Spider Composition Research Articles

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.

Contrasting landscape effects on species diversity and invasion success within a predator community

AbstractAimThe role of landscape variation in shaping invasion success remains unclear, particularly for exotic predators. We examined whether native and exotic predators responded similarly or differently to ecological change at the landscape scale.LocationVineyards and surrounding natural habitat in the Napa Valley, California, USA.MethodsWe surveyed spider communities in vineyards throughout the Napa Valley and used regression analysis to examine the effects of landscape context (proportion of vineyard monoculture and proportion of developed land surrounding vineyards) on spider species richness and on numbers and proportions of exotic spiders. We also compared spider composition and dominance of exotic spiders between vineyards and surrounding natural habitat using non‐metric multidimensional scaling (NMDS).ResultsThe success of the dominant exotic spider Cheiracanthium mildei was contingent on landscape context, and increased with proportions of vineyard and developed land in the surrounding landscape. Spider species richness and native spider abundance followed an opposing pattern. Cheiracanthium mildei was relatively unsuccessful near and in oak woodland, the primary type of natural habitat surrounding vineyards.Main ConclusionsResults show that anthropogenic landscape change can have contrasting effects on native and exotic predators and that landscape context can influence the relationship between habitat modification and invasion success. Results suggest that it may be possible to limit the spread of invasions by increasing the availability of habitat for native species.

Responses of ground-dwelling spiders (Araneae) to variable retention harvesting practices in the boreal forest

Green tree retention has been embraced as an alternative to large scale conventional clear-cutting aimed at managing the boreal forest in a more sustainable way. Ground-dwelling spider assemblages were studied 6 years after the application of variable retention harvesting practices (dispersed to 2%, 10% and 75% retention vs. aggregated retention) in deciduous and coniferous dominated stands as part of the Ecosystem Management Emulating Natural Disturbances (EMEND) project in the mixedwood boreal forest of northwestern Canada. Overall, spider composition was dominated by open-habitat species, especially by the wolf spider Pardosa moesta Banks, in all harvest treatments. However, an important shift in species composition was observed in unharvested controls. Distinctive ground-dwelling spider assemblages were observed in each forest cover-type and these were highly affected by the various harvest treatments, especially in coniferous forests. From the 164 species collected, 37 were indicators for various combinations of forest type, harvesting treatment and retention level. Species richness was affected as harvesting intensity increased; however, greater impact was observed in dispersed retention than in aggregated retention, and in coniferous stands compared to the same treatment in deciduous stands. Dispersed retention, even at 10%, seemed to mitigate adverse effects of harvesting on spider assemblages, but only in deciduous forests; however, important effects on biodiversity were observed in coniferous forests even after 75% retention. Retention of aggregated patches was useful in maintaining forest specialist species after harvesting, but was more effective when applied together with dispersed retention. Responses of spider assemblages to harvesting seemed to be highly correlated with environmental changes. Green tree retention preserves some of the heterogeneity, structural features and environmental conditions required by forest specialist species and thus becomes an effective management practice to maintain forest biodiversity, especially when both aggregated and dispersed retention practices are applied together. However, the effectiveness of these practices varies depending on the forest cover-type prior to harvesting and the amount of post-harvest residual. This suggests that a single prescription is insufficient for maintaining landscape heterogeneity, and that a combination of prescriptions is supported for a more sustainable management of the mixedwood boreal forest.

Is the reassembly of an arid spider assemblage following fire deterministic?

