Implications of Three Biofuel Crops for Beneficial Arthropods in Agricultural Landscapes

Production of biofuel feedstocks in agricultural landscapes will result in land use changes that may have major implications for arthropod-mediated ecosystem services such as pollination and pest suppression. By comparing the abundance and diversity of insect pollinators and generalist natural enemies in three model biofuel crops: corn, switchgrass, and mixed prairie, we tested the hypothesis that biofuel crops comprised of more diverse plant communities would support increased levels of beneficial insects. These three biofuel crops contained a diverse bee community comprised of 75 species. Overall, bees were three to four times more abundant in switchgrass and prairie than in corn, with members of the sweat bee (Halictidae) and small carpenter bee (Ceratina spp.) groups the most abundant. Switchgrass and prairie had significantly greater bee species richness than corn during the July sampling period. The natural enemy community at these sites was dominated by lady beetles (Coccinellidae), long-legged flies (Dolichopodidae), and hover flies (Syrphidae) which varied in their response to crop type. Coccinellids were generally most abundant in prairie and switchgrass, with the exception of the pollen feeding Coleomegilla maculata that was most abundant in corn. Several rare or declining coccinellid species were detected in prairie and switchgrass sites. Dolichopodidae were more abundant in prairie and switchgrass while Syrphidae showed no significant response to crop type. Our results indicate that beneficial insects generally responded positively to the increased vegetational diversity of prairie and switchgrass sites; however, when managed as a dedicated biofuel crop, plant and arthropod diversity in switchgrass may decrease. Our findings support the hypothesis that vegetationally diverse biofuel crops support higher abundance and diversity of beneficial insects. Future policy regarding the production of biofuel feedstocks should consider the ecosystem services that different biofuel crops may support in agricultural landscapes.     

Biocontrol potential varies with changes in biofuel–crop plant communities and landscape perenniality

We examined the potential local‐ and landscape‐level impacts of different biofuel production systems on biocontrol, an important service provided by arthropod natural enemies. Specifically, we sampled natural enemies with sweep nets and measured predation of sentinel pest eggs in stands of corn, switchgrass and mixed prairie in Michigan and Wisconsin (total n=40 for natural enemy sampling, n=60 for egg predation), relating them to crop type, forb cover and diversity, and the composition and heterogeneity of the surrounding landscape. Grasslands with intermediate levels of forb cover and flower diversity supported two‐orders of magnitude more natural enemy biomass, fourfold more natural enemy families, and threefold greater rates of egg predation than corn. Data suggest this was in part due to a general increase in biomass, richness and predation in perennial grasslands compared with corn, combined with a positive effect of intermediate levels of forb cover and flower diversity. Specifically, natural enemy biomass and family richness showed hump‐shaped relationships to forb cover that peaked in sites with 5–25% forbs, while egg predation increased with floral diversity. At the landscape scale, both natural‐enemy biomass and egg predation increased with the area of forest in the landscape, and egg predation almost doubled as the area of herbaceous, perennial habitats within 1.5 km of study sites increased. Our results suggest that floristically diverse, perennial grasslands support diverse and abundant predator communities that contribute to natural pest suppression. In addition, large‐scale production of biofuel crops could positively or negatively affect biocontrol services in agricultural landscapes through associated changes in the area of perennial habitats. Biofuel landscapes that incorporate perennial grasslands could support a variety of beneficial organisms and ecosystem services in addition to producing biomass.     

