Bengt Gunnarsson

Bird Predation On Spiders: Ecological Mechanisms And Evolutionary Consequences

Abstract Birds are common predators of arthropods in many ecosystems but their impact on spiders has not been assessed. Therefore, the experimental evidence for bird predation effects on spider populations was examined. In particular, the present review focuses on the questions: what are the ecological mechanisms and what are the evolutionary consequences? Data from 17 field experiments, mainly in forest ecosystems, showed that spider communities were often significantly affected by bird predation. Comparisons of experimental effects were based on the ratio of mean density on experimentally enclosed vegetation and on controls. In 27 tests, a significant effect was detected (mean ratio 3.03) but in 9 tests the effect was non-significant (mean ratio 1.03). Furthermore, field experimental studies on bird predation effects on certain spider species or certain genera were reviewed. In three investigations, significant predation effects were found on agelenid, linyphiid and theridiid spiders but there were no significant effects on lycosids. Selective bird predation on large individuals has been shown in studies on spider communities and single species. Data on bird predation effects on species richness were lacking although impact on large species was expected to be important. Three field experiments showed that different spider families may experience differences in bird predation pressure. An aviary experiment showed that frequently moving spiders had a higher risk of predation than sedentary individuals, but the evidence from field experiments supporting the hypothesis of high predation pressure on moving spiders was limited. This included sex-specific differences in size and movement, although at least one experiment showed that males had higher winter mortality than females. One experiment showed that bird predation can affect anti-predator behavior. In conclusion, the present evidence showed that bird predation on spiders in several contrasting forest ecosystems is strong. However, there are many hypotheses regarding bird predation on spider populations that should be examined in future field experiments.

Commercial harvest of logging residue in clear-cuts affects the diversity and community composition of ground beetles (Coleoptera: Carabidae)

Abstract Structural complexity of the habitat is known to influence the diversity and abundance of arthropod populations. Earlier studies have shown that presence of logging residue (slash) on the ground contributes to microhabitat complexity and removal of slash for biofuel in clear-cuts can have short-term (∼1 year) effects on ground-active beetle populations. This study examines the consequences of slash removal on carabids (Coleoptera: Carabidae) 5–7 years after harvest. Sampling of beetles was done by pitfall trapping in three matched clear-cut pairs, in spruce forest in central Sweden. The number and diversity (Berger–Parker dominance index) of carabid species were significantly higher in clear-cuts with slash harvest than in sites where slash was left on the ground. No difference in the overall rank-abundance pattern was found between clear-cuts with different slash treatments, but for species with certain habitat preferences the community was significantly altered. In all clear-cuts, slash removal caused a shift in dominance with an increase in generalist species and a decline in forest species. The results show that removal of slash may have long-lasting effects on the carabid community composition and structure. Hence, in forest landscapes with large-scale biofuel harvest, generalist carabid species may increase their abundance. Following the precautionary principle, it is suggested that every fifth clear-cut should be free of slash harvest.

Interspecific predation as a mortality factor among overwintering spiders

SummaryResults from field experiments indicate that predation occurs among spruce-living spiders during winter in SW Sweden. Field observations of natural activity showed that Philodromus spp and Pityohyphantes phrygianus together make up 80% of the spiders active on spruce in winter. They are therefore potential predators on other overwintering spiders. Laboratory experiments were performed at +4° C to assess the importance of such predation between spiders. Small spiders (length <2.5 mm) had a mean winter mortality of 58% when kept together with large spiders (≧2.5 mm) which had a mean mortality of 3% only. Among the small spiders the Erigninae spp seemed to be more vulnerable to predation than other taxonomic groups. Predation also occurred when large P. phrygianus were kept together, but such predation caused mortality of less importance to the spider populations than the mortality among small spiders. Differences in spider density and food availability did not change this pattern. Considerable weight increase occurred in subadult P. phrygianus when fed during winter. This suggests that winter foraging specimens increase their fitness. Interspecific predation among spiders is suggested to be an important mortality factor in natural populations at high spider densities in November and December, when the ambient temperature often is above 0° C and when the density of large spiders is not yet substantially reduced by bird predation.