AbstractFire is a common disturbance in many ecosystems, including arid Australia. Understanding whether fauna respond in a deterministic manner towards a single end‐point, or to multiple states, is of crucial importance for conservation management. Why different taxa or assemblages display single or multiple end‐points is also important to develop a synthetic theory of succession. To examine the post‐fire changes in assemblages of spiders, we established a chronosequence study in spinifex habitat of central Western Australia. Ground‐active spiders were pitfall‐trapped over nine months in sites representing experimental fires (0 and 0.5 years post‐fire) and wildfires (3, 5, 8 and 20 years post‐fire). There were significant non‐linear changes in species richness, evenness and composition of spiders with increasing post‐fire age. For all three measures, the assemblage appeared highly deterministic, converging towards the long unburnt state. Similarity in richness, evenness and species composition to the 20‐year‐old sites all increased with increasing time since fire (3–8 years). However, experimentally burnt sites did not neatly fit this sequence. We consider two alternative hypotheses to explain this second trajectory: inertia within the system or the rapid migration and recolonization from nearby surrounding unburnt areas. Analyses indicated that half of the 179 species had significant preferences for, or were restricted to, particular post‐fire ages. This suggests that adequate pyrodiversity, both in terms of post‐fire ages and/or scale and intensity of fires, may be important for the conservation of spiders in this habitat. However, owing to the high number of singletons and low indicator values, the significance of this result for conservation management remains equivocal. Despite this, the high degree of determinism provides hope that managers can develop a good predictive understanding of post‐fire successional changes in spider assemblages in arid Australia.

Spider Assemblages in the Overstory, Understory, and Ground Layers of Managed Stands in the Western Boreal Mixedwood Forest of Canada

Logging is the main human disturbance in the boreal forest; thus, understanding the effects of harvesting practices on biodiversity is essential for a more sustainable forestry. To assess changes in spider composition because of harvesting, samples were collected from three forest layers (overstory, understory, and ground) of deciduous and conifer dominated stands in the northwestern Canadian boreal mixedwood forest. Spider assemblages and feeding guild composition were compared between uncut controls and stands harvested to 20% retention. In total, 143 spider species were collected, 74 from the ground, 60 from the understory, and 71 from the overstory, and species composition of these three pools differed considerably among layers. Distinctive spider assemblages were collected from the canopy of each forest cover type but these were only slightly affected by harvesting. However, logging had a greater impact on the species composition in the understory and ground layers when compared with unharvested controls. Guild structure differed among layers, with wandering and sheet-weaving spiders dominant on the ground while orb-weaving and ambush spiders were better represented in the understory and overstory, respectively. Given the ecological importance of spiders and the expectation of faunal changes with increased harvesting, further efforts toward the understanding of species composition in higher strata of the boreal forest are needed.

Response of macroarthropod assemblages to the loss of hemlock (Tsuga canadensis), a foundation species

In eastern North American forests, eastern hemlock (Tsuga canadensis) is a foundation species. As hemlock is lost from forests due to the invasive hemlock woolly adelgid (Adelges tsugae) and pre-emptive salvage logging, the structure of assemblages of species associated with hemlock is expected to change. We manipulated hemlock canopy structure at hectare scales to investigate the effects of hemlock death on assemblages of ants, beetles, and spiders in a New England forest. Relative to reference hemlock stands, both in situ death of hemlock and logging and removal of hemlock altered composition and diversity of beetles and spiders, and logging increased the species richness and evenness of ant assemblages. Species composition of ant assemblages in disturbed habitats was non-random relative to the regional species pool, but we found no evidence that interspecific competition shaped the structure of ant, beetle, or spider assemblages, in either manipulated or intact forest stands. Environmental filtering by hemlock appears to maintain low levels of species richness and evenness in forest stands, suggesting that the loss of hemlock due to the hemlock woolly adelgid or human activities will not likely lead to extirpations of ant, beetle, or spider species at local scales.

