Distribution patterns of the litter macrofauna in agroforestry
and monoculture plantations in central Amazonia as
affected by plant species and management
Katrin Vohland
a, GoÈtz Schroth
b,*aMax-Planck-Institute for Limnology, Working Group Tropical Ecology, P.O. Box 165, PloÈn, Germany
bInstitute of Applied Botany, University of Hamburg, c/o Embrapa AmazoÃnia Ocidental, C.P. 319, 69011-970 Manaus-AM, Brazil
Received 3 June 1998; received in revised form 22 March 1999; accepted 22 March 1999
Abstract
Within heterogeneous land-use systems such as agroforestry or mixed tree crop plantations, the different morphological and physiological characteristics of the plants, together with their species-speci®c management, lead to a mosaic of different living conditions for the litter fauna. This may, in turn, in¯uence the processes of decomposition and nutrient cycling and, thus, the growth-conditions of the plants. We studied the effect of different plant species on the litter macrofauna by collecting macro-invertebrates in the litter layer of an agroforestry system composed of four regionally important tree crop species in central Amazonia: cupuac,u (Theobroma grandi¯orum), annatto (Bixa orellana), Brazil nut (Bertholletia excelsa) and peach palm (Bactris gasipaes), with the leguminous cover crop Pueraria phaseoloides and spontaneous grasses as soil cover. The agroforestry system was studied at two fertilisation levels and was compared with a peach palm monoculture. We found invertebrates belonging to 18 orders and 44 families. The number of fauna individuals per unit area differed signi®cantly between plant species within the agroforestry system. Both the number of individuals and the faunal biomass increased linearly with litter dry matter per unit area. The Shannon±Wiener index of family diversity showed a non-linear increase with increasing sampling area and litter mass in the samples, approaching saturation values between 2 and 2.5. The highest faunal abundance and diversity was found in the litter of the peach palm monoculture, apparently due to the stable and protected habitat provided by the ¯eshy offshoot remains from the palm harvests. The two fertiliser levels only differed with respect to two invertebrate groups, snails and isopods. The results indicate that the creation of a litter and/or mulch layer of at least 3 Mg haÿ1
, but preferably 6 Mg haÿ1
and the association of tree and cover crop species whose litter has a favourable effect on the fauna with species whose litter has a less favourable effect are suitable management tools for the conservation of an abundant and diverse litter fauna in plantations and agroforestry systems of the humid tropics.#1999 Elsevier Science B.V. All rights reserved.
Keywords:Agroforestry; Amazonia; Bactris; Bertholletia; Bixa; Faunal abundance; Faunal diversity; Litter; Perennial crops; Pueraria;
Theobroma
*Corresponding author. Tel.: +55-92-622-20-12; fax: +55-92-622-1100; e-mail: [email protected]
1. Introduction
Heterogeneous land-use systems such as agrofor-estry or mixed tree crop plantations are characterised by a more or less regular spatial arrangement of the plant species present. Alley cropping systems or plantations of perennial crops with a monospeci®c shade tree layer may contain only two principle plant species in a regular pattern, whereas tropical home-gardens may be highly diversi®ed species associations with a forest-like structure (Nair and Muschler, 1993). The different morphological and physiological char-acteristics of plant species such as their leaf mass, turnover and decomposition rates, together with spe-cies-speci®c management practices including shoot pruning, lead to distinct spatial patterns with respect to the quantity and quality of the litter layer in hetero-geneous land-use systems. Furthermore, crop species in such systems differ in the amount of ground shading which may in¯uence the activity of soil and litter fauna (Kang et al., 1994), in the redistribution of rain water passing through their canopy (Schroth et al., 1999b) and in the required fertilisation rates. Together, these factors create a mosaic of different living con-ditions for organisms dwelling in the litter layer. Badejo et al. (1995) showed that different types of tree mulch in¯uenced the microarthropod community in maize plots, apparently through their different chemical composition and their in¯uence on soil temperature and moisture conditions. Perfecto and Vandermeer (1996) reported changes in the diversity of the ant community in coffee plantations due to differences in shading and leaf litter. Accordingly, the spatial heterogeneity of the litter layer in mixed tree crop plantations should lead to small-scale differences in the composition and activity of the litter-dwelling faunal communities.
