Mixed effects of habitat fragmentation on species richness and community structure in a microarthropod microecosystem

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1 Ecological Entomology (2005) 30, Mixed effects of habitat fragmentation on species richness and community structure in a microarthropod microecosystem MARTIN HOYLE 1,2 and ALASTAIR R. HARBORNE 2 1 School of Biology, University of Nottingham, Nottingham, U.K. and 2 School of Biological and Chemical Sciences, University of Exeter, Exeter, U.K. Abstract. 1. Theory is unclear about the optimal degree of isolation of habitat fragments where the aim is to maximise species richness. In a field-based microecosystem of Collembola and predatory and non-predatory mites, moss patches of the same total area were fragmented to varying degrees. The habitat was left for several months to allow the communities to approach a new state of equilibrium. 2. The species richness (in particular of predatory mites) of a given area of habitat was greater when it was part of a large mainland area than part of an island, in agreement with theory. 3. Conversely, species richness and abundance were largely unaffected by fragmentation of a fixed area of island habitat. In this case, it is suggested here that the advantages of several small patches (e.g. reduced impact of environmental stochasticity, wider range of habitats overall) were equally balanced by the advantages of a single large patch (e.g. reduced effect of demographic stochasticity, wider range of habitats within a single patch, reduced edge effect), or that both effects were small. 4. The shapes of rank abundance curves were similar among the levels of fragmentation of a fixed area of island habitat, implying that fragmentation had little impact on community structure. Conversely, the species composition of non-predatory mites varied weakly, but significantly, by fragmentation. Key words. Beta diversity, conservation, habitat heterogeneity, metapopulation, SLOSS. Introduction After fragmentation, habitats undergo a process of community disassembly, or relaxation (Diamond, 1972). The number of species that will be lost in the future is the extinction debt (Tilman et al., 1994), which is partly due to demographic stochasticity. The island species richness is expected to decrease over time to its new equilibrium value. The degree of habitat fragmentation that maximises species richness may depend on the stage in the relaxation process. Correspondence: M. Hoyle, School of Biological and Chemical Sciences, Hatherly Laboratories, University of Exeter, Prince of Wales Road, Exeter EX4 4PS, Devon, U.K. m.w.hoyle@exeter.ac.uk Immediately after habitat fragmentation, the number of habitats and the proportion of species in common between the sub-populations will be critical (Higgs, 1981). Fragmented habitat dispersed over a large area is likely to capture a greater range of habitat types than a single large habitat patch of the same total area. As many species are restricted to particular habitats, several small patches may contain more species than a single large patch. The further apart the habitat patches, the greater the habitat diversity likely to be encompassed, and the greater the species richness (Simberloff, 1986). Some species require more than one habitat type (e.g. used by different life stages), and if movement is restricted among habitat patches, a single large patch may contain more species than several small patches. 684 # 2005 The Royal Entomological Society

2 Microarthropod habitat fragmentation 685 After relaxation, on the other hand, in addition to the number of habitats and the proportion of species in common between the sub-populations, metapopulation processes such as migration rates between patches and environmental and demographic stochasticity will influence the optimal number of patches. The species richness of several small habitat patches depends in part on the rate of accumulation of species extinctions. To estimate the time to extinction for a population, it is necessary to know the distribution of extinction times of sub-populations and the correlation of disturbances among sub-populations. The effect of demographic stochasticity is more pronounced for smaller populations, thus species should be more prone to extinction in a fragmented landscape than in a large remnant patch of the same total area (Burkey, 1989), suggesting that nature reserves should be designed to be as continuous as possible. On the other hand, environmental stochasticity such as fire, disease, invasions by non-native species, and drought may reduce the persistence time of populations in a single large patch (Mangel & Tier, 1993) and may counteract the effect of demographic stochasticity, making populations in fragmented systems less vulnerable to extinction