Spatial scale and the aggregation of stream macroinvertebrates associated with leaf packs

Freshwater Biology (1998) 39, 325–337

Spatial scale and the aggregation of streammacroinvertebrates associated with leaf packs

J O H N F. M U R P H Y, PA U L S . G I L L E R * A N D M A R G A R E T A . H O R A NDepartment of Zoology and Animal Ecology, University College Cork, Lee Maltings, Prospect Row, Cork, Ireland

*To whom correspondence should be sent.

S U M M A R Y

1. An experimental field study examined the aggregation of stream macroinvertebratesassociated with leaf packs over different spatial scales (several metres–km) (extent), atdifferent patch sizes (grain) and temporal scales (2 and 4 weeks).2. Standardized leaf packs were constructed and set in eighteen blocks of nine equallyspaced packs in glide areas over a 2 km stretch of a wooded stream. The distribution ofmacroinvertebrates colonizing the artificial leaf packs was investigated to examine theextent of both intraspecific and interspecific aggregation across leaf packs.3. All major colonizing taxa were intraspecifically aggregated across the leaf packs.Aggregation decreased with increasing patch size (grain) (from pack to block), and alsodecreased with decreasing spatial extent (from 2 km stretch to within-block scale) withpatch size held constant. Interspecific associations among all major taxa were notcommon on most occasions at the short temporal scale, although the proportion ofsignificant associations tended to increase somewhat over time and with spatial scale,but did not exceed 42% of all possible associations. The vast majority of significantassociations were positive rather than negative.4. The influence of heterogeneity in a number of environmental variables measured foreach leaf pack (accumulated detritus and sediment, leaf mass, flow and depth) on thedistribution of invertebrates was considered, but this could only partially explain thevariation in macroinvertebrate abundance across leaf packs.5. The roles of intrinsic aggregation and stochastic processes were examined asalternative explanations for the distribution patterns observed. It is apparent from thisstudy that intrinsic aggregation, in concert with resource partitioning, influences thecommunity structure of stream macroinvertebrates associated with leaf packs. Thesefindings may also have implications for the distribution of taxa in the benthos as awhole.

Introduction

It is well known that macroinvertebrates in streamsusually have aggregated spatial distributions, i.e. theyare non-randomly dispersed over the stream sub-stratum or among benthic samples or patches.Explanations for this aggregation have been based onthe influence of extrinsic factors such as water velocity,depth, substratum composition and the amount ofdetritus and periphyton in or on the substratum(Edington, 1968; Allan, 1975; Minshall & Minshall,

© 1998 Blackwell Science Ltd 325

1977; Culp, Walde & Davies, 1983; Orth & Maughan,1983; Erman & Erman, 1984; Teague, Knight & Teague,1985) and the potential impact of interactions such ascompetition (McAuliffe, 1984; Hart, 1985; Hemphill,1991; Kohler, 1992; Allan, 1995; Suhling, 1996) andpredation (Lancaster, Hildrew & Townsend, 1991;Hildrew, 1992; Malmqvist, 1993).

More theoretical approaches to understandingspatial distributions have been developed from the

326 J.F. Murphy, P.S. Giller and M.A. Horan

study of terrestrial insects associated with ephemeralresources that occur in discrete patches, e.g. dungbeetles on dung, carrion flies on carcasses (Ives, 1988,1991; Giller & Doube, 1994) and have led to theformulation of the aggregation model for coexistence(Atkinson & Shorrocks, 1981; Hanski, 1983; Shorrocks,Atkinson & Charlesworth, 1979, 1984). These studieshave found that the various insect species typicallyhave an aggregated spatial distribution, i.e. a largevariation in the number of individuals of each speciesper patch, even though patches are of similar oridentical quality. It is proposed that the aggregationin these systems is due, not to the species response toextrinsic environmental factors, but to intrinsic traitsof the species themselves, such as body size, the abilityto find patches (Ives, 1988; Hanski & Cambefort, 1991)or the propensity to colonize patches already colonizedby conspecifics (Holter, 1982). This is considered analternative to traditional resource partitioning as amechanism for explaining local species diversity andis characterized by strong intraspecific aggregation ofindividual species relative to interspecific aggregationamongst species across patches (Sevenster, 1996).

To the authors’ knowledge the only other applicationof this model to lotic freshwater habitats has been byTokeshi & Townsend (1987), who investigated patternsof distribution within an epiphytic chironomid com-munity on the apical stems of submerged macrophytes.Tokeshi (1994, 1995), however, subsequently suggestedthat random distribution was actually fairly commonin this community and strong aggregation was rare,and hence the pure aggregation model was not con-sidered appropriate as a means of explaining spatialsegregation. Tokeshi (1994, 1995) proposed that thestochastic dispersal and colonization of patches inthis lotic system may override any influence thataggregation could have on the chironomid community.

