Effects of microhabitat diversity and geographical isolation on oribatid mite (Acari: Oribatida) communities in mangrove forests

ARTICLE IN PRESS Pedobiologia 48 (2004) 245–255

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Effects of microhabitat diversity and geographical isolation on oribatid mite (Acari: Oribatida) communities in mangrove forests Shigenori Karasawaa,b,*, Naoki Hijiib a

Laboratory of Science Education, Graduate School of Education, University of the Ryukyus, Japan Laboratory of Forest Protection, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan

b

Received 18 June 2003; accepted 27 January 2004

KEYWORDS Arboreal microarthropod; Geographical isolation; Mangrove forest; Microhabitat diversity; Oribatid mite; Species diversity; Subtropical; Tidal flooding

Summary The effects of microhabitat diversity and geographical isolation on the structure of oribatid communities were studied in mangrove forests on the Ryukyu Islands of Japan. The study took place at three sites on two islands 470 km apart. Oribatid mites (Oribatida) were extracted from leaves, branches, bark of trunks (0–50, 50–100, and 100–150 cm high) and of knee roots, and from forest-floor soil and littoral algae, each defined as a microhabitat of oribatid mites. At the 0–50 cm height, the species composition of the oribatid communities on the knee-root bark and the bark of trunks of Bruguiera gymnorrhiza differed significantly from that on the other microhabitats. This difference was attributed to tidal flooding of the mangrove forests. Cluster analysis showed that oribatid communities in the same microhabitat at different sites tended to be more similar than those on different microhabitats at the same site. This implies that the species composition of oribatid communities in mangrove forests is more likely to be affected by factors responsible for microhabitat diversity (characterized specifically by the flooded trunks) than by geographical distance between the islands. & 2004 Elsevier GmbH. All rights reserved.

Introduction The Acari are often the dominant group of arthropods collected from forest floors and trees (Wallwork, 1983; Norton, 1994; Watanabe, 1997; Behan-Pelletier and Walter, 2000). Within this

group, oribatid mites (Oribatida) form the major taxonomic assemblage, with over 6000 described species worldwide (Balogh and Balogh, 1992). Feeding-habit diversification has been proposed as a hypothesis to explain the high diversity of oribatid mites (Luxton, 1972; Kaneko, 1988).

*Corresponding author. Laboratory of Forest Protection, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 4648601, Japan. Fax: þ 81-52-789-5518. E-mail address: [email protected] (S. Karasawa). 0031-4056/$ - see front matter & 2004 Elsevier GmbH. All rights reserved. doi:10.1016/j.pedobi.2004.01.002

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Microhabitat diversity is also suggested to be correlated closely with their diversity (Aoki, 1967; Hammer, 1972; Anderson, 1978). Recently, many studies have revealed that the structure and spatial diversity provided by trees are strongly associated ! 1978, 1985; with oribatid species diversity (Andre, Ito, 1986; Nicolai, 1986; Walter and Behan-Pelletier, 1993; Walter et al., 1994; Walter, 1995; BehanPelletier and Winchester, 1998; Winchester et al., 1999; Behan-Pelletier and Walter, 2000). Mangrove forests cover a wide range of coastal areas in tropical and subtropical regions. The Ryukyu Islands, southwest of Japan, have typical mangrove forests, in which unlike other terrestrial forests, the forest floor and bases of the mangrove trunks are flooded regularly. This flooding creates greater habitat diversity that should enhance the species diversity of oribatid mites and create unique communities. There is little information on the taxonomy of oribatid mites inhabiting mangrove forests or on the species composition and dynamics of these communities. Moreover, the Ryukyu region has experienced extensive topographic changes over the last 10 million years (Kizaki and Oshiro, 1980; Kizaki, 1997), which may have affected its faunal diversity (e.g., Yamane and Tano, 1983; Ota, 1998; Shimojana and Haupt, 1998). Nevertheless, the effect of geographical isolation on the oribatid mite species diversity has not been clarified (Nakatamari, 1983). This study identified the species composition of oribatid communities and examined the effects of habitat traits in mangrove forests and geographical isolation on the species diversity of these communities.

