Parkyn Leaf breakdown and colonisation by inverts Native vs Introduced trees

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New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 31 : 3013120028-83 30/97/3103 -0301 $7.00/0 T he Royal Society of New Zealand 1997 301

Leaf breakdown and colonisation by invertebrates in a headwaterstream: comparisons of native and introduced tree speciesS. M. PARKYN*M . J. WINTERBOURN

Department of ZoologyUniversity of CanterburyPrivate Bag 4800Christchurch, New Zealand*Present address: National Institute of Water &

Atmospheric Research Ltd, P. O. Box 11 115,Hamilton, New Zealand.

Abst rac t Leaf breakdown, colonisa t ion byinvertebrates, food choice by a facultative shredder,and composition and activity of the microflora ofleaves of three native and three introduced treeswere investigated in a small headwater stream,South Island, Ne w Ze aland. Leave s of each specieswere immersed and collected after 1, 32, 60, and95 days to measure mass loss and invertebratecolonisation. Additional leaves collected after 60days were used for food choice and respirationexperiments, and others taken after 95 days (somespecies) were examined with scanning electronmicroscopy. Leaf breakdown rates followed thesequence: elm > red beech > willow > mahoe >oak > mountain beech . Elm (introduced) and mahoe(native) supported the highest invertebrate densities,but shredders were most abundant on willow(introduced) and red beech (native) leaves. Inlaboratory choice trials the facultative shredderOlingajeanae showed a preference for elm and redbeech leave s which w ere fastest to break down andhad high respiration rates. We found that leaves ofintroduced trees canbe p referred by shredders andhe nc e , no strong associat ions were apparentbetween shredders and these native trees.

M96087Received 26November 1996; accepted 13 May 1997

K e y w o r d s l e a f b r e a kdown; inve r t e b r a tecolonisation; food choice; shredders; respiration;microflora

INTRODUCTIONTerrestrial vegetation that enters the stream channelprovides an important source of detrital food andhabitat for many stream invertebrates (Kaushik &Hynes 197 l ;Petersen& Cummins 1974; Anderson& Sedell. 1979; Rounick & Winterbourn 1983a).The strength of trophic and other linkages betweeninvertebrates and particular leaf species is not wellunderstood, although Ross (1963) suggested therewere close associations between some trichopteranspecies and forest types in North America.

N e v e r t h e l e s s , it is well es tabl i shed tha tinvertebrate shredders can have an important rolein processing leaf litter (Cummins et al. 1980;Collier & Winterbourn 1987; Bird & Kaushik1992), and that shredders (e.g., Anisocentropuskirramus: Nolen & Pearson 1993), scraper-grazers(e.g., Potamopyrgus jenkinsi: Hanlon 1981), andcollectors (e.g., Gamm arus pseudolimnaeus: Bird& Kaushik 1985) select particular kinds of leavesover others as food. This may be for a variety ofreasons including the structure, chemistry, andtoughness of the leaf, and also its degree ofm ic r ob ia l c ond i t ion ing and a ssoc ia t e ddecomposition (Petersen & Cummins 1974; Ironset al. 1988; Barlocher & Newell 1994).Little is know n abo ut the effects of litter inputsfrom introduced trees in New Zealand streamecosystems, or their value as foods for aquaticinvertebrates. Collier & W interbourn (1986 ) foundthat leaves of weeping willow (Salix babylonica)were heavily colonised and grazed by the snailPotamopyrgus antipodarum, and that larvae of thesh r e dd ing c a dd i s f ly Triplectides obsoletasignificantly increased the processing rate of willowleaves in a suburban Christchurch stream. Lester etal. (1994a) reported that larvae of another leaf-shredding caddisfly, Olinga feredayi preferredwil low (Salix fragilis) leaves that had be e n


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302 New Z ealand Journal of Marine and Freshwater R esearch, 1997, Vol. 31incubated in a stream for 56 days to periphyton,and also to willow leaves incubated for 7 and 28days. Hicks & Laboyrie (unpubl. data) contrasteddecomposition rates, carbon:nitrogen ratios, andinvertebrate colon isation of leaves from native NewZealand evergreen trees with two introduceddeciduous trees in a North Island stream. Theyfound that mahoe (Melicytis ramiflorus) and theintroduced trees, silver birch {Betula pendula) andalder (Alnus glutinosa), broke down much fasterthan the other native species, and that the numberand biomass of invertebrates were positively relatedto the mass loss rate of the leaves. No studies,however, have com pared the attractiveness of nativeand introduced tree leaf species to New Zealandstream invertebrates over time.

