Resident corticolous oribatid mites (Acari: Oribatida): Decay in community similarity with vertical distance from the ground 1

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1 14 (2): (2007) Resident corticolous oribatid mites (Acari: Oribatida): Decay in community similarity with vertical distance from the ground 1 Zoë LINDO 2 & Neville N. WINCHESTER, Department of Biology, University of Victoria, P.O. Box 3020, Victoria, British Columbia V8W 3N5, Canada, zlindo@uvic.ca Abstract: The decrease in community similarity was examined in corticolous oribatid mites (Acari: Oribatida) sampled along a 36-m vertical profile of 5 western redcedar trees in a temperate rainforest on Vancouver Island, Canada. Samples were collected every 2 m, and all adult oribatid mites were identified to species. When compared to species recorded from previous ground/canopy sampling efforts in the same trees, the 62 corticolous species unequivocally support the separation of these 2 communities at 4 m. All sampling heights contained canopy oribatid species, but only 0 4 m communities contained ground-dwelling oribatids. There was significant overall spatial autocorrelation and decay in community similarity with distance originating from species turnover at 4 m, suggesting limited range expansion of ground species into corticolous habitats. Community similarity, richness, and abundance of corticolous oribatid mite assemblages were not autocorrelated after 4 m above ground. Observed patterns at 4 m likely represent an environmental transition zone for ground-dwelling species, such as changes in moisture availability, rather than a physical dispersal barrier for individuals. We conclude that the trunk is not a dispersal corridor for ground species to colonize tree crowns and suggest that low similarity between nearest neighbouring sampling points, combined with the presence of immature and gravid oribatids, supports the assumption that corticolous oribatid mite assemblages are likely dispersal-limited residents. Keywords: community similarity, corticolous habitats, distance decay, oribatid mites, spatial autocorrelation, temperate rainforest. Résumé : La diminution de similarité entre communautés a été étudiée chez des acariens oribatides (Acari: Oribatida) corticoles échantillonnés le long d un profil vertical de 36 m sur 5 thuyas géants dans une forêt pluviale tempérée sur l île de Vancouver, Canada. Des échantillons ont été prélevés aux 2 m et tous les acariens oribatides adultes ont été identifiés à l espèce. Nous avons comparé les 62 espèces corticoles identifiées aux espèces trouvées lors d efforts d échantillonnage précédents à la fois près du sol et dans la canopée des mêmes arbres et cette comparaison supporte de façon non équivoque la séparation des 2 communautés à 4 m du sol. Des espèces oribatides de la canopée ont été trouvées à toutes les hauteurs alors que les oribatides du sol n ont été trouvés que dans les communautés en-dessous de 4 m. Dans l ensemble, il y avait une autocorrélation spatiale significative et une diminution de la similarité des communautés avec la distance qui avaient pour origine le renouvellement des espèces à 4 m. Cela suggère une expansion limitée des espèces du sol dans les habitats corticoles. La similarité des communautés ainsi que la richesse et l abondance des assemblages d acariens oribatides corticoles n étaient pas autocorrélés au-dessus de 4 m. Les patrons observés à 4 m représentent probablement plus une zone de transition environnementale pour les espèces du sol comme un changement dans la disponibilité en eau plus qu une barrière physique à la dispersion des individus. Nous concluons que les troncs ne constituent pas des corridors de dispersion pour que les espèces du sol puissent coloniser la cime des arbres. Nous suggérons que la faible similarité entre les points d échantillonnage les plus près combinée avec la présence d oribatides immatures et de femelles gravides supportent l hypothèse que les assemblages d acariens oribatides résidents sont probablement limités dans leur dispersion. Mots-clés : acariens oribatides, autocorrélation spatiale, diminution avec la distance, forêt pluviale tempérée, habitats corticoles, similarité des communités. Nomenclature: Marshall, Reeves & Norton, Introduction Recent canopy studies represent an area of ecological research that is contributing to the investigation of spatial patterns in biodiversity (Basset et al., 2003; Ozanne et al., 2003; Fagan et al., 2006). Canopy research has consistently described the presence of unique arboreal arthropod communities that are associated with tree crown microhabitats (Erwin, 1995; Stork, Adis & Didham, 1997; Basset, 2001, Basset et al., 2003). For example, suspended canopy soils in 1 Rec ; acc Associate Editor: Timothy T. Work. 2 Author for correspondence. temperate and tropical rainforests are habitats of accumulated organic matter (Moffett, 2000) with species-rich microarthropod communities often dominated by oribatid mites (Acari: Oribatida) (Behan-Pelletier et al., 1993; Winchester, Behan-Pelletier & Ring, 1999). Canopy/ground soil comparisons show that the arboreal oribatid mite community generally consists of oribatid mite assemblages that are dissimilar from those of the forest floor (see Behan-Pelletier & Walter, 2000), with the proportion of oribatid mite species in common between the 2 habitats being lower than 40% (Wunderle, 1992; Behan-Pelletier et al., 1993; Winchester, Behan-Pelletier & Ring, 1999; Lindo & Winchester, 2006).