Agroenergy Crops Influence the Diversity, Biomass, and Guild Structure of Terrestrial Arthropod Communities

Expanded production of contemporary bioenergy crops (e.g., corn) is considered a threat to the conservation of biodiversity, yet next-generation perennially based crops (switchgrass, mixed-grass?Cforb prairie) may represent an opportunity for enhancing biodiversity in agricultural landscapes. We employed a multi-scaled approach to investigate the relative importance of feedstock selection, forb content, patch size, and landscape-scale habitat structure and composition as factors shaping the diversity and abundance of terrestrial arthropod communities and the biomass of functional groups of arthropods associated with the provisioning of ecosystem services. Compared to intensively managed annual corn fields, switchgrass and mixed-grass?Cforb prairie plantings were associated with a 230% and 324% increase in arthropod family diversity and a 750% and 2,700% increase in arthropod biomass, respectively. Biomass of arthropod pollinators, herbivores, predators, and parasites were similarly the highest in mixed-grass?Cforb prairie, intermediate in switchgrass plantings, and the lowest in cornfields. Community-wide biomass and that of several functional arthropod groups were positively linked to increasing forest cover and land cover diversity surrounding biomass plantings, while pollinator and detritivore biomass was lower in smaller fields. Results not only suggest that the choice of biomass feedstock will play an important role in shaping within-field arthropod diversity but also indicate an important role for the composition of this surrounding landscape. Collectively, our results suggest that selection of perennially based biomass feedstocks along with careful attention to crop placement have important potential to enhance biodiversity conservation and the provisioning of ecologically and economically important arthropod-mediated ecosystem services in future agricultural landscapes.     

Crop identity and memory effects on aboveground arthropods in a long‐term crop rotation experiment

Agricultural landscapes are globally dominated by monocultures under intensive management. This is one of the main reasons for biodiversity loss and insect population decline in many regions all over the world. Agroecosystem biodiversity in these areas can be enhanced by cropping system diversification, such as crop rotations. Yet, long‐term studies on effects of crop rotations on aboveground agrobiodiversity are lacking. We set up a 10‐year long‐term crop rotation experiment in Central Germany and monitored the temporal dynamics of aboveground arthropods over a full cultivation period to investigate influence of current and preceding crop identity and cropping system diversification on activity density, species richness, and community structure. We found that species composition was strongly influenced by currently grown crop although effect on arthropods varied between species groups. Especially, winter oilseed rape strongly affects arthropod community structure. Interestingly, we were also able to show an influence of the preceding crops, indicating an ecological memory effect in the aboveground arthropod community. Our results show that crop identity of both currently and previously grown crops in crop rotations may lead to an increase in arthropod activity density and changes in species composition. Diversified crop rotations including appropriate crops can be an easily implemented tool to increase arthropod biodiversity and biomass at large spatial and temporal scales, particularly in areas dominated by a single crop (e.g., wheat, maize). Our results may help to design optimized crop rotations for large‐scale enhancement of insect biodiversity in agroecosystems.     

Global land-use change and the importance of zoophytophagous bugs in biological control: Coppicing willows as a timely example

Perennial plants, such as willows, miscanthus, and hybrid poplars, are promising bioenergy crops while lowering atmospheric CO₂. Increases in the acreage of perennial bioenergy crops will pose new challenges and opportunities for biological control. In this review, we suggest that zoophytophagous bugs could be expected to become increasingly important in biological control of arthropod pests in perennial bioenergy crops. The main reasons are: (1) perennial bioenergy crops provide suitable habitats for arboreal natural enemies like zoophytophagous bugs; (2) zoophytophagous bugs often increase in response to low disturbance frequencies; and (3) plant-feeding by zoophytophagous bugs will not likely affect plant biomass production in perennial crops. We review aspects of the biology of zoophytophagous bugs expected to be important in biological control of pests in perennial systems. We also present a predator–prey model investigating how alternative harvest methods affect biological control of herbivorous pests by zoophytophagous bugs in willow biomass plantations. Although there is good evidence that zoophytophagous bugs can provide pest control, more research is needed about factors affecting the dispersal and population dynamics of zoophytophagous bugs in agricultural landscapes.     