Female genitalia in Pityohyphantes phrygianus, a spider with a skewed sex ratio

The functioning of female genitalia in many arthropods, especially the site and mode of sperm storage, may have a strong impact on the pattern of sperm usage, leading to differential male fertilization success and even varying offspring sex ratios. The female genital morphology of the linyphiid spider Pityohyphantes phrygianus was investigated, as this species was shown to possess an intriguing mechanism of sex ratio determination, possibly related to genital morphology. By means of SEM and serial semi-thin sectioning, two distinctly different spermathecae plus an additional sperm sac, equipped with several valves, were found on each side of the genital apparatus. Sperm is stored under different conditions, as one of the spermathecae exhibits large gland cells of different types that discharge their products into the spermatheca through long secretory ducts, whereas the other spermatheca is surrounded by a simpler type of gland cell. Furthermore, there are folds instead of closed ducts leading to the spermathecae, and from the spermathecae to the oviduct. The latter fertilization-fold runs inside the copulatory chamber and enters the oviduct from the outside. The mechanisms for sex ratio bias in spiders are discussed. The occurrence of two types of spermatheca of which one is twisted, provides good evidence for the involvement of female genital morphology in sex ratio control of P. phrygianus offspring. Data are compiled on species for which information is available on sex ratio and female genital morphology. We put forward a hypothesis that in species with a skewed sex ratio, there is interaction between female genital morphology and behaviour, i.e. change of body position before, during or immediately after sperm transfer.

Effects of biodiversity and environment-related attitude on perception of urban green space

Green space in cities contributes to the quality of life for city dwellers, e.g., by increasing the opportunity for recreation. However, perception of urban green space is influenced by multiple factors. We investigated effects of biodiversity and environment-related attitudes on visual and auditory perceptions of urban green space. Field measurements of biodiversity were conducted in six sites across an urban gradient in Gothenburg, Sweden, and three categories of biodiversity—high, medium, low—were established. Households were sent a survey on aesthetic perception of urban green space, sound perception and the importance of trees and plants for the perception of bird species. Each respondent focused on the site that was located nearby. The environment-related attitudes comprised “Nature-oriented” and “Urban-oriented” persons and were based on participants’ own attitude estimations. It was shown that participants’ “subjective” aesthetic and sound-related perception of urban greenery were in line with the “objectively” measured subdivisions of high, medium and low biodiversity. So also were their estimations of the importance of trees and plants for perception of bird species in urban greenery, although differing only between high and medium/low biodiversity conditions. Persons rating themselves as highly nature-oriented were shown to give higher scores to urban green space aesthetics and to value greenery-related sounds higher, and to attach greater importance to trees and plants in their perception of bird species in urban greenery, than less nature-oriented persons. Highly urban-oriented persons compared to less urban-oriented persons did the same, but only regarding urban greenery-related aesthetics and sounds of nature. We conclude that environment-related attitudes influence perceptions of green space. Moreover, our findings support the idea that biodiversity per se also influences perceptions; people value green space significantly more with high than with low measured biodiversity. Urban planning needs to provide city inhabitants with green spaces that are species-rich, lush, varied and rich with natural sounds.

Effect of microhabitat complexity on the local distribution of arthropods in clear-cuts.

Abstract Structural complexity may affect activity and abundance of ground-dwelling arthropods. In this field experiment, the short-term effect on abundance of ground-active carabid beetles (Coleoptera: Carabidae) and lycosid spiders (Araneae: Lycosidae) from slash (logging residue) on the ground was studied. Arthropods were collected by pitfall trapping in three clear-cuts in southwest Sweden. The distribution of slash on the ground was manipulated, and two types of microhabitats were created adjacent to traps, i.e., either slash was cleared (bare ground traps) or slash was aggregated in heaps (slash-covered traps). Overall, carabids were significantly less abundant on the bare ground compared with the slash covered ground. For lycosids, no difference in abundance between the two microhabitat types was found. In reference plots, where no manipulation of microhabitat complexity was carried out, an overall positive relationship between structural complexity (slash height) and abundance of carabids was found. For lycosids, however, no correlations in reference plots were found. Bare ground microhabitats are mimicking structural complexity in clear-cuts after commercial slash harvest for biofuel. Large-scale removal of structural elements in clear-cuts might have short-term local effects, such as altered composition of arthropod communities.

Sex Ratio Variation in Sheet-Web Spiders: Options for Female Control?

