Leaf litter breakdown rates in boreal streams: does shredder species richness matter?

Freshwater Biology (2001) 46, 161–171

Leaf litter breakdown rates in boreal streams: doesshredder species richness matter?

MICAEL JONSSON, BJO8 RN MALMQVIST and PER-OLA HOFFSTENDepartment of Ecology and Environmental Science, Umea University, Sweden

SUMMARY

1. Leaf litter breakdown rates were assessed in 23 boreal streams of varying size(first–seventh order) in central and northern Sweden.2. Shredders were most abundant in small streams, while shredder species richnessshowed a hump-shaped relationship with stream order, with most species in fourthorder streams.3. In a partial least-squares regression analysis, year, water temperature, shredder spe-cies richness and shredder abundance were those factors correlating most stronglywith leaf breakdown rates. Shredder species richness was more strongly correlatedwith leaf litter breakdown rates than shredder abundance, and shredder biomassshowed no such correlation.4. These data suggest that shredder species richness is an important variable in termsof leaf litter dynamics in streams.

Keywords : detritus, leaf breakdown, shredders, species richness, streams

Introduction

Detritus constitutes the energy resource for decom-posing species and generates the major flow of en-ergy in all types of ecosystems (Polis & Strong, 1996).In temperate streams, a vast source of energy isderived from the surrounding vegetation throughleaf fall in autumn. Leaf litter input has been shownto affect higher trophic levels (Wallace et al., 1997),although the most apparent effect is found on thedetritivore level where both abundance and biomassare affected (Richardson, 1991).

After the leaves have entered the stream, rapidleaching of soluble organic compounds occurs, andthey are colonized by various microbes (Cummins etal., 1989). Although conditioned leaves are preferredby macroinvertebrates, the temporal nature of thebreakdown process is not strictly sequential (Gessner

et al., 1999). The great importance of microbial break-down in streams has been shown in many studies(e.g. Suberkropp & Klug, 1980; Maltby, 1992), butdecreases with increasing latitude and, at higher lati-tudes, macroinvertebrate breakdown outweighs mi-crobial breakdown (Irons et al., 1994). Microbial,macroinvertebrate and mechanical breakdown ratesare influenced by several factors, such as currentvelocity (Suberkropp & Klug, 1980), pH (Minshall &Minshall, 1978; Mulholland et al., 1987), temperature(Petersen & Cummins, 1974; Irons et al., 1994), lati-tude (Irons et al., 1994) and altitude (Fabre & Chau-vet, 1998).

According to the River Continuum Concept (RCC),small streams receive much larger quantities of leaflitter per unit area due to the proximity of the sur-rounding riparian vegetation, with the dense canopypreventing autochthonous production (Vannote et al.,1980). Downstream, as the channel widens, leaf litterinput per unit area decreases, whereas autochthonousproduction increases in importance, along with parti-cles produced in upstream breakdown processes. Thechange in leaf litter input quantity along a streamorder gradient regulates the populations of those or-

Correspondence: Micael Jonsson, Department of Ecology andEnvironmental Science, Umea University, SE-901 87, Umea,Sweden.E-mail: [email protected]

© 2001 Blackwell Science Ltd 161

M. Jonsson et al.162

ganisms depending on this input. In support of theRCC, the abundance and biomass of shredders havebeen found to be the greatest in small streams anddecrease as streams get larger (Minshall et al., 1983b;Grubaugh et al., 1997).

Despite the present, unprecedented global extinc-tion rate, only a few studies have attempted to assessthe ecological consequences of changed species rich-ness. More studies across a wider range of systemsand on different ecological processes are needed if weare to understand the function of species richness,and hence the effects of species loss. In terrestrialstudies, diversity has been found to maintain andincrease predictability (McGrady-Steed et al., 1997),reliability (Naeem & Li, 1997), invasibility (Symstad,2000), process efficiency (Heneghan et al., 1999) andproductivity and sustainability (Tilman et al., 1996). Alarge majority of the studies have been performed onterrestrial plant communities measuring nutrient up-take, changes in CO2 levels or gain in biomass. Incontrast, only a few studies on the function of speciesrichness have involved animal communities (Mikola& Setala, 1998; Heneghan et al., 1999; Jonsson &Malmqvist, 2000). To our knowledge, only a singlestudy has been performed on breakdown processes(Heneghan et al., 1999), but no field study using thisperspective has hitherto been carried out in aquaticsystems. The frequently very complex experimentaldesigns have rendered the results of many speciesrichness/ecosystem functioning studies difficult to in-terpret. Hence, most of them have generated equivo-cal results, so that there is still a pressing need to testavailable hypotheses on this relationship (Gaston &Spicer, 1998). These hypotheses are the diversity–sta-bility hypothesis (MacArthur, 1955), the rivet hypoth-esis (Ehrlich & Ehrlich, 1981), the redundancyhypothesis (Walker, 1992) and the idiosyncratic hy-pothesis (Lawton, 1994). Despite the fact that streamprocesses such as leaf litter breakdown are well stud-ied, little is known about the function of biodiversityin streams. The extensive knowledge concerning leafbreakdown and the organisms involved in this pro-cess, along with the ease by which stream organismscan be held in the laboratory, make this system anexcellent model for investigating the function of spe-cies richness.

