Ecology. 80(6). 1999. pp. 1873-1882© 1999 by Ihe Ecological Society of America

SOIL MICROARTHROPOD CONTRIBUTIONS TO DECOMPOSITION

DYNAMICS: TROPICAL-TEMPERATE COMPARISONS OF ASINGLE SUBSTRATE

L. HENEGHAN,1 '4 D. C. COLEMAN,1 X. ZOU,2 D. A. CROSSLEY, JR.,1 AND B. L. HAINES3

^Institute of Ecology, Ecology Annex, University of Georgia, Athens, Georgia 30602 USA''•Institute for Tropical Ecosystems Studies, P.O. Box 363682, University of Puerto Rico,

San Juan, Puerto Rico 00936department of Botany, University of Georgia, Athens, Georgia 30602 USA

Abstract. This study examined the effect of soil microarthropods on the decompositionof a single substrate (Quercus prinus L.) at two humid tropical forests (La Selva, CostaRica [LAS], and Luquillo Experimental Forest, Puerto Rico [LUQ]) and one temperateforest (Coweeta Hydrologic Station, North Carolina, USA [CWT]). In this litterbag ex-periment, naphthalene was applied to reduce the microarthropod population density fromhalf of three replicate plots established at each site. This enabled us to quantify the massloss contributed by the fauna (MLCF) at each site and permitted an analysis of the influenceof site-specific differences in the composition of the microarthropod assemblages on de-composition rates. We hypothesized that microarthropod regulation of the microbial pop-ulations involved in leaf litter decomposition would be stronger in humid tropical forests,which experience conditions of low climatic variability. In these conditions, there can bean enhanced degree of biotic interactions between microarthropods and their microbial foodsources. The elevated extent of these interactions should be expressed as a greater influenceof microarthropods at the tropical sites and could result in a site-specific effect of faunalassemblages on decomposition.

Decomposition of the oak litter proceeded faster in Puerto Rican and Costa Rican foreststhan in a temperate forest in North Carolina, USA. Microarthropods had little effect ondecomposition in the temperate forest, whereas their influence was pronounced at tropicalsites. Mass loss of litter from plots with reduced microarthropod populations was similarat the tropical sites. When plots with intact faunal communities were compared, differencesin the tropical sites were apparent, suggesting that there was a site-specific faunal contri-bution to decomposition at these sites.

Oribatid mites constituted a dominant component (41-64%) at each of the sites. Speciesrichness of oribatids and Fisher's alpha diversity were similar in each of the three sites.The Shannon index revealed a lower diversity at LUQ. Abundance of microarthropods waslowest at LAS. Species accumulation curves for each site, though similar in form, weredistinctive, as were diversity accumulation patterns in samples of increasing size. Therewas a positive relationship between species richness and the contribution of the fauna tolitter mass loss within each site. Thus, species diversity of decomposer fauna may haveimportant ecosystem consequences, particularly in warm moist tropical forests.

Key words: biological systems of regulation (BSR); Coweeta Hydrologic Laboratory; decompo-sition; diversity and ecosystem function; La Selva Biological Station; Luquillo Experimental Forest;microarthropods; tropical-temperate contrast.

INTRODUCTION

The decomposition of organic matter and the en-richment of the soil with the labile nutrients necessaryfor plant growth are a biological process operatingwithin the constraints imposed by a range of complexand interacting physical factors (Aber and Melillo1980, Berg and Staaf 1980, Dyer et al. 1990). Soil faunaare contributors to this process and to the maintenance

Manuscript received 2 July 1997; revised 18 May 1998;accepted 1 June 199S.

1 Present address: Environmental Sciences Program, DePaulUniversity, 2325 North Clifton Avenue. Chicago, Illinois60614-3207 USA. E-mail: lhenegha@wppost.depaul.edu

of soil fertility (Swift et al. 1979, Coleman and Cros-sley 1996). In the presence of microarthropods (e.g.,free-living Acari and Collembola), which are prevalentcomponents of this fauna, mass loss of newly senescedlitter increases, amounts of inorganic nitrogen may begreater, and primary productivity can be enhanced(Seastedt 1984, Setala and Huhta 1991). Since few ofthese animals directly consume decaying organic mat-ter, much of the regulation which they impose on de-composition is through their trophic interactions withthe microbial community (Moore et al. 1988, Lussen-hop 1992). As a consequence of these linkages of be-low-ground trophic cascades to ecosystem processes,

1873

1874 L. HENEGHAN ET AL. Ecology. Vol. 80. No. 6

alterations to the microarthropod assemblage structurehave been shown to have significant effects on soilrespiration and the leaching of a wide range of nutrientsfrom laboratory microcosms (Heneghan and Bolger1996).

