Recolonisation and distribution of spiders and carabids in cereal fields after spring sowing

Generalist predators are important for pest suppression during pest establishment because they may occur in the crop before and during pest arrival. However, different crop management practices can have a negative effect on predator populations. If so, there is a need for recolonisation by the predators to the crop fields. An important pest in Sweden is the bird cherry‐oat aphid, Rhopalosiphum padi, which migrates to cereal fields in spring. In turn, many cereal fields are spring sown and thus are disturbed by harrowing and sowing a short time before aphid arrival. In this study, three different questions about the populations of spiders and carabids in spring cereal crops were asked. First, does sowing in spring have a negative effect on the predators present in the fields? Second, if sowing has a negative effect on predator populations, do they recolonise the fields before pest arrival? Third, how are the predators distributed in the fields? We found that most carabids and most lycosid spiders were not affected by sowing and were distributed uniformly in the fields after sowing. Most linyphiid spiders were negatively affected by sowing, but then they recolonised the fields and were uniformly distributed in the fields after recolonisation. Thus, many spiders and carabids are present in the fields after sowing in spring and have the opportunity to suppress aphids during their establishment phase, which in turn may prevent a possible outbreak.     

Diversity and community composition of herbaceous plants in different habitat types in south‐east Cameroon

The composition of herbaceous vegetation was evaluated with the aim of characterizing forests at various ages of stand development. Herb stems were sampled in 250 4‐m² square plots distributed within six habitat types. A total of 36 herb species belonging to 15 families were recorded. Species richness did not significantly differ between habitat types. Most herb species occurred in all habitat types and were therefore generalists. However, a few indicator herb species were detected, and the results roughly suggested that herb species of the families Poaceae and Araceae were indicative of late successional forests; Zingiberaceae are indicative of early successional forests; and Commelinaceae, Costaceae, Cyperaceae and Marantaceae are indicators of flooded habitats. Species diversity and stem density of herbaceous plants did not change with forest succession as a decrease in abundance and frequency of occurrence of pioneer species in late successional forests was counterbalanced by the presence of generalist and late successional species. However, increasing proportions of dwarf stems in late successional forests translated to changes in the vertical structure of herbaceous plant communities. Herbivory pressure by gorillas did not have a notable effect on herbaceous plant community development. This study contributes to the definition of herbaceous ecological indicators of forest succession in different settings.     

The impact of spiders and high temperatures on cereal aphid (Rhopalosiphum padi) numbers in an irrigated perennial grass pasture in South Australia

Spiders were the most important group of aphid natural enemies in an irrigated perennial grass pasture. The Lycosidae and Linyphiidae were the only families encountered. An exclusion experiment found predation by spiders to be an important factor in controlling aphid numbers. Together with high temperatures, they maintained aphid numbers at a lower than expected level.     

Ecological impact of tobacco farming in miombo woodlands of Urambo District, Tanzania

This paper examines the ecological threat of tobacco farming in Urambo District, the leading producer of flue‐cured tobacco in Tanzania with other major producers being Tabora, Iringa and Chunya Districts. Structured interviews were conducted in four villages while 39 Modified‐Whittacker plots were laid in tobacco fallow lands for inventory of woody species to ascertain ecological performance and the impact of tobacco on species diversity, richness and standing stock functions. There was higher than expected species richness with a total of 115 tree and shrub species identified. Tobacco farming showed no significant negative effect on the floristic composition and stem density. However, the significantly reduced biomass and change in vegetation structure illustrate the potential loss in ecological function of the woodlands. Land clearing for tobacco planting account to an annual deforestation of 3.5% while on average a farmer requires 23 m³ of stacked wood only for curing per season which adds another 3% of deforestation. Shifting cultivation is no longer sustainable given the shortened fallow periods of 4 years. Improved barn structures, alternative sources of fuel like coal, tree planting, mixed cropping and cash crops that are environment friendly are recommended for ecological restoration.     

