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112 The Open Ecology Journal, 2010, Volume 3 Bucher et al. km north and west of the city of Berne in the Swiss plateau. woody habitat in the surrounding landscape and in the Maps showing the percentage of woody habitats within a degree of isolation from other woody habitats (Fig. 1; see 500 m radius were generated in ArcGIS version 9.2 (ESRI also Farwig et al. 2009). Woody habitats were divided into Redlands, CA). Official land-use data (Vector25, swisstopo, forest (dense broadleaf or mixed forest) and semi-open Wabern) were converted into a 10 × 10 m grid of either habitats (forest edge, hedgerows, orchards, single standing woody or other habitat types. For each grid, the percentage trees) because some arthropods are selective between the two (Table 2). The percentage of forest within a radius of 500 m varied from 0.2% up to 72.0% between the 30 sites. The percentage of semi-open habitat varied between 2.2% and 6.6%. The three levels of habitat isolation remained independent of habitat amount after division into forest and semi-open habitats (Forest: F2,27 < 0.01, p = 0.99; Levene statistic = 0.11, p = 0.90; Semi-open habitats: F2,27 = 0.71, p = 0.50; Levene statistic = 0.35, p = 0.71). The percentages of forest and of semi-open habitats showed no strong correlation (r = -0.058, p = 0.76).
Study Sites and Field Methods In March 2008, seven four-year old cherry trees (Prunus avium) were planted at each of the 30 sites. The trees were planted in a row on permanent grassland and were 3 m apart from each other. Before planting, arthropods overwintering on the trees were killed with winter oil (Syngenta Agro AG, Fig. (1). Independent variation of habitat amount and isolation 8157 Dielsdorf, Switzerland) to standardize the starting among the thirty study sites (Differences between means: F2,27 = condition of tree colonisation. A glue ring (7 cm width; W. 0.01, p = 0.99; Homogeneity of variances: Levene statistic = 0.10, p Neudorff GmbH KG, 31860 Emmerthal, Germany) was = 0.91). attached around the stem of three trees in each group, to prevent colonisation by walking arthropods. The glue rings of woody habitats was calculated within a 500 m radius. were renewed three times during the study period. Hides for Using these maps, suitable sites for tree planting were arthropods were attached to the stem in the canopy of all located in the field that formed a gradient from low to high trees. They consisted of a roll of corrugated cardboard with 5 percentages of woody habitats in the surrounding landscapes. cm diameter and a length of 15 cm. The top and sides were The sites had to be at least 750 m from existing sites, to sealed with foil, and the bottom left open for entering avoid strong overlap between landscape sectors. Independent arthropods. Arthropods use such hides as overwintering sites of the variation in the percentage of woody habitat the sites (Pekár 1999, Isaia et al. 2006). In late October 2008 the had three different levels of habitat isolation: Ten of the sites hides were collected and stored in a climate chamber at 5°C. were directly adjacent to forest to represent no isolation from Overwintering arthropods were counted and identified to woody habitat. Another ten sites had no forest within 100 m species level if possible (Table 1). We analysed five radius but were connected to forest by hedges or single frequently observed species each with a unique combination standing trees. The remaining ten sites were isolated from of dispersal mode and habitat preference (Table 2). any woody habitat by at least 100 m distance through open farmland. The sites varied independently in the percentage of
Table 1. Spiders and Insects Observed in the Cardboard Rolls, with Numbers of Individuals and Numbers of Sites at which the Species was Found. The Frequently Observed fly Triarthria setipennis is a Parasitoid of the Earwig Forficula auricularia and was found as Pupae in the Cardboard Rolls
Group Family Genus Species Individuals Sites
Araneae Anyphaenidae Anyphaena accentuata 27 16 Araneidae Aculepeira ceropegia 4 4 Larinioides cornutus 45 11 Nuctenea umbratica 75 25 Clubionidae Clubiona sp. 75 22 Dictynidae Dictyna arundinacea 13 3 Lathys humilis 3 3 Gnaphosidae Micaria sp. 1 1 Linyphiidae Erigone sp. 1 1
Colonisation in Fragmented Landscapes The Open Ecology Journal, 2010, Volume 3 113
(Table 1) Contd…..
