Sascha Buchholz

Ground spider assemblages as indicators for habitat structure in inland sand ecosystems

Open inland sand ecosystems harbour a specialised flora and fauna and are among the most endangered habitats in Central Europe. Land-use changes and lack of habitat dynamics are acknowledged as significant drivers for habitat loss and degradation. It is imperative for nature conservation to obtain criteria such as community structure and biodiversity of model groups to assess the conservation value of threatened habitats. By investigating the correlation between ground spider assemblages and habitat structure, the study aimed to find out the indicator potential of spiders in order to promote conservation objectives and management strategies for open inland sand ecosystems. Non-metric multidimensional scaling revealed four habitat groups with distinct spider assemblages that clearly reflected the whole variety of habitat structure types within the study area. Species distribution was constrained by biotic and abiotic gradients while the ecological traits of spiders differed significantly among the groups. Generalised linear models showed that abundances of particular species were significantly correlated with environmental factors and habitat structure, making them thus suitable as focal species to assess natural habitat modifications as well as success of management efforts. Based on these findings, we derived major aims for successful habitat management of inland sand ecosystems taking into account also the needs of arthropod conservation. Management should include both small and large reserves when aiming for higher levels of disturbance, and sand dynamics to prevent increasing scrub encroachment and to create a larger number of early succession stages.

Impact of the invasive moss Campylopus introflexus on carabid beetles (Coleoptera: Carabidae) and spiders (Araneae) in acidic coastal dunes at the southern Baltic Sea

Campylopus introflexus is an invasive moss in Europe and North America that is adapted to acidic and nutrient-poor sandy soils with sparse vegetation. In habitats like acidic coastal dunes (grey dunes) it can reach high densities, build dense carpets and modify habitat conditions. While the impact of the moss invasion on the vegetation is well analyzed, there is a lack of knowledge regarding possible effects on arthropods. In the present study we analyzed the impact of Campylopus introflexus on the ground-dwelling arthropods carabid beetles and spiders, as both taxa are known to be useful indicator taxa even on a small-scale level. In 2009 we compared species composition in a) invaded, moss-rich (C. introflexus) and b) native, lichen-rich (Cladonia spp.) acidic coastal dunes by using pitfall traps. A total of 1,846 carabid beetles (39 species) and 2,682 spiders (66 species) were caught. Species richness of both taxa and activity densities of spiders were lower in invaded sites. Species assemblages of carabids and spiders differed clearly between the two habitat types and single species were displaced by the moss encroachment. Phytophagous carabid beetles, web-building spiders and wolf spiders were more abundant in native, lichen-rich sites. Shifts in species composition can be explained by differences in the vegetation structure, microclimate conditions and most likely a reduced food supply in invaded sites. By forming dense carpets and covering large areas, the moss invasion strongly alters typical arthropod assemblages of endangered and protected (EU-directive) acidic coastal dunes.

Capture efficiency of pitfall traps is highly affected by sampling interval

Pitfall trapping is the most frequently used sampling method for the study of surface-dwelling arthropods such as carabid beetles (Coleoptera: Carabidae) or spiders (Araneae) (Uetz & Unzicker, 1976; Müller, 1984; Spence & Niemela, 1994; Lövei & Sunderland, 1996), even though it is the subject of much controversy and criticism (Curtis, 1980; Halsall & Wratten, 1988; Topping & Sunderland, 1992; Sunderland et al., 1995; Lang, 2000). Pitfall traps (Barber, 1931) are inexpensive, easy to handle, and samples are rich both in individuals and species (Topping & Sunderland, 1992; New, 1998; Santos et al., 2007). The traps, consisting mainly of plastic cups or glass jars, are sunk into the ground (Barber, 1931; Balogh, 1958; Grell, 1997). They usually contain a fluid to prevent arthropods from escaping, thus making the trap more effective. The fluid also works as a preservative (Gurdebeke & Maelfait, 2002; Jud & Schmidt-Entling, 2008). Many factors influence the efficiency of pitfall traps and the composition of their catches, such as choice of preservative (Pekár, 2002; Schmidt et al., 2006; Jud & SchmidtEntling, 2008), size (Brennan et al., 1999), shape (Luff, 1975), colour (Buchholz et al., 2010), and material (Waage, 1985) of the pitfalls or their covers (Buchholz & Hannig, 2009), and arrangement (Digweed et al., 1995; Melbourne, 1999; Ward et al., 2001). But to date, except for ants (Greenslade, 1973; Borgelt & New, 2006) the impact of different sampling intervals has been poorly analysed. In the literature, sampling intervals (mostly) range from weekly (Larsen et al., 2003; Gibson & New, 2007), through fortnightly (Desender & Bosmans, 1998; Schuldt et al., 2008), to monthly (Koivula et al., 1999; Lassau et al., 2005). Short sampling intervals are labour intensive and are therefore often very costly. They could also disturb the sampling sites, which could have a negative impact, e.g., on arthropods, plants, and breeding birds. On the other hand, during long sampling intervals, trap catchability could be highly affected by drying out of the preservative, overflowing due to rain, or by a large amount of rotten or damaged animals. Furthermore, the quality of the preservative on arthropods could change over time (Bombosch, 1962). Lastly, pitfall catches may be affected by a digging-in effect, as more individuals of certain soil arthropods tend to be caught in traps that have just been dug in. To date, this effect has only been demonstrated for Collembola (Joosse & Kapteijn, 1968), ants (Greenslade, 1973), and carabids (Digweed et al., 1995). The aim of this study, therefore, was to investigate the digging-in effect for a broad spectrum of arthropods usually captured in pitfalls traps and to compare this effect among the various arthropod groups.