than populations in continuous habitats. The magnitude of these effects will depend on the organism in question, the scale of the system and the degree of environmental correlation. If migration between reserve fragments is possible, the rescue effect (Brown & Kodric-Brown, 1977) may significantly increase species richness and abundance (Gilbert et al., 1998; Gonzalez et al., 1998). Single-species metapopulation modelling (Pelletier, 2000; Ovaskainen, 2002) suggests that the optimal degree of fragmentation of a given total area of habitat depends on the rate of migration among patches and the rates of mortality in habitat and non-habitat patches. With low or no migration, many small patches may be preferable to a single large reserve of the same total area (Pelletier, 2000; Hubbell, 2001). With migration, some studies suggest that an intermediate number of sub-populations maximises metapopulation persistence (Stacey et al., 1997; Pelletier, 2000 when the survival rate in the non-habitat patch is low; Ovaskainen, 2002), but others favour a single large reserve (Etienne & Heesterbeek, 2000). In a recent review of empirical studies, Debinski and Holt (2000) found a lack of consistency in the effects of habitat fragmentation on species richness and abundance. The evidence amongst microecosystem studies is also equivocal; extinction rates may be lower in unfragmented habitats (Forney & Gilpin, 1989; Burkey, 1997) or in fragmented habitat (Holyoak & Lawler, 1996) of the same total area/volume. Experimental tests of population dynamic processes have often used laboratory microcosms because of their tractability and short timescales of change, e.g. Forney and Gilpin (1989) and Burkey (1997). Here a moss microarthropod microecosystem is used. This has the advantage of being a completely natural system occurring in the field in both continuous and fragmented states. Thus experimental fragmentation merely reproduces patterns that occur naturally. Even though the generation times of the microarthropods are typically several months (Christiansen et al., 1992; Norton, 1994), the effects of habitat fragmentation on the fauna are measurable after just 6 months (Gilbert et al., 1998; Gonzalez et al., 1998; Gonzalez & Chaneton, 2002). Furthermore, predators suffer greater rates of extinction than non-predators (Gilbert et al., 1998), and relaxation continues for at least 1 year (Gonzalez, 2000). Although the moss microarthropod microecosystem is based on a far smaller spatial scale than a nature reserve, it has provided insights into the efficacy of wildlife corridors and the causes of the species area relationship. Connecting patches of moss habitat by moss corridors slows the rate of species extinction (Gilbert et al., 1998; Gonzalez et al., 1998), possibly by the rescue effect. The contributions of the metapopulation effect (higher colonisation and lower extinction rates on local mainland than on local island areas) to the species area relationship, relative to that of the combined effects of habitat heterogeneity and sampling are approximately equal after 6 months (Hoyle, 2004). With further relaxation of the fragmented moss microarthropod community, the metapopulation effect is expected to increase. Using the moss microarthropod microecosystem, a fieldbased study designed to test the effects of habitat fragmentation on species richness and abundance and community composition is presented. Fragmented and unfragmented patches were left for sufficient time to allow the communities to relax. It was found that mainland patches were more species rich than island patches. Species richness was unaffected and community structure only weakly affected by the degree of fragmentation. Materials and methods Eight replicate sets (blocks) of four treatments (Fig. 1) were cut from continuous moss [Isothecium myosuroides (Brid.) var. myosuroides] growing on eight large rocks in Snowdonia (U.K.), leaving bare rock in-between. The bare rock was considered to be an inhospitable environment for the majority of the moss taxa, restricting (but not eliminating) movement among patches. Treatments were arranged in a random order in June 2001, keeping a minimum distance of 10 cm of bare rock between islands (as Gilbert et al., 1998; Gonzalez et al., 1998) to minimise migration across the rock. Six months later the moss was removed in concentric rings of one unit of area (Fig. 1). Each patch was dismembered and left in a separate Tullgren funnel for 48 h. Emerging microarthropods were collected in an alcohol glycerol water (7:2:1) mixture, sorted into morphospecies (Table 1) and identified with help from relevant experts (see Acknowledgements). In the rest of this paper, the term species refers to these morphospecies, although note (Table 1) that most were single species. Although the choice of the duration of the experiment is partly arbitrary, it was set at 6 months because previous work (Gilbert et al., 1998; Gonzalez