Group Family Genus Species Individuals Sites
Philodromidae Philodromus sp. 51 23 Salticidae Heliophanus flavipes 1 1 Marpissa muscosa 15 9 Salticus scenicus 1 1 Synageles venator 36 12 Tetragnathidae Tetragnatha nigrita 1 1 Thomisidae Diaea dorsata 2 2 Xysticus sp. 2 2 Blattoptera Ectobius lapponicus 1 1 Cicadina Cicadellidae Balcanocerus larvatus 1 1 9 3 Coleoptera Carabidae Bembidion articulatum 3 2 Demetrias atricapillus 1 1 Chrysomelidae Oulema gallaeciana 1 1 melanopus 9 5 Coccinellidae Coccinella quinquepunctata 2 1 septempunctata 34 12 Cryptophagidae Ephistemus globulus 1 1 Curculionidae Anthonomus sp. 6 4 Magdalis ruficornis 1 1 Sitona lineatus 10 6 Lathridiidae Corticarina gibbosa 7 5 Staphylinidae Leucoparyphus silphoides 1 1 Oxytelus sculpturatus 1 1 Anobiidae Grynobius planus 1 1 Dermaptera Forficulidae Forficula auricularia 267 25 Diptera Muscidae Phaonia viarum 1 1 Tachinidae Triarthria setipennis 115 23 Heteroptera Anthocoridae Orius niger 2 2 Lygaeidae Gastrodes abietum 1 1 Heterogaster urticae 12 5 Rhyparochromus pini 1 1 Nabidae Himacerus mirmicoides 2 2 Hymenoptera Formicidae Myrmica sp. 1 1 Tetramorium caespitum 1 1 Chalcididae 8 6 Crabronidae Cerceris arenaria 1 1 Tenthredinidae 1 1 Vespidae Symmorphus sp. 1 1 Vespula vulgaris 1 1 Allodynerus rossii 2 1 Lepidoptera Noctuidae 4 4 Psocoptera 2 1 Thysanoptera 11 9 114 The Open Ecology Journal, 2010, Volume 3 Bucher et al.
Table 2. Dominant Mode of Dispersal and Habitat Preference their aerial dispersal they showed no significant differences of the Investigated Species (after Hänggi et al. 1995; in abundance between trees with and without glue rings Blandenier 2009) (Figs. 2I, M and Q; N. umbratica: t1,208 = 1.29, p = 0.20; L. cornutus: t1,208 = 0.67, p = 0.50; A. accentuata: t1,208 = 0.85, p Dispersal = 0.40). Isolation from woody habitats had no significant Species Habitat preference mode effect on the abundance of these spiders (Figs. 2J, N and R; N. umbratica: t3,26 = 0.88, p = 0.39; L. cornutus: t3,26 = -1.13, Forficula auricularia Walking Semi-open habitats and forest p = 0.27; A. accentuata: t3,26 = 0.80, p = 0.43). Synageles venator Walking Semi-open and open habitats The two orb weaving spiders N. umbratica and L. Nuctenea umbratica Ballooning Semi-open habitats cornutus prefer semi-open habitats (Table 2). The abundance of these two spiders decreased with an increase in the Larinioides cornutus Ballooning Semi-open and open habitats percentage of forest in 500 m radius (Fig. 2K, O; N. Anyphaena accentuata Ballooning Semi-open habitats and forest umbratica: t2,27 = -2.30, p = 0.03; L. cornutus: t2,27 = -1.87, p = 0.07). N. umbratica responded positively to higher amounts of semi-open habitats (Fig. 2L; t2,27 = 2.11, p = 0.04). No effect of the percentage of semi-open habitats on Data Analysis L. cornutus was found (Fig. 2P; t2,27 = -089, p = 0.38). The effect of glue rings was tested by comparing the The ghost spider A. accentuata lives in semi-open abundance of the five arthropod species between the 210 habitats and in closed forest (Table 2). In accordance with its trees. To analyse effects of the landscape parameters (habitat habitat preference the abundance of this spider increased isolation and habitat amount) on the same five species, with the percentage of forest and with the percentage of abundances were pooled per site. Tests were done using semi-open habitats (Fig. 2S, T; Forest: t2,27 = 2.65, p = 0.01; generalized linear models with quasi-Poisson error Semi-open habitats: t = 2.13, p = 0.04). distributions in R version (2.8.1) (R development core team 2,27 2008). The effect of glue rings on the jumping spider Synageles venator (Lucas, 1836) was tested using Pearson's DISCUSSION Chi-squared test, because the complete absence of the species from trees with glue rings did not allow generalized Habitat Isolation versus Habitat Amount linear model calculation (Bolker et al. 2008). Each of the five observed