Giller & Gee (1987) and Horne & Schneider (1995)highlight the fact that the scale of investigationinfluences patterns observed in the spatial dispersalof organisms. However, spatial scale is a functionof both the resolution of sampling (grain) and thearea encompassed by the study (extent) (Wiens,1989). The spatial resolution of analysis for manystudies on macroinvertebrate distribution to datehas been either the area of substratum sampled [e.g.Surber sampler (0.0625–0.0931 m2)] (Allan, 1975;Godbout & Hynes, 1982), or manipulative fieldexperiments where treatment patches [e.g. artificial

© 1998 Blackwell Science Ltd, Freshwater Biology, 39, 325–337

substrata (0.0150–0.693 m2)] (Rabeni & Minshall,1977; Reice, 1980) are exposed to colonization.The spatial extent of distribution studies variesconsiderably from individual surfaces on a rock(Rader & Ward, 1990) to individual rocks themselves(Poff & Ward, 1992), to whole reaches (Downes,Lake & Schreiber, 1993) and entire catchments(Corkum, 1992). Few, if any studies, have consideredthe effect of varying independently both the grainand extent of analysis on the perceived distribution.Wiens (1989) predicted that an increase in grainwould lead to a decrease in apparent heterogeneity,while an increase in spatial extent would lead toan increase.

Benthic accumulations of leaf litter in streams arean ephemeral, patchily distributed resource colonizedby macroinvertebrates, either as food (for shredders)or shelter. These patches provide a convenient experi-mental model with which to examine distributionpatterns in stream macroinvertebrates. The objectivesof this study were to investigate the degree of aggrega-tion of stream macroinvertebrates associated withartificial leaf packs over a range of spatial scales andto identify which processes may be most influentialin explaining the patterns observed, with particularattention being paid to the aggregation model forcoexistence as an alternative to resource partitioning.

A simple null hypothesis (H0) would be for taxa tobe distributed at random across the patches, i.e. noaggregation. Rejection of this could take several forms,depending on the relative degree of intraspecific andinterspecific aggregation among taxa. If intraspecificaggregation was low and interspecific aggregationhigh, then taxa would have similar (more or lessrandom) distribution patterns with similar but weakrelationships to environmental heterogeneity acrosspatches (H1a). If both intraspecific and interspecificaggregation are high, then all taxa are effectivelyaggregated in the same patches and thus show asimilar, but strong, relationship to environmentalheterogeneity among patches (H1b). If intraspecificaggregation is high and interspecific aggregation low,then taxa are aggregating independently and eitherintrinsic factors govern aggregation, as predicted bythe aggregation model for coexistence, stochasticfactors differentially influence species or taxa arepartitioning resources in relation to differences inenvironmental conditions among the patches (H1c),i.e. traditional resource partitioning.

Aggregation of stream macroinvertebrates 327

By standardizing conditions as far as possible acrossthe artificial leaf packs, an attempt was made to reducethe potential for resource partitioning and thus toinvestigate the extent to which intrinsic aggregationprocesses were operating. The experimental arrange-ment of the artificial leaf packs also allowed examina-tion of the effect of independently varying the grainand extent of the analysis on the observed distributionof the macroinvertebrates. It was predicted, followingWiens (1989), that with increased spatial extent therewould be an apparent increase in aggregation (intras-pecific and interspecific) and with increased grainthere would be an apparent decrease.

Materials and methods

Study site

This study was carried out along a 2-km section ofthe River Douglas, in north Co. Cork, Ireland (52°119N,8°149W). The catchment of the river is 17.76 km2, 42%of which is covered by a conifer plantation [mainlyPicea sitchensis (Bongard) Carr., Pinus contorta Douglasex Loudon and Pseudotsuga menziesii (Mirabel) Franco].Three of the river’s four tributaries originate on moor-land, dominated by purple moor grass (Molinia caeruleaL. Moench), before flowing through the forest. Theriparian vegetation of the study site is composedof native deciduous species such as birch (Betulapendula L.), beech (Fagus sylvatica L.), hazel (Corylusavellana L.) and oak (Quercus petraea L.) with anunderstorey of bramble (Rubus spp.). The underlyinggeology is Old Red Sandstone. The river at the studysite is ‘hard’ (640–1007 µeq/L) with a neutral to slightlyalkaline pH (7.01–7.38).

Experimental design

Mesh bags (15 3 20 cm, mesh size 10 3 3 mm) wereloosely filled with a similar volume of freshly abscisedbeech leaves collected from the ground near the stream.In total, 162 leaf packs were prepared and werethreaded onto lengths of marine twine in groups ofthree. Leaf packs were placed in the stream on 24November 1995, in eighteen blocks of nine-leaf packs.Within each block, the packs were arranged in a3 3 3 formation with 40–50 cm between each pack.All blocks were placed in glides within the studystretch, as similar as possible to one another to help


reduce environmental variation between leaf packs.Each group of three leaf packs was tied to two largecobbles and each leaf pack was firmly fixed to thestream bed by a steel tent peg.

Blocks were positioned in the stream in pairs.Within a pair, blocks were at least 2 m apart andpairs of blocks were placed in suitable areas ofglide over the 2 km study stretch. Four pairs ofblocks were placed in one large glide stretch with5 m between adjacent pairs. Another three pairs wereplaced in glides 300, 700 and 1500 m downstream ofthe first large glide The last two pairs were togetherin another large glide, a further 300 m downstream.After 2 weeks, nine blocks, one from each pair, wereretrieved from the stream. Each leaf pack wascarefully removed and placed in a 0.5-mm meshpond net positioned immediately downstream of thepack. The pack and its contents were then placedin a plastic bag and preserved with 70% ethanol.In the laboratory the entire pack was placed in atray, the beech leaves were washed, removed, driedat 60 °C for 48 h and then weighed to the nearestmilligram. The macroinvertebrates were sorted fromthe remaining detritus and sediment, identified andcounted. Detritus and inorganic sediment that hadaccumulated in the leaf pack over the study periodwere, in turn, separated by elutriation. This involvedgently swirling the sample and then carefully pouringthe floating detritus through a 150-µm sieve, allowingthe heavier inorganic sediment to settle in the sortingtray. Both fractions were dried at 60 °C for 48 h andindividually weighed to the nearest milligram (thedry mass of accumulated sediment was not recordedfor twenty-one of the eighty-one leaf packs on thefirst sampling occasion). After 4 weeks the remainingnine blocks were retrieved from the stream and thesame procedure was followed. King et al. (1987)found that natural leaf packs lasted, on average,, 1.7 months. Dobson (1991) and Malmqvist (1993)also considered a 1-month exposure period adequateto simulate accurately the longevity of natural packs.The stream depth was measured at the upstreamedge of each leaf pack. Current velocity wasmeasured at two points directly in front of eachpack, just above the substratum surface using anOTT (Messtechnik; Kempten, Germany) currentmeter, type C2910.1509. Both sets of measurementswere taken 1 week before collection. No spatesoccurred during the study period.