Contrasts of decomposition between temperate andtropical forests are hampered by the lack of compar-ability in methodologies, the paucity of comprehensivestudies, and the lack of a unifying conceptual frame-work (Swift and Anderson 1989). It is generally ac-cepted, however, that for any given quality of substrate,decomposition in the humid tropics proceeds more rap-idly than in the temperate region (Madge 1965, Laddet al. 1985). The determining factors of litter decom-position rates, namely climate, edaphic structure, re-source quality, fauna, and microbes, come into play inall terrestrial systems, though their relative importancemay vary along a latitudinal gradient. Lavelle et al.(1993) speculated that in the humid tropics, biologicalsystems of regulation, i.e., the mutualistic interactionsof fauna and microbes, are the paramount determinantsof decomposition dynamics for any one leaf type. Thisis because climatic variability is so greatly reduced inthe humid tropics that it represents a constant and nolonger acts as a constraint on biotic activity. This is inmarked contrast to temperate forests, where seasonalclimatic patterns strongly constrain the biota. In a sim-ilar vein, Couteaux et al. (1995) suggest that decom-position should be high at the transition between Med-iterranean and Atlantic climates, where favorable mois-ture and temperature occur simultaneously. Although,in temperate regions, modifications of microarthropodassemblages can influence the availability of N (e.g.,Seastedt and Crossley 1983, Heneghan and Bolger1996), differences in assemblage structure have notbeen shown to influence mass loss of decomposing lit-ter (Andren et al. 1995, Hoover and Crossley 1995).

We looked at the decomposition of a single substrate(Quercus prinus L.), in tropical and temperate sitesunder the influence of divergent microarthropod as-semblages. Assemblages of microarthropods are knownto diverge along a latitudinal gradient, with tropicalsites having more diverse assemblages than do tem-perate ones (Stanton 1979). Puerto Rico was one ofour tropical sites because it was assumed to harbor adifferent diversity of microarthropods compared withCentral American forests (Pfeiffer 1996). Thus in thisexperiment we explored three related issues: First, whatis the role of climate, substrate quality and biotic as-semblages in regulating the decomposition of the cho-sen substrate? This study is unique in that we trans-planted a single substrate, Q. prinus, collected at asingle watershed, and followed its decomposition si-multaneously at tropical and temperate locations. Sec-ond, are microarthropods more influential in decom-position dynamics in the tropics? To examine this wemanipulated the abundance of microarthropod popu-lations in litterbags containing the Q. prinus leaves by

using naphthalene, an arthropod repellent. The warmerand moister soils of the tropics provide optimal con-ditions for microbial growth and for the developmentof strong interactions between the microbes and thefauna which feed on them. We expected, therefore, ac-celerated rates of decomposition at the tropical sitesand an early and consistent faunal influence on the rateof mass loss. Third, is there a relationship between thestructure of the microarthropod assemblages and theircontribution to decomposition,'an important ecosystemfunction? We speculated that, as a consequence ofstronger fauna-microbial interactions, differences inthe microarthropod assemblages at the tropical sitescould result in site-specific differences in the rates ofdecomposition. If this contention is true, then whenmass loss from litterbags from the tropical sites arecompared, the differences between litterbags with mi-croarthropods would be greater than the differencesbetween the litterbags with reduced microarthropodpopulations.

The design of this experiment, where a single sub-strate is compared across sites which differ in microar-thropod diversity, makes it a potential test of the hy-potheses that relate species diversity to ecosystemfunction (Ehrlich and Ehrlich 1981, Schulze and Moo-ney 1993). We enumerated the microarthopod faunasat each of the sites to permit us to evaluate the role ofmicroarthropod assemblage structure in determiningdecomposition of the substrate. Estimates of the di-versity of oribatid mites, the dominant microarthropodgroup, are also presented.