3 686 Martin Hoyle and Alastair R. Harborne Rock 1 Moss > 10 cm Bare rock Mainland Total area (units) Fig. 1. The four experimental treatments were circular moss islands of one, two, and four units of area (1 unit ¼ 39 cm 2 ) and a circular patch of mainland moss of two units of area growing naturally on rock. Each rock was approximately 5 m from the next nearest rock. The order of treatments was random (here shown as progressing from more to less fragmented). After 6 months, moss was removed in concentric rings of one unit of area (indicated by white dotted lines). Only one (1) of the eight rocks (1 to 8) is displayed. et al., 1998) showed that this is sufficient to allow the effects of fragmentation to occur. Note that it is not possible to sample the fauna at the start of the experiment. Nevertheless, any initial differences in community composition are dealt with by replication in the statistical analysis. Moss samples were weighed before and after extraction to give the moss wet and dry weights. Fragmentation of fixed area Species richness and abundance in constituent patches in each treatment were summed within a given rock. The groups tested were: total microarthropod species, non-predatory mite species, and predatory mite species, and total microarthropod individuals, non-predatory mite individuals, Mesostigmata mite individuals, Prostigmata mite individuals, Collembola individuals, the six most common Cryptostigmata mite species (morphospecies 7, 8, 11, 12, 15, and 16: Table 1), a Prostigmata mite species (morphospecies 4), and two Collembola species (morphospecies 1 and 2). Generalised linear mixed-effects models ( glmmpql Venables & Ripley, 2002) with Poisson errors (suitable for count data) were implemented in R (Ihaka & Gentleman, 1996; Crawley, 2002), with moss dry weight as a covariate, and block as a random factor. Model simplification was performed by backward elimination from the maximal model to the minimum adequate model, and factor significance was gauged by a w 2 test of the increase in deviance after factor deletion. We might expect the smaller patches to be drier than the larger patches, due to their greater edge to area ratio, especially if it is a long time since the last rain. Furthermore, the moisture content of moss is likely to influence species richness and abundance. Therefore the above test was repeated on the moss wet weight using normal errors, with moss dry weight and patch fragmentation as covariates in a mixed-effects model (routine lme : Ihaka & Gentleman, 1996). Furthermore, to test for an edge effect in the moss patches, species richness and abundance were compared between the concentric rings of the two- and four-unit area patches using Poisson errors, with ring number and ring dry weight as covariates. To test whether the moss was drier towards the edge of the patch of area four units, a possible cause of any edge effect, the wet weight of the moss rings was analysed using normal errors with ring number and ring dry weight as covariates. Whilst comparison of species richness and abundance by fragmentation is important, this gives little information on community composition and no information on species turnover between habitats of differing fragmentation. For example, the species richness of two habitats could be equal, but the species composition might not overlap. Community composition was investigated graphically and statistically. Firstly, rank abundance graphs were plotted for each of the fragmentation levels. Rank abundance plots are preferable to diversity indices because no information is lost. Secondly, multidimensional scaling was performed on the abundance of all microarthropod species between the three levels of fragmentation, based on the Bray Curtis similarity coefficient (Bray & Curtis, 1957). The measure takes a maximum value of 1 when the species composition in two habitats is identical and a minimum value of 0 when the habitats have no species in common. The similarity (a measure of decreasing b diversity) between fragmentation levels j and k is defined as 1 P p i¼1 P p i¼1 X ij X ik X ij þ X ik where X ij is the abundance of the ith species in the jth fragmentation level, and where there are p species overall. The null hypothesis of no significant difference in b-similarity among the three levels of fragmentation was tested using analysis of similarities (ANOSIM; Clarke, 1993). ANOSIM contrasts observed similarities among groups of samples with those within groups. Permutation is then used to evaluate the significance of the statistic. Tests were applied to the standardised abundances so that the means and standard deviations for all levels of fragmentation were equal.