Analysis

The design of the experiment allowed the intraspecificand interspecific aggregation of macroinvertebratesassociated with leaf packs to be investigated at threespatial scales on both sampling occasions: Scale a,large spatial scale (large spatial extent), small patchsize (small grain), across all eighty-one individualpacks over the 2 km study stretch (n 5 81); scale b,large spatial scale, large patch size (large grain), acrossall nine blocks with the nine packs within each blockbeing pooled to form the ‘patch’ (n 5 9); scale c, smallspatial scale (small spatial extent), small patch size,across the nine packs within each of the nine replicateblocks (n 5 9, nine replicates). According to predic-tions then, aggregations at scale a would be expectedto be greater than at scale c (extent) and at scale b(grain). Wilcoxon’s signed ranks test was used tocompare changes in the magnitude of intraspecificaggregation between scales.

Taxa were not included in the aggregation analysisif their mean abundance per pack was less than 0.5for scale a analysis and 4.0 for scales b and c. Onlynine taxa were sufficiently abundant to be analysedat all three spatial scales on both sampling occasions.

Initially, ANOVA of the transformed [log(x 1 1)]numbers per pack was carried out on these nine taxa,and on all taxa combined, to investigate differencesin the abundance between blocks across the studyarea. The nature of the dispersal of these taxa, on allspatial scales, was then examined more closely usingIves’ (1988, 1991) measure of intraspecific aggregation:

JA 5 VA/N2 – 1/N,

where VA is the sample variance and N is the meannumber of the taxon A per sample (pack or block). JA

is a measure of the relative increase in the meannumber of conspecifics found in the same patch abovethat expected if their distribution was random acrosspatches. For example a value of JA 5 0.65 indicates a65% increase in the intraspecific crowding (Ives, 1991).The significance of the departure of JA from zero wastested using the χ2 test of dispersion.

Interspecific aggregation relationships amongstspecies pairs were analysed using Ives’ (1988, 1991)measure CAB, defined as:

CAB 5 CovAB/NM

where CovAB is the covariance between taxa A and


taxa B, N and M are the mean numbers of the twotaxa per sample (pack or block). This measure isanalogous to JA in that a value of CAB 5 0.65 indicatesa 65% increase in the number of heterospecifics foundin the same patch above that expected if taxa A andB were distributed independently of one anotheracross patches (Ives, 1991). Spearman rank correlationof sample abundances across the data set of the twotaxa concerned (per pack or block) was used to testthe significance of departure of CAB from zero (eitherpositive or negative associations). Both J and C meas-ures are independent of mean abundance (Ives, 1991),which allows aggregations to be compared acrossscales.

The relative strength of intra- and interspecificaggregation between taxa was assessed using thequantity AAB (Ives, 1991; Shorrocks & Sevenster, 1995),defined as:

AAB 5 [(JA 1 1)(JB 1 1)]/(CAB 1 1)2.

If AAB exceeds 1.0 then intraspecific aggregation isstronger than interspecific aggregation for the associ-ation between taxa A and B (Jaenike & James, 1991;Shorrocks & Sevenster, 1995). J, C and A values werecalculated at all three scales for the 2 and 4 weektime periods.

To ascertain the extent to which the measuredenvironmental variables explained the patterns ofdistribution of the macroinvertebrates over the studystretch, multiple regressions and stepwise multipleregressions were carried out on the log(x 1 1)-trans-formed data for total macroinvertebrate abundanceand for individual taxa over the eighty-one leaf packs.The stepwise procedure omits variables from theregression equation that do not make a statisticallysignificant contribution to the variability of thedependent variable.

Results

A total of seventy-six taxa were found in the beechleaf packs during the study period. There was asignificant increase in the number of individuals from2 to 4 weeks (Mann–Whitney U-test, P , 0.001, W 5

8937.5). The mean and the standard deviation of eachof the environmental variables measured for leaf packsfor both exposure periods is given in Table 1.

For both the 2- and 4-week periods, significantdifferences were found between blocks in the abund-


ances of the nine taxa examined, except for Isoperlagrammatica Poda and Hydropsyche instabilis Curtis after2 weeks (Table 2). The rank order of blocks in respectof abundance differed among taxa on both samplingoccasions and also the coefficient of variation wasgenerally greater for individual taxa than for all taxacombined (Table 2). This indicates that macroinverteb-rates as a whole were more evenly dispersed over thestream than were the individual taxa.

