The distribution and abundance of predatory arthropods overwintering on farmland

Soil samples (0·04 m^-2) were taken from December to January in 1981-82 and 1982-83 from five crop and two non-crop habitats on farmland in Hampshire to quantify the distribution and abundance of overwintering polyphagous predatory arthropods. Total numbers of overwintering polyphagous predators were greater in field boundaries, winter-sown cereals and established grassland than in the other four habitats. Many individual species overwintered in significantly higher densities in field boundaries than in any other habitat; others predominated in woodland, grassland and winter-sown cereals. The carabids Agonum dorsale and Demetrias atricapillus, probably the most important predators of cereal aphids, overwintered almost entirely in field boundaries.     

Effects of insecticide-treated and Lepidopteran-active Bt transgenic sweet corn on the abundance and diversity of arthropods

A field study was conducted over 2 yr to determine the effects of transgenic sweet corn containing a gene from the bacterium Bacillus thuringiensis (Bt) on the diversity and abundance of nontarget arthropods. The Bt hybrid (expressing Cry1Ab endotoxin for lepidopteran control) was compared with near-isogenic non-Bt and Bt hybrids treated with a foliar insecticide and with a near-isogenic non-Bt hybrid without insecticides. Plant inspections, sticky cards, and pitfall traps were used to sample a total of 573,672 arthropods, representing 128 taxonomic groups in 95 families and 18 orders. Overall biodiversity and community-level responses were not significantly affected by the transgenic hybrid. The Bt hybrid also had no significant adverse effects on population densities of specific nontarget herbivores, decomposers, and natural enemies enumerated at the family level during the crop cycle. As expected, the insecticide lambda-cyhalothrin had broad negative impacts on the abundance of many nontarget arthropods. One insecticide application in the Bt plots reduced the overall abundance of the natural enemy community by 21-48%. Five applications in the non-Bt plots reduced natural enemy communities by 33-70%. Nontarget communities affected in the insecticide-treated Bt plots exhibited some recovery, but communities exposed to five applications showed no trends toward recovery during the crop cycle. This study clearly showed that the nontarget effects of Bt transgenic sweet corn on natural enemies and other arthropods were minimal and far less than the community-level disruptions caused by lambda-cyhalothrin.     

The abundance of Ixodes ricinus ticks depends on tree species composition and shrub cover

SUMMARY The mainstream forestry policy in many European countries is to convert coniferous plantations into (semi-natural) deciduous woodlands. However, woodlands are the main habitat for Ixodes ricinus ticks. Therefore, assessing to what extent tick abundance and infection with Borrelia spirochetes are affected by forest composition and structure is a prerequisite for effective prevention of Lyme borreliosis. We selected a total of 25 pine and oak stands, both with and without an abundant shrub layer, in northern Belgium and estimated tick abundance between April and October 2008-2010. Additionally, the presence of deer beds was used as an indicator of relative deer habitat use. Borrelia infections in questing nymphs were determined by polymerase chain reactions. The abundance of larvae, nymphs, and adults was higher in oak stands compared to pine stands and increased with increasing shrub cover, most likely due to differences in habitat use by the ticks' main hosts. Whereas tick abundance was markedly higher in structure-rich oak stands compared to homogeneous pine stands, the Borrelia infection rates in nymphs did not differ significantly. Our results indicate that conversion towards structure-rich deciduous forests might create more suitable tick habitats, but we were unable to detect an effect on the infection rate.     

Species number, species abundance and body length of arboreal arthropods associated with an Australian rainforest tree

Abstract.

• 1 The species number, the abundance per species and the body length of arthropods foraging within the crowns of an over-storey rainforest tree from Australia, Argyrodendron actinophyllum (Sterculiaceae), were investigated by interception trap sampling and restricted canopy fogging. Emphasis was placed upon the interpretation of trap data. Arthropods were trapped continuously day and night, over a 2-year period and the final analyses examined the attributes of 759 species which represented 20,500 individuals.
• 2 The proportion of‘rare’species (Le. collected once) intercepted was high (35.7%), although lower than in other similar rainforest surveys. Neither the α log-series nor the log-normal distribution could be fitted to the relationship between number of species and number of individuals, since the number of rare species was much higher than predicted and the mode of the distribution could not be identified. The proportion of rare species was higher in fogging collections (452%) than in trap collections.
• 3 The data are compared with a study of Bornean arboreal beetles, obtained by fogging trees during a single sampling event. Several patterns were common to both data sets. However, the three-dimensional plot of the variables describing the structure of the arthropod community showed a notably rougher surface than in the case of Bornean beetles.
• 4 Although several factors may complicate the interpretation of the three-dimensional plots, long-term and continuous sampling may alter our perception of complex arthropod communities. This methodology is imperative for a proper understanding of arthropod community structure in rain forests.