4 Microarthropod habitat fragmentation 687 Table 1. Mite and Collembola morphospecies. Morpho-species Species identified Cryptostigmata mites 1 Ceratoppia bipilis (Peloppiidae ¼ Ceratoppiidae) (Hermann) 2 Chamobates borealis (Chamobatidae) (Tra gårdh) Chamobates cuspidatus (Chamobatidae) (Michael, 1884) Minunthozetes pseudofusiger (Mycobatidae) (Schweizer, 1922) 3 Phthiracarus longulus (Phthiracaridae) (Koch) 4 Sphaerozetes piriformis (Ceratozetidae) (Nicolet) 5 Tectocepheus sarekensis (Tectocepheidae) (Trägårdh, 1910) 6 Eueremaeus (possibly oblongus) (Eremaeidae) (Koch) 7 Quadroppia quadricarinata virginalis (Oppiidae) (Lions, 1982) 8 Achipteria nitens (Achipteriidae) (Nicolet, 1855) 9 Carabodes marginatus (Carabodidae) (Michael, 1884) Carabodes labyrinthicus (Carabodidae) (Michael) 10 Oribatula tibialis (Oribatulidae) (Nicolet, 1855) 11 Dissorhinaornata (Oppiidae) (Oudemans) RamusellaRamusella cf. Assimilis (Oppiidae) (Mihelcic, 1950) Suctobelba trigona (Suctobelbidae) (Michael) 12 Porobelba spinosa (Damaeidae) (Sellnick, 1920) 13 Caleremaeus monilipes (Caleremaeidae) (Michael) 14 Euzetes globulus (Euzetidae) (Nicolet) 15 Chamobates schuetzi (Chamobatidae) (Oudemans) 16 Carabodes willmanni (Carabodidae) (Bernini, 1975) 17 Odontocepheus elongatus (Carabodidae) (Michael, 1879) 18 Trichoribates trimaculatus (Ceratozetidae) (Koch) 19 Phthiracarus nitens (Phthiracaridae) (Koch) 20 Phthiracarus montanus (Phthiracaridae) (Perez-In igo) Plus five unidentified morphospecies. Mesostigmata mites 1 Paragamasus integer (Parasitidae) (Bhattacharryya, 1963) Paragamasus schweizeri (Parasitidae) (Bhattacharryya, 1963) 2 Zercon zelawaiensis (Zerconidae) (Sellnick, 1944) 3 Geholaspis longispinosus (Macrochelidae) (Kramer, 1876) Geholaspis mandibularis (Macrochelidae) (Berlese, 1904) 4 Paragamasus robustus (Parasitidae) (Oudemans, 1902) Pergamasus crassipes (Parasitidae) (Linnaeus, 1758) Pergamasus longicornis (Parasitidae) (Berlese, 1906) 5 Pergamasus septentrionalis (Parasitidae) (Bhattacharyya, 1963) 6 Cosmolaelaps claviger (Laelapidae) (Berlese, 1883) 7 Uropoda (Cilliba subgenus) sp. (Uropodidae) 8 Holoparasitus calcaratus (Parasitidae) (Koch, 1839) Holoparasitus inornatus (Parasitidae) 9 Uropoda misella (Uropodidae) Prostigmata mites 1 Bdellidae (species unknown) 2 Eupodidae (species unknown) 3 Eupodidae (species unknown) 4 Cryptognathidae (species unknown) Plus four unidentified morphospecies. Collembola 1 Pseudoisotoma sensibilis (Isotomidae) (Tullberg) 2 Xenylla boerneri (Hypogastruridae) (Axelson) 3 Orchesella villosa (Entomobryidae) (Geoffroy) Tomocerus minor (Entomobryidae) (Lubbock) 4 Neanura muscorum (Hypogastruridae) (Templeton) 5 Entomobrya nivalis (Entomobryidae) (Linnaeus) 6 Dicyrtomina minuta (Sminthuridae) (Fabricius) Lepidocyrtus curvicollis (Sminthuridae) (Bourlet) Plus three unidentified morphospecies.