These litter-dwelling organisms are of crucial importance for the growth conditions of the crop plants and the sustainable functioning of agroecosys-tems through their role in litter decomposition and concurrent nutrient release (Swift et al., 1979; Reddy, 1992; Henrot and Brussaard, 1997; Tian et al., 1997, 1998). An abundant and active litter fauna may thus help to ensure rapid recycling of nutrients from dead plant materials. This is particularly important under low-input conditions and on infertile soils. Of course, not all the fauna living in the litter layer is directly
involved in litter decomposition. Some groups are predators whose feeding activity may be important for regulating the abundance of other groups, includ-ing plant feeders and potential crop pests. So, the aim of agroecosystem design and management could be to increase the abundance of some groups (e.g., detriti-vores, fungivores) and to reduce the abundance of other groups (e.g., pest species), eventually using predators as biological control agents. This, however, would require much more detailed information about the ecological roles of the faunal groups involved and, especially, their interactions under the speci®c condi-tions of the respective agroecosystem than what is presently available, especially in the humid tropics. So, for the time being, a good principle for the design and management of tree crop plantations and other land-use systems in the humid tropics may be to aim for ahigh abundance and diversityof the litter-dwell-ing organisms, taklitter-dwell-ing care that potential crop pests do not become too numerous. This strategy seems to be adequate both for reducing the probability of pest outbreaks (Stamps and Linit, 1998) and for combating species loss through agricultural intensi®cation (Per-fecto and Vandermeer, 1996).
The aim of this study was to determine crop species and management combinations which favour a high abundance and diversity of the litter dwelling macro-fauna in heterogeneous land-use systems in the humid tropics. We collected and identi®ed the macro-inver-tebrates in the litter layer of an agroforestry system composed of four locally important tree crop species in central Amazonia, Brazil. Two fertilisation levels and a monoculture of one of the species were included in the study to assess the effects of management intensity and composition of the plant communities on the macrofauna. The study was part of a larger project on the recovery of abandoned land through site-adapted polyculture systems with perennial crops.
2. Methods
2.1. Study site
The study was conducted on the research station of Embrapa AmazoÃnia Ocidental near Manaus, Brazil (3880
S, 598520
2622 mm, air temperature of 268C and atmospheric humidity ca. 85% (mean values 1971±1993, O.M.R. Cabral and C. Doza, unpublished). The driest months are July to September, and the wettest months are February to April. The soil is a Xanthic Ferralsol according to the FAO/Unesco classi®cation (FAO/ Unesco, 1990) with a clay content of ca. 80% and a pH in water of 4.5 at 0±10 cm depth. Detailed soil data are given by Schroth et al. (1999a). The study site was cleared from primary rainforest in 1980. In 1981, an experiment with rubber trees (Hevea brasiliensis) was established, which was abandoned in 1986. The devel-oping secondary forest was manually cleared in 1992 and the vegetation was burnt on the site. The experi-mental plots were planted in the rainy season 1992/ 1993.