2 Lindo & Winchester: Dispersal limited corticolous mite communities Aoki (1973) described the portion of the oribatid mite community found in both ground and canopy systems as wandering forms, indicating that oribatid mites climb trees (Aoki, 1973) using the trunk, which acts as a dispersal corridor between the 2 habitats. However, Prinzing and Woas (2003) suggest that the similarity between canopy and forest floor communities is not explained by dispersing individuals. A recent study by Proctor et al. (2002) concluded that tree trunks were not highways of dispersal for mites to colonize the canopy, but habitats unto themselves. Despite previous studies of corticolous (bark-dwelling) habitats (André, 1984; Nicolai, 1993; Proctor et al., 2002; Prinzing, 2005), much of the vertical spatial patterning of bark arthropod communities is unknown. Ineffective or inefficient sampling methods for oribatid mites (Aoki, 1973; Moeed & Meads, 1983), lack of oribatid mite taxonomic resolution (Moeed & Meads, 1983; Heliövaara & Väisänen, 1986; Hanula & Franzreb, 1998; Majer et al., 2003), and/or sampling effort limited to heights within 2 m above the ground (Moeed & Meads, 1983; André 1984; Prinzing, 2001, Proctor et al., 2002; Majer et al., 2003) have been the major limitations in exploration of spatial patterning in corticolous oribatid mite communities. Additionally, many studies had no prior knowledge of the canopy/ground fauna assemblage with which to compare bark community assemblages (Moeed & Meads, 1983; Heliövaara & Väisänen, 1986; Majer et al., 2003). In systems with continuous corticolous epiphytic cover such as mosses, a high occurrence of soil/litter Collembola, Acari, and spiders in the canopy suggests continuity of the ground and canopy habitats (Basset, 2001). However, detection of discrete communities along a ground to canopy habitat gradient in the absence of corticolous epiphytic cover requires more than a description of the presence/absence of species. Emerging ecological models, spatial autocorrelation, and beta diversity analyses are areas of investigation used to infer mechanisms of dispersal and dispersal limitations or delineate the source area of colonizing species (Borcard, Geiger & Matthey, 1995; Hardy & Sonké, 2004), although these evaluations remain mostly theoretical. While positive spatial autocorrelation has been shown in many plant (Condit et al., 2002; Tuomisto, Ruokolainen & Yli-Halla, 2003) and animal (Poulin, 2003; Lloyd, MacNally & Lake, 2005; Mena & Vazquez-Domínguez, 2005) communities, it is unstudied in corticolous assemblages of oribatid mites. Here we document the vertical distribution of the corticolous oribatid mite assemblage on ancient western redcedar trees in a temperate North American rainforest and test for the presence of spatial autocorrelation. We discuss the low dispersal ability of corticolous oribatid mite species in light of patterns associated with their pair-wise assemblage similarity (beta diversity) and magnitude of spatial autocorrelation (species turnover) as measured by the change in beta diversity over distance. Methods Study area and sampling The study site is located in the temperate rainforest of the Walbran Valley on the southwest coast of Vancouver Island, British Columbia, Canada (48 39' n, ' w). The valley lies entirely within the Coastal Western Hemlock biogeoclimatic zone (Meidinger & Pojar, 1991) where the climate is characterized by wet, humid, cool summers and mild winters and a mean annual precipitation of 2991 mm is typical (Environment Canada, 2006). Conifers dominate this rainforest and include western hemlock (Tsuga heterophylla), Sitka spruce (Picea sitchensis), Amabilis fir (Abies amabilis), and western redcedar (Thuja plicata). The western redcedar trees sampled in this study are part of an ongoing project in the Walbran Valley (Lindo & Winchester, 2006). These trees are approximately 50 m in height, have trunk reiterations creating a multi-furcated crown that