METHODS

Sites

Three sites were used in the experiment: one tem-perate and two tropical forests. The temperate site wasat Coweeta Hydrologic Laboratory (CWT) in westernNorth Carolina (35°00' N, 83°30' W), which is a 2185-ha forested basin containing numerous small water-sheds. Watershed 18, where the experiment was carriedout, is at an elevation of 720 m and has precipitationamounting to 1700 mm/yr (Swank and Crossley 1988).The mean annual temperature is 13°C. The two tropicalsites chosen were Luquillo Experimental Forest (18°20'N, 65°49' W) in Puerto Rico (LUQ) and the La SelvaBiological Station (10°26' N, 83°59' W) in Costa Rica(LAS). The former is a lower montane rain forest witha mean precipitation of 3456 mm and a mean annualtemperature of 22.6°C. The soils are dominated by Zar-zal series that are deep oxisols of volcanic origin (Huf-faker 1995). La Selva is an Atlantic lowland rainforestwith a mean precipitation of 4000 mm and a meanannual temperature of 25.8°C (McDade and Hartshorn1994). The plot chosen was a small patch of secondaryforest dominated by Ochroma lagopus. The soils arealluvial, well drained and fertile. They are classified asa mixed, isohyperthermic, possibly andic, fluventic

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September 1999 SOIL FAUNA AND FOREST DECOMPOSITION 1875

Dystropept (Haggar and Ewel 1997). This site had beencleared as recently as 1991. Although there were somelow-rainfall months at both tropical sites, there wereno extended droughts during the year of this study.

Preparation of litterbags

Recently senesced leaves of Q. prinus were collectedat Coweeta Hydrologic Laboratory. Approximately 3g of air-dried litter were placed in fiberglass litterbagsmeasuring 10 X 10 cm. Three sets of bags were ovendried at 50°C to establish the relationship between air-dry and oven-dry mass. At each site, 144 litterbagscontaining Q. prinus were established in each of threeadjacent plots. Half of the litter bags were treated eachmonth (CWT) or biweekly (LAS and LUQ) with naph-thalene, a biocide that repels microarthropods. The rateof application was 100 g-m~2-mo"' (all sites) and thenaphthalene was mainly distributed around the litter-bags so that fauna would be repelled from the portionof the plots containing these bags. This is because thereis evidence from microcosm studies that naphthalenecan affect microbial activity (Seastedt and Crossley1983, Blair et al. 1989). Although there is no evidencethat application at the rates used in this study affectsmicrobial activity, this caveat must be borne in mindwhen the results are being assessed.

Before being placed in the field at LAS, the litterbagswere surface sterilized with sodium hypochlorite(NaCIO) according to the method of Newell and Fell(1982). This was imposed as a requirement for placingthe nonnative litter there. It was not expected to havelong-term effects on the microbial communities colo-nizing the litter (D. J. Lodge, personal communication).Although it would have been desirable to treat all litterin this manner, we suggest that to achieve the objectivesof the experiment the crucial matter is that there wassuccessful microbial colonization of litterbags at allsites. That this was achieved at all sites is substantiatedby the vigorous decomposition at all sites. Furthermore,other decomposition studies that we have conductedusing local litter types at these three sites show patternswhich are consistent with the conclusions we draw fromthe present study (Heneghan et al. 1998).

Each month, six litterbags (three from naphthalene-treated subplots and three from subplots where the an-imals had unrestricted access) were collected at randomfrom each plot at all sites. Litterbags were oven-driedat 50°C and weighed. The litter was subsequentlyground and subsamples ignited at 500°C to determinethe ash-free dry mass (AFDM).

Inventory of fauna

An extensive survey of microarthropods from CWT.LUQ, and LAS is currently being undertaken by us andother researchers (R Hansen and J. Longino, personalcommunication). Quality collections of these animalsare dependent upon easy access to the forest and uponthe efficiency of the extraction technique. The standard

