncd all thr indicator groups and xplaind 74% of th varianc for angiospr spcis richnss, 31% for spidrs, and 51% for tru bugs.

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8 204 A. Schwab t al. / Agricultur, Ecosysts and Environnt 93 (2002) Tabl 3 Sparan Rank ordr corrlation cofficints and significanc btwn 8 rankings of 18 hay adows in Mrishausr Randn, Switzrland a Angiospr spcis nubr Angiospr spcis divrsity Val. plant indx Spidr spcis nubr Spidr divrsity Val. spidr indx Tru bug spcis nubr Angiospr spcis nubr Angiospr divrsity 0.07 n.s. Val. plant indx n.s. Spidr spcis nubr 0.43 n.s n.s Spidr divrsity 0.25 n.s n.s n.s n.s. Val. spidr indx n.s n.s. Tru bug spcis nubr n.s n.s Tru bug divrsity 0.01 n.s n.s n.s n.s n.s n.s n.s. a n.s. = not significant <P < <P <0.01. P< Tru bug divrsity Tabl 4 Corrspondnc analyss of angiospr, spidr and tru bug spcis copositions of 18 hay adows in Mrishausr Randn, Switzrland a Axs Total inrtia Angiosprs Eignvalu Cuulativ % varianc of spcis data Spidrs Eignvalu Cuulativ % varianc of spcis data Tru bugs Eignvalu Cuulativ % varianc of spcis data a Eignvalus asur th iportanc of th axs (valus btwn 0 and 1) and th total inrtia is th total varianc in th spcis data as asurd by th chi-squar of th sapl-by-spcis tabl dividd by th tabl s total. Tabl 5 Parson-Product Mont corrlations cofficints and significanc btwn th nubr of indicator spcis (angiosprs, spidrs and tru bugs) and vgtation paratrs dscribing 18 hay adows in Mrishausr Randn, Switzrland a Angiosprs Spidrs Tru bugs Managnt intnsity Total plant covr n.s Plant bioass 0 20 c Plant bioass c PAR 20 c PAR 30 c n.s Angiospr spcis nubr (120 2 ) Plant covr of canopy coponnt n.s n.s. Plant bioass of canopy coponnt n.s Maxiu hight of flowring hrbs n.s a n.s. = not significant; 0.01 <P <0.05; <P <0.01; P<0.001.

9 A. Schwab t al. / Agricultur, Ecosysts and Environnt 93 (2002) Tabl 6 Multipl rgrssion odls prdicting spcis richnss of angiosprs, spidrs and tru bugs in 18 hay adows in Mrishausr Randn, Switzrland a Constant Variabl 1 Variabl 2 Angiosprs PAR 20 c Rgrssion cofficint Cuulativ r F-valu at nd F(1, 16) = 15.9, P < 0.001, S.E. = 8.4 Spidrs Plant bioass c Hight of canopy coponnt 3 Rgrssion cofficint Cuulativ r F-valu at nd F(1, 16) = 7.7, P < 0.01, S.E. = 5.6 F(2, 15) = 6.9, P < 0.01, S.E. = 5.1 Tru bugs Maxiu hight of flowring hrbs S.D. PAR 0 c Rgrssion cofficint Cuulativ r F-valu at nd F(1,16) = 23.4, P < 0.000, S.E. = 4.4 F(2, 15) = 17.1, P < 0.000, S.E. = 4.0 a Indpndnt variabls (paratrs of th vgtation structur). Fig. 2. Multipl rgrssion odls fitting actual data (obsrvd richnss), rgrssion lin and 95% confidnc intrval, (a) nubr of spidr spcis; (b) nubr of tru bug spcis. 4. Discussion Th rsults prsntd hr show a high lvl of concordanc btwn th indicator groups in thir ability to discriinat btwn th 18 sits (.g. corrspondnc analyss, Tabl 4). Th trnds in spcis coposition in rlation to land us history and prsnt-day anagnt of th sits ar siilar for th thr groups. For ach group, incrasd intnsity is associatd with rducd spcis richnss. Managnt intnsity has th strongst ffct on th nubr of angiospr spcis, xplaining 74% of th variability of spcis richnss. Th ainly foliag-dwlling tru bugs (51% of variation xplaind by anagnt) appar to b or affctd by rgular cutting than th spidrs (31%), rflcting th fact that narly 99% of all spidr individuals capturd wr soil-dwllrs. This conclusion is consistnt with th rsults of Otto and Dorn (1995) and Di Giulio t al. (2001). Managnt intnsity had rathr littl influnc on th actual spcis coposition of th counity, xplaining 18% of th variability for angiosprs, 29% for

