Distribution of ants and bark-beetles in crowns of ...

6

Distribution of ants and bark-beetles in crowns of tropical oaks Ulrich Simon, Martin Gossner and K. Eduard Linsenmair

ABSTRACT

The activity of arthropods m the uppermost and lower crowns of nine tropical oaks was studied from May to October 1997 in a montane forest of the Kinabalu National Park, Sabah, Borneo, Malaysia. The sampling covered a period from the end of the wet season to the beginning of the next wet season. Worker ants (Hymenoptera: Formicidae) and barkbeetles (Coleoptera: Scolytinae) were collected by arboreal pitfall traps and flight-interception traps, respectively. The number of individuals and species of both taxa did not differ significantly between the two canopy layers. Although bark-beetles showed no determinable differences in their occurrence across the strata studied, ß-diversity of ants among traps set in the upper part of the trees was significantly lower than in the lower traps, and ant subfamilies were not distributed evenly between the crown layers.

INTRODUCTION

Forests are layered or stratified ecosystems. This stratification results from both a non-uniform distribution of plant parts and from microclimate (Chazdon & Fetcher, 1984; Parker, 1995). Many studies have dealt with the vertical organization of plant assemblages (e.g. Terborgh, 1985; Oliver & Larson, 1990; Halle, 1995). Anima! assemblages are influenced, in turn, bythis stratification, which results partly from drastic differences between the upper and lower habitats in both temperate (Simon, 1995; Schubert, 1998) and tropical forests (SmiVt, 1973; Longino & Nadkarni, 1990; Nadkarni & Longino, 1990; Paoletti et al., 1991; Kato et al., 1994; Brühl et al., 1998). More precisely, there have been several sturlies comparing the arthropod fauna among forest layers (Sutton et al., 1983a; Basset, 1992c), but

only a few comparing the arthropods along a vertical gradient of the resources with which they are associated (Basset et al., 1999; Ch. 25). Only a few publications analyse the fine distribution of arthropods along these gradients (Davis & Sutton, 1998; Sirnon & Linsenmair, 2001). Trapsare advantageaus for studying the spatial distributions of arthropods, since they survey arthropod assemblages 'on the spot'. Repeated trapping allows an assessment of the assemblage composition at a defined site, and the impact on assemblage structure can be assumed to be low. Such an approach has seldom been used for studying the distribution patterns of arthropods within tree crowns in a tropical forest (Compton et al., 2000; Sirnon & Linsenmair, 2001). Setting up traps in different layers of the canopy allowed us to assess differences in the assemblages of ants and bark-beetles that may exist within tropical tree crowns. Ants are one of the most important insect groups in tropical rainforests both in terms of their number and their impact on the associated flora and fauna (Fittkau & Klinge, 1973; Hölldobler & Wilson, 1990a; Tobin, 1995). There aresturlies that indicate differences in the occurrence of ants in different layers of tropical forests (Longino & Nadkarni, 1990; Brühl et al., 1998). The first question to answer was whether there is a stratification of the ants that are active on the hark surface on a smaller scale within tree crowns. Beetles are highly diverse in the tropics (Erwin, 1982; Stork et al., 1997a). Beetle assemblages in tree crowns reflect forest types (Wagner, 2000), are said to be more or less tree specific (Stork & Brendell, 1990; Wagner, 2000) and offer a new Iook on mechanisms of community assemblage in tree crowns (Floren et al., 1998; Floren & Linsenmair, 1998a). 0degaard et al. (2000) found that previous estimates ofhost specificity ofbeetles, and hence of species richness, were too high. Scolytinae, in [59]

60

U. SIMON, M. GOSSNER & K. E. LINSENMAIR

/

particular, are very species rich in the tropics (Schedl, 1981; Kirkendall, 1993). Ecological requirements of the majority of tropical species are, in cantrast to temperate forests (Grüne, 1979; Szujecki, 1987), rather broad (Kirkendall, I993). There are indications that there is a stratified occurrence of species ofbark-beetles along the vertical extent of trees as weil as within the crowns of forest trees in tree species oftemperate forests (Capecki, 1969; Safranyik et al., 2000). This Ieads to the question whether bark-beetles also show differences in occurrence within tree crowns, that is, on a finer scale.