Response of ground-dwelling arthropods to a 'push–pull' habitat management system: spiders as an indicator group

Abstract Studies were conducted to assess the numerical response of ground‐dwelling arthropods to a habitat management system (‘push–pull’) developed to control maize stemborers using spiders (Araneae) as an indicator group. In this cropping system, maize is intercropped with a stemborer moth‐repellent (push) plant while an attractant trap crop (pull) is planted around this intercrop. Two study sites in western Kenya and one site at the Grain Crops Institute of the Agricultural Research Council in Potchefstroom, South Africa, were sampled. Treatments comprised a maize monocrop and an intercrop of maize and desmodium, Desmodium uncinatum Jacq., with Napier grass, Pennisetum purpureum (Schumach), as a trap crop around the field (‘push–pull’) in each site. Experiments were laid out in a completely randomized design with four replications at each site. Ground‐dwelling spiders were sampled using a combination of pitfall traps and soil samples. A total of 2175 spiders, 78 species in 18 families, were recovered in Kenya and 284 spiders, 34 species in nine families, were recovered in South Africa. Lycosidae was the most abundant family, accounting for >50% of all individual spiders and 27.6% by species richness. Spiders were significantly more abundant at the Kenyan sites than in South Africa while species diversity was significantly higher in South Africa than at the Kenyan sites. At all sites, spider abundance was significantly higher in the ‘push–pull’ than in the maize monocrop plots. However, the overall spider diversity was only significantly higher in the ‘push–pull’ than in the maize monocrop plots in South Africa. Moreover, species dominance did not differ between the two cropping systems at all sites. The results showed that the ‘push–pull’ system evidently enhances overall abundance of spiders, illustrating its potential in further pest control in the maize agroecosystems where spiders may often be one of the most important predatory groups.     

Changes in a Sudano-Sahelian rodent community after slash-and-burn farming (Gonsé Forest, Burkina Faso)

This study describes the recolonization by a Sudano‐Sahelian rodent community of an area slashed and burned in the middle of the dry season (Gonsé Forest in Burkina Faso). A series of abundance cycles occurred. The first began with the wet season and the arrival of a succession of four species: Taterillus gracilis, Tatera guineae, Nannomys sp. and Mastomys erythroleucus. Taterillus gracilis appeared to be the least demanding species. It was sexually active whatever the season and took advantage of reduced competition among species on the burned tracts. Tatera guineae also extended its spatial distribution but failed to reproduce. Nannomys sp. rapidly increased after the first rains. Although generally held to be the most prolific and opportunistic of the species involved, M. erythroleucus only found conditions amenable to its colonization late on. The timing of the disruption and whether or not it coincides with the reproductive period and the mobility phase of each species are decisive factors. This information is useful for a clearer understanding of the dynamics of each species and for evaluating the risk and probable locations of outbreaks of certain species.     

Species traits and catchment‐scale habitat factors influence the occurrence of freshwater mussel populations and assemblages

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Eggsac development rates and phenology of agrobiont linyphiid spiders in relation to temperature

Spider densities are often low after winter in annual crops, and crop management decimates spider populations several times per year. Population recovery rates and phenology depend on reproductive and development rates, which in turn are driven largely by temperature. We aimed to quantify the relationships between eggsac development rates and temperature in order to understand the relative value of different linyphiid species for the biological control of agricultural pests. Female adults of nine linyphiid species were collected from winter wheat in the UK over 3 years; Bathyphantes gracilis (Blackwall), Erigone atra (Blackwall), Erigone dentipalpis (Wider), Erigone promiscua O.P.‐Cambridge), Tenuiphantes tenuis (Blackwall) [formerly Lepthyphantes tenuis (Blackwall)], Meioneta rurestris (C.L. Koch), Oedothorax apicatus (Blackwall), Oedothorax fuscus (Blackwall), and Oedothorax retusus (Westring). These are agrobiont species that are dominant in agroecosystems. We tested how well development in the field can be predicted on the basis of laboratory experiments. We also built a simple phenology simulation model to test whether spider phenology in the field can be predicted by a general knowledge of the relationship between temperature and development rate. The relationships between temperature and development rates of eggsacs were not linear as described by a day‐degree model, but exponential as described by a biophysical model. Duration of the eggsac development period in the field was predicted accurately from laboratory experiments. We only found minor differences between development thresholds of eggsacs at constant temperatures compared with fluctuating temperatures. The phenology model predicted the phenology of L. tenuis and E. atra well, but the number of generations predicted for O. fuscus was not realised in the field. This suggests that development of this species may be affected by factors other than temperature. The methods used here could also be applied to other natural enemies, to determine whether their thermal biology is conducive to a role as biocontrol agents in disturbed agricultural systems.     