Global Patterns of Guild Composition and Functional Diversity of Spiders

The objectives of this work are: (1) to define spider guilds for all extant families worldwide; (2) test if guilds defined at family level are good surrogates of species guilds; (3) compare the taxonomic and guild composition of spider assemblages from different parts of the world; (4) compare the taxonomic and functional diversity of spider assemblages and; (5) relate functional diversity with habitat structure. Data on foraging strategy, prey range, vertical stratification and circadian activity was collected for 108 families. Spider guilds were defined by hierarchical clustering. We searched for inconsistencies between family guild placement and the known guild of each species. Richness and abundance per guild before and after correcting guild placement were compared, as were the proportions of each guild and family between all possible pairs of sites. Functional diversity per site was calculated based on hierarchical clustering. Eight guilds were discriminated: (1) sensing, (2) sheet, (3) space, and (4) orb web weavers; (5) specialists; (6) ambush, (7) ground, and (8) other hunters. Sixteen percent of the species richness corresponding to 11% of all captured individuals was incorrectly attributed to a guild by family surrogacy; however, the correlation of uncorrected vs. corrected guilds was invariably high. The correlation of guild richness or abundances was generally higher than the correlation of family richness or abundances. Functional diversity was not always higher in the tropics than in temperate regions. Families may potentially serve as ecological surrogates for species. Different families may present similar roles in the ecosystems, with replacement of some taxa by other within the same guild. Spiders in tropical regions seem to have higher redundancy of functional roles and/or finer resource partitioning than in temperate regions. Although species and family diversity were higher in the tropics, functional diversity seems to be also influenced by altitude and habitat structure.

Environmental conditions affecting spiders in grasslands at the lower reach of the River Tisza in Hungary

The aims of the present study were to reveal the differences between grassland types, and to identify the local and landscape parameters that influence the spider assemblages at the reach of the River Tisza. The rarefied number of spider species was negatively correlated with the proportion of forests in a radius of 500 m around each site. A negative correlation was found between the number of grassland specialist species and the proportion of the forests, but the number of forest species increased significantly with the neighboring forest area. The relative area of neighboring forests, the number of plant species and regular flooding played major roles in shaping of the species composition of spiders. The results of the present study emphasize the importance of the effect of habitat landscape properties on spider assemblages. The structural diversity of the landscape may influence species richness and composition of the habitats.

Spider, bee, and bird communities in cities are shaped by environmental control and high stochasticity

Spatially organized distribution patterns of species and communities are shaped by both autogenic processes (neutral mechanism theory) and exogenous processes (niche theory). In the latter, environmental variables that are themselves spatially organized induce spatial structure in the response variables. The relative importance of these processes has not yet been investigated in urban habitats. We compared the variance explained by purely spatial, spatially structured environmental, and purely environmental components for the community composition of spiders (Araneae), bees (Apidae), and birds (Aves) at 96 locations in three Swiss cities. Environmental variables (topography, climate, land cover, urban green management) were measured on four different radii around sampling points (< 10 m, 50 m, 250 m, 1000 m), while Moran's eigenvector maps (MEMs) acted as spatial variables. All three taxonomic groups showed weak spatial structure. Spider communities reacted to very fine-scaled environmental changes of lawn and meadow management and climate. Bird community composition was determined by woody plants as well as solar radiation at all radii, the scale of the influence varying among species. Bee communities were weakly explained by isolated variables only. Our results suggest that the anthropogenic structuring of urban areas has disrupted the spatial organization of environmental variables and inhibited the development of biotic spatial processes. The near absence of spatial structure may therefore be a feature typical of urban species assemblages, resulting in urban community composition mainly influenced by local environmental variables. Urban environments represent a close-knit mosaic of habitats that are regularly disturbed. Species communities in urban areas are far from equilibrium. Our analysis also suggests that urban communities need to be considered as being in constant change to adapt to disturbances and changes imposed by human activities.

Diversity and composition of arbustive spiders in an Atlantic Forest fragment and two adjacent areas