     

Effects of selective logging on tree species diversity and composition of Bornean tropical rain forests at different spatial scales

Reduced-impact logging (RIL) is known to be beneficial in biodiversity conservation, but its effects on tree diversity remain unknown. Pattern of tree diversity following disturbance usually varies with spatial scale of sampling (i.e., plot size). We examined the impacts of RIL on species richness and community composition of tree species at different spatial scales, and the scale (plot size) dependency of the two metrics; species richness versus community similarity. One 2-ha and three to four 0.2-ha plots were established in each of primary, RIL, and conventionally logged (CL) forest in Sabah, Malaysia. Species richness (the number of species per unit number of stems) was higher in the RIL than in the CL forest at both scales. The relationship between species richness and logging intensity varied with plot size. Species richness was greater in the RIL than in the primary forest at the 2-ha scale, while it was similar between the two forests at 0.2-ha scale. Similarly, species richness in the CL forest demonstrated a greater value at the 2-ha scale than at the 0.2-ha scale. Greater species richness in the two logged forests at the 2-ha scale is attributable to a greater probability of encountering the species-rich, small patches that are distributed heterogeneously. Community composition of the RIL forest more resembled that of the primary forest than that of the CL forest, regardless of plot size. Accordingly, species richness is a scale-dependent metric, while community similarity is a more robust metric to indicate the response of tree assemblage to anthropogenic disturbance.     

Effects of above-ground plant species composition and diversity on the diversity of soil-borne microorganisms

A coupling of above-ground plant diversity and below-ground microbial diversity has been implied in studies dedicated to assessing the role of macrophyte diversity on the stability, resilience, and functioning of ecosystems. Indeed, above-ground plant communities have long been assumed to drive below-ground microbial diversity, but to date very little is known as to how plant species composition and diversity influence the community composition of micro-organisms in the soil. We examined this relationship in fields subjected to different above-ground biodiversity treatments and in field experiments designed to examine the influence of plant species on soil-borne microbial communities. Culture-independent strategies were applied to examine the role of wild or native plant species composition on bacterial diversity and community structure in bulk soil and in the rhizosphere. In comparing the influence of Cynoglossum officinale (hound's tongue) and Cirsium vulgare (spear thistle) on soil-borne bacterial communities, detectable differences in microbial community structure were confined to the rhizosphere. The colonisation of the rhizosphere of both plants was highly reproducible, and maintained throughout the growing season. In a separate experiment, effects of plant diversity on bacterial community profiles were also only observed for the rhizosphere. Rhizosphere soil from experimental plots with lower macrophyte diversity showed lower diversity, and bacterial diversity was generally lower in the rhizosphere than in bulk soil. These results demonstrate that the level of coupling between above-ground macrophyte communities and below-ground microbial communities is related to the tightness of the interactions involved. Although plant species composition and community structure appear to have little discernible effect on microbial communities inhabiting bulk soil, clear and reproducible changes in microbial community structure and diversity are observed in the rhizosphere. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of long-term fire exclusion on tree species composition and stand structure in an old-growth Pinus palustris (Longleaf pine) forest