Intraspecific aggregation

All taxa that met the criterion for inclusion in theanalysis were significantly aggregated over the eighty-one leaf packs (scale a) on both sampling occasions(Table 3). The level of aggregation varied from 21.8%to 533.5% and from 12.5% to 661.6% increase in thelevel of intraspecific crowding within packs abovethat expected if they were randomly distributed, forthe 2- and 4-week samples, respectively. There was no

Table 1 The mean and the standard deviation of each of theenvironmental variables measured for each leaf pack for bothsampling periods

Mean 6 standard deviation

2 weeks 4 weeks

Leaf pack mass (g) 7.18 6 1.376 6.69 6 1.186Additional detritus mass (g) 0.73 6 0.505 1.26 6 0.631Additional sediment mass (g) 7.83 6 21.373 54.72 6 68.627Water depth (m) 0.29 6 0.044 0.26 6 0.047Water velocity (m s–1) 0.47 6 0.158 0.38 6 0.107

Table 2 Summary details of the ANOVA carried out on the log(x11)-transformed numbers per pack of the nine major taxa and ofall taxa combined, after 2 weeks and 4 weeks exposure. The coefficient of variation of the mean abundance of individuals per packin each block is also given for each taxon and all taxa combined on both sampling occasions

2 weeks 4 weeks

Coefficient of Coefficient ofMS: block MS: error F8,72 value Variation MS: block MS: error F8,72 value Variation

Baetis muticus L. 0.205 0.075 2.72** 0.326 1.396 0.128 10.92*** 0.711Chloroperla sp. 0.256 0.105 2.44* 0.306 0.626 0.111 5.62*** 0.514Isoperla grammatica 0.148 0.083 1.79 NS 0.257 0.330 0.068 4.85*** 0.453Leuctra hippopus Kempny 0.370 0.057 6.48*** 0.388 0.397 0.081 4.89*** 0.506Leuctra inermis Kempny 0.375 0.102 3.67*** 0.547 0.315 0.078 4.02*** 0.439Protonemura meyeri Pictet 0.522 0.069 7.52*** 0.459 0.133 0.057 2.33* 0.244Hydropsyche instabilis 0.075 0.070 1.08 NS 0.173 0.138 0.057 2.42* 0.272Orthocladiinae 0.229 0.057 4.01*** 0.357 0.266 0.071 3.72*** 0.382Chironominae 0.321 0.141 2.28* 0.801 0.282 0.070 4.04*** 0.393All taxa 0.055 0.024 2.33* 0.180 0.190 0.031 6.06*** 0.250

* P,0.05, ** P,0.01, *** P,0.001, NS not significant.


consistent change in aggregation among taxa betweenthe two sampling periods at this scale. From thetwenty-three taxa, of which the aggregation could becompared between sampling periods, nine showed anincrease in aggregation with time, eleven a decreaseand three showed no appreciable change.

The distribution of the taxa analysed over the nineblocks (scale b) showed significant aggregation onboth sampling occasions. The level of aggregationvaried from 2.4% to 62.3% and 5.5% to 70.8% increasein the level of intraspecific crowding, for the 2- and4-week samples, respectively (Table 3). Of the ninetaxa that could be compared at this scale, the degreeof aggregation increased over the study period forfour, decreased for three and remained similar for two.

At the smallest scale, within the blocks (scale c),aggregation values presented for each taxon were themeans of nine replicate blocks of nine packs. Thereplicate values for each taxon varied considerably(Table 3). Overall, 83% of values were significant andpositive and there were no significant negativeJ values. Aggregation intensity at this scale declinedwith time for six of the nine taxa examined.

When comparing the aggregation values acrossscales, for the two time periods combined, twenty-two of the twenty-four calculated J values were greaterat scale a than at scale b [aggregation decreasedwith increasing patch size (grain)] and the medianmagnitude of the decrease was statistically significant(W24 5 291.0, P , 0.001). Sixteen of the twenty-fourcalculated J values were greater at scale a than scale c


Table 3 Intraspecific aggregation J values (Ives, 1988) for each taxon at different scales of analysis (see Materials and methods forcriteria for inclusion)

Mean level of intraspecificaggregation across nine packs,within a block (Jc), calculated from

Level of intraspecific Level of intraspecific the nine replicate blocks. Number ofaggregation (Ja) across all aggregation (Jb) across the replicates (out of nine) with a81 leaf packs, within the nine blocks, within the significant Jc are in parenthesesstream stretch (scale a) stream stretch (scale b) (scale c)