The aim of the present study was to examinecolonisation of leaves of three native and threeintroduced trees in a New Zealand forest stream. Apreference for colonisation and feeding on nativespecies by stream invertebrates might indicate theexistence of "co-ev olved " associations of elementsof the indigenous biota , and therefore havemanagement implications for selection of riparianplantings.

METHODSStudy siteMiddle B ush Stream is a first-order tributary of theWaimakariri River, South Island, New Zealand. Itdrains a 28 ha catchment dominated by subalpinescrub, tussock, and bare scree in the University ofCanterbury field station at Cass and flows throughMiddle Bush, a ?>-A ha monotypic stand of moun tainbeech {Nothofagus solandri var. cliffortioides), Spotmeasurements made in March 1994 indicated thatthe stream had a pH of 6.7, conductivity of 100 (JScm "1 at 25C and alkalinity of 41 mg CaCO 3 H .Mean monthly water temperature ranges fromc. 4C (July) to 11C (January ) (Winterbourn 1976)and was c. 5C at the start of the present study. Alarge population of leaf-shredding invertebrates isfound in Middle Bush stream along with a diversefauna of predominantly detritivorous, collector-browsers (Winterbourn 1982).Leaf breakdownLeaves of three introduced (scarlet oak (Quercuscoccinea), English elm (Ulmus procera), andweeping w illow) and three native (mountain beech,mahoe, and red beech {Nothofagus fusca)) tree

species were collected soon after abcission. Theywere chosen on the basis of probable breakdownrates (Petersen & Cummins 1974; Webster &Benfield 1986; Linklater 1995). Thus, oak andmountain beech were assumed to break downslowly (time to 50% break down (T50) > 138 days),willow and red beech at medium rates (T 5 0 46-138days), and elm and m ahoe rapidly (T50 < 46 days).Leaves were a ir -dr ied for several weeks,weighed into 5 g lots and placed in 10 mm meshhair nets to allow invertebrate access. Mountainbeech leaves were threaded on to fine nylon fishingline to stop them from being w ashed from the nets.In April 1994, 16 leaf nets of each of the sixspecies were arranged randomly, in a 50 m reachof Middle Bush Stream with relatively homo-geneous current . They were te thered to pegsembedded in the substrate, and were positionedagainst rocks to simulate natural accumulations.Additional nets containing unweighed leaves ofeach species were also included for respirometry,sc a nn ing e l e c t r on m ic r osc opy ( S EM ) , a ndlaboratory feeding studies.

Leaf nets were retrieved after 24 h and 3 2, 60,and 95 days, yielding four replicates for eachincubation time. Those incubated for 24 h were usedto estimate initial weight losses by leaching. Theseleaves were washed onto a 2 mm m esh sieve, driedat 45 C for 24 h, and weighed to 1 m g. W eightloss by leaching was determ ined by subtracting theweights of the incubated leaves from weights of5 g sam ples of air-dried leaves that had been oven-dried (45C) for the same length of time. On eachsubsequent sampling date, leaves were separatedf rom de t r i tus , dr ied , and weighed. Largeinvertebrates were remov ed before the leaves werewashed; small invertebrates retained on a 50 ^immesh sieve were separated from silt and detritus ina Bogorov tray under a microscope (10-40x) .Invertebrates were identified using the keys ofWinterbourn & Gregson (1989).Scanning electron microscopyOn Day 60 (all leaf species) and Day 95 (elm, oak,and mountain beec h), leaf material was taken fromnets in the s t ream and prese rved in 2 .5%gluteraldehyde in 0. lA /N a cacody late buffer. In thelaboratory, they we re washed in buffer, post-fixedin 2% buffered osmium tetroxide, washed again,and dehydrated by passing through a graded alcoholseries (Rounick & W interbourn 1983b). After 12 hin 100% ethanol, leaf samples were dried in avacuum desiccator. Specimens were mounted on


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Parkyn & WinterbournB reakdown of native and introduced leaves 303aluminium stubs with carbon conductive paint andsputter-coated with 60 nm of gold. Both upper andlower leaf surfaces were examined qualitativelyusing a JEOL JSM 6100 SEM at magnifications upto 2300x.Microbial respirationLeaves of the six species removed from MiddleBush Stream after 32,60, and 95 days were washedgently to remove fine detritus and invertebrates.Whole leaves of mountain beech, red beech, andwillow, plus pieces of oak, elm, and mahoe leaveswere placed in 95 ml jars, filled with Middle BushStream water at 5C, and sealed with cork bungsencased in plastic film. T hree replicates of each leafspecies, and three control jars containing streamwater alone were incubated in the dark for 24 h at5C. Only o ne samp le of elm was possible after 95days because of the lack of remaining material.