method for extracting microarthropods, Tullgren fun-nels, relies upon establishing gradients of moisture andtemperature (Crossley and Blair 1991). Because of thisreliance on moisture gradients it was difficult routinelyto obtain samples from the tropics comparable in qual-ity to samples obtained at the temperate site, where wehave a large number of extractors set up in a climate-controlled room. To circumvent this problem, we dis-patched samples from LUQ by overnight mail in cooledcontainers, and these were extracted in our laboratoryin Georgia. Shipping samples from LAS could not beachieved as satisfactorily, so we used funnels there,which had been set up in an air-conditioned laboratory.Estimates of extraction efficiency were determined byheptane floatation (Walters et al. 1987) and were foundto be satisfactory. In this paper we present estimatesof the composition of the microarthropod fauna of thesites. Nine samples of the leaf litter, each measuring100 cm2, were taken from CWT on 30 March 1996,from LUQ on 16 June 1997, and from LAS on 8 April1997. The samples were taken along 50-m transects,and sampling was undertaken in such a way that in-dependent estimates of abundance and diversity couldbe made for each of the three plots at each site. Mi-croarthropods were separated as Collembola, Protura,and oribatid, mesostigmatid, and prostigmatid mites.Oribatids, which were a dominant component of thefauna at each site, were enumerated by morphospecies,and diversity estimates (Shannon [H1] and the log-se-ries [a] index) were made. Evenness was estimatedusing the Shannon evenness index (calculation of allthese metrics is given in Magurran [1988]). We utilized"jackknife" techniques to improve the estimation ofdiversity measurements (Magurran 1988). This tech-nique permits the estimation of a standard error foreach of the parameters measured.

We examined relationships between diversity esti-mates and the mass loss contributed by the fauna(MLCF) by simple linear regression. Relative mass losscontributed by the fauna was calculated as:

MLCF =treated litterbags — untreated litterbags

treated litterbags

where treated litterbags are those to which naphthalenehas been applied. We examined relationships betweenthe diversity of the oribatid mite assemblages andMLCF within-sites (using plot-specific diversity esti-mates and MLCF, i.e., three independent points at eachsite). Because of the low degrees of freedom (df = 1),statistical significance for these tests is very difficultto detect. We report these tentative relationships, how-ever, because of the overwhelming consistency of thetrends.

Statistical analysis

Mass remaining on each sampling date at each sitewas examined with repeated measures analysis of vari-ance. Although we carefully distributed naphthalene in

1876 L. HENEGHAN ET AL. Ecology. Vol. 80. No. 6

the appropriate subplots around individual litterbags,repeated measures analysis of variance provides a con-servative test of the null hypothesis that mass loss oflitter remained unaffected by the reduction of microar-thropods. Preplanned contrasts of decomposition fromtreated and faunated litterbags were performed usingrepeated measures analysis of variance. Proportions oflitter mass remaining were arcsine transformed beforeANOVAs (Sokal and Rohlf 1981).

Difference in abundance and diversity measurementsat the three sites were analyzed using analysis of vari-ance after ensuring that the data met the requirementsfor the test. All statistics were performed using SAS(SAS Institute 1988).

RESULTS

Decomposition

Over the course of the study, decomposition of Q.prinus was influenced by fauna at both tropical sitesbut not at the temperate site. When litterbags with faunawere contrasted with ones from naphthalene-treatedsubplots, there was a trend towards higher mass lossfrom litterbags with fauna at CWT toward the end ofthe first year's collections, though this effect was notsignificant (Fig. la. Table 1). Thirty-three percent ofits original mass had been lost. The mean contributionof the fauna as a proportion of the amount of mass lostfrom the nonfaunated litterbags was 28%, although thevariance in this estimate was high. The faunal influenceon decomposition at LUQ was initially minimal, butbecame conspicuous later in the sampling period (Fig.Ib, Table 1). After 300 d the mass loss was >75%, andthe difference in proportional mass loss between theuntreated and faunally impoverished bags was 37%.An interaction between month and treatment was noted,indicating differences in the trajectories with and with-out fauna (Table 1). Decomposition at LAS was con-sistently affected by the presence of fauna (Fig. Ic,Table 1). Within 30 d of placement in the field, massloss was greater in faunated litterbags; 60% of the sub-strate had decomposed at the end of 281 d, with aproportional contribution of 20% by the fauna. Theabsolute difference in mass loss between litterbags withand without fauna on each sampling date (Fig. 2) ranksthe contribution by the fauna to the decomposition dy-namics as LAS > LUQ > CWT, in terms of the speedof onset of influence and the consistency of the effectover time.