species showed a unique combination of dispersal mode and habitat preference. The responses to habitat isolation and/or habitat amount were RESULTS also unique for each species. The earwig F. auricularia was Walking Colonisers the only species significantly affected by habitat isolation. This corresponds with their walking dispersal, and with their The common earwig Forficula auricularia (Linnaeus, clear preference for woody habitats, which is absent in the 1758) is a walking disperser inhabiting semi-open habitats second walking disperser, the jumping spider S. venator. As and forest (Table 2). In accordance with their walking predicted, all three ballooning spiders did not respond to dispersal, earwigs showed significantly higher abundances habitat isolation, but to habitat amount. Other studies on on trees without glue rings (Fig. 2A; t1,208 = -5.99, p < 0.001). arthropods with high dispersal abilities show similar results As expected, earwig densities were strongly affected by (Krauss et al. 2003, Major et al. 2006). In accordance with isolation from woody habitats (Fig. 2B; t3,26 = 3.80, p < our results, the majority of spiders inhabiting arable fields 0.001). The response of earwigs to habitat amount was not are known ballooners, and respond to habitat amount significant (Figs. 2C, D; Forest: t2,27 = -0.10, p = 0.92; Semi- (Schmidt et al. 2008, Öberg et al. 2008). However, the open habitats: t2,27 = 1.18, p = 0.25). response of spiders in the canopies of mature orchards were The jumping spider S. venator is a walking disperser more variable, and included isolation effects on ballooning (Table 2), and was only found on trees without glue rings species (Herrmann et al. 2010). Thus, it appears that habitat (Fig. 2E; χ2 = 16, df = 1, p < 0.001). In accordance with its age and/or trophic interactions influence species’ response to occurrence also in open habitats (Table 2), S. venator habitat isolation. At the cherry tree groups of the current showed no relation to the different levels of isolation from study, abundance and species richness of trap-nesting wasps woody habitats (Fig. 2F; t3,26 = 1.28, p = 0.21). Neither the (Hymenoptera: Crabronidae, Pompilidae and Eumeninae) amount of forest (Fig. 2G; t2,27 = -0.52, p = 0.61) nor the were reduced by habitat isolation (Schüepp et al. 2010, see percentage of semi-open habitats (Fig. 2H; t2,27 = -0.87, p = also Holzschuh et al. 2009). This contradicts the idea that 0.39) had a significant effect on the abundance of this flying dispersers are not sensitive to habitat isolation. jumping spider. However, the reduced abundance of wasps at isolated sites may be due to complications in provisioning the nests with Aerial Colonisers adequate food rather than due to problems for adult wasps to reach the isolated sites. Additional studies will be necessary The three remaining spider species, Nuctenea umbratica to assess the generality of the dichotomy between isolation (Clerck, 1757), Larinioides cornutus (Clerck, 1757) and effects on walking dispersers and habitat amount effects on Anyphaena accentuata (Walckenaer, 1802), are known to flying dispersers, and the possible influences of additional disperse mostly by ballooning (Table 2). In accordance with factors such as habitat age and trophic interactions.
Colonisation in Fragmented Landscapes The Open Ecology Journal, 2010, Volume 3 115
Fig. (2). Abundances of the common earwig Forficula auricularia, the jumping spider Synageles venator, the two orb weaving spiders Nuctenea umbratica and Larinioides cornutus and the ghost spider Anyphaena accentuata in relation to (A, E, I, M and Q) presence and absence of glue rings, (B, F, J, N and R) habitat isolation, (C, G, K, O and S) percentage of forest, and (D, H, L, P and T) amount of semi- open habitats (*** p < 0.001, ** p < 0.01, * p < 0.05). Continuous lines indicate significant relationships (p < 0.05), dashed lines mark statistical trends (p < 0.1).