To unveil the nature of possible relationships, weestimated the effect of shredder species richness,along with a number of other potential predictors, on

leaf breakdown rates in streams belonging to a rangeof different stream orders. We predicted a hump-shaped relationship between shredder species rich-ness and stream order, since small streams are morevulnerable to drought, complete freezing and spatesthan larger ones (Malmqvist et al., 1999). Such ex-treme disturbances could negatively affect the sur-vival of animals living in small streams, and hencedecrease species richness. Large streams were alsoexpected to have low shredder species richness as aconsequence of a reduced input of leaf litter per unitarea as streams get larger. We hypothesized thatmass loss in the leaf packs would be strongly influ-enced by shredder abundance and biomass, but re-sults from an earlier experimental study (Jonsson &Malmqvist, 2000) suggested that shredder speciesrichness was also important.

Methods

The study sites were up to 700 km apart (60–66° N)in the northern to middle boreal zones of Sweden,where mixed coniferous forest is dominant (Anony-mous 1984). The streams had rocky beds partly cov-ered with aquatic mosses, except for first-orderstreams, which also had sandy areas. The riparianforest extended close to the waterline in all but thelargest streams (stream order 7), where riparian vege-tation was always ]10 m from the water. Alder,Alnus incana (L.), was the dominant streamside tree atall sites.

In 1997, the water level decreased markedly duringthe study period, whereas in 1998 there was an in-crease, or no change, in water levels across all sites.An early cold period in 1997, in combination with thedecreasing water levels, rendered all leaf packs at twosites (Kalix region) completely frozen. These packswere excluded from the analysis.

The field study was carried out during the autumn(October–November) of 1997 and 1998 in six regionsof central and northern Sweden (Fig. 1). To investi-gate variability in leaf litter input, shredder abun-dance, biomass and species richness, as well asabiotic factors, we chose streams of different orders.In each region, four or five streams of orders 1–7were selected. Stream order and map coordinates(national grid) were determined using 1:50000 maps.In each stream, a 50 m stretch of riffle was selectedand ten cages containing leaf packs were randomly

© 2001 Blackwell Science Ltd, Freshwater Biology, 46, 161–171

Breakdown rates and shredder species richness 163

Fig. 1 Locations of six regions studied in northern Sweden(circles) and the experimental sites belonging to differentstream orders within each of the regions. The small mapoutlines the Nordic countries with northern Sweden shownin black.

In the laboratory, animals and leaves from thecages were separated. The leaves were dried for 48 hat 50 °C, weighed and ashed followed by wettingwith distilled water and dried again for 48 h at 50 °Cto determine ash free dry mass (AFDM) (Benfield,1996). Leaf fragments larger than 1 cm were sepa-rated, washed clean from sand, silt and loose organicmaterial, dried, ashed and weighed. Shredders fromthe experimental leaf packs were preserved in 70%alcohol, counted and identified. For identification ofshredder species, keys by Brinck (1952), Hynes (1967),Lillehammer (1988), Wallace et al., (1990) and Nilsson(1996, 1997) were used. Species were classified asshredders using information published by Brinck(1949), Wallace et al. (1990), Gledhill et al., (1993) andNilsson (1997). Shredders were determined to species.The shredders from each cage were combined anddried for 48 h at 50 °C and weighed. Shredder speciesrichness (alpha diversity) at each site was determinedfrom the total number of species recorded from cagesat each site.