Total mass loss of Q. prinus litter, when comparedacross all sites, provided insight into the influence ofcontrasting assemblages on mass loss (Fig. 3a and b,Table 2). The low rate of decomposition at CWT con-trasted strongly with the rates at LUQ and LAS. De-composition trajectories from naphthalene-treated lit-terbags from LUQ and LAS had a remarkably similarpattern (Fig. 3a), with no significant difference betweenthem (Table 2). The mean mass loss from naphthal-

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70-

60-

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100 200 300 400

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£ 20

0-0

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80-

60-

40-

20-

100 200 300 400

-o— Untreated

••o—• Naphthalene

o—o

0 50 100 150 200 250 300Days

Fig. 1. Mass loss of Q. prinus leaves at: (a) Coweeta Hy-drologic Laboratory (CWT), North Carolina, USA (the lit-terbags were placed in the field on 21 December 1994): (b)Luquillo Experimental Forest (LUQ), Puerto Rico (the lit-terbags were placed in the field on 29 January 1995); (c) LaSelva Biological Station (LAS). Costa Rica (the litterbagswere placed in the field on 10 July 1995). Open diamond =litterbags teated with naphthalene. Open square = untreatedlitterbags into which fauna could freely migrate.

r

September 1999 SOIL FAUNA AND FOREST DECOMPOSITION 1877

TABLE 1. Results of repeated-measures analysis of variance (ANOVA) showing the F values and associated significancefor comparisons of amounts of dry mass remaining at three forested sites.

SiteCoweeta (CWT)

TreatmentTimeTime X treatmentError

Luquillo (LUQ)TreatmentTimeTime X treatmentError

La Selva (LAS)TreatmentTimeTime X treatmentError

df

199

40

199

39

199

39

ss

0.00032.0880.03051.9

0.0123.3020.050380.085

0.770.59630.0200.3774

MS

0.232030.00330.045

0.36680.00550.002

0.0662

0.009

F

0.0146.34

0.68

9.64166.88

2.55

15.66.842.13

P

0.930.00010.5441

0.0360.00010.022

0.02890.00010.06

enate-treated litterbags was 0.2% greater at LUQ.When faunated litterbags were compared, the differ-ence between the accelerated mass loss at LAS and therelatively slower rate at LUQ was apparent (Fig. 3b,Table 2). The mean mass loss from litterbags with faunawas 6.58% greater at LAS. After 281 d in the field, themass loss from litterbags with fauna was 67% at LUQand 64% at LAS.

Inventory of fauna

We have analyzed samples of the fauna extractedfrom the litter on the forest floor at all three sites (Ta-bles 3 and 4). Differences in taxonomic identity andspecies diversity were noted from all sites. Little over-lap existed in the species represented (L. Heneghan et

—0—CWT--A-- LUQ

# 4°-

2 20-

ir 10-o51 0

-10100 200

Days

300 400

FIG. 2. Faunal influence on mass loss of Q. prinus litterat the three sites. This shows the mean contribution of thefauna as a proportion of the li t ter mass remaining on eachsampling occasion.

al. unpublished data). Proportions of oribatid mites inall samples were high, ranging from 41 to 64%. Al-though the proportion of oribatid mites at La Selva waslowest of all three sites, there were elevated proportionsof mesostigmatid mites, many of which were uropo-dine, microbe-consuming species. Thus the largestfunctional component of the fauna of all sites was non-predatory mites. Collembola were the second mostdominant component of the fauna at CWT. They werethe third largest component at LUQ and LAS, wheremesostigmatid mites were the second most numerousgroup (Table 3). Abundance differed with oribatidmites from CWT, being >3 times more numerous thanat LAS, but similar to abundance at LUQ (Table 4).These differences in abundance were reflected in thevery rapid accumulation of additional species with in-creased abundance at LAS (Fig. 4a). Estimated oribatidspecies richness of the study area at all sites was similar(Table 4). This ranged from 60 species at LUQ to 69at LAS and 71 at CWT. Estimated richness at LAS was69 species. All sites had the same levels of diversitymeasured by the log series index, though LUQ had amarginally lower Shannon diversity score (Table 4).Both tropical sites had similar patterns in the diversityscores measured in progressively larger areas (Fig. 4b).Shannon diversity and species richness increased morerapidly at the smallest sample sizes at CWT (Fig. 4band c).

Relationship between diversity and decomposition

Within each site we found a positive relationshipbetween species richness and MLCF at each of the threeplots (Fig. 5). This was marginally significant at CWT(F = 183.04, df = 1, I, P = 0.059) and at LUQ (F =42.09, df = 1, 1, P = 0.097). The relationship betweenspecies richness and MLCF at LAS, though of a similarnature, was not significant (F = 5.55, df = 1, 1, P =0.255) (Fig. 5).