5 688 Martin Hoyle and Alastair R. Harborne (a) unit, and also between the island treatment of area two units and the mainland patch of area two units, using generalised linear mixed-effects models with Poisson errors. Any difference in species richness or abundance could have been caused partially by a difference in moss moisture content. Therefore moss wet weight was also compared between the mainland and island patches, with moss dry weight as a covariate, assuming normal errors. (b) In(species richness) (c) Mainland vs. Island Fragmentation Fig. 2. Species richness did not vary significantly by fragmentation for (a) all microarthropods combined, (b) predatory mites, and (c) non-predatory mites. The total number of species was counted from constituent patches within the same experimental rock. See Fig. 1 for clarification of the icons representing the degree of fragmentation of the patches. Error bars represent 1 SEM. A lower species richness and abundance in the island patches compared with the mainland patch might be anticipated, because the migration rate of microarthropods is presumably lower across bare rock than across moss. Hence, the total number of microarthropod species and individuals and the number of predatory and non-predatory mite species were compared between the island treatment of area one unit and the mainland core of area one Results Forty-two thousand microarthropod individuals of 69 morphospecies were recorded, mostly Acari (44%, only adults counted because juveniles are not extracted efficiently by the Tullgren method) and Collembola (55%, non-predatory) (Table 1). The Acari were Cryptostigmata (55%, non-predatory), Mesostigmata (2%, predatory), and Prostigmata (43%, predatory). On average, over all treatments and rocks, per unit area of moss, there were 94 adult Cryptostigmata individuals of eight species, four adult Mesostigmata individuals of two species, 74 Prostigmata individuals of two species, 211 Collembola individuals of three species, and two other microarthropod individuals of two species. Fragmentation of fixed area The species richness of predatory mites, non-predatory mites, and all microarthropods combined were unaffected by fragmentation of the fixed 156 cm 2 area of habitat (Fig. 2). Numbers of one of the Collembola species (morphospecies 1, Table 1) decreased significantly with decreasing habitat fragmentation (w 2 1 ¼ 18.3, P < ), whereas numbers of one of the Cryptostigmata mite species (morphospecies 11, Table 1) and the number of Mesostigmata mites increased significantly with decreasing habitat fragmentation (w 2 1 ¼ 15.5, P < and w 2 1 ¼ 8.6, P ¼ respectively). The abundances of the following taxonomic groups were independent of habitat fragmentation: total microarthropods, total non-predatory mites, total Prostigmata mites, total Collembola, five of the six Cryptostigmata mite species tested, the Prostigmata species, and one of the two Collembola species tested. For the tests of species abundance, experimental block was usually highly significant, indicating that the microhabitat varied significantly among rocks. Community structure, as measured by rank abundance, varied little among the three levels of fragmentation (Fig. 3). Furthermore, tests of b- diversity by fragmentation were non-significant for all microarthropods (ANOSIM measure of community dissimilarity based on Bray Curtis measure R ¼ 0.089, P ¼ 0.235) and predatory mites (R ¼ 0.018, P ¼ 0.364), but weakly significant for the non-predatory mites (R ¼ 0.357, P ¼ 0.024). Moss wetness increased with patch size (w 2 1 ¼ 5.5, P ¼ 0.019) (water content for patches of size 1, 2, and 4

6 Microarthropod habitat fragmentation 689 ln(abundance) Most fragmented Intermediate Least fragmented Fig. 3. log e (species rank abundance) of the three fragmentation treatments, each averaged over all rocks Rank units of area were 103%, 107%, and 137% of dry weight respectively). There was no significant trend in moss wetness by concentric ring (w 2 1 ¼ 1.4, NS) in the largest patch. Nevertheless, there was a strong edge effect in the largest moss patch: average total microarthropod species (19 in central core vs. 12 in outer ring, w 2 1 ¼ 8.8, P ¼ 0.003), total microarthropod individuals (450 vs. 292, w 2 1 ¼ 8.7, P ¼ 0.003), non-predatory mite species (9.6 vs. 6.6, w 2 1 ¼ 13.7, P < 0.001), and predatory mite species (4.6 