Statistical methods

Leaf mass loss from experimental leaf packs was usedas the dependent variable in partial least-squares(PLS) regression analysis (SIMCA-P 7.01, Umetri AB)with 11 independent variables. The critical value for amodel, or a single component, in a PLS regression isQ2\0.097, which corresponds to PB0.05 (SIMCASoftware Manual, 1996). In the analyses, all variables(Table 1) except for year, pH, latitude, longitude,stream order and species richness were log trans-formed better to fit a normal distribution.

Results

Twenty-six shredder species (primarily Trichopteraand euholognathan Plecoptera) were recorded, rang-ing between 1 and 11 species at the 23 sites (Ap-pendix 1). Abundance varied substantially amongsites. The PLS analysis resulted in a significant modelthat explained 15.4% of the variance of the indepen-dent variables (r2

x) and 70.5% of the variance of thedependent variable (r2

y). The total variation that couldbe predicted by the model (Q2

y) was 34.8%. Yearshowed the strongest relationship with leaf mass loss,followed by water temperature, shredder speciesrichness and shredder abundance (Fig. 2). Latitude,

placed in the stream. The trial began at the beginningof leaf fall and lasted for 28 days. Leaves of alderwere collected and dried for 48 h at 50 °C. Batches of4 g of leaves were placed in cages, constructed fromplastic netting (0.8 cm mesh size, to allow coloniza-tion by invertebrates) with tetrahedral shapes (:0.7 L). Rope (10 cm long) was tied to the cages andattached to iron rods driven into the bottom of thestreams. At the end of the study each cage wasretrieved by placing it in a handnet with 0.5-mmmesh. Measurements of width, pH and conductivitywere made at the start of the study. Water tempera-ture and current velocity were measured at the startand end of the study using a digital current velocitymeter equipped with a thermometer (mP-Flowtherm,Hontzsch Instruments, Waiblingen, Germany). Cur-rent velocity was measured 5 cm from the bottom,just upstream of each leaf cage. The standing crop ofbenthic coarse particulate organic matter (CPOM)was estimated at the end of the study by collecting allorganic material in seven 50×50 cm squares, ran-domly selected at each site, using a hand net with1-mm mesh. Animals in these samples weredisregarded.




Tab

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site

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Map

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1997


Fig. 2 Factor loadings in the PLS analysis. Independentvariables are white and the dependent variable black.Loadings represent the correlations between the variables andcomponent extracted.

Fig. 4 Stream order versus average shredder abundance(numbers per cage). y= −0.0858x+1.3693. Numbers indicateoverlapping data points.

Fig. 3 The relationship between stream order and shredderspecies richness observed. y= −0.321x2+2.494x+3.206.Numbers indicate overlapping data points.

Fig. 5 Stream order versus average benthic CPOM. Each dotrepresents one site and the numbers indicate overlappingdata points. The regression equation is y= −0.0845x+0.5407.

stream order and pH were also important in themodel.

Log leaf mass loss increased significantly with logshredder species richness (linear regression; r2=0.22;21 d.f.; PB0.05; y=0.286x−0.040). Shredder speciesrichness showed a quadratic relationship with streamorder (r2=0.38; 21 d.f.; PB0.01; Fig. 3) where highestshredder species richness was found in mid-orderstreams. Shredder abundance was negatively relatedto stream order (r2=0.24; 22 d.f.; PB0.05; Fig. 4), aswas benthic leaf CPOM (r2=0.37; 22 d.f.; PB0.01;

Fig. 5); i.e. small streams contained larger amounts ofleaf litter than larger ones. However, large variationin the quantities of benthic leaf CPOM was foundamong sites of the same stream order, especiallyamong low-order streams.

Discussion

Knowledge of the importance of species richness forecosystem processes in general is lacking (Chapin etal., 1998) and, for stream processes, virtually non-ex-istent. This is a serious shortcoming because under-



standing the function of species richness is directlylinked to the problem of biodiversity loss. There is apressing need to estimate the role of species richnessboth in experiments assessing process rates at variouslevels of species richness (Jonsson & Malmqvist, 2000)and in studies of natural ecosystems with varyingnumbers of species (Heneghan et al., 1999; Sankaran& McNaughton, 1999). Both approaches are necessarybecause the former allows critical experiments butlacks realism, while the latter has realism but pro-vides only correlative evidence.