After incubation, oxygen concentration wasmeasured with an oxygen meter (Yellow SpringsInstruments Model 57). Oxygen consumption byleaves and the i r assoc ia ted microf lora wascalculated from the difference in concentration ofoxygen in jars containing and lacking leaves.Leaves were blotted dry, photocopied on to mylarsheets, and surface area was determined with aLI-CO R leaf area meter. Leaves w ere then dried at45 C for 24 h and w eighed so that oxygen con-sumption could be expressed per gram dry weightof leaf, as well as per cm 2 of leaf surface area.Food choice experimentsThirty larvae of the conoesucid caddisfly Olingajeanae^, a facultative sh redder, were collected fromMiddle Bush Stream in July 1994 (mean caselengths SD = 10.4 1.0 mm). They were main-tained in the laboratory in Middle Bush Streamwater at 5C (the approximate temperature of thestream at time of collection). Twenty-four hoursbefore the start of the experiment, larvae wereplaced in a controlled temperature room at 15C toacclimatise them to experimental conditions. Nofood was supplied during this period.Nets containing unweighed samples of the sixleaf species were removed from Middle BushStream after 2 months. Discs (8.5 mm diam., ortAlthough Winterbourn & Gregson (1989) consideredthat O.jeanae was probably a synonym of O.feredayi,new research by J. Ward (Canterbury M useum)indicates that O. jeanae is a separate species and that itis the species found in Middle Bush Stream.

7 mm in the instance of the small mountain beechleaves) were cut from the leaves with cork borersand placed in a circle, in 8.5 cm diameter Petridishes filled with stream water to a depth of 1.2cm. Each dish contained one disc of each leaf typeand one caddis larva. All larvae were oriented inthe same direction but the leaf discs were rotatedwithin the dishes so that each leaf species occurredin a different position relative to the larva. Fivereplicates of each disc arrangement w ere used. Fivecontrols containing leaf discs in stream water butwith no caddis larva were run concurrently.After 48 h larvae and leaf discs were removedand the leaf discs were photocopied on to mylarsheets to enable surface area measurement with aLI-COR leaf area meter. The areas of experimen taldiscs were subtracted from those of control discsto determine leaf area losses.

Statistical analysisProcessing coefficients (k day"1) for weight lossof leaves were calculated as:-k = ln(%fl/100)/* (Petersen & Cummins 1974):where -k is the daily expo nential rate of weight loss,%R is the percentage of leaf material remaining,and / is the duration of the experiment in days.Differences in processing coefficients and weightloss (log transformed) among leaf species weretested with ANOVA followed by Du ncan 's M ultipleRange tests.Significant differences between the occurrencesof the most abundant species of invertebrates (logtransformed) in leaf nets on each sampling date,were analysed using ANOVA and Tukey 's post hoctest. Resp iration data (log transformed) for each leafspecies on three sampling dates were also testedfor significant differences using ANOVA andTukey 's post hoc test to compare differences amongmeans.

The non-parametric Kruskal-Wallis test andmul t ip le cont ras t te s ts were used to de tec tdifferences between the preferences of larvae forleaf types, as ANOVA techniques are inappropriatefor ana lys ing pre fe rence exper iments wheremultiple food types are offered to consumers(Barlocher & Newell 1994).RESULTSLeaf breakdownLeaching weight losses in the first 24 h (Table 1)were highest for willow (25% ), mahoe (22% ), and


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304 New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 31Fig. 1 Under-surfaces of leavesincubated in Middle BushStream, New Zealand. A, wil-low leaf after 60 days showingbacterial colonisation; B, oakleaf after 95 days showing fewfungal hyphae; C, mountainbeech after 60 days showingwaxy cuticle covered in fungalhyphae; D, mountain beech af-ter 95 days when the waxy cuti-cle was largely removed and fun-gal hyphae had penetrated theleaf. Scale bars: A, B = 10 um;C, D = 100 um. (B = bacteria; F =fungal hyphae; and W = waxy cu-ticle.)