vs. 2.1, w 2 1 ¼ 7.7, P ¼ 0.006). Mainland vs. Island The average number of microarthropod species was significantly greater in the mainland compared with the island moss for one unit of area (17.9 vs species, w 2 1 ¼ 4.9, P < 0.027), and two units of area (22.9 vs species, w 2 1 ¼ 7.8, P ¼ 0.005). The number of predatory mite species was significantly greater in the mainland for one unit of area (5.4 vs. 3.4 species, w 2 1 ¼ 10.3, P ¼ 0.001), and two units of area (4.6 vs. 3.6 species, w 2 1 ¼ 6.7, P ¼ 0.010). These differences were not due to variable moisture content, as this did not vary significantly between the mainland and island (although wetness increased with patch size, see above). In contrast, the number of microarthropod individuals and non-predatory mite species did not differ significantly for either area. There was little evidence of an edge effect for the island patch of area two units. Total microarthropod species, non-predatory mite species and predatory mite species did not vary by ring, but there were fewer microarthropod individuals towards the edge (w 2 1 ¼ 8.0, P ¼ 0.005). Discussion The most important findings of this study are as follows: (1) species richness, abundance, and community composition were largely unaffected by fragmentation of a fixed area (156 cm 2 ) of moss habitat (even though the most fragmented patches were drier), and conversely (2) the species richness (in particular of the predatory mites) of a given area of habitat was greater when it was part of a large mainland area (>> 156 cm 2 ) than part of an island. The first result suggests either that the advantages of several small patches (e.g. reduced impact of environmental stochasticity, wider range of habitats overall) were equally balanced by the advantages of a single large patch (e.g. reduced effect of demographic stochasticity, wider range of habitats within a single patch, reduced edge effect), or that both effects were small. In particular, it would be instructive to design a further experiment with a greater range of patch sizes, and to run the experiment for longer to allow relaxation to continue to a greater extent. Then species richness might be found to depend on habitat fragmentation. Surprisingly, there was an edge effect even though moss wetness did not vary within a patch (although any moisture gradient is likely to depend on the time of last rain). There may have been some other environmental influence at the edge of the patch that was unfavourable to the fauna. The shapes of the rank-abundance curves

7 690 Martin Hoyle and Alastair R. Harborne were similar among the levels of fragmentation, implying that fragmentation had little impact on community structure. Conversely, species composition did vary weakly, but significantly, by fragmentation for the non-predatory mites. The results of the mainland vs. island comparison agree with the findings of Gilbert et al. (1998), Gonzalez et al. (1998) and Gonzalez and Chaneton (2002) that mainland moss contains more microarthropod species than island moss, possibly due to the rescue effect (Brown & Kodric- Brown, 1977) (and not due to moss wetness, as this covariate was non-significant). The results also agree with empirical (Gilbert et al., 1998) and theoretical (Diamond, 1984; Schoener, 1989) studies, that habitat fragmentation adversely affects predators more than non-predators. However, there was only weak non-significant evidence that the mainland moss contained more microarthropod individuals, possibly because there were many more nonpredatory than predatory mite individuals, and the nonpredatory mites were less affected by fragmentation. The conclusions of the metapopulation modelling studies mentioned in the Introduction depend on the level of migration between fragmented patches. It was assumed that the minimum inter-patch distance of 10 cm, created at the beginning of the experiment, was sufficient to restrict or prevent dispersal between moss fragments, as a gap of 5 cm apparently represents a substantial dispersal barrier for these organisms (Gilbert et al., 1998; Gonzalez et al., 1998). However dispersal is one of the least known aspects of the biology of soil microarthropods (Norton, 1994). Cryptostigmata mites have been recorded moving approximately 3 cm per day in soil (Berthet, 1964) and Collembola 1.4 cm per week (Sjogren, 1997). A high rate of