The present study preliminarily investigates the diversity and composition of arbustive spider assemblages occurring in an early stage (arbustive), an intermediate stage (early secondgrowth), and a mature stage area (Atlantic Forest fragment) in Torres municipality, Rio Grande do Sul. Spiders and other arthropods (potential prey) were caught during the day with beating trays inside ten 3 m2 plots randomly placed inside each area, during two field trips in 2008. An individual-based rarefaction (with confidence intervals) was utilized to access differences in spider richness between areas, spider assemblage composition variation among the three succession areas was accessed by a MANOVA via randomization test, and the correlation between arbustive spider composition and the composition of potential preys was analyzed by a Mantel test. There were no significant differences in arbustive spider abundances between areas. Mature stage area presented higher arbustive spider richness compared to early stage area and intermediate stage area. The three habitats showed different compositions of arbustive spider assemblages, and the composition of arbustive spiders was not correlated with the composition of the potential preys. Higher richness of arbustive spiders in the mature area is probably due to a more diverse plant community. Compositions found in the three habitats indicate that many mature-forest species could not be capable of using matrix habitats. It seems that the mature patch is surrounded mainly by an inhospitable matrix, with little overlap in spider species composition between the habitats showing more similar vegetation composition (mature and intermediate). Our findings outpoint to a habitat-selective spider assemblage in this Atlantic Forest fragment highlighting the importance of protecting Atlantic forest fragments to conserve arbustive spider assemblages. Moreover, intermediate stage areas contiguous to forest fragments could be important to some forest dwelling species and also needs attention. Key words: fragmentation, matrix-tolerant species, forest succession, forest structure.

Spider, bee and bird communities in cities are shaped by environmental control and high stochasticity

Spatially organized distribution patterns of species and communities are shaped by both autogenic processes (neutral mechanism theory) and exogenous processes (niche theory). In the latter, environmental variables that are themselves spatially organized induce spatial structure in the response variables. The relative importance of these processes has not yet been investigated in urban habitats. We compared the variance explained by purely spatial, spatially structured environmental, and purely environmental components for the community composition of spiders (Araneae), bees (Apidae), and birds (Aves) at 96 locations in three Swiss cities. Environmental variables (topography, climate, land cover, urban green management) were measured on four different radii around sampling points (<10 m, 50 m, 250 m, 1000 m), while Moran's eigenvector maps (MEMs) acted as spatial variables. All three taxonomic groups showed weak spatial structure. Spider communities reacted to very fine-scaled environmental changes of lawn and meadow management and climate. Bird community composition was determined by woody plants as well as solar radiation at all radii, the scale of the influence varying among species. Bee communities were weakly explained by isolated variables only. Our results suggest that the anthropogenic structuring of urban areas has disrupted the spatial organization of environmental variables and inhibited the development of biotic spatial processes. The near absence of spatial structure may therefore be a feature typical of urban species assemblages, resulting in urban community composition mainly influenced by local environmental variables. Urban environments represent a close-knit mosaic of habitats that are regularly disturbed. Species communities in urban areas are far from equilibrium. Our analysis also suggests that urban communities need to be considered as being in constant change to adapt to disturbances and changes imposed by human activities.

The role of dispersal from natural habitat in determining spider abundance and diversity in California vineyards

Many generalist predators do not persist in crops year-round, and must colonize crops on a seasonal basis from natural vegetation. We monitored the spider community in the air, on the ground, and in the foliage of California vineyards and neighboring oak-woodlands, to assess whether dispersal from natural habitats influences the abundance and species diversity of spiders in agroecosystems. Early in the growing-season, low spider numbers in vineyard foliage indicated that few spiders overwintered in the vineyard. Aerial dispersal did not, however, occur primarily in the spring when generalist predators are likely to be important in pest suppression, but in mid-summer, while dispersal activity on the ground did not change over time. Aerial collection traps showed greater similarity to spider composition in vineyard foliage than ground collection traps, indicating that spiders in vineyards disperse more by air than over ground. Dwarf spiders (Linyphiidae) dominated spider composition on aerial traps but were not abundant on the vine, while other spider species showed an opposing pattern, suggesting that while spiders may have entered vineyards by air, other factors may have also contributed to spider composition in vineyards. These results are discussed with respect to control of vineyard pests and the establishment of spiders in vineyard agroecosystems.