METHODS Sampiing site and study trees The study was carried out in the Mount Kinabalu National Park, Malaysian Federal State of Sabah, in the north-western part of Borneo. The park covers an area of approximately 75370ha (Ismail & Din, I995). Emergent oak trees of the genus Quercus occur only around Sayap Ranger Station at the western boundary ofthe park (6° 10' N, 116° 35' E, altitude c. 1000 m above sea Ievel). The forest in this area is characterized as a 'lower montane forest' by Kitayama (I992) or as a 'hill dipterocarp forest' by Rahman et al. (I995). Nine trees of the genus Quercus were investigated. They grew along the valley of the river Wariu, near Sayap Ranger Station (Fig. 6.1). All of them were tall trees (Table 6.I ), which emerged 10 to 20 ni above the main canopy layer. The first branching of the study trees occurred within the upper closed layer of tree crowns in the forest, between I7 and 28m above the ground. Allthese trees had leaves with a sunken midrib and 5-10 secondary nerves, a leafmargin minutely toothed in the upper half and acorns of the same shape. According to Cockburn ( 1972), these traits are typical for Quercus subcericea A. Camus (I933), a species that occurs in lowland and lower montane forest from 300 to 2600 m asl but is supposed to reach a height of 2(T m. Accordingly, an unequivocal identification of the trees at the species Ievel has proved impossible, as inflorescences could not be obtained. Voucher specimens ofleaves and acorns have been collected and are deposited in the collection of one of us (U. S. ). Most tree sites were situated on the flat banks of Kemantis Creek or the Wariu river (Trees I, 2, fr-9). Tree 3 grew on the flat top of a small watershed between

CREEK MINODTUHAN

Fig. 6.1. Situation map ofthe study site, redrawn from a local hiking map, with indication of the study trees (Bl to B9).

Kemantis Creek and the Wariu river (Fig. 6.I) trees 4 and 5 grew on more or less steep s1opes (30° inclination) on the east-faced sides ofthe Wariu river bed. lnsect collecting and processing Two different types of trap were used in this study (see also Sirnon & Linsenmair, 200I). First, flightinterception traps made of gauze with funnels of cotton attached above and below (modified after Basset et al., I997b) were used. The gauze was Im long and 50cm wide. Two intersecring rectangles of gauze were crossed rectangularly to make up the central barriers. The upper

DISTRIBUTION OF ANTS AND BARK-BEETLES IN CROWNS OF TROPICAL OAKS

61

Table 6.1. Characteristics ofstudy trees and height ofthe two typesoftrapset up in the crown ofstudy trees Tree

Tree height (m)

DBH(m)

FIT-U(m)

FIT-L (m)

APT-U (m)

APT-L(m)

BI B2 B3 B4

44.0 38.5 35.0 35.0

2.1 1.4

35.0 27.0

1.5 1.3

37.0 27.5 27.0 24.5

29.0 21.5 17.0 19.5

B5

36.0 34.0 36.0 45.0 35.0

1.8

26.5 23 .5 28.5 28.0 27.5 37.0

27.0 22.5 16.5 17.0 21.0 20.5 19.5 26.5 14.5

29.0 29.0 30.0 38.5 25.0

20.0 22.0 21.0 27.5

B6 B7 B8 B9

1.5 2.0 2.2 1.4

24.0

17.0

DBH, diameterat breast height; FIT, flight-interception trap; APT, arborea1 pitfall trap; U, upper crown; L, 1ower crown.