Arthropod but Not Bird Predation in Ethiopian Homegardens Is Higher in Tree-Poor than in Tree-Rich Landscapes

Bird and arthropod predation is often associated with natural pest control in agricultural landscapes, but the rates of predation may vary with the amount of tree cover or other environmental factors. We examined bird and arthropod predation in three tree-rich and three tree-poor landscapes across southwestern Ethiopia. Within each landscape we selected three tree-rich and three tree-poor homegardens in which we recorded the number of tree species and tree stems within 100 × 100 m surrounding the central house. To estimate predation rates, we attached plasticine caterpillars on leaves of two coffee and two avocado shrubs in each homegarden, and recorded the number of attacked caterpillars for 7–9 consecutive weeks. The overall mean daily predation rate was 1.45% for birds and 1.60% for arthropods. The rates of arthropod predation varied among landscapes and were higher in tree-poor landscapes. There was no such difference for birds. Within landscapes, predation rates from birds and arthropods did not vary between tree-rich and tree-poor homegardens in either tree-rich or tree-poor landscapes. The most surprising result was the lack of response by birds to tree cover at either spatial scale. Our results suggest that in tree-poor landscapes there are still enough non-crop habitats to support predatory arthropods and birds to deliver strong top-down effect on crop pests.     

Perennial biomass feedstocks enhance avian diversity

Federal mandates to increase biofuel production in North America will require large new tracts of land with potential to negatively impact biodiversity, yet empirical information to guide implementation is limited. Because the temperate grassland biome will be a production hotspot for many candidate feedstocks, production is likely to impact grassland birds, a group of major conservation concern. We employed a multiscaled approach to investigate the relative importance of arthropod food availability, microhabitat structure, patch size and landscape‐scale habitat structure and composition as factors shaping avian richness and abundance in fields of one contemporary (corn) and two candidate cellulosic biomass feedstocks (switchgrass and mixed‐grass prairie) not currently managed as crops. Bird species richness and species density increased with patch size in prairie and switchgrass, but not in corn, and was lower in landscapes with higher forest cover. Perennial plantings supported greater diversity and biomass of arthropods, an important food for land birds, but neither metric was important in explaining variation in the avian community. Avian richness was higher in perennial plantings with greater forb content and a more diverse vegetation structure. Maximum bird species richness was commonly found in fields of intermediate vegetation density and grassland specialists were more likely to occur in prairies. Our results suggest that, in contrast to corn, perennial biomass feedstocks have potential to provide benefits to grassland bird populations if they are cultivated in large patches within relatively unforested landscapes. Ultimately, genetic improvement of feedstock genets and crop management techniques that attempt to maximize biomass production and simplify crop vegetation structure will be likely to reduce the value of perennial biomass plantings to grassland bird populations.     

Functional spatial scale of community composition change in response to windthrow disturbance in a deciduous temperate forest

Community dynamics in local habitats are affected by landscape characteristics such as the area and connectivity of surrounding habitats at a functional spatial scale where the community responds to landscape structure. However, the functional spatial scale at which community composition is affected by landscape structure has never been explored. We assessed the functional spatial scales of composition change in birds and in three types of arthropod communities (canopy, forest-floor and flying ones) with regard to landscape heterogeneity resulting from a large typhoon in a temperate forest of Japan. We examined the effects of tree-fall disturbance on the communities at various spatial scales, with special attention to compositional evenness. The spatial scale of the best-fitting model, which was selected from models fitted to the disturbance area at stepwise spatial scales, was interpreted as the community-specific functional spatial scale. The composition of all communities studied was all significantly dependent on gap area. The functional spatial scale was highest in birds (370 m in radius), intermediate in flying arthropods (90 m) and lowest in canopy and forest-floor arthropods (10 m). This result may reflect typical dispersal ability and the spatial range of resource use in the community. Compositional changes in each community were consistent with theory regarding traits and responses of component taxa, although the enhancement of evenness was observed only in the arthropod communities. These results imply that management and reserve selection based on functional spatial scales can be effective in the conservation of biodiversity and ecosystem services at the community level.     