This study indicates that several factors correlatewith, and potentially influence, the rate at which leaflitter is processed in freshwater streams. The strongrelationship with year probably reflects between-yearvariation in several unmeasured factors, includingice, and the levels of temperature and precipitationpreceding our experiment. Water temperaturestrongly influences the rate of leaf litter breakdown(e.g. Petersen & Cummins, 1974; Suberkropp et al.,1975; Irons et al., 1994), mainly through its influenceon microbial (Webster & Benfield, 1986) and, to alesser extent, macroinvertebrate (Hart & Howmiller,1975) processes. Shredder species richness was thebiotic variable showing the highest loading (i.e.strongest correlation with component extracted). Apositive effect of shredder species richness on leafbreakdown rates has been observed in laboratoryexperiments (Jonsson & Malmqvist, 2000). In naturalstreams, a multitude of factors, including predationand population increase of the remaining shredderspecies, might obscure the effect of reduced speciesrichness observed in controlled laboratory experi-ments. A positive relationship between richness (oforibatid mites) and oak leaf breakdown was observedin a terrestrial study across a large climatic gradient(Heneghan et al., 1999). No attempt to identify themechanisms was presented, although it was assumedthat local differences in the relationships betweenmites and micro-organisms were involved.

Two mechanisms are currently suggested to ex-plain how higher diversity could favour ecosystemfunctioning (Naeem et al., 1999). These are ‘the sam-pling effect’ (Aarsen, 1997; Huston, 1997; Tilman etal., 1997) and ‘the complementarity effect’ (Naeem etal., 1994; Lawton et al., 1998). The sampling effect isstatistical and is due to the increasing probability ofincluding species with a marked influence on processrates. We refute this mechanism as we did not see

any correlation between shredder biomass and leafbreakdown rate that might suggest the presence ofsuch dominating species.

The complementarity effect is thought to operatethrough an increasingly more efficient use of avail-able resources, with an increasing number of specieshaving slightly different niches. In an East Africandecomposition system, involving carcasses and asso-ciated scavengers (primarily vultures), species showdifferent arrival time, aggregation, beak and bodysizes (Kruuk, 1967). Analogously, shredders mighthave subtle differences in life history, morphologyand other features which make their niches comple-mentary. It is also quite possible that shredder speciesfacilitate for one another by affecting the leaves me-chanically, chemically or indirectly via the micro-biota. We suggested in an earlier paper that suchfacilitation could be important (Jonsson & Malmqvist,2000). We also suggested that intraspecific competi-tion might be stronger than interspecific competitionso that interactions in leaf packs might be weakerwhen neighbouring individuals belong to differentspecies than if they belong to the same species. Thus,in a more diverse shredder community, behaviouralinteractions might be less and hence more time couldbe devoted to feeding, resulting in a faster rate ofdecomposition. We have observed aggressive be-haviour among leaf-eating insects and found signs ofphysical interactions such as mutilated legs, cerci andantennae (Malmqvist, 1993). The effects observed inour previous laboratory experiment (Jonsson &Malmqvist, 2000) are more likely to be of interferencenature than via the resource, which in the experimentwas abundant. The literature suggests that competi-tion might be significant among shredders in streams(Smock et al., 1989; Richardson, 1991; Malmqvist &Oberle, 1995; but see Reice, 1991). However, the na-ture of the competition seems rarely known.

Shredder abundance showed a positive relation-ship with leaf mass loss, which agrees with previ-ously published studies (e.g. Benfield & Webster,1985; Fabre & Chauvet, 1998). However, the influenceof abundance was weaker than that of species rich-ness, partly contradicting the species redundancy hy-pothesis, which suggests that high abundances cancompensate for low species richness leaving ecosys-tem functions unchanged as long as all functionalgroups are represented (Walker, 1992). Shredderbiomass showed no significant relationship with



breakdown rate. A high leaf breakdown rate, in termsof remaining large leaf fragments, would suggesthigh shredder biomass. However, since shredders, toa variable extent, fragment leaf litter in addition tothat ingested (Cummins, 1973), leaf litter breakdownrates may show a low correlation with shredderbiomass.