accumulate organic matter, and form discrete patches of suspended soil that contain species-rich oribatid mite assemblages (Lindo & Winchester, 2006). The diameter of the trunks at breast height (DBH) ranges from 2.13 to 2.72 m (mean DBH = 2.52 m ± 0.27 SD), and the average trunk is devoid of limbs and reiterations below 30 m (mean limb height = 33 m, minimum = 18 m, maximum = 45 m). The bark of western redcedar typically ranges from 10 to 25 mm thick, is fibrous with longitudinal fissures, and can be easily removed from live trees in small amounts without injury to the tree. The bark is consistently free from accumulations of lichens, moss, and organic debris all the way up the trunk except where trunk reiterations, limb junctions, and other protuberances allow the accumulation of organic matter to form suspended soils. Sampling was conducted July 23 26, Single rope climbing methods (Lindo & Winchester, 2006) were used to access a range of heights along the trunk of 5 western redcedar trees. Bark samples were removed along the north side of the trees from a cm area to a depth of 1 cm using a paint scraper and collected in plastic bags. Samples were collected every 2 m from the trunk base (0 m) to a maximum height of 36 m. Where trunk reiterations occurred, only the largest of the trunks was sampled. A total of 95 bark scraping samples were collected and extracted in Berlese funnels within 3 d of collection for 48 h (Norton & Kethley, 1988). Following extraction, sample dry weights were measured and the number of microarthropods per gram of dry weight was recorded. Extracted microarthropods were sorted into the following taxonomic groups: mites (Acari), springtails (Collembola), and other microarthropods, which included pseudoscorpions (Pseudoscorpiones), beetles (Coleoptera), wingless parasitic wasps (Hymenoptera), millipedes (Myriapoda), and spiders (Araneae). The Acari were further identified to suborder (Mesostigmata, Prostigmata, and Oribatida), and adult oribatid mites were identified to species (Marshall, Reeves & Norton, 1987). Representative specimens were slide mounted using Hoyer s medium (Krantz, 1978), and a reference collection is deposited at the Canadian National Collections in Ottawa, Canada. Statistical analyses Standardized abundance of microarthropod groups and oribatid mite species richness was evaluated along the vertical height gradient of trees using one-way analysis of variance (ANOVA) and least significant difference (LSD) 224

3 ÉCOSCIENCE, vol. 14 (2), 2007 multiple comparison tests of each height. For these analyses, a conservative 1% significance level was used to adjust the probability of type I error arising from lack of independence of sampling points due to spatial autocorrelation. While unsophisticated, this adjustment provides assurance of α = 0.05 even when autocorrelation reduces the effective sample size by more than an order of magnitude (Dale & Fortin, 2002). Actual spatial autocorrelation of the oribatid mite communities along the vertical stratification of the trunks was evaluated using standardized species abundance of replicate height samples. We compared a community compositional similarity matrix based on Bray Curtis similarity of square root transformed oribatid mite species abundances with a geographical distance matrix based on absolute Euclidean distances of the trunk samples using a Mantel test (Manly, 1997) in Primer 5 (Primer-E Ltd., Plymouth, UK). The Mantel test produced a rank correlation coefficient (r m ) that corresponded to the average magnitude of spatial autocorrelation of the community composition similarity over all sampling heights. The significance of the matrix correlation coefficient was assessed by a randomization test with permutations. In order to examine the patterns and significance of spatial autocorrelation across different heights, a Mantel correlogram and a distance decay plot were constructed. In the Mantel correlogram, data were divided into distance classes