10 206 A. Schwab t al. / Agricultur, Ecosysts and Environnt 93 (2002) spidrs, and 17.5% for bugs. Howvr, so spidr spcis ar prootd by intnsiv cutting and a high nubr of spcis wr xclusivly found in sits of th sa anagnt typ. Conclusions about th valu of grassland sits dpnd on th critria usd for valuation (Tabl 2). Bàldi and Kisbndk, 1997, argu that an valuation should tak account of th consrvation iportanc of th spcis prsnt. Significant corrlations wr found btwn th ranking of th nubr of angiospr spcis and that of th two consrvation indics (i.. val. plant indx and val. spidr indx, Tabl 3). Th nubr of angiospr spcis ay, thrfor, b considrd a rasonabl prdictor of th consrvation valu of hay adows. As a first stp towards a robust and rapid thod for valuating grasslands basd on th vgtation structur, paratrs with a significant influnc on spcis richnss wr idntifid. Tabl 6 shows that angiospr spcis richnss was clarly rlatd to th PAR. Howvr, fro a practical point of viw it dos not ak sns to rplac th traditional plant spcis list with an indirct asur such as PAR if it provids no significant ti saving. For invrtbrats, howvr, indirct thods ar potntially vry usful, as th faunistic assssnts ar ti-consuing, wathr dpndnt and dand xprt knowldg. Th spcis richnss of spidrs was influncd ost strongly by th dnsity of th canopy, whil tru bug richnss was also affctd by plant spcis coposition. Th rsults suggst that th spcis richnss of spidrs tnds to b low if th lowr part of th plant canopy is dns. This finding is consistnt with any othr studis showing that th divrsity of th spidr counity is strongly influncd by habitat structur (Lowri, 1948; Hatly and MacMahon, 1980; Robinson, 1981; Downi t al., 1995; Andrlik-Wsingr t al., 1996). Canopy paratrs can, hnc, b usd as prdictors of spcis richnss of spidrs, spcially at th faily lvl (Duffy, 1966). Th larg group of Linyphiida was influncd by th cobination of th avrag canopy hight and angiospr covr of canopy coponnt 3 (72% of th variation in Linyphiida spcis richnss xplaind), phasizing th iportanc of hight and dnsity of th vgtation for this faily. Th Gnaphosida in contrast, wr ainly influncd by th PAR 0 c (ost probably as a asur of th closnss of th vgtation), th lgu bioass and th axiu hight of th flowring hrbs (84% variation xplaind). For th tru bugs, th hight of th canopy and a asur of th canopy dnsity appard to b iportant. Th ngativ corrlation of axiu hight of flowring hrbs can b xplaind by th fact that th prsnc of tallr angiospr spcis is gnrally associatd with incrasd canopy dnsity, which rstricts th ovnt of flying inscts such as tru bugs. Th standard dviation of th PAR on th ground is a paratr xprssing th horizontal variability of th vgtation canopy of th habitat. An htrognous canopy offrs or gaps than a dns, hoognous on and ay b th rason for th positiv corrlation btwn S.D. PAR 0 c and th nubr of tru bug spcis. Th statistical partitioning of th data (Borcard, 1992) into structural (vgtation canopy) and anagnt coponnts showd that about 60% of variation (Fig. 3) in spcis richnss can b xplaind by anagnt (18% of th varianc of th angiospr data, 29% of th spidrs and 17.5% of th tru bugs), and by vgtation structur (33, 11 and 40% rspctivly). Th intraction tr, anagnt vgtation structur, xplaind a furthr 8 14% of th varianc. Canopy paratrs with significant ipact on th abundanc of th indicator groups wr th nubr of angiospr spcis in 120 2, hight of flowring hrbs, grass and hrb bioass, PAR, hight of th canopy and surroundings. Th rsults prsntd hr show that canopy paratrs xplain at last 50% of th variation in spcis richnss, a rsult coparabl with that obtaind in othr studis (Hatly and MacMahon, 1980; Barthl, 1997; Dnnis t al., 1998). Paratrs rlatd to canopy dnsity ar th ost iportant for xplaining th spcis richnss of th indicator groups. An valuation thodology basd on both th nubrs of angiospr spcis and slctd structural paratrs would provid accptabl stiat of abov-ground biodivrsity. Such a thodology would b uch fastr than convntional biodivrsity assssnts. In our study, it took an avrag of 30 in for on prson to ak a list of th angiospr spcis prsnt at on sit, th rcording of plant hight took only 1 in, whil

