Communities of Oribatida associated with litter input in western red cedar tree crowns: Are moss mats ‘magic carpets’ for oribatid mite dispersal? Zoë Lindo Department of Biology, University of Victoria, PO Box 3020, Victoria, British Columbia, V8W 3N5, Canada. Present address: Department of Biology, McGill University, 1205 Docteur Penfield, Montreal, Quebec, H3A 1B1, Canada. E-mail: [email protected] Oribatid mite abundance, species richness, and community composition in annual litter fall were compared between the high canopy of an ancient temperate rainforest and the forest floor to evaluate whether litterfall, including moss debris, is a dispersal vector for these organisms. Oribatid mites were extracted from litterfall collected from canopy (30 m) and ground (1 m) litter traps associated with six western red cedar trees in the Walbran Valley on the south- west coast of Vancouver Island, Canada, over 3, 6, and 12 months. Total annual litter input was not significantly dif- ferent between canopy and ground traps, as high amounts of litter were associated with both habitats. Litter compo- sition differed between the two habitats and cumulative input over larger spatial scales may prove to be appreciably different. Fifty-seven species of oribatid mites were associated with total litterfall collected in canopy and ground traps over 12 months. Species richness over the entire sampling period was similar between canopy and ground habitats, but oribatid mite species composition differed significantly, and is most likely related to litter composition and the ini- tial source of litter. Oribatid mite abundance (number of individuals per gram dry weight) associated with litterfall was low compared to suspended soil habitats, and not significantly different between litter accumulation in ground and canopy traps. Nevertheless, a general trend of high litter input and high species richness associated with litterfall in canopy habitats, combined with high disperser survivorship, suggests dispersal vectors such as moss mats are impor- tant for maintaining arboreal oribatid mite communities. Key words: Canopy, oribatid mites, litterfall, dispersal vector n ancient western red cedar trees (Thuja plicata D. Don) to increase in abundance (Hubbell, 2001). In fragmented, I(Cupressaceae) of North American temperate rainforests, isolated, or patchily distributed habitats like suspended soils, suspended soils form high above the forest floor (ca. 35 m) I would expect dispersal to be especially limiting, and may and occur as discrete patches of habitat ranging in surface explain the arboreal specificity in forest oribatid mite com- area from 100 to 20,000 cm2 (Lindo & Winchester, 2006). munities. Suspended soils are habitat islands of accumulated organic The mechanism of oribatid mite colonization of canopy matter and epiphytes separated from one another within a habitats is unknown. Colonisation of canopy habitats by tree crown by a barren bark matrix and between trees by the ground-dwelling oribatid mites is unlikely based on ground/ atmosphere (Moffet, 2000). These habitats contain an abun- canopy comparison studies, and recently, the trunk has been dant and species-rich community of arboreal micro-arthro- dismissed as a potential dispersal corridor (Proctor et al., pods dominated by oribatid mites (Oribatida), many species 2002; Beaulieu et al., 2006; Lindo & Winchester, 2007b). A of which are undescribed and not found on the forest floor more parsimonious explanation is that canopy oribatid mites (Lindo & Winchester, 2006). Arboreal specificity in micro- colonize new canopy habitats from other canopy sources. arthropod communities and in particular oribatid mites has The proposed mechanisms of canopy-to-canopy dispersal been well documented (see Behan-Pelletier & Walter, 2000) include cursorial transport (Beaulieu et al., 2006), active and canopy/ground comparison studies show that the num- (Norton, 1980) and accidental (Krivolutsky & Lebedeva, ber of oribatid mite species in common between the two 2004a, b) phoresy, and aerial plankton (Karasawa et al., habitats is typically less than 40% (Wunderle, 1992a; Behan- 2005). Pelletier et al., 1993; Winchester et al., 1999; Lindo & Another possibility for aerial transport is via dispersal Winchester, 2006). Among the factors affecting the diversity vectors, such as moss and lichen propagules, leaf litter, and abundance in arboreal oribatid mite communities are branch tips, and twigs. The importance of a particular vector tree species, elevation (Fagan et al., 2005), and the availabil- depends on the rate at which dispersers are carried to a ity of habitat (Lindo & Winchester, 2007a). Dispersal events, recipient habitat (propagule load) and on the survivorship colonization history, local and regional scale structural com- during transport. Dispersal vectors could have high propag- plexity, and stability of suspended soil environments are also ule loadings and high survivorship of dispersers due to less- hypothesised as major determinants of arboreal