The centerpiece of the study was an agroforestry system with four locally important tree crop species: Peach palm (Bactris gasipaes, Arecaceae) for the production of heart of palm (palmito); cupuac,u ( Theo-broma grandi¯orum, Sterculiaceae), a small tree related to cacao whose fruit pulp is widely used in (and increasingly outside) the region for the prepara-tion of juice, ice cream and sweets; the Brazil nut tree (Bertholletia excelsa, Lecythidaceae) which, beside the well-known nuts, produces excellent wood; and annatto (Bixa orellana, Bixaceae) which is widely cultivated in the tropics for its non-toxic red dye. Plot size was 3248 m. The trees were grown in rows
with 4 m spacing between the rows (Fig. 1). A row with peach palm (at 2 m spacing within the row) alternated with a mixed row of cupuac,u and Brazil nut (at 6.7 m spacing between the trees within the row), a row of annatto (at 4 m spacing within the row) and again a mixed row of cupuac,u and Brazil nut, after which the next row of peach palm followed. Between the trees, Pueraria phaseoloides (tropical kudzu, Fabaceae) was sown as a cover crop or developed from residual seed from the former rubber plantation. The agroforestry system was studied at two ferti-lisation levels, full fertiferti-lisation according to local experiences, and 30% of this fertilisation level (`low input plots'). The low input plots received no N fertiliser since May 1996, about one year before the study. For comparison, a peach palm monoculture was included in the study, which was planted at 22 m
spacing and fertilised at the same rate per tree as the peach palm in the agroforestry plots with full
fertilisa-tion. In November/December 1996, three months before the study, the fully fertilised plots and the monoculture, but not the low input plots, were limed with 2.1 Mg haÿ1of dolomitic lime. The plots were arranged in a randomised complete block design with three replications.
The Brazil nut trees were the largest of the four tree species with a height of ca. 6±8 m. The cupuac,u trees had a height of ca. 3 m and a shrubby growth habit. Both species have relatively scleromorphic leaves. Annatto was cut back annually at 1.5 m height and had approximately the same size as the cupuac,u trees. Its litter consisted not only of the soft leaves, but also of branches and many fruit shells. From the peach palms, the palmito was harvested by cutting the off-shoots three times per year when they reached 8 cm diameter at 1 m height. The palmito was extracted directly in the ®eld, and the remains consisting of the older and outer parts of the offshoots were left on the soil where they formed most of the litter layer of this species. The last palmito harvest before the litter sampling had been in October 1996 (412months before the sampling).
2.2. Sampling
Litter samples were collected from 28 February to 11 March 1997, during the second half of the rainy season. In each plot, we collected one litter sample from each of the tree species present, i.e. peach palm,
cupuac,u, Brazil nut and annatto in the six agroforestry plots, and peach palm in the three monoculture plots. In the agroforestry plots, we also collected one litter sample per plot from an area covered byPuerariaat 2±3 m distance from the neighboring trees, and one sample from a similar area covered by spontaneous grass growth. In the monoculture plots, there was little ground vegetation because of the shade and presum-ably the root competition from the palms.
To obtain a representative sample for the litter of a given tree, we sampled a triangular area under the tree with the trunk at one corner and the other two corners at 50 cm distance from the trunk and at 50 cm distance from each other. Sampling area was 0.11 m2per tree. For the peach palm, this sampling scheme seemed less appropriate because of the close spacing of the palms within the rows (2 m). So, we collected here a rec-tangle of 100 cm length in the direction of the palm row by 50 cm width, with the respective palm at one corner. The sampling area was 0.5 m2per tree. The larger sampling area was used in this case because of the much more heterogeneous distribution of the coarse palm litter in comparison with the litter of the dicot trees. Similar sampling areas were used for the cover crop and the grass which also had a heterogeneous litter because of the variable growth of the soil cover species themselves and some in¯uence from the litter of surrounding trees.
The sampling areas were marked and the borders cut with a knife. The whole litter was rapidly put into a bowl to avoid losses of mobile fauna, and the macro-fauna was sorted out by hand and stored in ethanol. Wet weight and humidity (by drying at 808C for 48 h) of the litter were determined and the dry weight calculated. The fauna was identi®ed to the family level. Then, it was dried at 608C and weighed. Ants and spiders were not quanti®ed in the samples, because we considered them too mobile and too spotty in their distribution to be assessed with this sampling method. However, there were very few individuals of both groups in the samples.