Taxa 2 weeks 4 weeks 2 weeks 4 weeks 2 weeks 4 weeks

Baetis muticus 0.384 * 0.809 * 0.102 * 0.502 * 0.384 (9) 0.654 (9)B. rhodani Pictet 0.767 * 1.364 * – – – –Ecdyonurus sp. 1.399 * 1.172 * – – – –Rithrogena semicolorata Curtis 1.006 * 0.473 * – 0.187 * – 0.327 (6)Amphinemura sulcicollis Ris 0.396 * 0.499 * – 0.163 * – 0.312 (6)Brachyptera risi Morton 4.013 * 1.412 * – – – –Chloroperla sp. 0.482 * 0.084 * 0.084 * 0.258 * 0.465 (9) 0.347 (8)Isoperla grammatica 0.252 * 0.313 * 0.056 * 0.196 * 0.370 (7) 0.215 (8)Leuctra hippopus 0.283 * 0.533* 0.138 * 0.243 * 0.296 (6) 0.321 (8)L. inermis 0.705 * 0.416 * 0.273 * 0.508 * 0.508 (7) 0.293 (8)Protonemura meyeri 0.356 * 0.211 * 0.205 * 0.055 * 0.263 (9) 0.189 (9)Agapetus sp. 5.335 * 1.017 * – 0.343 * – 0.545 (6)Hydropsyche siltalai Dohler 0.645 * 0.293 * – – – –H. instabilis 0.218 * 0.234 * 0.024 * 0.070 * 0.281 (8) 0.227 (9)Rhyacophila dorsalis Curtis 0.584 * 0.125 * – – – –Sericostoma personatum Spence – 1.076 * – – – –Silo palipes Fabricius – 1.318 * – – – –Limnephilidae (early instar) 0.285 * 0.322 * – 0.059 * – 0.359 (6)Elmis aenea (larva) Muller – 0.178 * – – – –Hydraena gracilis Germar – 1.719 * – – – –Dicranota sp. – 1.191 * – – – –Simuliidae 1.514 * 0.584 * – 0.231 * – 0.614 (8)Orthocladiinae 0.317 * 0.356 * 0.125 * 0.145 * 0.280 (9) 0.286 (9)Chironominae 3.283 * 0.282 * 0.623 * 0.151 * 0.682 (6) 0.209 (9)Tanypodinae 0.246 † 0.589 * – – – –Turbellaria 0.871 * 6.616 * – – – –Potamopyrgus jenkinsi Smith 3.598 * 3.433 * – – – –Hydracarina – 0.697 * – – – –Gammarus duebeni Liljeborg 1.040 * 1.459 * – 0.708 * – 0.978 (8)

*An aggregation value significantly different from zero (χ2 test of dispersal; P , 0.05; n 5 81 at scale a; n 5 9 at scale b and scale c).†An aggregation value significantly different from zero at the P , 0.10 level.–Indicates that the taxon was not sufficiently abundant for analysis.

(aggregation decreased with decreasing spatial extent)and the median magnitude of the decrease was statist-ically significant (W24 5 221.5, P , 0.006) (Table 3).Varying the spatial extent did not affect the aggrega-tion values to the same extent as varying the grain,particularly after 2 weeks. This pattern was mostobvious when both spatial extent and grain weredecreased, i.e. comparing scale b to c, and yet twenty-three of the twenty-four calculated J values increasedwith the magnitude of the increase being highlysignificant (W24 5 284.0, P , 0.001) (Table 3).


Interspecific aggregation

Analysis of interspecific aggregation between taxaacross all eighty-one packs (scale a), for the firstsampling period, showed that 26.5% of the taxa com-binations had significant positive aggregation and7.1% had significant negative aggregation (Table 4).At this spatial scale and time, significant interspecificassociation values (CAB) varied from a 43.7% decreaseto a 149.1% increase in the level of heterospecificcrowding, above that expected if both taxa were


Table 4 Summary percentages of the prevalence of significant interspecific aggregation among taxa at the three different scales andon both sampling occasions. Significance was tested using Spearman rank correlation of taxon abundances per sample (pack orblock). At scale c the percentage values are calculated from all the replicates (324 after 2 weeks and 945 after 4 weeks)

Across all 81 packs within Across the 9 blocks within Across the 9 packs withinthe stream stretch (scale a) the stream stretch (scale b) a block (scale c)

2 weeks 4 weeks 2 weeks 4 weeks 2 weeks 4 weeks

Percentage of interspecific aggregation 33.6 41.9 5.6 24.8 9.5 11.3values, Cab (Ives, 1988), significantlydifferent from zeroPercentage of Cab values significantly 26.5 40.4 5.6 24.8 8.0 10.7positivePercentage of Cab values significantly 7.1 1.5 0.0 0.0 1.5 0.6negative

independently dispersed over the packs. However,64.4% of pairwise comparisons exhibited distributionsthat were independent of each other. The percentageof significant positive associations increased for the 4-week sample to 40.4% but the percentage of significantnegative associations decreased to 1.5% leaving 58.1%of taxa pairs independently aggregated (Table 4). Thesignificant interspecific association values ranged froma 26.7% decrease to a 178% increase in taxa pair co-occurrence, from that expected if both taxa weredispersed independently of one another.

Increasing the patch size from pack to block (scaleb) resulted in a considerable reduction in the propor-tion of significant positive interspecific associationsand the elimination of any significant negative associ-ations for both sampling periods (Table 4). The propor-tion of significant positive associations increasedbetween the 2- and 4-week exposure periods. Therange of the significant association values increasedbetween the 2- and 4-week periods at this scale froma 13.7–16.2% increase to a 6.6–30.3% increase in thenumber of heterospecifics in the same block above thatexpected if both taxa had independent distributions.

Analysis of the interspecific aggregation of taxawithin the blocks (scale c) for both samples showedthat the majority of replicate CAB values calculated forall taxa combinations were not significantly differentfrom zero (Table 4). However, 58.3% and 52.4% ofpairwise comparisons for the first and second sampleperiods, respectively, did have one or more significantreplicate CAB values.

Overall, taxa were aggregated independently of oneanother, particularly so at scales b and c and anysignificant associations between taxa tended to bepositive, and only rarely negative. Values for the ratio


of intraspecific to interspecific aggregation AAB werecalculated at each of the three scales for all speciespairs among the nine most abundant taxa (as listed inTable 5) for both the 2- and 4-week sample periods.Values were consistently greater than unity, indicatingthat, for these taxa, intraspecific aggregation wasstronger than interspecific aggregation.