Materials and methods Study area The study was carried out at two sites on Okinawa Island and at one site on Iriomote Island which lie about 470 km apart within the Ryukyu Island group. On Okinawa, one site (OK-1) was situated on the left bank of the Okukubi River at the town of Kin (261110 N, 1271520 E). There, the air temperature averaged 22.61C, and the precipitation was 1877 mm in 2002 according to the Okinawa Meteorological Observatory. The vegetation was a typical mangrove forest ca. 4.5 m high, consisting of dominant Bruguiera gymnorrhiza (L.) Lam. and the rare Rhizophora stylosa Griff. At OK-1, the tree species sampled was B. gymnorrhiza. The other site (OK-2) was situated ca. 100 m southeast of OK-1,

S. Karasawa, N. Hijii

and the vegetation was a typical bank forest 10– 15 m high. Pinus luchuensis Mayr and Casuarina equisetifolia Forster were dominant and intermingled with each other, and both species were sampled. On Iriomote, the site (IR) was located on Funaura Bay (261230 N, 1231450 E). In 2002, the air temperature averaged 23.71C, and the precipitation was 1727 mm in 2002 according to the Okinawa Meteorological Observatory. The vegetation was a dense mangrove forest about 6 m high, which consisted of dominant B. gymnorrhiza (the tree species sampled at the site) and R. stylosa.

Sampling Samples at OK-1 and IR were collected in autumn (21 October–5 November 2001), winter (26 December 2001–7 February 2002), spring (16 April–9 May 2002), and summer (1 July–22 August 2002). At OK2, samples were collected only in summer and winter of 2002. A 15 m  15 m plot was set up at each site. Samples were collected in each plot from leaves, branches, bark of trunks (0–50, 50–100, and 100–150 cm high), and bark of knee roots, from forest-floor soil and littoral algae. Each of these sampling substrates was defined as a microhabitat of oribatid mites. Samples were collected in the bank forest between 10:00 and 14:00, and in the mangrove forests at the lowest tide level between 09:00 and 18:00. On each sampling occasion, one tree was selected in each plot, and ten 10–50-cm-long shoots were taken from the tree at a height of between 100 and 200 cm above ground level. Each tree was marked after sampling so as not to be sampled again. The shoot samples of B. gymnorrhiza from OK-1 and IR were divided into leaves and branches. Trunk-bark samples were collected from five trees selected in each plot at each time. A bark sample of about 20 cm  20 cm was taken at each of the three heights from each tree at all sites. Samples of knee-root bark were obtained from 5 to 10 knee roots of B. gymnorrhiza randomly selected at OK-1 and OK-2. Three soil samples were taken with a metal sampler (5 cm  5 cm wide  4 cm deep) at OK-1 and OK-2, while only 500-cm3 algal sample was collected from the soil surface at OK-1.

Extraction of mites Animals were washed off leaves, branches, and shoots with dilute soap detergent, and collected by filtering the solution through a 35-mm nylon mesh. Animals were extracted from the other samples by using Tullgren funnels placed under 40-W electric

ARTICLE IN PRESS Structure diversity and oribatids in mangrove forests

bulbs for 72 h. All oribatid mites were sorted and counted, and only adults were identified to the species level. After extraction of animals, all bark samples were dried in an oven (1001C, 24 h) and weighed to allow calculation of abundance per gram substrate.

Measuring the level of tidal flooding The level of tidal flooding in the mangrove forests was measured on three trees randomly selected at each of OK-1 and IR. At high tide, the level was measured on each tree six times (8, 10–13, and 16 December 2002) at OK-1 and four times (8–11 February 2002) at IR. From the relationships between those measurements and the tide levels reported by the Okinawa Meteorological Observatory in 2002, seasonal changes in the level of flooding were determined.

Analyses The density of animals is expressed as the number of individuals per 100-g dry-weight sample or per 100-cm3 soil or algal sample. The mean density and mean numbers of species are expressed as the mean (7SD) density and species number over four (OK-1, IR) or two (OK-2) sampling occasions. The species diversity of oribatid communities is expressed by two indices: the H index (Morisita, 1996), modified from Shannon-Wiener’s index (H 0 ) to minimize the effect of sample size, and Simpson’s diversity index (1  D) (Lande, 1996), where H ¼ H 0 þ fðS  1Þ=Ng½1 þ fðS  1Þ=Ng2 ; H0 ¼ 