funnel was attached to a box with a transparent Iid via a plastic tube in order to trap arthropods trying to escape by flying upwards. At the base of the bottom funnel was a sample jar with a hole in its cover. This type of trap combines the advantages of a window trap and a Malaise trap. It samples mainly flying insects and is one of the most efficient methods for sampling beetles (Schubert, 1998; Sirnon & Linsenmair, 2001). Arboreal pitfall traps were also used, modified from the design ofWeiss (1995), who used them on spruce trunks in a temperate forest in Germany. Small plastic buckets were cut in half longitudinally, each half supporting a small plastic bag filled with killing agent. The half-bucket and plastic bag were pressed to the surface of the hark by an elastic band, ensuring a tight fit of the rim of the plastic bag to the hark, and smooth movement ofbark-dwelling arthropods into the trap. The width of the pitfall trap was about 12 cm. All sampling jars were filled with an aqueous solution of 1% copper sulphate, combined with a small amount of ~etergent. This agent kills arthropods very quickly and, although an irritant, is nontoxic to adult humans. The disposition of the traps in the crowns was always similar: one flight-interception trap and one arboreal pitfall trap in the uppermost reachable parts of the tree c~owns, and a similar pair of traps at the Ievel of the first branching of the trees. The absolute height of the traps above ground varied with the height of the oak trees studied and the distJflce to the first branch (Table 6.1). /

We climbed the oaks using single-rope techniques (Perry, 1978). Traps were surveyed twice a month during the period from 4 April 1997 to 15 September 1997. Damaged traps were repaired during sampling and eventually replaced. lnsects were sorted to order, and other arthropods to various higher taxa (see Sirnon & Linsenmair, 2001). We present data on ants only from the arboreal pitfall traps; ants were sorted to morphospecies, and identification to genus Ievel was made using Bolton (1994). Data for the subfamily Scolytinae in this paper were obtained only from flight-interception trap catches, which were sorted to morphospecies. Data analyses We used the Jaccard index for the estimation of ß-diversity: the lower the Jaccard value, the higher the ß-diversity (see Magurran, 1988). We used this index to compare ant and scolytine assemblages between trees, and within trees between the two studied layers. For statistical analyses we used the program STATISTICA 5.1 (Statsoft, 1997), except for the Wilcoxon-Wilcox test (see below). We used the Wilcoxon matched-pairs test to compare insect abundance in the upper and lower parts of the trees. Comparisons ofWilcoxon-tests with t-tests (asymptotic efficiency) showed that the loss of efficiency is low, as the tests have 95.5% of the power of the comparable parametric test (Weber, 1986). To compare the differences in species composition between the upper and the lower parts of the tree, we calculated a sign-test based on


62

100

80

Q)

60

Ol

Q..

40

20

0 81

82

83

84

85

86

87

88

89

I• Dolichoderinae l l D Formicinae 0 Myrmicinae l l D Ponerinae I Fig. 6.2. Proportion of ant individuals collected on each study tree, detailed for each subfamily.

presence/ absence data, in which species were regarded as replicates. To detect differences in the similarity values Uaccard index) between crown parts, we performed a Friedman test and, as a post-hoc test, the Wilcoxon-Wilcox test (Köhler et al., 199 5). RESULTS

Ants In total, 494 ant specimens representing 71 morphospecies were collected with the arboreal pitfall traps. Twenty-three species (32.4%) were singletons, and another 23 species occurred only as pairs. Accordingly, one third of all ant morphospecies was represented by three or more individuals. Four subfamilies of Formicidae were collected, with Myrmicinae dominating the number of individuals collected (43%), followed by Dolichoderinae (28%), Formicinae (23%) and Ponerinae (6%). The distribution of species richness was slightly different. Although Myrmicinae still dominated (41%), Formicinae were better represented (35%). The Dolichoderinae represented 17% of species and the Ponerinae 7%.