Habitat Preference forest preference showed the opposite, negative response to forest cover. While all three ballooning spiders are common With respect to habitat preference of the ballooning in semi-open habitats, only N. umbratica and A. accentuata species, the only species that preferred closed forest had higher densities in landscapes with high amounts of responded positively to the percentage of forest in the semi-open habitats. It is likely that the preference for various surrounding landscape. In contrast, both species without 116 The Open Ecology Journal, 2010, Volume 3 Bucher et al. open habitats such as wetlands and fallows by L. cornutus Bolker, BM, Brooks, ME, Clark, CJ, Geange, SW, Poulsen, JR, Stevens, blurred a possible influence of the (quantitatively less MHH & White, JS (2008) Generalized linear mixed models: a practical guide for ecology and evolution. Trends in Ecology and important) semi-open habitats. Differential effects of habitat Evolution, 24, 127-35. amount according to species habitat preference are known Debinski, DM & Holt, RD (2000) A survey and overview of habitat from spiders in arable fields. While species with a strong fragmentation experiments. Conservation Biology, 14, 342-55. preference for arable habitats respond negatively to the Ewers, RM & Didham, RK (2006) Confounding factors in the detection of amount of perennial habitats on a landscape scale, the species responses to habitat fragmentation. Biological Reviews, 81, 117-42. majority of species are generalists and respond positively to Fahrig, L (2003) Effects of habitat fragmentation on biodiversity. Annual the amount of perennial habitats (Pfiffner & Luka 2000, Review of Ecology Evolution and Systematics, 34, 487-515. Schmidt et al. 2008). 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In contrast, all three species Herrmann, JD, Bailey, D, Hofer, G, Herzog, F & Schmidt-Entling, MH of ballooning spiders even tended towards higher densities (2010) Spiders associated with the meadow and tree canopies of on trees with glue rings compared to trees without glue rings. orchards respond differently to habitat fragmentation. Landscape As ballooning spiders have limited or no control over their Ecology, 25, 1375-84. Holzschuh, A, Steffan-Dewenter, I & Tscharntke, T (2009) Grass strip landing location, it could have been expected that numerous corridors in agricultural landscapes enhance nest-site colonization individuals need to climb the trees after landing in a nearby by solitary wasps. Ecological Applications, 19, 123-32. location, which would have led to reduced densities on trees Isaia, M, Bona, F & Badino, G (2006) Comparison of polyethylene bubble with glue rings despite flying dispersal. However, the wrap and corrugated cardboard traps for sampling tree-inhabiting abundance of spiders is influenced by competitors and spiders. Environmental Entomology, 35, 1654-60. Jacobs, W & Renner, M (1998) Biologie und Ökologie der Insekten. Gustav antagonists such as ants (Wise 1993), which were largely Fischer Verlag, Stuttgart. restricted to trees without glue rings (Stutz & Entling Krauss, J, Steffan-Dewenter, I & Tscharntke, T (2003) Local species revised). Thus, the presence of ants may have countered a immigration, extinction, and turnover of butterflies in relation to trend towards lower spider densities on trees without glue habitat area and habitat isolation. Oecologia, 137, 591-602. Kruess, A & Tscharntke, T (1994) Habitat fragmentation, species loss, and rings. biological control. Science, 264, 1581-4. In summary, it proved highly useful to study habitat Lindenmayer, DB & Fischer, J (2007) Tackling the habitat fragmentation panchreston. 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However, fields and adjacent semi-natural habitats. Agriculture Ecosystems & examples from the literature suggest that additional factors Environment, 78, 215-22. Pekár, S (1999) Some observations on overwintering of spiders (Araneae) in such as habitat age and trophic interactions can affect the two contrasting orchards in the Czech Republic. Agriculture response of organisms to fragmentation and require further Ecosystems & Environment, 73, 205-10. study. R development core team (2008) R: a language and environment for statistical computing. R foundation for statistical computing. Saunders, DA, Hobbs, RJ & Margules, CR (1991) Biological consequences ACKNOWLEDGEMENTS of ecosystem fragmentation: a review. Conservation Biology, 5, 18- 32. We thank Raphael Didham, Peter Hambäck and two Schmidt, MH, Thies, C, Nentwig, W & Tscharntke, T (2008) Contrasting anonymous reviewers for comments on an earlier version of responses of arable spiders to the landscape matrix at different spatial scales. Journal of Biogeography, 35, 157-66. the manuscript, Debra Bailey and Beatrice Schüpbach who Schüepp, C, Herrmann, JD, Herzog, F & Schmidt-Entling, MH (2010) contributed to the landscape analyses and all farmers who Differential effects of habitat isolation and landscape composition provided land for study sites. The study was supported by on wasps, bees, and their enemies. Oecologia, DOI the Swiss National Science foundation under grant number 10.1007/s00442-010-1746-6. 3100A0-114058 to Felix Herzog and Martin (Schmidt-) Stutz, S & Entling, MH. Effects of the landscape context on aphid-ant- predator interactions on cherry trees. Biological Control Entling. (Submitted). 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Received: September 16, 2010 Revised: October 21, 2010 Accepted: October 22, 2010
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