oribatid er changes in environmental conditions during transport. mite species richness (Southwood, 1996). There is limited evidence for such a passive dispersal mech- Dispersal is a dynamic biological process that is a driver anism in arboreal oribatid mites (Wunderle, 1992b; Karasa- of many ecological theories such as island biogeography wa et al., 2005), but horizontal movement within canopy sys- (MacArthur & Wilson, 1967), metapopulation (Hanski, 1999) tems is probable, as is transport to lower canopy habitats or and metacommunity dynamics (Wilson, 1992), and neutral the forest floor by litterfall. models (Bell, 2000; Hubbell, 2001; Chave et al., 2002). Under The degree to which dispersal and dispersal limitations low rates of dispersal, species are more aggregated, and contribute to the dynamics and structure of communities rare, local, or endemic species can survive in high abundance advocates the need to elucidate the mechanisms of disper- because of lack of competitors and adequate ecological time sal, to quantify dispersal rates, as well as to quantify the spa- Trends in Acarology [2009] M.W. Sabelis & J. Bruin (eds.) 143 Zoë Lindo Table 1 Results of repeated measures ANOVA for oribatid mite species richness (average number species per trap), and abundance of micro- arthropods (total number individuals per g dwt litter; mean ± SD) collected in litter traps at 30 m and 1 m above the forest floor associated with six western red cedar trees after 12 months. Given are P-values for different sources of variation with degrees of freedom in parenthe- ses. Canopy (30 m) Ground (1 m) Habitat (1,10) Time (2,20) Time × Habitat (2,20) Oribatida species richness 19.17 ± 2.8 13.50 ± 5.1 0.007 <0.001 <0.001 Oribatida abundance 0.542 ± 0.32 0.506 ± 0.20 0.123 0.482 0.038 Prostigmata abundance 0.249 ± 0.09 0.224 ± 0.32 0.701 0.001 0.923 Mesostigmata abundance 0.082 ± 0.05 0.095 ± 0.17 0.686 0.260 0.125 Collembola abundance 10.407 ± 11.51 1.562 ± 0.60 0.096 0.039 0.069 Other micro-arthropod abundance 0.195 ± 0.28 0.139 ± 0.04 0.435 0.038 0.297 Total micro-arthropod abundance 11.475 ± 11.38 2.527 ± 0.86 0.234 0.044 0.152 tial scale in which dispersal occurs. The objectives of this I used repeated measures ANOVA to test for the effect of study were: (1) measure and compare annual amounts of lit- time and habitat on litter input in canopy and ground litter ter input into canopy and ground habitats, and (2) observe traps, abundance of major micro-arthropod groups, and ori- oribatid mite species and abundances associated with annu- batid mite species richness. These analyses were performed al litter input in these two habitats to evaluate whether lit- using Statistica 7.0 (StatSoft Inc., 2004) with a significance terfall is a dispersal vector for these organisms. level of α = 0.05. Community composition of adult oribatid mites in canopy- and ground-collected litterfall was evaluat- ed using non-metric multidimensional scaling (NMDS) MATERIALS AND METHODS (Clarke, 1993) which arranged the samples (traps) with The study site was located in the temperate rainforest of the respect to the rank order of similarity in community compo- Walbran Valley on the southwest coast of Vancouver Island, sition based on Bray-Curtis similarity of √x-transformed orib- British Columbia, Canada (48°39’N, 124°35’W). The valley atid mite species abundance data. The final ordination of a lies entirely within the CWH biogeoclimatic zone (Meidinger priori trap placement was assessed for significance of ran- & Pojar, 1991) where the climate is characterized by wet, dom occurrence based on analysis of similarities (ANOSIM) humid, cool summers and mild winters, and where a mean with 10,000 randomized permutations using habitat (30 m annual precipitation of 2,990 mm is typical for this area canopy, 1 m ground) and collection time (3, 6, 12 months) as (Environment Canada, 2006). Conifers are dominant in this factors (Primer, 2001). rainforest and include western hemlock (Tsuga heterophylla (Rafn.) Sarg.), Sitka spruce (Picea sitchensis (Bong) Carr.), Amabilis fir (Abies amabilis (Dougl.) Forb.), and western red RESULTS cedar (Thuja plicata D. Don). The six western red cedar trees Total amount of litterfall over 12 months in canopy vs. ground used in this study were approximately 50 m tall, with the traps was not statistically different (F1,10 = 3.079, P = 0.110), diameter of the trunks at breast height ranging from 2.13 to although litterfall was consistently higher in canopy litter traps 3.65 m (mean ± SD = 2.71 ± 0.52 m). These trees are estimat- (Fig. 1). There was a significant effect of time on amount of lit- ed to be 800-1,200 years old, and well developed suspended terfall collected, with the most litterfall collected between 3 2 soils ranging in size from 0.1 to 2.0 m are abundant within and 6 months (F2,20 = 14.212, P<0.001). Litterfall primarily con- tree crowns at heights greater than 20 m where trunk reiter- sisted of moss, cedar bark, litter and twigs, hemlock cones and ations and major limb junctions take place.