D¼

X

nX

ðni =NÞ log2 ðni =NÞ;

o ni ðni  1Þ =NðN  1Þ;

where N is the total number of individuals, S the total number of species, and ni the number of individuals of the ith species in a community. Cluster analysis was conducted to examine the similarity of oribatid communities among the microhabitats surveyed (in eight categories of microhabitat and three tree species). A similarity dendrogram among the oribatid communities was drawn by using Morisita’s similarity index (Cl ) (Morisita, 1959)  X  Cl ¼ 2 nAi nBi =½ðlA þ lB ÞNA NB ;

247 i nAi ðnAi  1Þ =NA ðNA  1Þ; hX i nBi ðnBi  1Þ =NB ðNB  1Þ; lB ¼

lA ¼

hX

where NA ðNB Þ is the total number of individuals, and nAi ðnBi Þ the number of individuals of the ith species in community AðBÞ: Cl equals 1 for complete similarity and 0 for no similarity between the two communities. The index was calculated from the mean density (per 100-g dry-weight sample or per 100-cm3 soil or algal sample) for each species in each microhabitat. Because no oribatid mites were collected from the forest-floor soil at OK-1, that site was not included in this analysis. The mean density, mean number of species and mean species diversity of oribatid mites were compared between samples from the four types of bark microhabitat by using Kruskal-Wallis test, and between samples from the flooded bark (kneeroot bark and the bark of the trunk at 0–50-cm height of B. gymnorrhiza) and those from the other types of trunk bark by using Wilcoxon’s rank-sum test.

Results Level of tidal flooding Fig. 1 shows the seasonal changes in the mean and highest levels of seawater at high tide in 2002. The monthly mean tidal flooding level of the trunk of B. gymnorrhiza at OK-1 ranged from 29 to 51 cm above ground level, with the highest level at 94 cm. The monthly mean at IR was between 19 and 44 cm above ground level, and the highest level was 86 cm. Thus, at both OK-1 and IR, the knee-root bark and the bark of the trunk at 0–50-cm height on B. gymnorrhiza were often flooded, the bark of the trunk at 50–100-cm height was occasionally flooded, and that at 100–150-cm height was rarely flooded.

Density of oribatid mites The mean densities of oribatid mites collected from each microhabitat over the study period are given in Table 1. The mean densities found on the leaves and branches of B. gymnorrhiza at OK-1 were higher than those found on the same microhabitats at IR. The littoral algae in the mangrove forest at OK-1 had the lowest oribatid density (0.670.6 100 cm3; n ¼ 4), and the forest-floor soil in the bank forest (OK-2) had the highest (82.2788.4 100 cm3; n ¼ 2) (Table 1). No oribatid mites were

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S. Karasawa, N. Hijii

both the knee-root bark and the bark of the trunk at 0–50-cm height on B. gymnorrhiza at both OK-1 and IR. This was significantly lower than what was observed on the trunk bark at other heights with more than four species (Wilcoxon’s rank-sum test, Po0:001).

Species diversity As with the trend in the number of species, the mean values of the diversity indices (H and 1  D) were highest in the oribatid community from the forest-floor soil in the bank forest (Table 1). At both OK-1 and IR, the mean H values were significantly lower on the knee-root bark and the bark of the trunk at 0–50-cm height (mean H  p0:28) than those on the other types of trunk bark (mean H  41:50; Wilcoxon’s rank-sum test, Po0:001). A similar trend was found for the index 1  D (Table 1).

Similarity of oribatid communities among microhabitats

Figure 1. Seasonal changes in the mean and highest levels of seawater at high tide.

found in the forest-floor soil at OK-1. Among the bark microhabitats, the knee-root bark at IR had the highest mean density (351.37269.6 100 g1 dry wt.; n ¼ 4), and the bark at 0–50-cm height at IR had the lowest (32.0738.0 100 g1 dry wt.; n ¼ 4). However, there was no significant difference in the densities among the four types of bark microhabitat (Kruskal-Wallis test, P40:05).