Frequent fire is an integral component of longleaf pine ecosystems, creating environmental conditions favoring survival and growth of juvenile pines. This study examined stand structure, species composition, and longleaf pine regeneration in an old-growth tract of longleaf pine forest (Boyd Tract) experiencing long-term (>80 yr) fire exclusion in the Sandhills of North Carolina. Sampling of woody stems (i.e., 2.5 cm diameter at breast height) and tallies of longleaf pine seedlings were carried out in plots established randomly on upland, mesic areas and lowland, xeric areas within the Boyd Tract. Dominant woody species in mesic plots were black oak, hickories, and large, sparse longleaf pines. Xeric plots had high densities of turkey oak with the large longleaf pines, as well as higher frequencies of smaller longleaf stems. These differences between areas were associated with higher clay content of upland soils and higher sand content of lowland soils. Age-class frequency distributions for fire-suppressed longleaf pine following the last wildfire at the Boyd Tract approximately 80 yr ago contrasted sharply with data from an old-growth longleaf tract in southern Georgia (Wade Tract) that has been under a long-term frequent fire regime. Post-burn recruitment for the Boyd Tract wildfire appears to have been initially high on both site types. Longleaf pine recruitment diminished sharply on the mesic site, but remained high for 60 yr on the xeric site. Currently, longleaf pine regeneration is minimal on both site types; several plots contained no seedlings. Sharp contrasts in longleaf pine dominance and stand structure between the Boyd and Wade Tracts demonstrate the importance of large-scale disturbance, especially hurricanes and fire, in shaping the structure and function of longleaf pine ecosystems of the southeastern United States. In particular, long-term exclusion of fire on the Boyd Tract has altered stand structure dramatically by permitting hardwoods to occupy at high densities the characteristically large gaps between longleaf stems that are maintained by fire and other disturbances.     

3种松树林林分改造树种的生长格局

对3 种松林下的黎蒴、白花油茶、竹节树苗木的生长进行了研究.3 种苗木的地径和冠幅生长量以及黎蒴和油茶的树高生长量排序为湿地松林分>有凋落物加勒比松林分>无凋落物加勒比松林分,而竹节树在有凋落物加勒比松林下的树高生长略大于湿地松林,二者显著大于无凋落物加勒比松林.无凋落物的加勒比松林因为去除地表凋落物后减少了养分的输入,土壤有机质和氮含量减少,导致苗木生长下降.总体来看,竹节树的地径和冠幅的生长量最大,黎蒴树高的生长量最大,二者是改造松林的优良树种,而油茶生长慢.     

Competition and facilitation between tree species change with stand development

Processes governing tree interspecific interactions, such as facilitation and competition, may vary in strength over time. This study tried to unveil them by performing dendrometrical analyses on black spruce Picea mariana, trembling aspen Populus tremuloides and jack pine Pinus banksiana trees from pure and mixed mature boreal forest stands in the Clay Belt of northwestern Quebec and on the tills of northwestern Ontario. We cored 1430 trees and cut 120 for stem analysis across all stand composition types, tree species and study regions. Aspen annual growth rate was initially higher when mixed with conifers, but then progressively decreased over time compared to pure aspen stands, while jack pine growth rate did not differ with black spruce presence throughout all stages of stand development. When mixed with aspen, black spruce showed a contrary response to aspen, i.e. an initial loss in growth but a positive gain later. On the richer clay soil of the Quebec Clay Belt region, however, both aspen and spruce responses in mixed stands reversed between 37 and 54 years. Overall, our results demonstrate that interspecific interactions were present and tended to change with stand development and among species. Our results also suggest that the nature of interspecific interactions may differ with soil nutrient availability.     

Hibernation of predatory arthropods in semi-natural habitats

Non-crop habitats provide important resources for natural enemies. Many natural enemies hibernate in non-crop habitats, from which they may colonise arable fields in the spring. Spring colonisation ensures annual repopulation of the crop with natural enemies, allowing them to keep pace with the development of pest populations. The availability of non-crop habitats can, therefore, be crucial to successful conservation biological control. We quantified the density of overwintering natural enemies near organic Brussels sprout crops in five different non-crop habitats (short grassy field margin, herbaceous field margin, herbaceous field margin under a tree line, ditch and forest). Soil and litter samples of non-crop habitats were taken at two sites. One site was located in an open agricultural landscape, the other in a landscape dominated by mixed forest. Insects belonging to Staphylinidae, Araneae, Carabidae, Coccinellidae and Dermaptera were the most abundant. Mean densities of predatory arthropods were higher in the open agricultural landscape (290 predators m⁻²) than in the forested landscape (137 predators m⁻²). Herbaceous habitat types supported the highest densities of overwintering predators (up to 400 predators m⁻²), whereas densities in the forest were lowest (10 predators m⁻²). These results indicate that herbaceous non-crop habitats are important refugia for predators and may play a vital role in conservation biological control. Handling Editor: Arne Janssen.     