elm (13%) leaves. Weight loss of red beech was4% but no measurable leaching was found for oakand mountain beech.Processing coefficients among species differedsignificantly (ANOVA, P < 0.001) with leaveslosing weight in the sequence: elm > red beech >willow > mahoe > oak > mou ntain beech (Table 1).Breakdown rates of oak, mahoe, and willow werenot significantly different from each other andneither were those for red bee ch, willow, and m ahoe(Table 1). Times for 50% weigh t loss (T 50 ) showedsimilar trends to processing coefficients, ranging

from 19 days (elm) to 92 days (mountain beech)(Table 1).

Weight losses of leaves could be attributed inpart to consumption by invertebrates. After 32 days,oak, mountain beech, and red beech showedevidence of chewing, whereas elm did not appearto have been eaten by shredders. Over the courseof the study, mahoe leaves became very thin; theprimary mode of consumption w as surface grazingresulting in skeletonisation, rather than chew ing atleaf margins. Mountain beech and oak leaves didnot become skeletonised, but the margins of oakand willow leaves had been chewed by shreddersafter 6 0 and 95 days, respec tively.Leaf surfaces examined qualitatively after 60days in Middle Bush Stream were heavily colonised


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Parkyn & WinterboumBreakdown of native and introduced leavesFig. 1 (continued.)

305

Table 1 Weight loss by leaching (24 h) , time to 50 % weight loss (days), and process ing coefficients(k , day"1) for the leaves of six tree species. Du ncan's Grou ping shows differences in processingrates between s pecies. Those with the same letter are not significantly different.SpeciesNothofagus solandri var.cliffortioides (mountain beech)Quercus coccinea (oak)Melicytus ramiflorus (mahoe)Salix babylonica (willow)Nothofagus fusca (red beech)Ulmus procera (elm)

TypeNativeIntroducedNativeIntroducedNativeIntroduced

% leached(24 h)0022254

13

T5 0(days)925137312319

k(day-1)-0 .0055-0 .0099-0 .0135-0 .016-0 .0225-0 .0269

Duncan'sgroupingaabbebecdd

by bacteria (Fig. 1A), especially where internaltissues were exposed. The tetraradiate spores andhyphae of aquatic hyphomycetes were seen on theouter surfaces of all species, although few were

found on oak leaves after either 60 or 95 days (Fig.IB) . In contrast, mountain beech leaves werecolonised by large numbers of hyphae most ofwhich did not seem to penetrate the leaves; instead


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30 6 New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 31

Q .

f j 32 daysI 60 daysWit 95 days

Introduced NativeLeaf species

Fig. 2 Mean respiration rates ( 1 SE, n = 3) for eachleaf species after 32, 60, and 95 days in Middle BushStream, New Zealand. A, area-specific respiration (|igOi cm~2h"'); B, mass-specific respiration (mg O2 gh~'). (E = elm; W = willow; O = oak; M = mahoe; RB =red beech; and MB = mountain beech.)they appeared to be only loosely attached to thewaxy cuticle (Fig. 1C). After 95 days , however, thewaxy cuticle of mountain beech leaves had largelydisappeared facilitating penetration of hyphae(Fig. ID ). Elm leaves had also decomp osed further,but most oak leaves w ere largely intact despite thelarge number of bacteria apparent on their surfaces.Microbial respirationOxy gen uptake rates for oak and elm (on a surfacearea basis) were consistently lower and higher,

respectively, than for the other leaf types, and elmwas significantly higher than oak after 32 da ys andh igher than a l l o ther species a f te r 60 days(P < 0.0001, Tuk ey's post hoc test, Table 2, Fig. 2).Wh en oxygen consumption wa s expressed per gramof leaf material, mahoe had significantly highermicrobial activity than oak on all sampling dates,and significantly higher activity than mountainbeech on 32-day and 60-day m aterial and red beechon 60-day material (P < 0.0001, Tukey's post hoctest, Table 2, Fig. 2). However, the biomass:surfacearea ratio of mahoe leaves was the low est of the sixspecies considered. No significant differences werefound between species after 95 days.Invertebrate colonisationThirty-six taxa of invertebrates were found in leafnets. Of these, five could be categorised as eitherobligate or facultative shredders: the caddisfliesZelandopsyche ingens an d O.jeanae, the stonefliesZelandobius sp. and Austroperla cyrene, andHydraenidae (Coleoptera) as indicated by previousstudies in Middle Bush Stream (W interbourn 1982).Chironomidae was the most abundant taxon on allsampling dates fol lowed by Oligochaeta, thestonefly Spaniocerca zealandica, and Scirtidae(Coleoptera).