The RCC predicts that species richness in streamswould follow a hump-shaped pattern with increasingorder due to variability in water temperature (DTmax)(Vannote et al., 1980). The greater the variability inwater temperature, the more species would havetheir temperature optima within its range. Other ex-planations are certainly possible and, for whateverreason, hump-shaped relationships have been confi-rmed by studies in stream continua (e.g. Minshall etal., 1985; Oberdorff et al., 1993). However, anthropo-genic disturbances and land use have changed manystream systems leading to divergence from predic-tions made by the RCC, which is based on relativelypristine stream systems in the temperate regions ofNorth America (Giller & Malmqvist, 1998). In agree-ment with the RCC, our field study showed a hump-shaped species richness pattern, though onlyshredder species richness was investigated. While theabundance of shredders decreased with stream size,probably tracking the decrease in benthic leaf CPOM,shredder species richness peaked at stream order 4.As the lowest shredder species richness was observedin streams of orders 1 and 7, abundance and speciesrichness were obviously not related. The reason forthe lack of a peak in shredder species richness inlow-order streams might be because such streams areexposed to a higher degree of disturbances, includingspates, complete freezing and drought. Shredderabundance has been shown to recover faster thanspecies richness after experimental insecticide distur-bance (Whiles & Wallace, 1992) and after abnormallyhigh discharge (Minshall et al., 1983a). Good coloniz-ers can temporarily reach a higher abundance thanbefore the disturbance as a result of reduced compet-itive and predatory pressures (Hurlbert, 1975). Thus,patterns of shredder abundance and species richnesscan vary widely along a stream order gradient due tothe history of disturbance.

This study supports our previous laboratory exper-iment by indicating a clear association between spe-cies richness and leaf breakdown rate and has someimportant implications. It agrees with terrestrial stud-

ies that have attributed the sensitivity of ecosystemprocesses to a decline in biodiversity (Naeem et al.,1999). Importantly, the effect is manifest within afunctional feeding group, which suggests that specieswithin such a group are not redundant. This findingis in contrast to the redundancy hypothesis, whichpredicts that other species will compensate loss of aspecies within the same functional group and that,therefore, major effects would be expected only whenthe last species in a group disappears (Walker, 1992).Some discussion of the functional feeding group is,however, required. Species are frequently and jus-tifiably put into a limited number of categories for thepurposes of modelling or for the approximate inter-pretation of community composition. This procedureoften performs well (e.g. Hawkins & Sedell, 1981).Functional feeding groups are, however, also usedout of convenience although it is well known thatfeeding of freshwater macroinvertebrates is oftenhighly variable and recognized to differ with respectto species, ontogeny, geographical locality, seasonand even sex (e.g. Malmqvist et al., 1991). It is indeedunlikely that any two species show identical foodpreferences or feeding strategies. Recently, Ledger &Hildrew (2000a,b) found that species of nemouridstoneflies, normally attributed to the shredder cate-gory, in acidic conditions broadened their food rangefrom leafy detritus to include algae and biofilmgrazed from stones. Such feeding opportunism isindeed likely to be widespread in streams. On theother hand, the functional group concept as devel-oped for freshwater insects is not defined strictly bywhat is eaten but rather derived from how food isacquired on the basis of morpho-behavioural mecha-nisms (Cummins & Merritt, 1996). Obviously, theappropriate functional feeding group can be equivo-cal and, hence, redundancy within functional groupsis moot. A closer look at the use of functional groupsin other studies of biodiversity function might wellprove also to suffer from similar issues of generaliza-tion. Our usage of ‘shredders’ in the present papercomprises taxa we normally find in leaf packs, whichgrow well on leaves offered to them in laboratory andwhich also are referred to as shredders in the litera-ture (although, see Ledger & Hildrew, 2000a,b).

If most shredder taxa were to be lost from a stream,the potential effects would be a reduced breakdownrate of leaves and a proportionally greater break-down by microbes. Accumulation of organic material



would be expected to increase as a smaller proportionof the litter would be comminuted into fragments andfaeces, reinforced by the fact that retention is nega-tively related to particle size. As unlimited accumula-tion is unlikely in the long run, due to physicallimitation of suitable retention sites, the timing oftransport might change in a manner less reflectingshredder phenology than hydrographic episodes.Further consequences might be a restriction inboth energy flow to higher trophic levels and exportof fines to downstream reaches (cf. Wallace et al.,1982). Further studies are required to test these pre-dictions.

Acknowledgments

We thank Alan Hildrew, Simon Rundle and twoanonymous reviewers for valuable comments on themanuscript and Peter Rivinoja for technical assis-tance. Financial support was provided by theSwedish Foundation for Strategic Environmental Re-search (MISTRA), Helge Ax:son Johnson Foundationand the Swedish Council for Forestry and Agricul-tural Research (SJFR).

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(Manuscript accepted 26 April 2000)




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Documents

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