Number of species of oribatid mites A mean of three oribatid species were found on the leaves and branches of B. gymnorrhiza at OK-1 while only one was observed at IR. The forest-floor soil in the bank forest (OK-2) had the largest number of species (35.5714.8; n ¼ 2) (Table 1). The mean number of oribatid species was o2 on

The cluster analysis classified the oribatid communities from the various microhabitats into five groups, according to the similarity of species composition (Fig. 2). The canopy group was composed of the communities recorded from all samples of leaves and branches, all of which were dominated by Dometorina sp. and Humerobates varius Ohkubo. The second group was drawn mostly from the trunk bark at all sites, and consisted of the communities from the samples at 50–100 and 100–150-cm height on B. gymnorrhiza at OK-1 and IR, and from all trunk-bark samples from P. luchuensis and C. equisetifolia in the bank forest (OK-2). The communities in the trunk-bark group were dominated by Scheloribates sp., H. varius, Incabates sp., Peloribates barbatus Aoki, Hexachaetoniella hardyi Balogh (OK-2), and Licneremaeus sp (OK-2). The third group was restricted to the community from the forest-floor-soil samples at OK-2, where Oppiella nova Oudemans, Xylobates sp., and Hermannia kanoi Aoki were dominant. The fourth was the flooded-trunk group, consisting of the communities from the knee-root bark and the bark of the trunk at 0–50-cm height on B. gymnorrhiza at both OK-1 and IR. Only one species, in the Oripodidae, was predominant in this group. The last group stemmed from the littoral-algal samples at OK-1, where Fortuynia marina Hammen dominated.

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249

Table 1. Mean density ðMÞ; number of species ðSÞ; and species diversity (H and 1  D) of oribatid mites in all microhabitats over the study period (per 100 g dry weight of sample) (mean 7 SD) Forest type Tree species (site) Mangrove forest B. gymnorrhiza (OK-1)

B. gymnorrhiza (IR)

Bank forest C. equisetifolia (OK-2)

P. luchuensis (OK-2)

a

Microhabitat

M

S

H*

1D

Leaf Branch 100–150 cm trunk-bark 50–100 cm trunk-bark 0–50 cm trunk-bark Bark of knee root Littoral algaea Forest-floor soil

51.9726.9 53.6721.3 67.4729.1 46.0740.3 105.8768.6 65.1715.4 0.670.6 0

3.072.0 3.371.0 4.571.9 4.371.0 1.870.5 1.870.5 0.871.0 0

0.4970.41 1.4270.43 1.7970.57 1.6170.31 0.2470.20 0.2670.21 0.2170.41 F

0.1570.12 0.6570.11 0.6670.14 0.5870.13 0.0870.08 0.1070.09 0.0870.17 F

Leaf Branch 100–150 cm trunk-bark 50–100 cm trunk-bark 0–50 cm trunk-bark Bark of knee root

5.477.8 13.3713.5 58.7733.2 42.1747.7 32.0738.0 351.37269.6

0.870.5 1.071.4 7.571.3 5.572.1 1.370.5 1.570.6

0 (one species) 0.4070.79 2.1770.18 2.1171.15 0.2870.55 0.0670.11

0 (one species) 0.2170.42 0.6970.09 0.6370.31 0.1770.33 0.0270.03

Shoot 100–150 cm trunk-bark 50–100 cm trunk-bark 0–50 cm trunk-bark

314.6714.8 61.6760.7 76.2778.8 101.8782.7

4.570.7 6.074.2 6.074.2 8.072.8

1.6470.26 1.8770.66 1.6370.09 2.1770.43

0.6070.13 0.6370.11 0.6070.06 0.7270.19

100–150 cm trunk-bark 50–100 cm trunk-bark 0–50 cm trunk-bark

74.5726.0 58.5716.5 82.5739.4

8.074.2 10.570.7 12.572.1

2.5070.55 3.0170.00 3.2270.83

0.7970.04 0.8470.00 0.8570.10

Forest-floor soila

82.2788.4

35.5714.8

5.0470.37

0.9670.02

3

Per 100 cm substrates.