2.3. Data analysis
For every sample, the water content of the fresh litter and the litter dry weight were determined and the number of fauna individuals per m2and per g of dry litter was calculated. The data were checked for
normality of distribution, homogeneity of variances and independence of means and variances, and the fauna data were log-transformed because of positive correlations between means and variances (Little and Hills, 1978). To analyse the spatial structures of litter and litter fauna within the agroforestry plots, an analysis of variance was conducted for a randomised complete block/split plot design with the fertilisation level as main plot treatment and the plant species as subplot treatment for the different sampling positions within the agroforestry systems, without considering the monoculture treatment (Little and Hills, 1978). To investigate if the system context of a plant species had an in¯uence on the litter and litter fauna, the sampling positions in the peach palm monoculture plots were then compared with the peach palm positions from the agroforestry plots at high and low fertilisation by ANOVA for a randomised complete block design. In case of signi®cance of the F-test at p< 0.05, the means were compared by least signi®cant difference tests at the same level of signi®cance. From the families present in each sample, the Shannon±Wiener index of diversityHwas calculated (`family diversity' in Fig. 4).
3. Results and discussion
3.1. Litter quantity and humidity
The amount of litter per m2 did not vary signi®-cantly between the plant species within the agrofor-estry system, although on the average, Brazil nut had the highest and grass had the lowest litter mass (Table 1). The peach palm in the monoculture had a much higher litter mass than in the agroforestry plots, obviously because of the higher number of palms per unit area. The difference was signi®cant at p< 0.06 (Table 2).
nut. The litter of the peach palm monoculture had the highest water content from all sampling positions, the difference to the peach palm litter in the agroforestry plots being signi®cant (Table 2). This was because the offshoot pieces tended to be larger and the litter layer was more shaded in the monoculture than in the agroforestry plots. The fertiliser level and the spe-cies±fertiliser interaction had no effect on these vari-ables (data not shown).
3.2. Abundance and distribution of the litter fauna
The fauna collected in the litter samples belonged to 19 orders and 44 families. The number of families
encountered in each litter type is given in Tables 1 and 2. Diversity patterns are discussed below. The most numerous groups were beetles and millipedes with 10 families each and bugs with eight families. Termites were absent from the samples. Table 3 gives the scienti®c and common names of the encountered groups as well as information on typical size and feeding habits as far as available.
In the agroforestry plots, the number of individuals per unit ground area and the faunal biomass per unit area differed signi®cantly between plant species, with annatto and peach palm at the higher end and cupuac,u at the lower end of the observed range (Table 1). The same trend was observed for the number of individuals
Table 1
Comparison of litter and fauna under tree and ground cover species in an agroforestry system in central Amazonia (means of high and low fertilisation levels)
Litter mass (g mÿ2)
Litter humidity (% of DM)
Faunal densitya Faunal biomassa Number of
familiesb
(individual mÿ2) (individual gÿ1) (mg mÿ2) (mg gÿ1) Peach palm 314 170b 108a 0.366 330 1.28 27
Annatto 400 208b 147a 0.396 557 1.69 16
Brazil nut 427 131b 117ab 0.260 724 1.59 12
Cupuac,u 306 117b 39c 0.129 294 0.65 7
Pueraria 382 326a 106ab 0.325 679 1.88 28
Grass 262 351a 42b 0.195 246 1.31 24
F 0.50 7.35 3.49 2.06 1.67 1.39
p 0.773 <0.001 0.020 0.113 0.187 0.269
Note: The fertiliser effect and the fertiliser±species interactions were non-significant in all cases. Numbers followed by the same letter in one column are not significantly different atp< 0.05 (LSD test). The number of families is given for all six replicate samples together.
aThe statistical analysis of the fauna data was carried out after log-transformation.
bThe sampling area differed between tree species, see discussion of diversity patterns in the text.