1878 L. HENEGHAN ET AL. Ecology. Vol. SO. No. 6

*^_.Q

100-

90-

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—•—CWT--A-- LUQ--0-- LAS

£ 80-

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FIG. 3. Contrast of mass loss of Q. prinus from litterbags(ash-free dry mass): (a) that had been treated with naphthaleneto reduce the microarthropod population; (b) that had no re-strictions to access by fauna

DISCUSSION

Fauna! influence within sites

Previous studies on the decomposition of Q. prinusleaves at Coweeta had demonstrated that they disappearslowly (>50% remaining after 2 yr) and that there isonly a minimal faunal influence on decomposition rates

(4%) (Cromack 1973, Seastedt 1984). Seastedt et al.(1983) showed that, despite a large increase in the col-onization of litterbags by microarthropods in the sec-ond year, the effect of microarthropods on decompo-sition rates remained the same in both years. We there-fore confirm that this low-quality substrate is relativelyrecalcitrant in the temperate forest, and that the pres-ence of a diverse and abundant microarthropod assem-blage does little to influence its loss.

Observations correlating increased microarthropodabundance with mass loss from a variety of substratesare commonplace regarding both tropical and temper-ate sites (Crossley and Hoglund 1962, Reddy 1995).Evidence of a role for arthropods in accelerating massloss in the tropics comes from the limited number ofstudies where arthropod abundance has been manipu-lated (Swift and Anderson 1989). Reddy and Venka-taiah (1989) manipulated the abundance of microar-thropods in decomposing Eucalyptus leaves in an In-dian forest and reported that decomposition of the leaflitter proceeded more rapidly in litterbags with a coarsemesh compared to loss from fine-mesh litterbags andlitterbags which had been suspended above the ground.

Overall site comparisons

Our study tests the generally held notion that re-source quality exerts an influence on decompositiononly within the limits imposed by the physical milieuof climate and edaphic factors (Swift and Anderson1989. Lavelle et al. 1993, Couteaux et al. 1995). Thatclimate governs the overall rates of decomposition ata geographical scale is apparent from the relationshipbetween actual evapotranspiration and decompositionrates (Meentemeyer 1978, Dyer et al. 1990, Berg et al.1993). Evidence that edaphic characteristics are influ-ential in decomposition dynamics comes from studieswhere litter, transplanted between sites, shows signif-icantly altered rates at the different sites (e.g., Tanner1981). Coleman et al. (1990) showed that the differ-ential grazing intensity of decomposers among thethree temperate ecosystems included in a reciprocal-transplanting experiment generally resulted in in-creased decomposition rates of three leaf types of vary-ing quality.

We have shown in this study that mass loss of Q.prinus. both with the full decomposer assemblage andin the absence of microarthropods, proceeds more rap-

TABLE 2. Contrast between sites for amount of mass loss from bags with naphthalene andwithout naphthalene applications.

Treatment Contrast ssMass loss from naphthalene-treated litterbags

Tropical and temperature sites contrastedTropical sites contrasted

Mass loss from untreated litterbagsTropical and temperate sites contrastedTropical sites contrasted

1.160.000063

1.550.07

368.40.02

946.8847.58

0.00010.89 NS

0.00010.0001

September 1999 SOIL FAUNA AND FOREST DECOMPOSITION 1879

TABLE 3. Abundance and percentage of animals in principal microarthropod groups from litter at the three study sites.Values represent cumulative totals from nine 100-cm: samrples.

CWTt

AnimalsAcari

OribatidsMesostigmataAstigmataProstigmata

CollembolaProtura

Abundance(total 2323)

1430202

11192450

38

%

61.568.690.488.27

19.371.63

LUQtAbundance

i Total 2497)

160147144

184194

3

%

64.1218.87

1.77.377.760.01

LAS§Abundance(total 1436)

5924364432

3320

%

41.2230.363.062.22

23.120

t Sample taken 30 March 1996.+ Sample taken 16 June 1997.§ Sample taken 8 April 1997.

idly in the tropical sites than in the temperate site. Totalmass loss at the tropical sites is comparable («60ft).despite differences in mean annual temperature and :o-tal precipitation at the sites (Heneghan et al. 19vS).The differences between temperate and tropical siresare obvious, indicating that overall climatic constraintsare paramount. However, within the tropical region eth-er factors, including faunal assemblage structure, ip-pear as determinants of decomposition.