Discussion Characteristics of oribatid communities in the mangrove forests In all, 27 species of oribatid were recorded from all microhabitats in the mangrove forests (OK-1, IR) (Table 2). This value was smaller than that obtained from the forest-floor soil in the bank forest (OK-2) (Table 1) and from other studies conducted in terrestrial forests on the Ryukyu Islands (Aoki et al., 1979; Ito and Aoki, 1999). This lower species number is due to the fact that there were no oribatid mites in the forest-floor soil of the mangrove forestsFnormally their dominant habitat. It is unlikely that many oribatid mites can dwell in the forest-floor soil of mangrove forests, as they are regularly flooded by seawater and therefore, consistently salty. Most of the oribatid mites collected in the mangrove forests belong to the Brachypylina and

are included in the families Selenoribatidae and Fortuyniidae (Table 2). Species of the Brachypylina have been recorded in various arboreal environments (Aoki, 1973; Behan-Pelletier and Winchester, 1998; Behan-Pelletier and Walter, 2000; Ichisawa, 2001), whereas species belonging to the Selenoribatidae and Fortuyniidae are restricted to littoral habitats (Hammen, 1960, 1963; Strenzke, 1961; Schuster, 1963; Grandjean, 1966, 1968; Luxton, 1967; Aoki, 1974; Luxton, 1992). Thus, in this study the oribatid communities of mangrove forests are characterized taxonomically by species derived from both arboreal and littoral environments. Only five of the total 51 oribatid species found in the forest-floor soil in the bank forest (OK-2), the usual habitat of oribatid mites, were also collected from the bark of trunks of B. gymnorrhiza (Table 2). In general, arboreal oribatid communities differ from soil communities (Aoki, 1973). Many studies have revealed that each arboreal microhabitat, such as canopy, trunk, and epiphytes, supports ! 1978, 1985; Ito, different oribatid fauna (Andre,

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S. Karasawa, N. Hijii

Figure 2. Similarity dendrogram of oribatid communities in all microhabitats.

Cohort

Family

Species

Microhabitat Littoral algae

Cosmochthoniidae

MIXONOMATA

Steganacaridae Euphithiracaridae Trhypochthoniidae Liodidae Oppidae Thyrisomidae Selenoribatidae

DESMONOMATA BRACHYPYLINA

Fortuyniidae Cymbaeremaeidae Haplozetidae Oribatulidae Oripodidae

Palakalummidae Sheloribatidae

Humerobatidae Oribatellidae Galumnidae

Cosmochthonius reticulatus Grandjean Notophthiracarus hamatus (Hammer) Rhysotritia ardua (C. L. Koch)a Trhypochthonius sp. Liodes sp.a Oppidae sp. Banksinoma sp. Arotrobates granulatus Luxton Selenoribatidae sp. Alismobates reticulatus Luxton Fortuynia marina Hammen Scapheremaeus sp. Incabates sp.a Peloribates barbatus Aoki Dometorina sp. Oribatulidae sp. Oripoda sp. Oripodidae sp. Truncopes moderatus Aoki et Ohkuboa Parakalumma sp. Scheloribates sp.1 Scheloribates sp.2 Scheloribatidae sp.1 Scheloribatidae sp.2 Humerobates varius Ohkubo Oribatella calcarata (C. L. Koch) Pergalumna sp.a

0–50-cm bark

50–100cm bark

100–150cm bark

Branch

Leaf

n

n n

n n n n n n

n n n

n n n

n

n n

n n n

n n

n

n

n

n

n

n n

n n

n n n

n

n

n

n

n

n n n

n n n

n

n

n

nPresent. a

Five species of the total 51 species found in the forest-floor soil in the bank forest (OK-2).

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ENARTHRONOTA

Bark of knee root

Structure diversity and oribatids in mangrove forests

Table 2. Oribatid fauna collected from the mangrove forests on Okinawa (OK-1) and Iriomote (IR) islands in the Ryukyu Islands

251

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1986; Nicolai, 1986; Walter and Behan-Pelletier, 1993; Walter et al., 1994; Walter, 1995; BehanPelletier and Winchester, 1998; Winchester et al., 1999; Behan-Pelletier and Walter, 2000). The cluster analysis (Fig. 2) illustrates that, in the mangrove and bank forests, the component species of oribatid communities in the canopy group (leaves, branches, and shoots) differed from those in the trunk-bark group. Furthermore, the species in both groups differed from those in the floodedtrunk group, to a similar extent by which they differed from the species in the forest-floor soil and littoral algal groups. Thus, the flooded trunks in mangrove forests provide a specific microhabitat for oribatid mites that is different from the canopy and the soil.