Table 2
Comparison of peach palm (Bactris gasipaes) litter and fauna in agroforestry with high and low fertilisation and in monoculture in central Amazonia
Litter mass (g mÿ2)
Litter humidity (% of DM)
Faunal densitya Faunal biomassa Number of families (individual mÿ2) (individual gÿ1) (mg mÿ2) (mg gÿ1)
Polyculture low 394 167b 115 0.290 230 0.57 19 Polyculture high 234 172b 101 0.443 429 2.00 23 Monoculture 605 444a 295 0.452 1353 2.11 27
F 6.31 7.17 3.87 0.90 4.13 1.80
p 0.058 0.048 0.116 0.475 0.106 0.277
Note: Numbers followed by the same letter in one column are not significantly different atp< 0.05 (LSD test). The number of families is given for all three replicate samples together.
Table 3
Groups of fauna encountered in litter samples from agroforestry and monoculture plots in central Amazonia, with typical length of adult individuals and feeding habit according to literature information and own observations
Order Common name Family Length (mm) Feeding habita Na Ref.b
Oligochaeta Earthworms Glossoscolecidae 30 geo 44 11 Archaeognatha Jumping bristletails Meinertellidae 4 sap fung graz phy 26 2, 17 Chilopoda Centipedes Geophilomorpha 8±12 pred 9 3 Symphyla Symphylans Scutigerellidae 1 graz? r l 40 8 Diplopoda Millipedes Polyxenidae <5 fung? graz? l? 65
Siphonotidae 5 fung? graz? l? 77 Siphonophoridae 4 sap? fung? 3
Spirobolidae 3 l w 7 18
Spirostreptidae 15 l w 13 7, 15
Chelodesmidae 12 l w 69 18
Cryptodesmidae 4 graz? l? 1 18
Fuhrmannodesmidae 3 2
Platyrhacidae 50 w 22 18
Pyrgodesmidae 4 s l graz 327 1 Blattodea Cockroaches Blattidae 7 sap l 6 3, 9 Coleoptera Beetles Chrysomelidae 4 phy r 1 3, 6 Curculionidae 7 p r fung 2 3, 6 Eucnemidae 8 fung w? 17 3, 6 Elateridae 7 sap pred phy 32 3, 6
Hydrophilidae 5 sap 1 3, 6
Micromalthidae 5 w 1 3, 6
Scolytidae 3 w 1 3, 6
Silvanidae pred 1 3, 6
Staphylinidae 3±4 sap fung phy pred 9 3, 6 Tenebrionidae 5±15 fung w 15 3, 6
Diptera Maggots ? (Larvae) 9 sap 31 3, 4
Thysanoptera Thrips Phlaeothripidae 2 fung phy pred 6 3, 13, 16 Heteroptera Bugs Corcidae 5 pred phy 2 3, 14
Cydnidae 7 r 5 3, 14
Gelastocoridae 2±4 pred 5 3
Largidae phy 1 3, 14
Lygaeidae 5 pred phy 6 3, 14 Pentatomida 7 pred phy 15 3, 14
Podopidae 4±6 phy 1 3, 14
Scutelleridae 5 phy 1 3
Auchenorryncha Cicada CercoÂpidae 4 phy 17 3 Sternorrhyncha Aphid Phylloxeridae 2 phy 1 3
Hymenoptera Ant Formicidae 2.5 l 19 3
Dermaptera Ear-wig Labiidae 5 l 2 3
Lepidoptera Caterpillar ? (Larvae) 10 phy 19 3 Opiliones Harvestman Palpatores 2 pred 2
Isopoda Woodlice Scleropactidae 7 sap fung l 426 10, 12 Philosciidae 3 sap fung l 106 12 Gastropoda Snails (with shells) 7 fung phy 86 5
aAbbreviations: Nindividuals ecountered in this study; geogeophageous; sapsaprophagous; fungfungivorous; grazgrazing;
lleaf litter; wwood (coarse litter); rroots; phyphytophagous; predpredaceous
bReferences: 1 (Adis, 1986), 2 (Adis and Sturm, 1987), 3 (Borror et al., 1981), 4 (Brauns, 1954), 5 (Burch and Pearce, 1990), 6 (Costa et al.,
per unit litter weight, although the differences were not signi®cant in this case (Table 1). The faunal biomass per unit area and per unit litter weight were highest under Brazil nut, annatto andPueraria, but the differences were not signi®cant (Table 1).