The distribution of ant individuals and species varied among the trees. In five trees, Myrmicinae were the mostabundant group (trees B2, B3, B4, B5, B9), whilst Dolichoderinae (trees B 1 and B8) and Formicinae (tree B7) were most abundant in two and one tree, respectively. In tree B6, a more or less equal number of Dolichoderinae, Formicinae and Myrmicinae occurred (Fig. 6.2). Myrmicinae were the most species-rich subfamily on six trees, whereas Formicinae dominated in the other study trees (Table 6.2). Numbers of species of ants did not differ significantly between the two strata (Table 6.2; Wilcoxon matched-pairs test, two-tailed, z = 0.948, p = 0.343). As shown in Fig. 6.3, there is a slight, but statistically nonsignificant, decrease in species number in the lower traps. Within each of the subfamilies only the species numbers of the Formicinae differed significantly (Table 6.2; Wilcoxon matched-pairs test; z = 1.960; p = 0.049), showing a preference for the upper area of the canopy. Neither the number of individuals of Formicinae nor the species and individual numbers of the other subfamilies differed statistically (all with p > 0.05) across layers.


63

Table 6.2. Number ofant species collected in arboreal pitfall traps in different trees and heights, detailed by subfomilies Study tree

Dolichoderinae

Uppercrown BI

3

B2

Formicinae

M yrmicinae

Ponerinae

5

2

I

4

4

0

B3

3

3

6

0

B4

I

B5 B6

3 2

3 7

4 4

I 0

5

B7 B8

4 2

5 I 3

2 I 0 0

8

4

B9 Lower crown

BI

0

0

B2

4

6 6

I 0

5 5 6

0

6 3

0 0

B3

B4 2 3 3 3 0

B5 B6 B7 B8

B9

2 3 2 3 6 2 2

80 ~-------------------------------------------------------------------,

70 60

~ 50

·~

29

a.

Ul

0

40

~

.c

§

z

30

20 10

Sum af species

Uppertraps

Lower traps

Only in upper traps

Only in lower traps

I• Dolichoderinae 111111 Formicinae D Myrmicinae B Ponerinae I Fig. 6.3. Number of ant species collected, detailed by subfamilies and different sampling situations.

ln both traps

64


Table 6.3. Number ofindividuals caught in arboreal pitfalltraps for the Jivemost abundant ant species, detailed by study tree and sampling height Study tree BI B2 B3 B4 B5 B6 B7 B8 B9 p value"

Techrwmyrmex sp. 1

Technomyrmex sp. 4

Pizeidole sp.l

Camponotus sp.l

Vollen.hovia sp. 6

u

L

u

L

u

L

u

L

u

L

0 1 0 1 6 38 0 5 0

7 0 1 0 4 0

0 0 9 3 0 0

0 7 1 3

4 0 0 0 4 17 0 4 5

0 0 0 0

3 0

4 0 0 0 3 2 4 2

0

1 0

0 0 2 16 2 5 0 1 0

4 1 0 0 2 1 11

3 0.834

0.893

4 0 0 3 0.612

3 1 0 3 2

0.575

1 0 2

Not testable

U, upper crown; L, lower crown. "Wilcoxon matched-pairs test.

On1y four species were frequent enough (i.e. occurring on six trees or more) to be tested with a matched- pairs test (Table 6.3). None of these species showed significant preferences for one of the two sampling strata. Since the Myrmicinae Vollenhovia sp. 6 occurred only on five trees, the matched-pairs testwas not applied. However, more individuals ofthis species were collected in the upper layer, suggesting a preference for this part of the crown. We tested possible ant preferences for height in a single tree with a sign-test using species as replicates. A significant difference (p < 0.05) existed only in one tree (B5), with more individuals found in the upper traps. A Friedman test of Jaccard indices calculated for data between the two layers and within each study layer showed one of the groups to be significantly different (p = 0.020). It appeared that the indices for the samples from arboreal pitfall traps of the upper layer (APT-U) against the lower layer (APT-L), and the indices among trees in the lower layer (APT-L), were not statistically different from each other (WilcoxonWilcox test: /f(3; 36;0.05) < 19.0; p > 0.05). The indices calculated among trees in the upper layer (APT-U versus APT-U) were significantly different from both of theother sets of comparisons (Fig. 6.4; /f(3; 36;0.05l > 19; p < 0.05).