11 A. Schwab t al. / Agricultur, Ecosysts and Environnt 93 (2002) Fig. 3. Statistical partitioning of th paratrs xplaining ost of th cological variation. th PAR profils took 30 in. Thus, just ovr on hour was rquird to rcord an avrag sit, and th survy of 18 sits took 2.5 days. This contrasts vry favourably to th 21 days that Hänggi (1989) ndd for th assssnt of th spidr fauna in 10 sits. 5. Conclusions Th us of bioindicators is a usful approach to valuating biodivrsity (Paoltti, 1999) and has th potntial to ak a ajor contribution to optiizing diffrnt faring systs. Howvr, siplr and or ti-fficint thods ar ndd for valuating th succss of initiativs to nhanc biological divrsity in th agricultural landscap. Basd on th findings of this study, a rapid valuation thodology ay b rcondd, coposd of thr ain typs of inforation. Th first is a list of th angiospr spcis prsnt, sinc, it is th bst singl indicator of th biodivrsity at a sit. Th scond lnt is inforation about th structur of th canopy, likly to considrably iprov prdictions about th divrsity of invrtbrats. Finally, it is iportant to includ inforation about th surrounding land us, sinc, this can strongly influnc local spcis richnss, prhaps through arial photographs. Acknowldgnts W ar gratful to Jaklina Stigr and Ralph Hckann for th idntification of spidr and tru bug spcis, rspctivly. W thank Abros Hänggi for his hlp with idntifying crtain difficult spcis and for any usful discussions about consrvation valuation. W also thank any fild and laboratory hlprs and othrs fro th Swiss Fdral Rsarch Station for thir assistanc in coplting this papr. W ar gratful to th farrs of Mrishausn for allowing us to work in thir filds. Th work was fundd by th Swiss National Scinc Foundation, Th Swiss Fdral Rsarch Station for Agricultur and Agrocology, Zurich-Rcknholz providd support and infrastructur. Rfrncs Adis, J., Probls of intrprting arthropod sapling with pitfall traps. Zool. Anzigr. 202, Andrlik-Wsingr, G., Pfadnhaur, J., Albrcht, H., Kühn, N., Matthis, A., Tötz, P., Einfluss strukturllr und floristischr Ausprägungn von Rainn in dr Agrarlandschaft auf di Spinnn (Arana) dr Krautschicht. Vrh. Dr. Gs. Für. Ökologi 26, Bàldi, A., Kisbndk, T., Orthoptran assblags as indicators of grassland naturalnss in Hungary. Agric. Ecosyst. Environ. 66,

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