Faunal density and biomass under peach palm did not differ signi®cantly between the cropping systems, although the monoculture had a much higher mean density and biomass per unit area than the agroforestry plots corresponding to its higher litter mass. Among the peach palm litter samples, the lowest faunal den-sity and biomass per unit litter weight were found in the agroforestry plots with the lower fertilisation level (Table 2).
Comparing the litter from all plant species, both the number of individuals and the faunal biomass per unit ground area were linearly related to the litter dry weight per unit area. The relationship was closer for faunal biomass than for faunal density (Fig. 2). The regression curves in Fig. 2 met thex-axis at a litter mass of 210±220 g mÿ2, suggesting that at lower values little macrofauna would have found suitable living conditions in the litter. Accordingly, several litter samples of cupuac,u, the species with the lowest litter mass in this study, contained no macrofauna at all. This indicates that under the conditions of this study, it is necessary to maintain a litter (or mulch) layer of at least 300 g mÿ2(or 3 Mg haÿ1) in a tree crop plantation to avoid the loss of many or most macro-invertebrates from the litter layer. Twice this
amount would be necessary to increase the abundance of the litter macrofauna to levels comparable to those encountered in the peach palm monoculture (Fig. 2).
3.3. Distribution of individual fauna groups
In addition to these overall abundance patterns, some macrofauna groups showed speci®c preferences for certain litter types, especially for those of peach palm,Puerariaand annatto. With an average of 25% of the fauna individuals, the peach palm samples from both poly- and monocultures contained signi®cantly more sapro-fungivores than the other samples (4% on the average), indicating moist living conditions within the decomposing offshoot remains. Diptera larvae, a group which prefers humid to wet environments because many of its members are fungus-feeding, were present in roughly half the litter samples from peach palm andPueraria, but in none of the samples from cupuac,u, Brazil nut and grass (Fig. 3). Earth-worms (Oligochaeta) were most abundant inPueraria, peach palm and annatto litter, but were also absent from cupuac,u and Brazil nut litter (Fig. 3).
The millipedes were also most abundant in peach palm, annatto and Pueraria (low fertilisation) litter (Fig. 3). They belonged to several different groups: the second most abundant and constantly occurring family were the pyrgodesmids (21% of all collected individuals), rather small-sized polydesmid millipedes which are probably grazers. In general, the highest
densities of pyrgodesmids were found in the low fertilisation plots under peach palm in both mono-culture and agroforestry systems and underPueraria. A further small sized millipede group, the presumably fungus-feeding polyxenids, reached its highest abun-dance in the litter of the peach palm monoculture but was absent from cupuac,u and grass litter. Peach palm litter also offered a preferred habitat for larger sized millipede families, such as chelodesmids, pla-tyrhacids, spirobolids and spirostreptids. Some of these are known to feed on coarse litter, such as woody debris.
The symphyla, which belonged all to a single species (Hansiella orientalis), did not occur in the litter of Brazil nut and cupuac,u and that of the lower fertilised annatto, but throughout the rest of the sys-tem. Beetles (Coleoptera) were also absent from cupuac,u litter and, with the exception of one sample with Staphylinidae, from the Brazil nut litter.
A few groups had other preferences. Bugs and thrips occurred almost exclusively in grass litter, whereas caterpillars were most frequent under Brazil nut.