Gauging megadiversity with optimized and standardized sampling protocols: A case for tropical forest spiders

Characterizing and monitoring biodiversity and assessing its drivers require accurate and comparable data on species assemblages, which, in turn, should rely on efficient and standardized field collection. Unfortunately, protocols that follow such criteria remain scarce and it is unclear whether they can be applied to megadiverse communities, whose study can be particularly challenging. Here, we develop and evaluate the first optimized and standardized sampling protocol for megadiverse communities, using tropical forest spiders as a model taxon. We designed the protocol COBRA‐TF (Conservation Oriented Biodiversity Rapid Assessment for Tropical Forests) using a large dataset of semiquantitative field data from different continents. This protocol combines samples of different collecting methods to obtain as many species as possible with minimum effort (optimized) and widest applicability and comparability (standardized). We ran sampling simulations to assess the efficiency of COBRA‐TF (optimized, non‐site‐specific) and its reliability for estimating taxonomic, phylogenetic, and functional diversity, and community structure by comparing it with (1) commonly used expert‐based ad hoc protocols (nonoptimized, site‐specific) and (2) optimal protocols (optimized, site‐specific). We then tested the performance and feasibility of COBRA‐TF in the field. COBRA‐TF yielded similar results as ad hoc protocols for species (observed and estimated) and family richness, phylogenetic and functional diversity, and species abundance distribution. Optimal protocols detected more species than COBRA‐TF. Data from the field test showed high sampling completeness and yielded low numbers of singletons and doubletons. Optimized and standardized protocols can be as effective in sampling and studying megadiverse communities as traditional sampling, while allowing data comparison. Although our target taxa are spiders, COBRA‐TF can be modified to apply to any highly diverse taxon and habitat as long as multiple collecting techniques exist and the unit effort per sample is comparable. Protocols such as COBRA‐TF facilitate studying megadiverse communities and therefore may become essential tools for monitoring community changes in space and time, assessing the effects of disturbances and selecting conservation areas.     

The digging‐in effect on ant studies with pitfall traps: influence of type of habitat and sampling time

Pitfall traps are among the most common sampling methods used for the study of ants. There are many types of traps and many possible ways of using them. The various methodologies may introduce biases in sampling. One possible bias may be caused by the digging‐in effect (DE), resulting in higher catches of ants immediately after traps are set in the ground which subsequently decline. In this study, we performed two experiments to verify the consequences of the DE for ants in a Mediterranean ecosystem. In the first experiment we distinguished between two types of habitats: closed and open (i.e., with or without shrub or tree cover). The second experiment was carried out in a homogeneous pasture where the time of prevalence of the DE was verified, investigating the duration of the effect. The results indicate differences between communities in the first 24 h after setting of the traps, which had disappeared after 48 h. This does not dismiss the possibility of certain species being affected either positively (increase in captures) or negatively (decrease). Changes in species composition, determining whether the DE was manifested or not, differed among habitat types.     

Effects of hunger level and nutrient balance on survival and acetylcholinesterase activity of dimethoate exposed wolf spiders

The influence of two nutritional factors (food quantity and quality) on the responses of a wolf spider, Pardosa prativaga (L.K.), to a high dose of the insecticide dimethoate, was investigated in a fully factorial experimental design. Spider groups with different (good and bad) nutrient balance were created by feeding them fruit flies of either high or low nutrient content for 28 days. Both groups were then split into satiated and 14 days starved subgroups. Each of these was further divided into insecticide treated and control halves. Survivorship and acetylcholinesterase (AChE) activity measured on the survivors were used as response variables. Survivorship after topical dimethoate exposure (LD₅₀; 48 h) was influenced by spider body weight, nutrient balance, and starvation. Furthermore, AChE activity was significantly inhibited by dimethoate exposure. A significant interaction between nutrient balance, starvation, and dimethoate exposure revealed synergistic effects of starvation and nutrient imbalance on AChE inhibition by dimethoate in surviving spiders. These results show that the tolerance of non-target arthropods to dimethoate may vary depending on the nutritional history of the animal.     