Tropical site comparisons

We have shown that there was no detectable differ-ence in the amount of decomposition when litterbigswithout microarthropods were contrasted at tropicalsites. However, there were strong differences betw eenthe tropical sites when decomposition from littertigsto which the microarthropods had access was contrast-ed. This confirmed our hypothesis that decompositionin the tropics can be strongly influenced by the sire-specific characteristics of the local biota.

The two tropical sites were broadly similar in rhetrophic group diversity of their resident microarthrorodpopulations. We chose oribatid mites for diversity es-timations, since they were the most abundant microar-thropod group present at all locations. Furthermore, wehoped that diversity estimates made for oribatid mireswould reflect the diversity of the other microarthrorodgroups that operate on somewhat similar spatial indtemporal scales to the oribatids, e.g., Prostigmata indsome Collembola.

The positive relationship between oribatid species

richness and MLCF, though not always significant, issimilar in form in all cases. It is strongest at CWT,where oribatid mites comprise the largest portion ofthe total microarthropod fauna, and weakest at LAS.where oribatid mites comprise <50% of the fauna. Dif-ferences in slopes between the sites suggest that thereare different consequences of species addition or de-letion across each landscape (Fig. 5). A diminution ofspecies at CWT had Hale effect on mean faunal con-tribution. LUQ is ranked next in the effect of species-richness alterations on mean mass loss. The fauna atLAS, which is most species-poor on small spatialscales, is the most vigorous in terms of consequencesof increased or reduced species richness. The findinghere of the relationship on local scales between diver-sity and process can be checked against diversity es-timates of other components of the fauna as these be-come available.

The stronger contribution from the fauna to massloss at LAS compared to LUQ, even though the twosites have generally similar levels of diversity, pointsto there being an, interaction between the microarthro-pod species and microbial decomposers that is specificto these divergent assemblages. This may be related todifferences in the abundance of the fauna or to thespecies-specific trophic behavior of the animals. Theresponse of microbes to faunal feeding is dependentupon grazing intensity (Hanlon and Anderson 1979).with increased microbial respiration occurring with in-creased grazing up to an optimal level, and thereafterdiminishing. The present work is a demonstration of

TABLE 4. Means (± 1 SE) and ANOVA results for jackkr.ir'e estimates of abundance and diversity of oribatid mite assemblagesin l i t ter at the three audy sites (cumulat ive area = 0.- m- from samples taken on a siagle date (dates as in Table 3).

EstimateSpecies richness (5)Abundance.Shannon (H')Shannon evenness (£)Log series (a)

CWT71.339311

3.6370.74

12.86

± 3.* ">(± 0.i 0.i 0

,52A

D8.94A

.055A

,009AB

.81A

LUQ60.— =232" =

3.206' =0.7-5 =11.97 =

. 97.04A

:).I52B

!).023A

_.103A

LAS68.77 ± 4.37»

738.55 ± 39.73s

3.603 ± 0.081*0.67 ± 0.02^

13.67 ± 2.175''1

ANOVA resultF,.,4 = 1.72.F,,4 = 29.93

.F,.;4 = 5.25,FU4 = 4.61.Fi:4 = 0.09.

P = 0.2. P = 0.000 1P = 0.012P = 0.02P = 0.91

Note: Different superscript letters indicate significant iiffer-snces (Tukey's hsd).

1880 L. HENEGHAN ET AL. Ecology, Vol. 80. No. 6

o-C

60-,

50-

40-

30

20

10

0

a La Selva (LAS)* Coweeta (CWT)A Luquillo (LUQ)

0 200 400 600 800 1000 1200Number of individuals

0.2 0.4 0.6Area sampled

60jc

50-c/:

t 40"W'"o 30"oJD| 20-•£

10-

0

Windsor 1996). This, in turn, may result in the in-creased influence of the microarthropod on decompo-sition amount.