Bark-beetles The flight-interception trap catches included 420 specimens of Scolytinae, ofwhich 414 were assigned to 164 morphospecies (six specimens were too damaged). The majority (72.5%) of morphospecies were represented by on1y one or two specimens (91 singletons, 28 pairs). The number of morphospecies among study trees was not evenly distributed (x 2 = 36.81; df = 8; p < 0.001). Tree B3 had more morphospecies than average, and trees B6, B7 and B9 had fewer morphospecies than average (Fig. 6.5). The number of morphospecies of bark-beetle did not differ significantly between the two sampling heights (Tab1e 6.4; Wilcoxon matched-pairs test, z = 0.118; p = 0.910). Themost common bark-beetles (n > 10) did not show significant preferences for either of the two sampling heights, as was the case with the ants. A Friedman test of Jaccard indices calculated for within-tree samples and between-tree samples showed no significant differences (p > 0.05; Fig. 6.6). In a last analysis, we performed a sign test to estimate the preference of the species assemblage of bark-beetles for sampling heights, at the single tree Ievel. In two trees, the species assemblage was significantly different between the upper and the lower crown. In tree B4, more species were collected in the lower


b

a

0.5

Fig. 6.4. Cornparison oftheJaccard indices of the ants within heights and between heights. The srnall box indicates the median value; the !arge box is the 25- 75% confidence interval; a and b indicate significant differences (Friedrnan test p < 0.02; Wilcoxon-Wilcox text p < 0.05); APT, arboreal pitfall trap; U, upper crown; L, lower crown.

a

0.4

X

Q)

-o 0.3 .!::

"E ca (.) (.) ca --,

65

0.2 0 0

0.1

0

0.0 APT-U I APT-U

APT-U I APT-L

APT-L! APT-L

40

"' Q)

·c::s Q)

c.

0"' 30 Q;

..c

E ::J

z

20

81

82

83

84

85

86

87

88

89

Fig. 6.5. Nurnber of species ofbark-beetles collected per tree for trees BI to B9. The rnean nurnber of species per tree is indicated by a dotted line.

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Table 6.4. Number ofspecies ofbark-beetle collected by jlight-interception traps in study trees, detailed by sampling height Study tree

BI B2 B3 B4 BS B6 B7 B8 B9

Upper crown

Lower crown

18

26 21 31 29 23 7 6

13

30 10

21 17 10 22 14

particularly when entire nests are sarnpled. Floren & Linsenrnair (1997b) presented data on 500 to 5000 individuals of ants frorn a single Aporusa tree crown (Euphorbiaceae) in south-east Asia, and Adis et al. (1998a) collected 948 specirnens of ants in a single crown of Goupia glabra Aubl. (Celastraceae) in an Amazonian upland forest. In our studies, however, the nurnber of ants was higher than in studies using baits (ltino & Yamane, 1994; U. Sirnon & R. Bertele, unpublished data). Arboreal pitfall traps generate data on the intensity of activity of ants. This intensity of activity was very low at both sarnpling heights. An observational study of the activity of ants on sirnilar trees also indicated low activity. Baits exposed on these trees (tuna, sugar) resulted in a drarnatic increase of ant activity within a short tirne-span (around I 0 rnin; U. Sirnon & R. Bertele, unpublished data). Under undisturbed circurnstances, the density of active ants in oak tree crowns rnay be relatively low; high densities of ants on the study oak trees occurred only where there was disturbance or during recruitrnent after detection of prey.

13

12

The p-value of a Wilcoxon matched-pairs testwas 0.910.

traps (n = 35; z = 3.381; p < 0.001), andin tree B6, species preferred the upper crown (n = 19; z = 2.294; p = 0.022). For the three other trees p values were not significant.

Ant distribution within tropical oak tree crowns The cornposition of the ant subfarnilies in our sarnples was sirnilar to the results for arboreal ants presented by Brühl et al. ( 1998) for another study site (Poring Hot Spring) at Mount Kinabalu National Park. Forrnicinae

DISCUSSION

Samp1ing ofbark-dwelling arborea1 ants The nurnber of ants collected was low cornpared with the nurnber of ants sarnpled by insecticide fogging,

0.28

.,.. ,

..., ..................:.. ····· ......................... ···················:··· . ..