Assessment of species diversity from species abundance distributions at different localities

We show how the spatial structure of species diversity can be analyzed using the correlation between the log abundances of the species in the communities, assuming that two communities at different localities can be described by a bivariate lognormal species abundance distribution. A useful property of this approach is that the log abundances of the species at two localities can be considered as samples from a bivariate normal distribution defined by only five parameters. The variances and the correlation can be estimated by maximum likelihood methods even if there is no information about the sampling intensity and the number of unobserved species. This method also enables estimation of over-dispersion in the sampling relative to a Poisson distribution that allows sampling adjustment of the estimate of β-diversity. Furthermore, we also obtain a partitioning of species diversity into additive components of α-, β- and γ-diversity. For instance, if the correlation between the log abundances of the species is close to one, the same species will be common and rare in the two communities and the β-diversity will be low. We illustrate this approach by analysing similarities of communities of rare and endangered species of oak-living beetles in south-eastern Norway. The number of recorded species was estimated to be only 48.1% of the total number of species actually present in these communities. The correlations among communities dropped rather quickly with distance with a scaling of order 200 km. This illustrates large spatial heterogeneity in species composition, which should be accounted for in the design of schemes of such devices for assessing species diversity in these habitat-types.     

The ubiquity of intraguild predation among predatory arthropods

Intraguild predation (IGP) occurs when one predator species attacks another predator species with which it competes for a shared prey species. Despite the apparent omnipresence of intraguild interactions in natural and managed ecosystems, very few studies have quantified rates of IGP in various taxa under field conditions. We used molecular analyses of gut contents to assess the nature and incidence of IGP among four species of coccinellid predators in soybean fields. Over half of the 368 predator individuals collected in soybean contained the DNA of other coccinellid species indicating that IGP was very common at our field site. Furthermore, 13.2% of the sampled individuals contained two and even three other coccinellid species in their gut. The interaction was reciprocal, as each of the four coccinellid species has the capacity to feed on the others. To our knowledge, this study represents the most convincing field evidence of a high prevalence of IGP among predatory arthropods. The finding has important implications for conservation biology and biological control.     

Structure,composition and species diversity in an altitude-substrate matrix of rain forest tree communities on Mount Kinabalu,Borneo

We studied forest structure, composition and tree species diversity of eight plots in an environmental matrix of four altitudes (700, 1700, 2700 and 3100 m) and two types of geological substrates (ultrabasic and non-ultrabasic rocks) on Mount Kinabalu, Borneo. On both substrate series, forest stature, mean leaf area and tree species diversity (both 4.8 cm and 10 cm diameter at breast height [dbh]) decreased with altitude. The two forests on the different substrate series were similar at 700 m in structure, generic and familial composition and tree species diversity, but became dissimilar with increasing altitude. The decline in stature with altitude was steeper on the ultrabasic substrates than on the non-ultrabasic substrates, and tree species diversity was generally lower on ultrabasic substrates than on non-ultrabasic substrates at 1700 m. The forests on non-ultrabasic substrates at higher altitudes and those on ultrabasic substrates at the lower altitudes were similar in dbh versus tree height allometry, mean leaf area, and generic and familial composition at 1700 m. These contrasting patterns in forest structure and composition between the two substrate series suggested that altitudinal change was compressed on the ultrabasic substrates compared to the non-ultrabasic substrates. Tree species diversity was correlated with maximum tree height and estimated aboveground biomass, but was not with basal area, among the eight study sites. We suggest that forests with higher tree species diversity are characterized by greater biomass allocation to height growth relative to trunk diameter growth under more productive environment than forests with lower tree species diversity.     

Predator diversity and abundance provide little support for the enemies hypothesis in forests of high tree diversity

Predatory arthropods can exert strong top-down control on ecosystem functions. However, despite extensive theory and experimental manipulations of predator diversity, our knowledge about relationships between plant and predator diversity--and thus information on the relevance of experimental findings--for species-rich, natural ecosystems is limited. We studied activity abundance and species richness of epigeic spiders in a highly diverse forest ecosystem in subtropical China across 27 forest stands which formed a gradient in tree diversity of 25-69 species per plot. The enemies hypothesis predicts higher predator abundance and diversity, and concomitantly more effective top-down control of food webs, with increasing plant diversity. However, in our study, activity abundance and observed species richness of spiders decreased with increasing tree species richness. There was only a weak, non-significant relationship with tree richness when spider richness was rarefied, i.e. corrected for different total abundances of spiders. Only foraging guild richness (i.e. the diversity of hunting modes) of spiders was positively related to tree species richness. Plant species richness in the herb layer had no significant effects on spiders. Our results thus provide little support for the enemies hypothesis--derived from studies in less diverse ecosystems--of a positive relationship between predator and plant diversity. Our findings for an important group of generalist predators question whether stronger top-down control of food webs can be expected in the more plant diverse stands of our forest ecosystem. Biotic interactions could play important roles in mediating the observed relationships between spider and plant diversity, but further testing is required for a more detailed mechanistic understanding. Our findings have implications for evaluating the way in which theoretical predictions and experimental findings of functional predator effects apply to species-rich forest ecosystems, in which trophic interactions are often considered to be of crucial importance for the maintenance of high plant diversity.     