Only few groups seemed to respond to the fertilisa-tion level. The Gastropoda, which were represented by snails with shells, occurred in the low fertilisation treatment only under peach palm and Pueraria, whereas in the fully fertilised and limed plots, they
could be found in high numbers all over the system, with the exception of the annatto litter (Fig. 3). As snails are known to have high demands of calcium for building their shells, it is likely that the liming of the plots was the factor in¯uencing this distribution. The plant suckers (Heteroptera, Auchenorrhyncha, Ster-norrhyncha) and phytophagous insects (Thysanoptera) occurred in the cupuac,u and Brazil nut litter only under full fertilisation, but not under low fertilisation. In the other litter types, this group was found at both fertilisation levels.
The isopods were the most abundant group in the study with 34% of all collected animals, occurring in high numbers at nearly all sampling sites. Two species of differing size could be easily distinguished, the larger one belonging to the Scleropactidae ( Circonis-cussp.) and the smaller one to the Philosciidae. The smaller species only occurred in the low fertilisation treatment, with the exception of the peach palm litter where it occurred in both fertilisation treatments. The larger species occurred at both fertilisation levels, but was also more frequent in the lower fertilisation treatment. The reason for this distribution is not clear. Competitive exclusion of the isopods by organisms which preferred the higher fertilisation treatment seemed to be contradicted by the generally high abundance of this group in the samples.
3.4. Diversity of the litter fauna
Diversity values depend on the sampling area which differed between the plant species and cropping sys-tems in this study (larger sampling area for more heterogeneous litter types, see Section 2). Moreover, there was a strong effect of the litter quantity on the number of fauna individuals in the samples (Fig. 2). As a consequence, comparisons of mean faunal diver-sity at the different sampling positions would have been biased in favour of larger samples with a higher litter mass per unit area, irrespective of any speci®c effect of the litter type on the faunal diversity. To identify such speci®c effects, the family diversity was plotted against the litter sampling area (Fig. 4(a)) and the litter dry matter (Fig. 4(b)). To see how much the diversity index increased with increasing sampling area and litter mass for each litter type, both the mean diversity per sample and the diversity of the combined replicate samples for each litter type were included in the graphs. As the fertiliser level had no in¯uence on the faunal diversity (data not shown), only mean values of both fertilisation levels are shown.
All data points together approximated saturation curves of increasing diversity with increasing size of the samples (Fig. 4(a) and (b)). The lower sample sizes (i.e., means of individual samples of cupuac,u, Brazil nut and annatto litter) lay in the increasing part of the curve, whereas the highest values (i.e.,
com-bined replicates of Pueraria, peach palm and grass litter) seemed to approach a saturation value of the diversity index somewhere between 2 and 2.5.
Despite this common pattern for all data points together, Fig. 4 shows pronounced differences in fau-nal diversity between the investigated plant species. At the lower end of the curves, the average faunal diver-sity in a sample of cupuac,u litter was far lower than in a sample of Brazil nut and, especially, annatto litter collected from a similar area (Fig. 4(a)) and with an approximately similar litter quantity (Fig. 4(b)). The difference between annatto and cupuac,u was signi®-cant atp< 0.05. The diversity values of the combined samples of the six trees per species, on the other hand, were very similar (H1.5; Fig. 4(a) and (b)), because
increasing the sample size increased the diversity much more for cupuac,u than for annatto. This indi-cates that on a small scale, the litter fauna of cupuac,u was sparse and rather species-poor, but the fauna differed between litter samples collected from differ-ent trees of this species, so that the population of cupuac,u trees had a relatively diverse litter fauna. The small-scale diversity of the litter fauna of annatto, in contrast, was relatively high, but as the fauna differed little between samples from different trees of this species, the diversity of the litter fauna of the popula-tion of annatto trees was only little higher than that of the population of cupuac,u trees. For Brazil nut, the situation was intermediate.