Female biased primary sex ratios are known in many social spiders with highly structured populations and strong inbreeding. However, the primary sex ratio of the solitary spider Pityohyphantes phrygianus is also skewed, producing one-third males and two-thirds females, whereas the sex ratio is even in the related Linyphia triangularis. The female mating behaviours and genitalia were compared, revealing differences in, e. g. apparatus for sperm storing and in mating position; P. phrygianus takes a near perpendicular position before copulation starts. After completion of mating in P. phrygianus, experimental manipulations of the female’s position for 24 h significantly changed the proportion of males in the first brood. This suggests that a biasing mechanism acting in the female’s spermathecae-ducts is at least partly responsible for the sex ratio bias. However, the sex ratio in first and second broods averaged an overall mean of one-third males. The results suggested that females in the wild have options, which are not used, to direct the primary sex ratio away from one-third males. This deviation from Fisherian sex ratio may be caused by selfish genetic elements and/or meiotic drive.

Skewed primary sex ratio in the solitary spider Pityohyphantes phrygianus

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Bumblebees in the city: abundance, species richness and diversity in two urban habitats

Bumblebees are well known for their contribution to the ecosystem service of pollination. In urban areas, green space management beneficial to pollinators can be an important step in sustaining large urban bee populations. The abundance, number of species and diversity of bumblebees (Bombus spp), as well as the abundance of honeybees (Apis mellifera), were studied in 13 urban gardens (including allotments) and 13 ornamental flowerbeds (in parks and green spaces) in the city centre of Gothenburg, Sweden. In total, 12 species of bumblebees were observed. Species richness was significantly higher in gardens than in flowerbeds, but diversity (Berger–Parker and Simpson indices) was higher in flowerbeds than in urban gardens. The abundance in gardens was significantly higher and approximately twice that found in flowerbeds. The number of honeybee individuals was positively correlated with the abundance of bumblebees. Neither species richness nor abundance of bumblebees was affected by site size. However, a high flowering frequency positively affected the total number of bumblebee and honeybee individuals at the sites. We conclude that urban gardens contribute to sustaining a high abundance of bumblebees in the city centre, and indirectly facilitates small scale urban food production. A pollinator-friendly management of urban green space with plentiful flowering may promote a community of bumblebees with high abundance and diversity.

Expansion of deciduous tall shrubs but not evergreen dwarf shrubs inhibited by reindeer in Scandes mountain range

Summary One of the most palpable effects of warming in Arctic ecosystems is shrub expansion above the tree line. However, previous studies have found that reindeer can influence plant community responses to warming and inhibit shrubification of the tundra. We revisited grazed (ambient) and ungrazed study plots (exclosures), at the southern as well as the northern limits of the Swedish alpine region, to study long‐term grazing effects and vegetation changes in response to increasing temperatures between 1995 and 2011, in two vegetation types (shrub heath and mountain birch forest). In the field layer at the shrub heath sites, evergreen dwarf shrubs had increased in cover from 26% to 49% but were unaffected by grazing. Deciduous dwarf and tall shrubs also showed significant, though smaller, increases over time. At the birch forest sites, the increase was similar for evergreen dwarf shrubs (20–48%) but deciduous tall shrubs did not show the same consistent increase over time as in the shrub heath. The cover and height of the shrub layer were significantly greater in exclosures at the shrub heath sites, but no significant treatment effects were found on species richness or diversity. July soil temperatures and growing season thawing degree days (TDD) were higher in exclosures at all but one site, and there was a significant negative correlation between mean shrub layer height and soil TDD at the shrub heath sites. Synthesis. This study shows that shrub expansion is occurring rapidly in the Scandes mountain range, both above and below the tree line. Tall, deciduous shrubs had benefitted significantly from grazing exclosure, both in terms of cover and height, which in turn lowered summer soil temperatures. However, the overriding vegetation shift across our sites was the striking increase in evergreen dwarf shrubs, which were not influenced by grazing. As the effects of an increase in evergreen dwarf shrubs and more recalcitrant plant litter may to some degree counteract some of the effects of an increase in deciduous tall shrubs, herbivore influence on shrub interactions is potentially of great importance for shaping arctic shrub expansion and its associated ecosystem effects.