Some other studies have demonstrated that increasedcomplexity of detrital assemblages affects an ecosys-tem-level function. In a laboratory system replicatingthe complexity of a coniferous forest soil, Setala et al.(1991) showed that a more complex fauna, which in-cluded a number of soil biotic elements (nematodes,tardigrades, microarthropods), mineralized more nutri-ents. Other studies examining many substrate typeshave similarly shown an effect of food web complexityon decomposition rates or N dynamics (Couteaux et al.1996, Vedder et al. 1996). We show that a manipulationof microarthropods can have an impact on decompo-sition. This accords well with observations on theunique contribution to decomposition and nitrogen mo-bilization of organisms which are commonly assignedto single functional groups (Faber and Verhoef 1991,Siepel and Maaskamp 1994).

The new paradigm relating biodiversity to aspects ofecosystem function is unusual in being often regardedas meaningless in its broad propositions, but is regu-larly confirmed by experimental manipulations. Hustonand Gilbert (1996) state, "There seems to be little sci-entific basis for arguing that biodiversity per se . . . isimportant for ecosystem functioning." However, manystudies where diversity has been manipulated and theconsequences for an ecosystem function subsequentlyevaluated have found a relationship between the two(Ewel et al. 1991, Naeem et al. 1994, Tilman andDowning 1994). Other studies, both comparative sur-

LAS:v = 0.050.t-1.126 r2 = 0.847LUQ: y = 0.019.x + 0.042 r2 = 0.670CWT: y = O.OOS.r - 0.103 r2 = 0.983

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Cumulative sample area (m2)

FIG. 4. (a) Species accumulation curves for the threestudy sites, (b) Species area curves for the three study sites,(c) Changes in diversity (measured with Shannon index) withincreasing area sampled at the three study sites.

pattern. The elucidation of mechanism, however, willrequire the direct observation of the trophic ecology ofthe assemblages. Particularly relevant here will be acomparison of the extent of fungivory at the three sites.The high moisture and warm temperatures which favorfungal growth may in turn increase the preponderanceof fungal grazers in tropical systems (Levings and

g 0.6 H

oc<u0.4-

0.2 ̂

0-

-0.2

• CWT

10 20 30 40 50Species richness

60

FIG. 5. Relationship between species richness at each ofthe three replicated plots at each site and the difference be-tween mass loss from naphthalene-treated and untreated lit-terbags on the date of diversity estimate.

September 1999 SOIL FAUNA AND FOREST DECOMPOSITION 1881

veys and experimental manipulations, in contrast, re-port no relationship (Wright 1996, Wardle et al. 1997).

It is important that the ambiguity of the generalclaims about diversity and function be tempered bydetailed knowledge of the systems being examined.Each taxonomically defined set of organisms may par-ticipate in a multitude of ecosystem processes. Lodgeet al. (1996), for instance, identify 11 key ecosystemprocesses that require the involvement of microorgan-isms. The extent to which taxonomic diversity inter-sects with functional diversity clearly will depend uponthe processes examined and the scale at which the in-teractions are measured. We conclude that discussionsof the linkages between diversity and ecosystem func-tion be leavened by a consideration of the interactionof the fauna and their local environment. Furthermore,it is apparent that two assemblages that appear quitesimilar, as in the case of LAS and LUQ microarthro-pods, may function in a way not easily predicted fromstatic measurements of diversity.

The results that we report have two important func-tional implications. First, the presence of site-specificfauna! effects on decomposition argues for greater con-sideration of the effect of modified land-use practiceson the decomposer organisms in tropical ecosystems.If feedback between perturbed detrital foodwebs andecosystem processes occurs, this indirect consequencewill have a substantial effect on tropical soil fertility.Second, in scenarios of global climate change whereelevations of atmospheric trace gases are linked tomodifications of temperature and precipitation overbroad geographical regions, the climatic constraints ondecomposer activity may be relaxed. This can, as wehave demonstrated here, result in a greater site-specificeffect of micoarthropods on decomposition.

ACKNOWLEDGMENTSThis research was supported by NSF grant DEB-94I68I9

to the University of Georgia. We thank members of the Huer-tos project at La Selva, Dr. John J. Ewel, Ankila Hiremath,Seth Bigelow, and Selvino Villegas. The Organization forTropical Studies (OTS) facilitated the work at La Selva andprovided help with processing the necessary permits. Wethank Dr. John Connolly for discussion on the statistical treat-ment of the data. We are very grateful for comments on thismanuscript from John Johnston, Christien Ettema, and TomBolger. The manuscript benefited from thoughtful commentsfrom the editor and two anonymous referees. The task ofpreparing the data was made much easier by excellent helpin the laboratory from Katharine Dowell.

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