.....................

0.24

0.20 ... ..................... ·:· ~ '0 .5 0.16

...... ... ............... .............. .......... .. .. ................

.. ............... .

-e

~

J

0.12 ... ..................... ........................... ''' ··································· ······ ····· ··

0.08 ............. . .

D

D

D

0.04 ..... .... ..... . 0.00 ..__ _ _ _ _..__ _ _ _ _..__ _ _ _ _..__ _ _ ____.

FIT-U- FIT-U

FIT-L- FIT-L

FIT-U- FIT-L

Fig. 6.6. Comparison oftheJaccard indices for bark-beetles within heights and between heights. The small box indicates the median value; the !arge box is the 25-75% confidence interval; FIT, flight-interception trap; U, upper crown; L, lower crown. There were no significant differences (p > 0.05, Friedman test) .


and Myrmicinae were species rich, and Ponerinae, in contrast to samples ohtained from the soil and litter layer, were species poor. Surprisingly, no species of Pseudomyrmicinae was sampled, even though species of the genus Tetraponera are typical inhahitants of lower vegetation, either of tree crowns (Brühl et al., 1998) or in the lower canopy (Itino & Yamane, 1994). However, typical genera of tree-inhahiting groups of ants were collected. Götzke (1993), for example, found Ponerinae and species of Pachycondyla and Ponera to he tree-dwelling just as we did. Floren ( 199 5) reported the genus Diacamma from tree crowns. No statistical difference either in species numher or in individual numher of ants could he detected hetween the two sampling heights. Only one species, Vollenhovia sp. 6, showed a tendency to prefer the upper crown. We conclude that worker ants of species dwelling in tree crowns apparently use the whole tree crown (and perhaps partly or completely the trunk) without any discrimination hased on height or exposure. These results were corrohorated for worker ants in a companion study (U. Sirnon and R. Bertele, unpuhlished data). Ants on the study oaks apparently exhihit a generalist foraging strategy. In contrast, alates may need particular structural and microclima tic fea tures for their estahlishment. The spatial distrihution of alates will he discussed elsewhere. The ß-diversity of ants in the upper crowns of study trees was significantly lower than in the lower crowns. Although the total numher of species in upper traps was higher than in lower traps (48 to 44) there were many lower trapsthat shared no common species among themselves. Whereas nesting is restricted mainly to the lower parts of the trees (U. Sirnon & R. Bertele, unpuhlished data), foraging activity of worker ants was not significantly different hetween the two study heights. The reduced ß-diversity in the upper crown may indicate a microclimatic Iimitation of ant occurrence in these emergent trees. Sampling bark-beetles in tree crowns Scolytinae are often neglected in studies on heetle diversity. This is hecause of the difficult taxonomy of the taxon,, and the comparatively low proportions of the suhfamily ohtained in insecticide-fogging samples (e.g. Davies et al., 1997; Wagner, 1997). In our study, hark-heetles presented at least a tenth of all heetle specimens collected hy the flight-interception traps.

67

We think that this high proportion reflects the sampling method: flight-interception traps in tree crowns catch hark-heetles arriving for reproduction as weil as Scolytinae leaving after hatching. We cannot exclude the possihility that a proportion of the hark-heetles caught are just passing hy on dispersal. How !arge a proportion these 'tourists' represent could only he estimated hy studying the hark-heetles in the wood of the study trees itself (see Büchs (1988) 'hark emergence eclector'). Distribution ofbark-beetles within tree crowns In our study, we expected that there would he different microclimatic conditions within the lower part of the closed canopy compared with those in the exposed upper parts of these emergent trees. Although no measurements of climatic data were availahle, the distrihution of mosses, Iichens and vascular epiphytes was different across study heights. There were more mosses and vascular epiphytes in the lower parts, and more crustaceous Iichens in the upper crowns, indicating, at least, differences in the air and/ or the hark moisture Ievels (U. Sirnon & S. Unsicker, unpuhlished data). Accordingly, we expected differences in species composition, or at least in the individual numhers of Scolytinae caught within the two contrasting layers. Overall, no such patternwas detected. This indicates that the exposed, and most likely heated, outer parts of the Quercus crowns that we studied were not avoided hy hark-heetles. This is reflected in the similar values for ß-diversity that we ohtained for within-tree and hetween-tree assemhlages of Scolytinae. In the upper parts of the crowns, hark-heetles must he adapted to or at least he tolerant of dry and hot conditions. Conclusions: tree crowns as single units? With regard to information on the trees and insect taxa presented in the present contrihution, we might conclude that tree crowns represent a single unit used hy arthropods without discrimination hy height or exposure. However, several pieces of evidence are counterindicative. •