Environmental explanations of aggregation

Following stepwise multiple regressions, the onlyenvironmental variable that had a significant effect onthe overall abundance of organisms across leaf packsfor the 2-week sampling period data was water vel-ocity, explaining only 11.3% of the variation. For the4-week sampling period, mass of additional detritus,stream depth and water velocity were shown to havea significant effect, and together these variablesaccounted for 37.2% of the variation in overall abund-ance of organisms across leaf packs (Table 5). After2 weeks exposure, four of the nine most abundanttaxa were shown to have distributions unrelated toany of the variables. Water depth, velocity and massof additional sediment significantly influenced thedistribution of Chloroperla sp., Protonemura meyeri andOrthocladiinae. Leuctra hippopus and Chironominaeabundance in leaf packs however, was affected by themass of additional detritus (Table 5). In no case didthe percentage of the variation accounted for by theselected variables exceed 31.4%. After 4 weeks expo-sure only Protonemura meyeri had a distribution inde-pendent of the measured environmental variables. Themass of additional detritus was the only variablehaving a significant effect on the abundance of eachof the other eight taxa. Leaf pack mass, water velocity,


Table 5 Multiple regressions and stepwise multiple regressions for all taxa combined and for the nine most abundant taxa, for thefirst and second exposure periods. The coefficient of determination (R2) is the percentage of the total variability in abundance oforganisms per leaf pack attributable to the measured environmental variables as defined by the regression equation. Thedependent variable (Y) in the stepwise multiple regression equations is the log(x 1 1)-transformed abundance of organisms perleaf pack. The selected independent variables are stream velocity (V), stream depth (D), mass of additional detritus (Ad), mass ofadditional sediment (Sed), mass of leaf pack (PM)

14 days 28 daysMultiple regressions Stepwise multiple regressions Multiple regression Stepwise multiple regressionCoefficient of Coefficient of Coefficient of Coefficient ofdetermination determination determination determination(R2) (R2) (R2) (R2)

Significance Equation Significance Equation

All taxa 0.173 NS 0.113 Y 5 2.07 1 0.35V 0.413 P , 0.001 0.372 Y 5 2.43 1 0.17Ad–0.01D 1 0.53V

Baetis muticus 0.071 NS – – 0.274 P , 0.001 0.219 Y 5 0.93 1 0.38AdChloroperla sp. 0.323 P , 0.001 0.291 Y 5 0.55 1 0.02D 0.314 P , 0.001 0.271 Y5 0.20 1 0.19Ad

1 0.01Sed–0.63V 1 0.1PMIsoperla grammatica 0.023 NS – – 0.191 P , 0.006 0.182 Y 5 0.58 1 0.19Ad

1 0.6VLeuctra hippopus 0.157 NS 0.093 Y 5 0.75 1 0.18Ad 0.240 P , 0.001 0.222 Y 5 0.14 1 0.18Ad

1 0.08PMLeuctra inermis 0.121 NS – – 0.324 P , 0.001 0.303 Y 5 0.09 1 0.19Ad

1 0.08PM 1 0.8VProtonemura meyeri 0.357 P , 0.001 0.314 Y 5 0.64 1 1.25V 0.144 P , 0.037 – –Hydropsyche instabilis 0.087 NS – – 0.147 P , 0.032 0.132 Y 5 1.3 1 0.16Ad

–0.001SedOrthocladiinae 0.289 P , 0.002 0.243 Y 5 1.29 1 0.77V 0.294 P , 0.001 0.259 Y 5 2.14–0.002Sed

–0.004Sed 1 0.17Ad–0.01DChironominae 0.176 NS 0.161 Y 5 0.44 1 0.32Ad 0.242 P , 0.001 0.241 Y 5 1.13 1 0.18Ad

1 0.69V–0.01D

–, No independent variables were selected by the regression.

mass of additional sediment and water depth alsohad a significant influence on certain individual taxa(Table 5). Coefficient of determination values, whilston average greater than after 2 weeks, still did notexceed 30.4%.

Multiple regression equations, which did notexclude any of the variables, were calculated for alltaxa at both the 2- and 4-week sample periods andyielded coefficient of determination values of 17.3%(F5,53 5 2.22, not significant) and 41.3% (F5,75 5 10.56,P , 0.001), respectively. Multiple regressions were alsocarried out individually on the nine most abundanttaxa (Table 5). The coefficient of determination valuesindicate that the measured environmental variableswere weak explanatory variables of the patterns ofabundance of six of the nine major taxa after thefirst exposure period. After 4 weeks, however, themeasured variables accounted for a significant propor-tion of the variation in abundance for all nine taxa,


although the maximum R2 value attained for anytaxon did not exceed 35.7% (Table 5).

Discussion

The results of this study show that stream macro-invertebrates colonizing ephemeral patches of leafpacks are distributed among the patches in a non-random manner, therefore the original null hypothesescan be rejected. This aggregated pattern of distributionis evident at each of the three spatial scales of investi-gation and over the two periods of exposure. Almostall taxa analysed exhibited strong intraspecificaggregation, while interspecific aggregation amongtaxa was rare on most occasions, thus the data supportthe alternative hypothesis H1c. Whether the aggrega-tion is due to intrinsic factors, as suggested under theaggregation model for coexistence, stochastic factorsas postulated by Tokeshi (1994), or taxa are partitioning


resources in the traditional sense in relation to differ-ences in environmental conditions among the patches,is considered below.