Influence of fine woody debris on spider diversity and community structure in forest leaf litter

Exploitation of forests for biofuel results in variable amounts of fine woody debris (FWD) being left on the forest floor. Such practices have the potential to affect the diversity and taxonomic composition of litter-dwelling organisms. The ability to predict the consequences of harvesting biofuel will be strengthened by knowledge of how variation in FWD affects litter organisms in non-harvested forests. A field experiment with three treatments [Control (no manipulation), FWD Removal, and FWD Addition] was carried out in replicated open plots in an unexploited forest in order to determine how the amount of fallen FWD affects the abundance, diversity and community composition of spiders, selected because of their importance as predators in the leaf-litter food web. Manipulation of FWD started in August, and by the following July total spider density in the FWD Addition treatment was 1.6× that of FWD Removal plots. Spider density in FWD Removal plots was 30% lower than the Control treatment, with no statistically significant difference between Control and FWD Addition. Manipulating FWD had no statistically significant effects on standard indices of spider diversity. However, multivariate community-level analyses revealed statistically significant differences in spider community structure between the FWD Removal and FWD Addition treatments. Two dominant genera of web builders contributed the most to this effect of manipulating FWD. Our results, and the findings of studies of woody debris in younger forests, suggest that the relatively small effects of FWD in our field experiment may reflect the age of the forest, with effects on the fauna likely being larger in forests younger than the one we studied.

Towards an ecologically-sustainable forestry in the Atlantic Forest

Since pristine Atlantic Forest remnants are vanishing, and biological reserves are in short, conservation of biodiversity will largely depend on proper management of the anthropogenic matrix. Here, we test (1) the effectiveness of ecologically-managed tree monocultures in maintaining Araucaria Forest biodiversity, (2) how this effectiveness change among taxa, and (3) we discuss management principles that can be used by the forestry industry in order to contribute positively to biodiversity conservation. The study was conducted in the São Francisco de Paula National Forest, southern Brazil, an environmental mosaic composed mostly of patches of Araucaria Forest and ecologically-managed monocultures of Araucaria, Pinus and Eucalyptus. Using standardized sampling methods in these four main habitats, we have recorded the richness and species composition of small mammals, birds, leaf-litter frogs, butterflies, galling insects, spiders, opiliones, flatworms, woody plants, epiphytic angiosperms, epiphytic ferns, lichens, and fruit-body producing fungi. Overall, we recorded 506 species in Araucaria Forest, 181 (36%) of which were exclusive of this habitat while 325 (64%) could be found in at least one monoculture. Distribution patterns of species richness and number of records across taxonomic groups showed that a large biodiversity can be found inside ecologically-managed plantations of Araucaria, Pinus, and Eucalyptus. For all studied taxa, except for epiphytic angiosperms and fruit-body producing fungi, more than half of the Araucaria Forest species could be found living on monocultures. We discuss how the actual management practices of the forest industry can be improved to contribute positively to the conservation of the Atlantic Forest biodiversity.

Multitrophic effects of nutrient addition in upland grassland

Although the effects of nutrient enhancement on aquatic systems are well documented, the consequences of nutritional supplements on soil food webs are poorly understood, and results of past research examining bottom-up effects are often conflicting. In addition, many studies have failed to separate the effects of nutrient enrichment and the physical effects of adding organic matter. In this field study, we hypothesised that the addition of nitrogen to soil would result in a trophic cascade, through detritivores (Collembola) to predators (spiders), increasing invertebrate numbers and diversity. Nitrogen and lime were added to plots in an upland grassland in a randomised block design. Populations of Collembola and spiders were sampled by means of pitfall traps and identified to species. Seventeen species of Collembola were identified from the nitrogen plus lime (N+L) and control plots. Species assemblage, diversity, richness, evenness and total number were not affected by nutrient additions. However, there was an increase in the number of Isotomidae juveniles and Parisotoma anglicana trapped in the N+L plots. Of the 44 spider species identified, over 80% were Linyphiidae. An effect on species assemblage from the addition of N+L to the plots was observed on two of the four sampling dates (July 2002 and June 2003). The linyphiid, Oedothorax retusus, was the only species significantly affected by the treatments and was more likely to be trapped in the control plots.The increased number of juvenile Collembola, and change in community composition of spiders, were consequences of the bottom-up effect caused by nutrient inputs. However, despite efforts to eliminate the indirect effects of nutrient inputs, a reduction in soil moisture in the N+L plots cannot be eliminated as a cause of the invertebrate population changes observed. Even so, this experiment was not confounded by the physical effects of habitat structure reported in most previous studies. It provides evidence of moderate bottom-up influences of epigeic soil invertebrate food webs and distinguishes between nutrient addition and plant physical structure effects. It also emphasises the importance of understanding the effects of soil management practices on soil biodiversity, which is under increasing pressure from land development and food production.