Contrasting responses of arable spiders to the landscape matrix at different spatial scales

Aim Animal communities can be influenced by the composition of the surrounding landscape through immigration. Depending on habitat preferences, however, the effect of the landscape matrix can be positive or negative and can vary with scale. We tested this idea with arable spiders and tried to infer dispersal distances from relationships between local density and landscape composition at different spatial scales. Location Thirty‐eight landscapes around the cities of Göttingen and Giessen, Germany. Methods Spiders were captured with pitfall traps in one field of winter wheat in each landscape. Landscape composition around the fields was characterized at 11 scales from 95 m to 3 km radius by land‐use mapping and subsequent GIS analysis. Correlation tests were performed between landscape composition and local densities or species richness. Results In both study regions, local species richness was enhanced by non‐crop habitats on a landscape scale. The overall densities of wolf spiders (Lycosidae), long‐jawed spiders (Tetragnathidae), crab spiders (Thomisidae), and dwarf sheet spiders (Hahniidae) increased significantly in landscapes with high percentages of non‐crop habitats. Out of the 40 species tested, 19 responded positively to the percentage of non‐crop habitats in the surrounding landscape, and five responded negatively. Depending on the species, the spatial scales with the highest explanatory power ranged from 95 m to 3 km radius around the study fields, potentially reflecting dispersal distances. Main conclusions Arable spider species showed contrasting responses to the landscape context with respect both to the direction and to the spatial scale of the relationship. The variation in landscape requirements among species ensures high spider densities in a wide range of situations, which contributes to ecosystem resilience. However, species richness of arable spiders depends on heterogeneous landscapes with high percentages of non‐crop habitats.     

Effects of Tree Genotypic Diversity and Species Diversity on the Arthropod Community Associated with Big‐leaf Mahogany

Despite potential interactive effects of plant species and genotypic diversity (SD and GD, respectively) on consumers, studies have usually examined these effects separately. We evaluated the individual and combined effects of tree SD and mahogany (Swietenia macrophylla) GD on the arthropod community associated with mahogany. We conducted this study within the context of a tree diversity experiment consisting of 74 plots with 64 saplings/plot. We sampled 24 of these plots, classified as monocultures of mahogany or polycultures of four species (including mahogany). Within each plot type, mahogany was represented by either one or four maternal families. We surveyed arthropods on mahogany and estimated total arthropod abundance and species richness, as well as abundance and richness separately for herbivorous and predatory arthropods. Overall tree SD and mahogany GD had positive effects on total arthropod species richness and abundance on mahogany, and also exerted interactive effects on total species richness (but not abundance). Analyses conducted by trophic level group showed contrasting patterns; SD positively influenced herbivore species richness but not abundance, and did not affect either predator richness or abundance. GD influenced predator species richness but not abundance, and did not influence herbivore abundance or richness. There were interactive effects of GD and SD only for predator species richness. These results provide evidence that intra‐ and inter‐specific plant diversity exert interactive controls on associated consumer communities, and that the relative importance of SD and GD may vary among higher trophic levels, presumably due to differences in the underlying mechanisms or consumer traits.     

The response of riparian vegetation to flood‐maintained habitat heterogeneity†

Abstract Riparian environments are subject to the scouring and depositional effects of floods. Riparian vegetation and substrates are scoured during high flows, while litter and sediment is deposited downstream. In the Prosser and Little Swanport River catchments in south‐east Tasmania, vascular plant species were surveyed in large riparian relevés. Within these relevés, 1 × 1 m subplots were placed in both flood‐scoured and depositional environments. Species composition was compared between these three datasets, to investigate the importance of floods in determining species richness and species composition of riparian vegetation. Species richness and diversity were highest in areas experiencing flood scour. Herbs appear particularly reliant on the creation of gaps for colonization, and some major riparian shrub species may also require disturbance to maintain their abundance. The depositional environment tended to favour shrubs and graminoids. Given that differences in species composition are related to flood‐induced features of the riparian environment, the regulation of these rivers might reduce the diversity of the riparian vegetation downstream of dams.     