Factors affecting aggregation

Several factors may have an important influence onthese distribution patterns. However, they mustexplain both the intensity of the aggregation and thefact that it is largely independent among the taxa.

(i) Active factors. The macroinvertebrate distributionpattern across the leaf packs may have been due totaxa actively responding to microhabitat differencesbetween the leaf packs. This would imply coloniza-tion/emigration dynamics with active selection andrejection of packs in the search for favoured conditions.The propensity of independent aggregation couldthen be explained by traditional resource partitioningamong taxa.

After 2 weeks exposure, multiple regressionrevealed that abiotic differences between the leaf packs(velocity and depth) tended to have a significant effecton the distribution of different taxa, but after 4 weeksmass of additional detritus seemed to be more influen-tial. Malmqvist (1993) studied the colonization ofartificial alder leaf packs over a 5-week period andalso found that the mass of additional detritus had asignificant influence on the abundance of colonizingindividuals. However, the measured environmentalvariables explained a relatively small proportion ofthe variation in the abundance of individual macroin-vertebrate taxa across patches. While a significantamount of the variation was explained by the variablesfor more taxa with increased exposure time in thepresent study, the fact that the range of the coefficientof determination values for the nine taxa did notincrease with time suggests that the explanatory powerof the measured environmental variables may havereached their maximum. Ultimately though, around60% of the variation in macroinvertebrate abundanceper pack was left unexplained.

In the present study, all the leaf packs were ofsimilar initial quality and volume but with time theybecame less uniform in the amount of entraineddetritus and sediment. It could be argued that watervelocity measurements were at too coarse a scale tobe relevant to the organisms in the leaf packs or thatother differences in microhabitat quality between leaf


packs, which were not quantified in this study, suchas categories of detritus rather than total detritalbiomass (Drake, 1984) or sediment particle size(Minshall & Minshall, 1977; Culp et al., 1983; Jowett& Richardson, 1990; Holomuzki, 1996), may haveinfluenced the distribution of the taxa at each scale.They may also have diverged with respect to unmeas-ured leaf pack attributes, such as fungal conditioningof the beech leaves or the nutritional quality of thedetritus. These inequalities between leaf packs wouldhave influenced macroinvertebrate distribution to agreater extent as the experiment progressed, but theyare unlikely to provide a complete explanation for thestrength of the observed patterns.

(ii) Intrinsic aggregation. The aggregation model forcoexistence (Atkinson & Shorrocks, 1981; Ives, 1988,1991) states that if species using the same type ofresource differ in their distribution over patches insuch a way that interspecific associations are reducedrelative to intraspecific associations, then species coex-istence is facilitated (Sevenster, 1996). The ratio AAB

assesses the relative strength of the two forms ofaggregation and hence the potential for the data toconform to the above model. The values calculatedfrom this study suggest that intraspecific aggregationis, in fact, more influential than interspecific aggrega-tion in the distribution of the stream benthos amongthe leaf packs. This held true even at the larger spatialextent (scales a and b), where positive associationsbetween taxa were relatively common. The aggrega-tion model proposes that individuals aggregate accord-ing to the intrinsic traits of the taxa, not traditionalresource partitioning. In the present context, differen-tial mobility between taxa and variations in the tend-ency of taxa actively to enter the drift (and to returnto the stream bed) could lead to species-specific colon-ization/emigration rates between leaf packs and hencepotentially result in independent distribution patterns(Hildrew, 1996). Such intrinsic clumping has beenconsidered to lead to a ‘conventional’ partitioning ofresources (Winterbottom, Orton & Hildrew, 1997), butthis is not strictly the same as traditional resourcepartitioning, where taxa actively select and rejectpatches in search of favoured environmental condi-tions. Individuals could also tend to associate posi-tively with conspecifics to facilitate intraspecificinteractions, as has been suggested in terrestrial sys-tems to increase mating opportunities for example


(Holter, 1982), although this obviously does not applyto aquatic larval stages. Intraspecific aggregationshave also been considered as a result of individualsintrinsically avoiding patches containing hetero-specifics so as to reduce competition for ‘enemy-freespace’ (Jeffries & Lawton, 1984).

The aggregation model, however, does not take intoaccount dispersal by the individuals after the initialcolonization of a patch. In studies on terrestrial insects,once the female lays the eggs the distribution patternis set and there is no dispersal between patches, so theinitial strong, independent aggregation is maintainedthroughout the larval period. This is not true forstream macroinvertebrates as, depending on theintrinsic mobility of taxa, they are capable of redistribu-tion throughout the exposure period. Also it shouldbe considered that, from the point of view of thecolonizing taxa, the leaf packs on the stream bed arenot resource islands in an uninhabitable matrix to thesame extent as is the case in similar terrestrial studiesdiscussed earlier. Most of the macroinvertebrates maywell be found in similar or greater numbers in thesurrounding benthos as in the leaf packs. On this basisit would seem unlikely that lotic taxa would maintainas strong a tendency towards intraspecific aggregationand avoidance of interspecific association as is foundin the terrestrial insect assemblages. Yet this studyshows the opposite, with a range of intraspecificaggregation intensities similar to those reported interrestrial studies (Giller & Doube, 1994; Kouki &Hanski, 1995).