and a normalized Mantel r m for each distance class was calculated. The degree of spatial autocorrelation within each distance class was assessed separately for significance by permutation and a Bonferroni correction for multiple significance tests (testwise α = 0.05/5 = 0.01). In the distance decay model, all pair-wise comparisons of Bray Curtis similarity were plotted against absolute vertical distances. Results Sixty-two species representing 44 genera and 33 families were identified from 2291 adult oribatid mites collected from all bark scraping samples from 5 western redcedar trees (Table I). Average oribatid, prostigmatid, and mesostigmatid mite abundance (no. individuals g dwt substrate 1 ) and average total microarthropod abundance showed similar trends of decreasing abundance with increasing height and a distinct decrease in abundance above 4 m (Figure 1). This trend was significant for oribatid mite abundance at 0, 2, and 4 m compared to other heights (ANOVA: F 18, 76 = 1.987, P = 0.021). Average species richness of oribatid mites was significantly greater at 0 m when compared to all other heights and generally decreased with height from ground (ANOVA: F 18, 76 = 7.193, P < 0.001) (Figure 2). There was a significant overall relationship between Bray Curtis similarity and geographical distance when all samples were included in a global Mantel test (r m = 0.313, P = 0.002), but the Mantel correlogram failed to reveal the source of significant spatial autocorrelation (Table II). However, the distance decay model demonstrated how the 0-, 2-, and 4-m sampling points contributed to the overall Bray Curtis pair-wise comparisons (Figure 3). Discussion Arboreal specificity in temperate microarthopod communities and in particular oribatid mites has been well doc- Table I. Oribatid mite species (Acari: Oribatida) from corticolous habitat of western redcedar trees in the Walbran Valley, Vancouver Island, British Columbia, Canada. Family Species* 0 4 m 6 36 m Palaeacaridae Palaeacarus hystricinus 5 0 Brachychthoniidae Synchthonius crenulatus 0 1 Synchthonius sp Eobrachychthonius sp. 6 0 Liochthonius sp Liochthonius sp Liochthonius sp Phthiracaridae Archiphthiracarus sp Archiphthiracarus sp Oribotritiidae Mesotrita nuda 1 0 Maerkelotritia sp. 1 7 Epilohmanniidae Epilohmannia sp Nanhermanniidae Nanhermannia elegantula 1 0 Hermanniidae Hermannia gibba 3 0 Neoliodidae Platyliodes macroprionus 0 1 Gymnodamaeidae Gymnodamaeus sp. 0 3 Hungarobelbidae Hungarobelba sp. 2 0 Damaeidae Epidamaeus sp. nr. floccosus Belba (Belba) sp. 0 1 Cepheidae Conoppia sp. 6 0 Eupterotegaeus rhamphosus Eupterotegaeus sp. nr. rostratus Eremaeidae Eueremaeus acostulatus 0 1 Eueremaeus chiatous 0 24 Eueremaeus marshalli 0 39 Tenuialidae Tenuiala sp. 2 0 Liacaridae Liacarus sp. 1 nr. bidentatus 5 0 Liacarus sp. 2 nr. robustus 1 0 Peloppiidae Ceratoppia sp Ceratoppia sp Carabodidae Carabodes hoh 1 0 Tectocepheidae Tectocepheus velatus Oppiidae Oppiella nova 66 0 Oppiella sp Moritzoppia sp Quadroppiidae Quadroppia quadricarinata 19 0 Suctobelbidae Suctobelbella sp. 2 nr. longicuspis 8 2 Suctobelbella sp Suctobelbella sp Suctobelbella sp Suctobelbella sp Suctobelbella sp Autognetidae Autogneta sp. nr. longilamellata 2 0 Thyrisomidae Banksinoma lanceolata 1 0 Cymbaeremaeidae Scapheremaeus sp. 1 0 Achipteriidae Achipteria sp. nr. curta 7 0 Dentachipteria sp Anachipteria acuta 3 2 Phenopelopiidae Eupelops sp. 1 0 Scheloribatidae Scheloribates sp Parapirnodus hexaporosus 0 14 Oribatulidae Phauloppia sp. 0 5 Zygoribatula sp. 1 2 Oribatellidae Oribatella sp Chamobatidae Chamobates sp. 7 0 Ceratozetidae Ceratozetes pacificus 9 0 Melanozetes crossleyi 38 0 Sphaerozetes winchesteri 0 4 Sphaerozetes sp Sphaerozetes sp Mycobatidae Cyrtozetes sp. 0 8 Mycobates corticeus Total *Species are numbered to be consistent with Lindo and Winchester,