Diversity of canopy and understorey spiders in north-temperate hardwood forests

1 We characterized and compared diversity patterns of canopy and understorey spiders (Arachnida: Araneae) on sugar maple ( Acer saccharum Marsh.) and American beech ( Fagus grandifolia Ehrh.) in hardwood forests of southern Québec, Canada.
2 We sampled canopies of 45 sugar maple and 45 American beech trees and associated understorey saplings in mature protected forests near Montréal. Samples were obtained by beating the crown foliage at various heights and by beating saplings around each tree.
3 Eighty-two species were identified from 13 669 individuals. Forty-eight species and 3860 individuals and 72 species and 9809 individuals were collected from the canopy and the understorey, respectively.
4 Multivariate analyses (NMDS ordination and NPMANOVA) showed the composition of canopy and understorey assemblages differed significantly, and canopy assemblages differed between tree species. Rank-abundance distribution models fitted to the canopy and understorey data indicated that different mechanisms structure the assemblages in both habitats. Three abundant spider species were significantly more common in the canopy; ten species were collected significantly more often in the understorey.
5 The forest canopy was shown to be an important reservoir for spider diversity in north-temperate forests.     

Influence of landscape structure and farming practice on body condition and fecundity of wolf spiders

Structurally complex landscapes and organic management have been shown to augment natural pest enemy populations on arable land. Here, body condition and reproductive capacity of wolf spiders were studied to see if these traits can explain the larger populations in these environments. Females of Pardosa spiders (Araneae: Lycosidae) were collected at 7 organically and 7 conventionally managed fields situated in landscapes with different proportions of perennial crops, annual crops, forest, and different numbers and sizes of fields. Body condition (relative female weight) and fecundity (number of offspring and relative egg sac weight) were measured for each captured spider. In contrast to the hypothesis, Pardosa females caught in fields situated in landscapes dominated by large fields of annual crops had superior body condition. Farming practice had no effect on either body condition or fecundity measures. It is suggested that increased spider body condition in homogeneous landscapes may be due to less competition for available resources, although temporal variation should be included before drawing final conclusions on spiders’ body condition in the agricultural landscape.     

Diversity of Central European urban biota: effects of human‐made habitat types on plants and land snails

Aim Urbanization is associated with strong changes in biodiversity, but the diversity of plant and animal assemblages varies among urban habitats. We studied effects of urban habitats on the diversity of vascular plants and land snails in 32 large cities. Location Central Europe, Belgium and the Netherlands. Methods The species composition of all vascular plants that had not been planted by humans, and all land snails, was recorded in seven 1‐ha plots within each city. Each plot contained one urban habitat type representing a different disturbance regime: historical city square, boulevard, residential area with compact building pattern, residential area with open building pattern, park, early successional and mid‐successional site. For each plot, we obtained temperature and precipitation data. The effects of climate and habitat types on species composition were quantified using ordination methods with an adjusted variation partitioning algorithm. Differences in species composition among urban habitats were described using statistically determined diagnostic species, and differences in alpha, beta and gamma diversity were quantified. Results A total of 1196 plant and 87 snail species were recorded. Habitat type explained higher proportions of the total variation in both plant and snail species composition (11.2 and 8.2%, respectively) than did climate (4.6 and 6.3%). For both taxa, the main differences in species composition were observed between strongly urbanized sites in city centres and early successional and mid‐successional sites. For vascular plants, the number of species was lowest in city squares and boulevards, and highest at successional sites and in residential areas with compact building patterns. Beta diversity of vascular plants calculated for the same habitat types among cities was highest for squares and successional sites. The number of snail species was lowest in city squares and at early successional sites, and highest at mid‐successional sites. The highest beta diversity of snail assemblages among cities was observed within the city square and early successional habitat types, and the lowest within residential area habitat types. Main conclusions Urban habitats differ notably in the diversity of their vascular plant flora and land snail fauna. Understanding the habitat‐related biodiversity patterns in urbanized landscapes will allow projections of future impacts of urban land‐use changes on the biota.