Landscape mosaic of Araucaria forest and forest monocultures influencing understorey spider assemblages in southern Brazil

Abstract This study investigates how abundance, diversity and composition of understorey spiders were influenced by four different forest habitats in a southern Brazilian Araucaria forest. The study area encompasses a landscape mosaic comprised of Araucaria forest, Araucaria plantation, Pinus plantation, and Eucalyptus plantation. Understorey spiders were collected by beating the vegetation inside three patches of each forest habitat. To assess possible predictors of spider assemblage structure, several patch features were analysed: potential prey abundances, estimation of vegetation cover, diversity index of vegetation types, patch ages, patch areas, and geographical distance between patches. To assess the influence of high‐level taxa approaches on spider assemblage patterns, analyses were carried out individually for family, genera and species levels. Additionally, Mantel tests were carried out in underlying similarity matrices between each taxon. Significant differences in spider abundances among forest habitats were found. Pinus plantations showed the highest abundance of spiders and Eucalyptus plantations showed the lowest abundance. Spider abundance was significantly influenced by patch ages, geographical distance and vegetation cover. Expected numbers of families, genera and species did not vary among forest habitats. Spider composition of two Eucalyptus patches differed from the other forest patches, probably due to their low vegetation cover and isolation. Genera composition was the best correlate of species composition, showing that a higher‐level surrogate can be an alternative to the species approach. The understorey spider diversity in this managed area could be maintained when suitable habitat structures are provided, thus ensuring the connectivity between different habitat types. Further studies should focus on individual species responses to the conversion of native forest to monocultures.

Comparison of Polyethylene Bubble Wrap and Corrugated Cardboard Traps for Sampling Tree-Inhabiting Spiders

In this paper, we studied the use of polyethylene bubble wrap and corrugated cardboard bark traps to sample spiders inhabiting tree trunks, with special reference to test whether they function the same way despite the differences in the spider community imposed by the different tree species and surroundings and times of sampling. The survey was carried out from July 1997 to July 1998 in Turin, Italy, in a green urban area on three poplars and four American basswoods. Differences between the two methods were tested in terms of abundance, diversity, dominance, mean body length of specimens, and proportion of juveniles, calculating Spearman’s correlations. Three-factor analysis of variance, multiresponse permutation procedure, and indicator species analysis were used for further analysis. Results obtained by the two traps were correlated following a similar trend over sampling time. Bubble polyethylene seemed to be more effective than corrugated cardboard at trapping higher number of specimens. The dimension and total amount of interstices and the different microhabitat conditions of temperature and humidity seemed to be the main factors influencing spider composition in terms of the selected variables. On the basis of our results, polyethylene bark traps are recommended for sampling spiders living on trees. It is important to consider seasonality in experimental design, autumn being the period with the highest abundance of spiders but a lower level of diversity.