It is also possible that the macroinvertebrate distri-bution pattern over the leaf packs could be a reflectionof the local distribution patterns in the benthos. Thelocal benthos may be an important source of indi-viduals, particularly of the more mobile ‘crawling’taxa. A patchy and independently aggregated distribu-tion of taxa in the benthos could lead to a similaraggregation in the associated leaf packs. While inver-tebrates are known to be aggregated in the streambed, there is no evidence to suggest that they areaggregated largely independently of one another, asthey are in the leaf packs in the present study. Evenif this were true, however, one would still need toexplain why the taxa were aggregated independentlyin the stream bed in the first instance. An explanationof the present results based simply on direct coloniza-tion from the immediately surrounding benthos seems


unlikely and other factors are more likely to be playinga role.

(iii) Passive factors. Passive factors, whereby extrinsicprocesses act stochastically on the distribution ofspecies, could also be important. Tokeshi & Townsend(1987) proposed a mechanism in which the stoch-asticity of dispersal influences the distribution ofchironomid larvae on aquatic macrophytes. This modelhighlights how the heterogeneity and unpredictabilityof the lotic system causes the larvae to be continuallyredistributing, resulting in a gradient of aggregationintensities ranging from a quasi-random to a stronglycontagious distribution, therefore enhancing speciescoexistence by reducing the possibility of interspecificinteractions (Tokeshi, 1994).

In the present study intraspecific aggregation wasapparent at all scales, while random dispersal occurredonly rarely and at the smallest spatial scale. The rangeof dispersal patterns found in this study was not asdiverse as that characteristic of the epiphytic chirono-mid community (Tokeshi, 1994), where a substantialproportion of taxa had true random distributions.This suggests that, in the present study, the streamcommunity as a whole is less susceptible to stochasticprocesses, such as drift, than the chironomid assem-blage. Also, drift distances and redeposition for sometaxa are a function of intrinsic behavioural traits ofthe taxa and hence may not be a truly stochasticprocess (e.g. Lancaster, Hildrew & Gjerløv, 1996). Asmentioned previously, such differential mobility couldlead to independent distribution patterns. However,stochastic processes may still have a large influenceon other taxa that have less control over entry intoor redeposition from the drift and may redistributeindividuals among the leaf packs in a more stochasticmanner, contributing to the range of intraspecificaggregation values presented in this study. The relativeimportance of the drift as a source of colonists and asa means of redistribution was not assessed in thisstudy but it has been reported that up to 80% of theindividuals arriving on denuded substrate trays (atrandom or actively) were from the drift, and that taxaexploiting patchily distributed resources such as leafpacks are among the most likely to enter the drift(Townsend & Hildrew, 1976; Mackay, 1992). Overall,though, the ‘stochastic patch dynamics’ model cannoteasily explain why the majority of taxa were largely


aggregated independently of one another or whynegative associations between taxa were so rare.

Effects of scale

With only two sampling periods, temporal changeswere difficult to interpret. There was no consistentchange, across all taxa, in the degree of intraspecificaggregation at any spatial scale between the twosampling periods. The distribution of the macroinvert-ebrates among the leaf packs varied between spatialscales of investigation as predicted by Wiens (1989).As the patch size (grain) was increased, with spatialextent remaining constant, the strength of the intras-pecific aggregation decreased. This may be related toa decrease in the spatial variance as a greater propor-tion of the spatial heterogeneity is contained withinthe patch and lost to the resolution of the analysis(Elliott, 1971; Wiens, 1989). Again, as predicted, whenspatial extent was increased, with patch size heldconstant, intraspecific aggregation tended to increase.Wiens (1989) proposed that, with increasing spatialextent, greater spatial heterogeneity is included in theanalysis and hence between-patch variance increases.While similar support for the predictions of Wiens(1989) has been documented in terrestrial systems(Giller & Doube, 1994), to the authors’ knowledge, thepresent study is among the first in freshwater systems.

The prevalence of interspecific aggregation of taxaamong the leaf packs varied across spatial scales andin time. For both time periods and at each spatial scaletaxa were aggregated predominantly independentlyof one another, but as patch size was increased, withspatial extent held constant, the prevalence of positiveinterspecific associations decreased. An increase in thespatial extent of the analysis, with patch size heldconstant, resulted in a substantial increase in thepercentage of positive associations. Both these patternssuggest that over a large spatial scale the macroinverte-brates tend towards a positive overlap in their distribu-tion, but at the local scale they tend to be distributedindependently of one another. Negative interspecificassociations were rare and decreased to negligibleproportions on the second sampling occasion. Overallthe prevalence of positive interspecific aggregationamong taxa, while remaining low, did increase withtime and spatial scale.

In conclusion, it is apparent from this study thatintrinsic aggregation, in concert with resource parti-


tioning, could be an important structuring force instream macroinvertebrate assemblages associated withleaf packs, as suggested by Moutka (1990) for a filterfeeding assemblage. The relative importance of bothprocesses could not be assessed in this study due tothe fact that they were quantified independently ofone another. If intrinsic traits play a role in thedistribution of taxa in leaf packs, they may also beimportant for the distribution of taxa in the benthosas a whole. While the nature of these traits is unclearat present, further investigations in this area may beuseful in developing an understanding of the structureand function of the zoobenthic community.

Acknowledgments

We would like to thank an anonymous referee forhelpful comments on an earlier draft.

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(Manuscript accepted 4 October 1997)

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Spatial scale and the aggregation of stream macroinvertebrates associated with leaf packs