The total number of invertebrates (Fig. 3), andthe total num ber of shredders (Fig. 4) per gram dryweight of leaf remaining, showed a general increaseover time. Highest numbers of invertebrates werefound on elm (introduced) and mahoe (native)leaves, whereas shredders were most abundant onwillow (introduced) and red beech (native) leaves.Few shredders were found on elm and mahoe.Shredder density peaked on willow leaves after 60days, and was associated with high weight loss ofthe leaves and evidence of leaf chewing.Abundances of six common taxa and totalinvertebrates on leaves of each tree species were

Table 2 Summary of significant differences among respiration rates of leavesof three nativemahoe (M), red beech (RB), mountain beech (MB), and threeintroduced tree specieselm (E), willow (W), oak (O)after 32 and 60 daysin Middle Bush Stream, New Zealand. No significant differences were foundafter 95 days. Species are listed from highest respiration rate (left) to lowest(right) and those with different subscripts are significantly different (Tukey'spost hoc test, P < 0.0001, n = 3).Exposure(days) Respiration (area specific) Respiration (mass specific)32 Ea RBa MBa Ma W ab O b60 Ea Wb RBb M b MBb Ob M a RBa Eab W ab O bc MBCM a Wa Eab Ob RBb MBb


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Parkyn & W interboum Breakdown of native and introduced leaves80

307Mahoe (native)Elm (introduced)

Willow (introduced)Red beech(native)

40 60Time (days)

Fig . 3 Total num bers of invertebrates per gram leaf re -maining (mean 1 SE) for six leaf species after 32, 60,and 95 days. (DW = dry weight.)

aaS4a

Mahoe (native)Elm (introduced)

Willow (introduced)Red beech (native

2- "Oak (introduced)"Mou ntain beech (native)

40 60Tims (days)

Fig . 4 Total num bers of shredders per gram leaf dryweight rem aining (mean + 1SE) for six leaf species after32 , 60, and 95 days. (DW = dry weight.)

compared on each sampling date . Signif icantdifferences were found among leaf species in theabundances of O. jeanae and Hydraenidae after 60days (P < 0.01), Scirtidae after 95 days (P < 0.01),and oligochaetes after 32 and 60 days (P


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308Native species

15 T Mahoe

New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 31Introduced species

Elm 32 days 60 days^ 9 5 days

Fig. 5 Mean abundances (+1SE , n = 4) of shredder species inleaf nets of each of the six spe-cies, collected after 32,60, and 95days inMiddle Bush Stream, NewZealand. (Zi = Zelandopsycheingens; Oj = Olingajeanae; H =Hydraenidae; Ac = Austroperlacyrene; and Z = Zelandobius sp.)

HShredders

surface area than mountain beech and oak (1.8 and0.8%, respectively) (P < 0.001), with elm beingeaten in almost every dish. Small proportions ofmahoe discs (5%) and willow discs (3%) wereconsum ed, but these we re not significantly differentfrom the other species.DISCUSSIONAs anticipated, leaves of the six tree species brokedown at different rates. Elm and red beech lostweight fastest, mountain beech and oak were

s lo w es t , w h er eas w i l l o w and m a h o e hadintermediate breakdown rates.Pat terns of inver tebrate colonisat ion alsodiffered amo ng leaf species, but we found no strongevidence of co-evolved linkages between shreddersand native tree species. In the field experiment, sixinsect species that were known to be at leastfacultative shredders occurred on all six leaf species.Hydraenidae and O. jeanae, the most abundantshredder taxa, had their h ighest densi t ies onin troduced wil low. In laboratory food-choiceexperiments with leaves incubated for 60 days in