4 Lindo & Winchester: Dispersal limited corticolous mite communities Figure 1. Average microarthropod abundance (number individuals g 1 dwt bark + SE) collected from heights along the trunks of 5 western redcedar trees in the Walbran Valley, Vancouver Island, Canada. umented (Wunderle, 1992; Behan-Pelletier & Winchester, 1998; Fagan et al., 2006, Lindo & Winchester, 2006), but where along the vertical gradient between the ground and tree crown this specificity occurs remains unresolved. We found that abundances of total microarthropods, oribatid, prostigmatid, and mesostigmatid mites all demonstrate a separation in corticolous communities between 4 and 6 m above the ground. This trend is also evident in oribatid species richness and oribatid community similarity. All sampling heights contained cosmopolitan and known canopy species, but only the 0- to 4-m communities contained known ground-dwelling oribatids. There was high oribatid species richness below 4 m (49 species), including 29 species which were exclusive to the 0- to 4-m height class, and many corresponding to previously documented grounddwelling fauna such as Dentachipteria sp., Liacarus sp. 2, and Eupelops sp. Above 6 m there was a high overlap of species on bark with species of suspended soil habitats in the high canopy and low community overlap with ground assemblages (see Lindo & Winchester, 2006). Of the 13 species found strictly at or above 6 m, 11 had been previously recorded as canopy specialists, although Eueremaeus acostulatus and Suctobelbella sp. 6, which are rare in this system, have 226

5 ÉCOSCIENCE, vol. 14 (2), 2007 Figure 2. Average species richness of oribatid mites (+ SE) collected at heights along the trunks of 5 western redcedar trees in the Walbran Valley, Vancouver Island, Canada. Line is power law function (y = x ). Table II. Mantel r values for distance classes of corticolous oribatid mite communities along the vertical horizon of western redcedar tree trunks. P-values are based on permutations and compared to Bonferroni adjusted a = Distance class (m) r m P Figure 3. Distance decay model based on pair-wise Bray Curtis percent similarity of corticolous oribatid mite communities plotted against absolute vertical distance between pairs, with 0-, 2-, and 4-m comparisons highlighted for clarity. been recorded as singletons on the forest floor (Lindo & Winchester, 2006). Species having higher abundance on the bark than in suspended soils may be deemed corticolous; these include Mycobates corticeus, Moritzoppia sp., Synchthonius sp. 2, and Eupterotegeaus sp. nr. rostratus. The distinct difference in oribatid mite abundance, species richness, and community composition occurring at 4 m supports the canopy specificity argument. We suggest that sampling at heights below 4 m, as many previous bark studies have done (Moeed & Meads, 1983; André, 1984; Prinzing, 2001, Proctor et al., 2002; Majer et al., 2003), does not accurately represent the entire bark assemblage and may create a biased interpretation of similarity by overestimating the number of species in common between ground and canopy. The patterns observed also refute claims that tree trunks are dispersal corridors between ground and canopy habitats for oribatid mites in this system (Moeed & Meads, 1983; Hanula & Franzreb, 1998). Corridors facilitate movement of individuals (Gonzalez et al., 1998, Tewsbury et al., 2002) and provide connectivity of 2 or more large habitat areas (Beier & Noss, 1998). High community similarity and low species turnover, which typify distance decay models along dispersal corridors (Garcillán & Ezcurra, 2003) were not observed in the spatial patterns of community similarity in corticolous oribatids. Although there was significant spatial autocorrelation of community composition in corticolous oribatid mites with height, the overall similarity between sampling points was low (mean similarity of community composition across all pairs of heights = 38%) and was considerably lower for sampling points below 4 m compared with sampling points above 4 m (mean = 15%). Upon close examination of the decay in similarity