•

The ß-diversity of ants is lower in the upper crown, indicating differences in the composition of the faunal assemhlages. The mechanism of colonization and recolonization of tree crowns is mainly hy chance (Floren & Linsenmair, 1997h; Floren et al., 1998). However,

68

•

•


within single trees the species assemblages ofbarkbeetle in the tree crown was demonstrably different. Consequently, the random (re-)colonization of tropical tree crowns often results in a (random?) subdivision of the assemblage. Basset ( 1991 b, 1992c) reported a nonuniform distribution ofherbivory and herbivores in Argyrodendron actinophyllum F. Muell., an Australian rainforest tree, and Lowman (1985, 1995) found conspicuous differences in the spatial distribution ofherbivory in rainforest tree crowns. Sirnon & Linsenmair (200 I) showed that, at the ordinal Ievel, the arthropod assemblages of the same tree species were different. They also found significant differences in the occurrence of ants in the upper and lower crown of the same study trees, mainly revealed by samples from flight-interception traps. Since in this study, ant workers, collected by pitfall traps, showed no statistical difference in activity between upper and lower crowns, the reason for the difference observed by Sirnon & Linsenmair (2001) may weil be that alates prefer the lower parts of the tree crowns, probably because of the increased amount of accumulated litter and the more equable microclimate.

Ricklefs (2000, p. 84) stated that for the study of tropical plant assemblages: 'when the abundance of many species averages one individual per hectare, local communities of species within hectare plots have little meaning'. In our study, from two thirds to three quarters of the morphospecies of ant and bark-beetle were singletons. Ricklefs (2000, p. 84) concluded that 'old concepts of ecological communities and the sampling strategies accompanied them, should be discarded'. If his Statement is taken to include insect trapping among the 'old ... strategies' then we must beg to disagree. We do agree that more satisfying data could be obtained with a

more extended sampling protocol including more study sites on a !arger scale, as reported by Pitman et al. (1999) for Amazonian tree species. In our ~pinion, a trapping protocol as discussed above does provide relevant insights into the composition of canopy arthropod assemblages, the processes occurring within them and their dynamics. We do not know if the bark-beetles we collected have their breeding habitats at the crown heights that we studied or whether the nests of the ants encountered were close to the trap positions. An unknown number ofindividuals ofboth taxaarealso likely tobe 'tourists'. For this study, the issue was of secondary importance. The conspicuous differences in faunal composition at the ordinallevel between the upper and the lower crown of the study trees found by Sirnon & Linsenmair (200 I) was not reflected within these two numerically and ecologically important taxa.

ACKNOWLEDGEMENTS We are grateful to the Director of Sabah Parks, Datuk Ali Lamri, for permission to work in Kinabalu Park, and to Mr Francis Liew and Mr Wong for their support. Ansow Gunsalam of the Botanical Department of Kinabalu Park helped us to find the study trees araund Sayap Ranger Station. Peninsus Guliong and his family provided great hospitality at Sayap Ranger Station. Many thanks to our Malaysian worker Soimin Magindol, who surveyed and maintained the traps over a period of more than 4 months with great reliability. Furthermore, three of our students, Heinrich Bardorz, Reinhard Hertele and Sibylle Unsicker, tagether with Jan Pfütze helped us to start the project and with surveying and dismantling the traps. The fieldwork was supported by travel grants of the Tropical Canopy Programme (1994-1998) of the European Science Foundation.