microarthropod dispersal across the bare rock would reduce or abolish differences in individual densities/species richness among levels of patch fragmentation. However, the observation that the mainland moss contained more species than the island moss suggests that dispersal was indeed restricted across the rock. If it is assumed that there was at least some migration between patches, then no evidence was found to support Hubbell (2001) (although an examination of his Unified Theory in relation to fragmentation has not yet been fully explored), which favoured several small patches at low migration levels. Additionally, little evidence was found to support theoretical models (Stacey et al., 1997; Pelletier, 2000; Ovaskainen, 2002) favouring an intermediate number of sub-populations, or (Etienne & Heesterbeek, 2000) favouring a single large patch. However, such models generally assume more generations and more habitat patches than were available in this study. Furthermore, if the range of patch fragmentation and the statistical power of the experiment were greater, and if the experimental treatments were left for longer to allow relaxation to continue to a greater extent, then some supporting evidence might be found. For the taxonomic groups significantly affected by fragmentation, the direction of the relationship was species specific (the number of individuals of one species increased with decreasing patch fragmentation, whereas the numbers of two others decreased). There was strong evidence that the number of predatory Mesostigmata individuals decreased with increasing fragmentation, as predicted by theory (Diamond, 1984; Schoener, 1989). There were fewer individuals of one Collembola species for the least fragmented treatment, possibly because there were more predatory Mesostigmata mite individuals in this treatment, and Collembola are known to be particularly vulnerable to predation by predatory mites (Mitchell, 1977). The nonpredatory hard-bodied adult Cryptostigmata mites seemed unaffected by fragmentation, possibly because they are less mobile than the predatory mites (thus requiring only a small home range), or because they are less vulnerable to predation than soft-bodied Collembola. Despite their small size, generation times of microarthropods are actually quite long relative to the 6-month duration of this experiment (Christiansen et al., 1992; Norton, 1994). This suggests that the effect of demographic stochasticity in this study was probably limited, implying that species richness is independent of habitat fragmentation (Hoyle & Gilbert, 2004). However, although habitat patches at the scale of a microecosystem may experience less spatial variation in the weather and other environmental influences compared with patches at the scale of the nature reserve, environmental stochasticity may still be important, thus favouring several small patches. It is difficult to quantify the degree of microarthropod habitat diversity in moss. Despite using just one species of moss, on the microscale there may be differences in microclimate among the patches due to differences in the contours and aspects of the rocks (Alpert, 1991). Habitat diversity within a moss patch depends on the degree of habitat specialisation of the microarthropods, and may depend on the ratio of the area of the region sampled to the size of the organism. Species may require a range of different habitats (e.g. used by different stages, or for oviposition sites for gravid females), and hence will be found only in moss patches that encompass this range of habitats, perhaps the cause of the generally highly significant block effect. In summary, species richness and abundance and community composition of both predators and non-predators were mostly unaffected by fragmentation of a fixed area of habitat. Nonetheless, species richness was greater in an area of mainland than in an equal area of island habitat. These findings may generalise to many other communities of species. Acknowledgements We thank R. Norton, P. Martinez, H. Klompen, M. Skorupski, and W. Welbourn for mite identification, B. Cave for Collembola identification, A. Smith for moss identification, A. Sylvester and M. Roberts for help with microarthropod counting, the National Trust in Snowdonia for access to their land for this experiment

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