Response of carabid beetles (Coleoptera: Carabidae) and spiders (Araneae) to coastal heathland succession

We analyzed the impacts of succession and grass encroachment on carabid beetle and spider assemblages in a coastal heathland. Further, indicator species for different successional stages (grey dunes, dwarf-shrub heath, grassy heath, heath with shrubs, birch forest) were identified, and their relations to habitat parameters were analyzed. The study was conducted on the Baltic island of Hiddensee, Germany. Ground-dwelling arthropods were sampled using pitfall traps along a successional gradient containing five stages. Ordination by nonmetric multidimensional scaling (NMDS) and analysis of similarities (ANOSIM) revealed a clear separation of species composition among the successional stages. Both in carabid beetles and spiders, most indicator species were obtained in the youngest stage (grey dunes) and fewest in the intermediate stages (grassy heath, heath with shrubs). Also the proportion of endangered species was highest in grey dunes. Based on our results, conservation management of coastal heathlands should preserve a mosaic of different successional stages with a clear preference on younger stages (grey dunes and dwarf-shrub heath).

Effects of a major tree invader on urban woodland arthropods.

Biological invasions are a major threat to biodiversity; however, the degree of impact can vary depending on the ecosystem and taxa. Here, we test whether a top invader at a global scale, the tree Robinia pseudoacacia (black locust or false acacia), which is known to profoundly change site conditions, significantly affects urban animal diversity. As a first multi-taxon study of this kind, we analyzed the effects of Robinia dominance on 18 arthropod taxa by pairwise comparisons of woodlands in Berlin, Germany, that were dominated by R. pseudoacacia or the native pioneer tree Betula pendula. As a negative effect, abundances of five arthropod taxa decreased (Chilopoda, Formicidae, Diptera, Heteroptera, Hymenoptera); 13 others were not affected. Woodland type affected species composition of carabids and functional groups in spiders, but surprisingly did not decrease alpha and beta diversity of carabid and spider assemblages or the number of endangered species. Tree invasion thus did not induce biotic homogenization at the habitat scale. We detected no positive effects of alien dominance. Our results illustrate that invasions by a major tree invader can induce species turnover in ground-dwelling arthropods, but do not necessarily reduce arthropod species abundances or diversity and might thus contribute to the conservation of epigeal invertebrates in urban settings. Considering the context of invasion impacts thus helps to set priorities in managing biological invasions and can illustrate the potential of novel ecosystems to maintain urban biodiversity.

Losing uniqueness – shifts in carabid species composition during dry grassland and heathland succession

Abstract Dry sand ecosystems, such as dry grasslands and heathlands, have suffered habitat loss and degradation due to land‐use changes and are today among the most endangered habitats in Central Europe. To evaluate the impact of degradation processes on habitat quality, we investigated how succession from sparse vegetated sand ecosystems to grass‐invaded and tree‐dominated ecosystems and the environmental parameters associated with it influences carabid assemblages. We also determined to what extent typical xerophilic species assemblages still exist. Pitfall trapping at 28 study sites in northwestern Germany yielded 111 carabid species that were grouped using Kendalls W coefficient of concordance. Ordination revealed that the differences between the four species groups resulted from vegetation cover and soil humidity, indicating that carabid distribution clearly reflects degradation processes. Our results suggest that areas in which succession proceeds were unsuitable for assemblages typical of dry grasslands and heathlands. In all, 35 species are lost due to succession from dry grassland and heathland to grass‐invaded and tree‐dominated sites. We discuss implications for habitat management and restoration, since dry sand ecosystems comprise a very high number of specialized and endangered species.