Table 3 Significant differences observed among densities of the most abun dantinvertebrate taxa on leav es of six tree species after 32 , 60, and 95 days in MiddleBush Stream, New Zealand. For each sampling, leaf species are listed fromhighest invertebrate density (left) to lowest (right) and those with differentsubscripts are significantly different (Tukey's post hoc test, * = P < 0.05, **= P < 0.01, n = 4). Abbreviations for tree names as in Table 2. No significantdifferences in densities were found for Chironomidae, Spaniocerca zealandica,or total invertebrates on any sampling day.TaxaScirtidaeOligochaetaOlingajeanaeHydraenidae

Exposure(days)9532606060

P********

M awawaW a

Leaf speciesO a Wab R B a b Eb MBbM a b Eab R B a b M B ab ObM a b R B a b Wab M B a b ObRBab = Oab Eab Mab MBbM B ab R B a b Mab Eb Ob


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350300-250-200-150100-500

Native speciesMahoe

Parkyn & WinterbouraBreakdown of native and introduced leavesFig. 6 Mean abundances (1 SE,n = 4) of common non-shredderspecies in leaf nets of each of thesix species, collected after 32, 60,and 95 days in Middle BushStream, New Zealand. (Ch =Chironomidae; S = Scirtidae; Sz= Spaniocerca zealandica; and Ol= Oligochaeta.)

309

S 250-1.a 200'I 150'n| j 100-

50 -0

15 0100-50 -0

Ch SRed beech

Ch S SzMountain beech

S SzInvertebrates

300-250-20 015 010 050 -

0250-200-15 0100-5 0 :

0

150-100-5 0-

0

Introduced species

Elm 32 daysE2 60 days 95 days

sWillow

Oak

S SzInvertebrates

0.14-^ 0.12-u

IA- 0.08-a| 0.06-u.2 0.04-a 0.02- i ab

w RBIntroduced NativeLeaf species

Fig. 7 Mean leaf disc surface area loss (1 SE, n = 30)of the six species after feeding by Olingajeanae larvaefor 48 h in laboratory experiments at 15C (mean areaof control discs = 0.54 and 0.33 cm2 (mountain beech).(E = elm; W = willow; O = oak; M = mahoe; RB = redbeech; and M B = mountain beech.)

Middle Bush Stream, O. jeanae showed a strongpreference for introduced elm and native red beechleaves. In contrast, mountain beech leaves werehardly eaten at all despite the experimental caddisflylarvae originating from a stream in monospecificmountain beech forest. Compared with mountainbeech leaves, leaves of red beech and especiallyelm were very soft, and the latter had significantlyhigher oxygen consumption rates per unit surface

area that the other five leaf types. Interestingly,however, very few O. jeanae larvae were found onelm leaves in the field, perhaps because the leaves"folded up" and became slimy as decompositionprogressed. This may have made them physicallymore difficult to eat than the flat, less slimy leafdiscs used in laboratory trials, and indicated thattheir palatability was not influenced solely bydegree of microbial conditioning.Similarly, Hanlon (1981) concluded that leafstructure was a major factor influencing foodselection by the f reshwater gastropod Pota-mopyrgusjenkinsi (= P. antipodarum; Ponder 1988)as "soft" leaf species (e.g., willow Salix fragilis)were preferred to "hard" leaves (e.g., oak Quercusrobur) resulting in faster growth. Rounick &Winterbourn (1983a) found that larvae of thecaddisfly Z. ingens preferentially selected mountainbeech leaves that had been in water the longest, andconcluded this was because they were the softestleaves rather than the most strongly colonised bymicrobes. O. feredayi also fed preferentially onwillow leaves that had been in a stream the longest(Lester et al. 1994a), however, the authors suggestedthat palatability may have been enhanced by theleaching of compo unds from leaves, or an increasein microbial co lonists, rather than softness.

Differences in the concentration s of tannins andother secondary plant compounds may affect the


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310 New Ze aland Journal of Marine and Freshw ater Resea rch, 1997, Vol. 31breakdo wn rates of leaves of various species (Stout1989) , al though Ostrofsky (1993) found norelationship between leaf processing rate and tanninconcen t ra t ion . The f eed ing mechan isms anddigestive capabilities of detritivores, and theirability to detoxify or tolerate "defensive" chemicalsin leaves is also known to influence food selection(Suberkropp et al. 1976; Barlocher 1985; Arsuffi& Suberkropp 1989). Both Campbell et al. (1992)and Lester et al. (1994a) found rapid leaching ofpolyphenolics including tannins when leaves weresubmerged in stream water , and the phenoliccontent of willow leaves fell from 12% to almost0% in 8 weeks (Lester et al. 1994). Our findingthat wil low leaves were colonised by largenumbers of shredding hydraenid beetles after 60days, could therefore reflect a substantial declinein phenolic content at this time, and could helpexplain the high densities of invertebrates reportedby Col l ie r & Win terbourn (1986) on Salixbabylonica leaves after 41 and 58 days in twoChristchurch rivers.