Comparison of Polyethylene Bubble Wrap and Corrugated Cardboard Traps for Sampling Tree-Inhabiting Spiders

In this paper, we studied the use of polyethylene bubble wrap and corrugated cardboard bark traps to sample spiders inhabiting tree trunks, with special reference to test whether they function the same way despite the differences in the spider community imposed by the different tree species and surroundings and times of sampling. The survey was carried out from July 1997 to July 1998 in Turin, Italy, in a green urban area on three poplars and four American basswoods. Differences between the two methods were tested in terms of abundance, diversity, dominance, mean body length of specimens, and proportion of juveniles, calculating Spearman’s correlations. Three-factor analysis of variance, multiresponse permutation procedure, and indicator species analysis were used for further analysis. Results obtained by the two traps were correlated following a similar trend over sampling time. Bubble polyethylene seemed to be more effective than corrugated cardboard at trapping higher number of specimens. The dimension and total amount of interstices and the different microhabitat conditions of temperature and humidity seemed to be the main factors influencing spider composition in terms of the selected variables. On the basis of our results, polyethylene bark traps are recommended for sampling spiders living on trees. It is important to consider seasonality in experimental design, autumn being the period with the highest abundance of spiders but a lower level of diversity.

Influence of climate changes on animal communities in space and time: the case of spider assemblages along an alpine glacier foreland

AbstractThe impact of global warming in space and time is described for species assemblages of wandering spiders along the alpine glacier foreland of the Forni Valley (Northern Italy). We tested the effect of environmental variables (e.g. elevation, age of glacier retreat, vegetation cover, debris cover) on species richness and on species composition of spiders. Age of glacier retreat was the only significant variable influencing spider species assemblages in the valley. A spatially structured distribution of species and species assemblages along the chronosequence of glacier retreat was evidenced. The threshold abruptly differentiating two groups of species richness and species composition fell between sites deglaciated 100 and 155 years before the analysis. Latitudinal shifts towards the poles in species ranges at the global scale in response to climatic changes are known, and an altitudinal shift in species range should be expected for spiders at the local scale of the Forni Valley. Such a shift is present in spider species assemblages, although not as an expected gradual change in species richness and composition, but with a threshold effect after one century of glacier retreat. We discuss our results in the light of plausible future scenarios due to global warming, the consequence of further glacier retreats onto spiders, and caveats for monitoring studies.

Simplifying assessment of forest management practices for invertebrates: How effective are higher taxon and habitat surrogates for spiders following prescribed burning?

Taxonomic impediments require us to know if simpler approaches such as using higher taxa or habitat surrogates can help assess the needs of invertebrates when managing forests. This study tests whether higher taxon (families for species) and habitat (vegetation structure for species and families) surrogates might assist in determining the effects of prescribed burning on spiders. Spiders were sampled at sites representing a chronosequence of increasing ages since burning (0, 3, 6 and 9 years) by pitfall trapping and vacuuming from understorey vegetation in the South-west Botanical Province of Australia; a global biodiversity hotspot. Reassembly followed a similar pattern for both the richness of species and families. Immediately following burning, mean richness of families and species was low (8.75 and 14.5, respectively). By 3 years, however, there were significant increases in familial and species richness (13.25 and 23.75, respectively). Thereafter, taxon richness remained constant. Multiple regressions suggested habitat variables potentially influencing family and species richness were litter depth, bare ground (%), the density of dead vegetation (0–100 cm high), and the density of live vegetation (40–140 cm). The first three of these variables correlated significantly with time since burning, suggesting these components of habitat structure are useful predictors of taxon richness for spiders following fire. For assemblage composition, the congruence in post-fire response patterns between taxonomic ranks was highly significant, although the amount of variation explained was low (Rho < 0.55). In the first 9 years post-fire, there was a significant shift in the taxonomic composition of spiders at family, but not specific, rank. Family composition at recently burnt sites differed significantly from that at sites burnt 9 years previously. Correlated with assemblage composition were litter depth, bare ground, and the density of live vegetation (0–40 cm above the ground). However, the best correlation obtained by any combination of habitat variables with assemblage composition (Rho < 0.317) was less than that obtained by using family level data. We conclude, therefore, that where resources are limited and there is a need to determine the effects of forest management practices on invertebrates using surrogates, identifying spiders to higher taxa is better than relying on habitat structure.