Flooded trunk as a microhabitat for oribatid mites The oribatid communities on the flooded bark (knee-root bark and the bark of the trunk at 0– 50-cm height) of B. gymnorrhiza had fewer species and a lower species diversity than those on the other types of bark (Table 1). This may reflect the fact that the knee root and the lowest part of the

S. Karasawa, N. Hijii

trunk are flooded periodically as the tide rises. Terayama (1989) reported that few ants were collected from the trunk at 0–50-cm height or from detritus on the forest floor in a mangrove forest. In our study, however, the densities of oribatid mites on the flooded bark were similar to, or even greater than, those on the other trunk-bark microhabitats (Table 1). This result suggests that only a few oribatid species, which are adapted to that environment, can inhabit such microhabitats. Of the eight oribatid species found on the flooded bark, one (Oripodidae sp.), which is probably an undescribed species, was restricted to that habitat type (Table 2). Beck (1969) reported that large numbers of Rostrozetes ovulum (Berlese), which is common in forest soils in the northern USA and southeastern Canada (Norton and Palmer, 1991), survived inundation in a Brazilian flooded forest. In our study, however, the Oripodidae sp. did not occur in the forest-floor soil or on the littoral algae. Therefore, species richness in these microhabitats seems to have been reduced because oribatids, in general, are not marine detritivores. At the same time, the flooded trunks create a unique habitat that apparently contains a unique oribatid species that enhances the species diversity of oribatid mites in mangrove forests (Fig. 3).

Figure 3. Schematic diagram of the distribution pattern of oribatid communities in mangrove and bank trees in the Ryukyu Islands.

ARTICLE IN PRESS Structure diversity and oribatids in mangrove forests

Effects of microhabitat diversity and geographical isolation on the structure of oribatid communities The oribatid communities on the leaves and branches of B. gymnorrhiza were more similar to those on the same microhabitats at the other sites than to those on different microhabitats at the same site. These communities were also similar to those of C. equisetifolia, whose leaf morphology is different from that of B. gymnorrhiza. Communities on the trunk bark at various heights also shared greater similarity with those in the same microhabitat at other sites than those in other microhabitats at the same site (Fig. 2). Thus, oribatid communities in the same microhabitats at different sites tend to be more similar than those in different microhabitats at the same site (Fig. 3). This points to the adaptations many species appear to have for living in arboreal and littoral habitats, perhaps reflecting the different structures, air movement patterns, and general microclimates of microhabitats (Hammen, 1960, 1963; Behan-Pelletier and Walter, 2000). A few geohistorical studies of the Ryukyu Islands have indicated that it was probably during the early Pleistocene (ca. 1.5–1.3 Ma) that the last landbridge connection of Okinawa Island with Iriomote Island occurred (Kizaki and Oshiro, 1980; Kizaki, 1997). Ichisawa (2001) reported that arboreal oribatid mites were dispersed by wind while Behan-Pelletier and Winchester (1998) pointed out that active dispersal by random movement is an important mode of colonization of canopy habitats by oribatid mites. Coulson et al. (2002) found that it is feasible for oribatid mites to directly disperse over great distances on, or in, seawater, without the need to be attached to driftwood. However, since the fossils of oribatid mites were recorded from Devonian (ca. 400 Ma) (Norton et al., 1988), the mangrove fauna of oribatid mites may have already been established when the islands were connected. As a result similar characteristics of the oribatid communities may have remained until the present day. Since we have no direct evidence of dispersal between distant islands and no geographical information about the establishment of the mangrove forests, we could not detect the effects of geographical isolation on the community characteristics on either island. In conclusion, the structure of oribatid communities in mangrove forests is more likely to be affected by factors responsible for habitat diversification (characterized specifically by the flooded

253

trunks of mangrove trees) than by geographical distance between the islands.

Acknowledgements We thank Dr. M. Shimojana of the School of Education of University of the Ryukyus for valuable comments and criticism; Mr. T. Sasaki of the Museum at University of the Ryukyus for supporting the field work; and Dr. H. Harada of the School of Educational and Human Science of Yokohama National University, Drs. J. Aoki and K. Ichisawa of the Kanagawa Prefectural Museum, and Mr. N. Ohkubo of the Mie Plant Protection Office for valuable advice on oribatid mites; and anonymous referees for valuable comments.

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