Is pitfall trapping a valuable sampling method for grassland Orthoptera

Common methods to assess diversity and abundance of Orthoptera are sweep netting, transect counts and box-quadrat sampling. Pitfall trapping, by contrast, is rarely used, and the value of this method is still being questioned. In 2008, we studied Orthoptera species richness and abundance in five vegetation types along a gradient of dune succession on the Baltic Sea island of Hiddensee (NE Germany) by comparing transect-count and pitfall-trapping data. Using transect counts, 12 species were detected in the study area. With pitfall traps, three chorto- and thamnobiont Ensifera species (C. dorsalis, M. roeselii and T. viridissima) were not caught at all, and it was only in low-growing and sparsely-vegetated grey dunes that all present species were detected. With pitfall traps, the proportion of present species recorded strongly declined with increasing height and density of the vegetation type. Assuming that transect counts are a good proxy for relative Orthoptera densities, densities ascertained by pitfall traps are strongly biased by vegetation structure and locomotive behaviour of the species. More than 80% of all individuals were caught in sparsely-vegetated grey dunes. Frequency patterns of the species also differed. Using pitfall traps, especially chortobiont species were significantly underrepresented. Qualitative and quantitative sampling of Orthoptera using pitfall traps seems only reasonable in habitats with low and sparse vegetation and a high proportion of geobiont species.

Invasive moss alters patterns in life-history traits and functional diversity of spiders and carabids.

Invasive plants can modify terrestrial habitats and affect the natural faunal composition. In acidic coastal dunes the invasive moss Campylopus introflexus can form dense carpets that largely replace native vegetation. As shown in a previous study, moss invasion affects habitat structure and ground-dwelling arthropod diversity. We suggested that including the functional diversity concept in the analysis of moss invasion impacts may offer further insights. We used pitfall trap data to compare trait composition and functional diversity of spiders and carabids in (a) invaded, moss-rich (C. introflexus) and (b) native, lichen-rich (Cladonia spp.) acidic coastal dunes. Moss invasion induced shifts in the trait values body size and feeding preference (carabids) and hunting mode (spiders): Species were smaller in native sites, and the percentages of web-building spiders and phytophagous beetles were reduced in invaded sites. Furthermore, moss invasion led to a more heterogeneous trait composition for spiders, and changed functional diversity of both arthropod groups, although with the opposite effects: While spiders were functionally more diverse in invaded sites, moss invasion reduced carabid beetles’ functional diversity. We also observed changes in the relationship between species richness and functional diversity that indicate a high functional similarity for spiders but a lower one for carabid beetles in native grey dunes. C. introflexus invasion not only alters the arthropod diversity and assemblage structure of endangered acidic coastal dunes but also interferes at a functional level. These results provide further insight into the way plant invasions might alter the structure and function of ecosystems.

Disentangling urban habitat and matrix effects on wild bee species

In face of a dramatic decline of wild bee species in many rural landscapes, potential conservation functions of urban areas gain importance. Yet effects of urbanization on pollinators, and in particular on wild bees, remain ambiguous and not comprehensively understood. This is especially true for amenity grassland and extensively managed wastelands within large-scale residential housing areas. Using Berlin as a study region, we aimed to investigate (a) if these greenspaces are accepted by wild bee assemblages as foraging habitats; (b) how assemblage structure of bees and individual bee species are affected by different habitat (e.g., management, flower density) and urban matrix variables (e.g., isolation, urbanization); and (c) to what extent grassland restoration can promote bees in urban environments. In summer 2012, we collected 62 bee species belonging to more than 20% of the taxa known for Berlin. Urbanization significantly affected species composition of bees; 18 species were affiliated to different levels of urbanization. Most bee species were not affected by any of the environmental variables tested, and urbanization had a negative effect only for one bee species. Further, we determined that restoration of diverse grasslands positively affected bee species richnesss in urban environments. We conclude that differently structured and managed greenspaces in large-scale housing areas can provide additional foraging habitats and refuges for pollinators. This supports approaches towards a biodiversity friendly management within urban regions and may be of particular importance given that anthropogenic pressure is increasing in many rural landscapes.

Simulating small-scale climate change effects—lessons from a short-term field manipulation experiment on grassland arthropods

Climate change is expected to cause major consequences on biodiversity. Understanding species‐specific reactions, such as species shifts, species declines, and changes in population dynamics is a key issue to quantify large‐scale impacts of climate change on biotic communities. As it is often impossible or at least impracticable to conduct large‐scale experiments on biotic responses to climate change, studies at a smaller scale may be a useful alternative. In our study, we therefore tested responses of grassland arthropods (carabid beetles, spiders, grasshoppers) to simulated climate change in terms of species activity densities and diversity. We conducted a controlled field experiment by changing water and microclimatic conditions at a small scale (16 m2). Roof constructions were used to increase drought‐like conditions, whereas water supply was enhanced by irrigation. In all, 2 038 carabid beetles (36 species), 4 893 spiders (65 species), and 303 Orthoptera (4 species) were caught using pitfall traps from May to August, 2010. During our experiment, we created an artificial small‐scale climate change; and statistics revealed that these changes had short‐term effects on the total number of individuals and Simpson diversity of the studied arthropod groups. Moreover, our results showed that certain species might react very quickly to climate change in terms of activity densities, which in turn might influence diversity due to shifts in abundance patterns. Finally, we devised methodological improvements that may further enhance the validity of future studies.