patterns the overall rate of similarity decay with distance appears driven by pair-wise comparisons of 0-, 2-, and 4-m sampling points. Decreasing community similarity with increasing distance is derived from 2 factors: geographic distance related to dispersal abilities of species present and community association with spatial environmental gradients (Nekola & White, 1999; Briers & Biggs, 2005). However, separating the 2 mechanisms of spatial autocorrelation can be difficult (Tuomisto, Ruokolainen & Yli-Halla, 2003). Dispersal limitation can lead to differences in community composition even in areas of similar environments, and geographic distance often explains much of the variation in species composition (Qian, Ricklefs & White, 2005; Fellis & Esch, 2005). Spatial patterns of corticolous oribatid mites may be attributed to dispersal limitation, but other unmeasured environmental variables may also be important. We suggest that the patterns observed at 4 m represent a transition zone for ground-dwelling species rather than a physical dispersal barrier for all species. This transition zone may represent a biome shift in demographic or environmental requirements (such as moisture availability) of individual species (Risser, 1995; McDonald et al., 2005). As such, we cannot rule out spatial patterns being governed by factors other than dispersal coupled with geographical (ground/canopy) distance. For instance, moisture, temperature, or the availability of food have been suggested by Prinzing (2005) as factors exerting influence on corticolous microarthropod communities. While bark is a more extreme environment than either soil or suspended soil habitats (Prinzing, 2005), compensatory redistribution among microhabitats on exposed bark may enable mites to maintain viable moisture and temperate requirements (Prinzing, 2001). Oribatid mites are potentially able to complete whole lifecycles in epistratum environments as egg-laying can be carried out in old exuviae, under detritus, and in crevices (Butcher, Snider & Snider, 1971). Our observations confirm the presence of juveniles and gravid females (Z. Lindo, unpubl. data), suggesting that corticolous oribatid mites might form resident populations in this habitat. 227

6 Lindo & Winchester: Dispersal limited corticolous mite communities Conclusion The corticolous environment of western redcedar is a suitable habitat for microarthropods, including 62 species of oribatid mites, but it is not a dispersal corridor for oribatid species emigrating from ground to canopy habitats. Decay in community similarity with vertical distance from the ground and overall spatial autocorrelation originates from species turnover at 4 m. This shift in oribatid mite community assembly suggests limited range expansion of ground species into corticolous habitats, but our knowledge of the species present in this system and their previous ground/canopy distributions unequivocally supports a distinct canopy community. Low similarity between nearest neighbouring sampling points, in combination with a lack of spatial autocorrelation above 6 m, and the presence of immature and gravid oribatids suggests that corticolous oribatid mite assemblages may form dispersal-limited resident populations. Acknowledgements The authors gratefully acknowledge K. Jordan (arbornautaccess@hotmail.com) for his contribution in climbing methodology, bark and canopy sampling, and general field expertise and A. Sastri and V. Behan-Pelletier for comments on drafts of this manuscript. This research was supported by National Science and Engineering Research Council of Canada grant no (Z. Lindo) and grant no (N. N. Winchester). Literature cited André, H. M., Notes on the ecology of corticolous epiphyte dwellers 3. Oribatida. Acarologia, 25: Aoki, J.-I., Soil mites (oribatids) climbing trees. Pages in M. Daniel & B. Rosicky (eds.). Proceedings of the Third International Congress of Acarology. Prague. 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