Allan (1995) considered that inver tebratedetritus feeders unquestionably prefer microbiallyconditioned leaves over uncolonised ones, and thepreference of O. jeanae for elm leaves, which hadthe highest respiration rates per unit surface area,is consistent with this contention. However, noobvious dif ferences in microbial communitycomposition were observed between leaf speciesin Middle Bush Stream, and on all leaves bacteriarather than fungi appeared to be the main agen ts ofmicrobial decompositon. Furthermore, microbialactivity differed little among leaf species, with theexception of elm , thereby limiting its value as a foodchoice determinant.

In gene ral, there were no consistent invertebratecolonisation preferences for leaves of either nativeor introduced tree species. Nevertheless, highabundances of invertebrates were found on someleaf species, in particular introduced willow andnative red beech. In laboratory trials, the shredderO. jeanae fed preferentially on soft, conditionedleaves of introduced elm and red beech, whereasthe toughest leaves (mountain beech, oak) werevirtually uneaten.Our field results indicate that riparian inputsfrom different tree species can, to some degree,affect community composition and abundance andthere is evidence that New Zealand shredders canprefer in troduced to nat ive species . This isconsistent with comparative ecological studies

undertaken in the South Island, where only smalldifferences in invertebrate commu nity co mpo sitonwere found between phys iograph ica l ly com-parable s t reams in nat ive and exotic coniferforests (Ha rding & W interbourn 1995; Friberg eta l. 1997) and popula t ions o f the sh redd ingstonefly Austroperla cyrene were largest inconiferous forest streams indicating the absenceof an obligatory feeding link with indigenousriparian forest.

Sof t- leaved t ree species , especial ly thoselacking phytochemical deterrants are likely to bepreferred as foods by shredders. However, ourfindings should not be interpreted to mean that theplanting of soft-leaved riparian trees is necessa rilyan optimal strategy for enhancing populations ofdetritivorous insects. For instance, willow leaveswere colonised by high densities of both shreddersand non-shredders in our study indicating they m aybe an adequate food source and hab itat, but Lesteret al. (1994b) found that invertebrate densities andbiomasses decreased in willow (Salix fragilis) lined,compared to open sections, of some Otago stream s,and shredder densities and biomasses did notincrease despite the significantly higher input ofcoarse paniculate organic matter (CPOM). Theau th o r s a t t r i b u t ed t h i s t o a d ec r eas e i ncolonisation sites available to invertebrate s fromthe physical intrusion of willow tree roots intointerstitial spaces in the substrate, thus negatingthe benef i t s o f increased CPOM inpu ts . Inaddition, many deciduou s trees have short, pulsedinputs of leaves which break down fast, and areunlikely to be present in stream s for much of theyear when growing larvae are present (C umm inset al. 1989). A combination of r iparian plantspecies that provide detr i tal inputs over anextended period, and whose leaves decomposeat various rates, is likely to be a better strategyfor maintaining appropriate food resources foraquat ic de t r i t ivores , f aunal d iver s i ty , andproductivity.

ACKNOWLEDGMENTSWe thank Nellie, Vince, and Joanna Parkyn for help withfieldwork, and Jan Mackenzie for invaluable assistancewith scanning electron microscopy. Thanks to AshleySparrow who kindly provided statistical advice andKevin Collier and John Quinn for encouragement andconstructive comments on the manuscript. We appreciatethe helpful comments of two anonymous referees.


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Parkyn & WinterbournBreakdown of native and introduced leaves 311REFERENCESAllan, J. D. 199 5: Stream ec ology: structure and functionof running waters. London, Chapman & Hall.388 p.And erson, N. H.; S edell, J. R. 1979: Detritus processingby macroinvertebrates in stream ecosystems.

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Parkyn Leaf breakdown and colonisation by inverts Native vs Introduced trees

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