Using species abundance distribution models and diversity indices for biogeographical analyses

We examine whether Species Abundance Distribution models (SADs) and diversity indices can describe how species colonization status influences species community assembly on oceanic islands. Our hypothesis is that, because of the lack of source-sink dynamics at the archipelago scale, Single Island Endemics (SIEs), i.e. endemic species restricted to only one island, should be represented by few rare species and consequently have abundance patterns that differ from those of more widespread species. To test our hypothesis, we used arthropod data from the Azorean archipelago (North Atlantic). We divided the species into three colonization categories: SIEs, archipelagic endemics (AZEs, present in at least two islands) and native non-endemics (NATs). For each category, we modelled rank-abundance plots using both the geometric series and the Gambin model, a measure of distributional amplitude. We also calculated Shannon entropy and Buzas and Gibson's evenness. We show that the slopes of the regression lines modelling SADs were significantly higher for SIEs, which indicates a relative predominance of a few highly abundant species and a lack of rare species, which also depresses diversity indices. This may be a consequence of two factors: (i) some forest specialist SIEs may be at advantage over other, less adapted species; (ii) the entire populations of SIEs are by definition concentrated on a single island, without possibility for inter-island source-sink dynamics; hence all populations must have a minimum number of individuals to survive natural, often unpredictable, fluctuations. These findings are supported by higher values of the α parameter of the Gambin mode for SIEs. In contrast, AZEs and NATs had lower regression slopes, lower α but higher diversity indices, resulting from their widespread distribution over several islands. We conclude that these differences in the SAD models and diversity indices demonstrate that the study of these metrics is useful for biogeographical purposes.     

Shrubs and species identity effects on the distribution and diversity of ground-dwelling arthropods in a Gobi desert

Disentangling the relationship between shrub vegetation and ground-dwelling arthropods at multiple levels of taxonomic resolution is essential to developing a suitable management strategy for the conservation of shrub-associated arthropod biodiversity in shrubland ecosystems. Using Gobi desert (dominated by shrub species Nitraria sphaerocarpa and Reaumuria soongorica) occurring widely in inland arid areas of northwest China as a model system, we sampled ground-dwelling arthropods by pitfall trapping method under canopies of both shrubs and in intershrub bare areas during spring, summer and autumn. Our aim was to determine whether the presence and species identity of shrubs influence the distribution of ground-dwelling arthropods and whether the influence of shrub presence and species identity differs among trophic and taxonomic groups. At the community level, total arthropod abundance and species richness were significantly greater under shrubs than in intershrub bare habitats, whereas more arthropods were captured under N. sphaerocarpa than under R. soongorica. At the trophic group level, the abundance of predator and decomposer arthropods was significantly greater under shrubs than in intershrub bare habitats, whereas herbivore and decomposer arthropods were more abundant under N. sphaerocarpa than under R. soongorica. At the family level, the abundance of Carabidae, Gnaphosidae, Karschiidae, Tenebrionidae and Thomisidae was consistently much greater under shrubs than in intershrub bare habitats, whereas that of Formicidae, Philodromidae and Tettigoniidae did not differ between the shrub microhabitats and the intershrub bare habitats. However, the abundance of Curculionidae under R. soongorica and the abundance of Geotrupidae under N. sphaerocarpa were significantly lower than that in intershrub bare habitats. N. sphaerocarpa was commonly preferred by arthropods in the families of Curculionidae and Tenebrionidae, whereas R. soongorica was commonly preferred by arthropods in the family Geotrupidae, but shrub species identity did not affect the abundance of the remaining families. Our results suggest that shrubs and species identity play important roles in structuring ground-dwelling arthropod communities, but the response of arthropods differed among trophic or taxonomic groups. This study may have important implications for the conservation of invertebrate biodiversity in Gobi desert ecosystems.