A similar tendency as for cupuac,u and annatto could be observed for the two soil covers,Puerariaand grass (Fig. 4). Comparing the mean diversity values of the individual samples, in both cases with a sampling area of 0.5 m2and a mean litter dry matter of ca. 200 g per sample, the litter fauna under Pueraria was more diverse than that of the grass vegetation, but for the combined litter of the six replicate samples the faunal diversity of the grass litter was even slightly higher than that of thePuerarialitter. So, while the almost monospeci®c Pueraria soil cover appeared to offer more favorable living conditions for a diverse popula-tion of litter fauna on the scale of a 0.5 m2sample, probably due to its denser shade and the more N-rich litter, this fauna changed little in composition between different sampling points in the ®eld. The grass litter fauna, on the other hand, had a lower diversity at a small scale, but the faunal composition changed from spot to spot, presumably re¯ecting the variability of the spontaneous vegetation itself. For a larger plot, there would presumably be little difference in faunal diversity between spontaneous vegetation and intro-duced cover crop. However, investigations into the consequences of the differing small-scale diversity of the fauna on litter decomposition processes might be worthwhile.
The highest faunal diversity was found in the litter of the peach palm monoculture (Fig. 4). Here, the faunal diversity in a 0.5 m2 sample with 300 g of
litter was higher than that of samples from a sixfold larger area from the agroforestry plots. Several of the rarest groups (with only a single individual) were only detected in the litter of the peach palm monoculture, such as the beetle families Chrysomelidae, Elateridae and Micromaltidae, and the bug families Gelastocer-idae and PentatomGelastocer-idae. This favourable effect of the peach palm litter on the fauna was apparently due to the moist and ¯eshy offshoot remains at different stages of decomposition which effectively protected the inhabiting fauna from solar radiation and desicca-tion. Diplopods were especially prominent in this litter. In the primary forest of the region, fallen and decomposing logs on the forest ¯oor play a compar-able role for diplopods and other sensitive fauna groups (K. Vohland, personal observation). The fauna communities in the three replicates from the peach palm monoculture were very similar, indicating that a 0.5 m2 (or 300 g) sample of litter from a single
sampling point already contained most of the faunal diversity to be expected in larger samples from more different sampling points (Fig. 4).
4. Conclusions
This study showed that the litter-dwelling macro-fauna of a mixed tree crop plantation in humid tropical Amazonia was characterised by a mosaic of species distribution and diversity. Sampling positions only a few meters apart differed in the composition and abundance of their litter fauna, depending on the species of tree crop or ground vegetation whose litter dominated at the respective point in a plot. Such `single-tree patterns' of the distribution of soil biota (and the processes of litter decomposition) have also been observed in the rainforest of French Guyana (Charpentier et al., 1995). The analysis of such pat-terns may be of use in the identi®cation of plant species and management measures which favour an abundant and diverse litter fauna in tropical agro-ecosystems.
The close relationships between litter mass and faunal abundance in the litter layer indicate that a powerful management tool for maintaining an abun-dant litter fauna in tree crop plantations would be to provide an adequate amount of litter on the soil, in agreement with earlier recommendations (Lavelle et al., 1995). This objective can be attained through the inclusion of tree or cover crop species that produce a high litter mass, or through mulching. An important litter quality parameter with respect to the fauna seemed to be the ¯eshy nature of the peach palm offshoot remains which protected sensitive fauna groups from irradiation and desiccation. Also, a legu-minous cover crop seemed to have a more favourable effect on the litter fauna than spontaneous grass vegetation.
more important management tools than the fertilisa-tion level for the development of an abundant and diverse litter fauna in humid tropical plantation sys-tems.
Acknowledgements
This study was carried out within an experiment funded by the German Ministry of Education, Science and Research (BMBF) together with the Brazilian Conselho National de Pesquisa Cienti®ca (CNPq) as part of the SHIFT programme. The ®rst author was holding a scholarship from the German Academic Exchange Service (DAAD) while conducting this work. We thank C. Schmidt for identi®cation of iso-pods, J. Adis and U. Scheller for identi®cation of symphyla, F. Aragau for help in the sample collection and E. Barros for helpful discussions during the pre-paration of the paper.
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