Bird and bat predation services in tropical forests and agroforestry landscapes

Understanding distribution patterns and multitrophic interactions is critical for managing bat‐ and bird‐mediated ecosystem services such as the suppression of pest and non‐pest arthropods. Despite the ecological and economic importance of bats and birds in tropical forests, agroforestry systems, and agricultural systems mixed with natural forest, a systematic review of their impact is still missing. A growing number of bird and bat exclosure experiments has improved our knowledge allowing new conclusions regarding their roles in food webs and associated ecosystem services. Here, we review the distribution patterns of insectivorous birds and bats, their local and landscape drivers, and their effects on trophic cascades in tropical ecosystems. We report that for birds but not bats community composition and relative importance of functional groups changes conspicuously from forests to habitats including both agricultural areas and forests, here termed ‘forest‐agri’ habitats, with reduced representation of insectivores in the latter. In contrast to previous theory regarding trophic cascade strength, we find that birds and bats reduce the density and biomass of arthropods in the tropics with effect sizes similar to those in temperate and boreal communities. The relative importance of birds versus bats in regulating pest abundances varies with season, geography and management. Birds and bats may even suppress tropical arthropod outbreaks, although positive effects on plant growth are not always reported. As both bats and birds are major agents of pest suppression, a better understanding of the local and landscape factors driving the variability of their impact is needed.     

Structural complexity of arthropod guilds is affected by the agricultural landscape heterogeneity generated by fencerows

Intensive farming imposes harsh conditions impeding the persistence of most arthropod species within crop fields. Hence, arthropods surviving the unfavourable conditions prevailing within crop fields may disperse towards nearby uncropped margins, such as fencerows. Here, we evaluate the influence of landscape heterogeneity on the abundance of different guilds, particularly herbivores and their natural enemies. Said heterogeneity mostly derives from fencerow network density. Hence, we developed an approach based on fitting linear‐mixed models to elucidate the effects of landscape heterogeneity and field position (fencerows and crop interiors) on arthropod diversity. Mixed models were fitted to arthropod data obtained by pitfall trap samplings in 36 crop fields. Arthropod communities were structurally and functionally more complex along fencerows than within nearby crop interiors. Arthropods abundance was modulated by landscape heterogeneity, increasing the abundance of natural enemies as the landscape heterogeneity increased. On the contrary, herbivores abundance decreased as landscape heterogeneity increased. Consequently, the ratio between herbivores and natural enemies also decreased as landscapes became more heterogeneous. Natural enemies with larger body sizes, mostly carabid beetles, were more sensitive to landscape homogenisation. Our study reveals that, despite the coarse‐grained landscapes in the Rolling Pampa, fencerow density appears as a key factor for structuring complex arthropod guilds in intensively farmed agricultural mosaics. In landscapes with higher density of fencerows, arthropods tend to concentrate along them, thus increasing the community structural complexity as well as the predation pressure over herbivores. This structural complexity of upper trophic levels enhances the ‘top‐down’ regulation of herbivore populations, consequently decreasing the probability of pest outbreaks within crop fields.     

Cascading ecological effects of landscape moderated arthropod diversity

Understanding the mechanisms regulating the diversity and distribution of arthropods is essential to understanding food web interactions and ecosystem functioning. Local arthropod diversity is known to be linked to features of surrounding landscapes, including the area of human‐developed land. Yet, how such landscape moderation of diversity affects processes within local sites remains understudied. We report on a study that 1) measured the impacts of human development surrounding old‐field habitats of arthropods on arthropod food web structure within those habitats and 2) determined if these shifts were associated with cascading impacts on the plant community. We sampled the arthropod community in 16 old‐fields that span an urban‐rural gradient throughout southern New England, USA. In each field, we also established paired mesocosms enclosing vegetation, one of which allowed arthropod herbivory while the other excluded such interactions, to isolate impacts of arthropod herbivory on three functional groups of plants: grasses, goldenrod and non‐goldenrod forbs. Biomass of both herbivorous and predatory arthropods were positively related to the proportion of natural area surrounding a field early in the growing season (June). This relationship persisted later into the season for predatory arthropods (through July), but not for herbivorous arthropods. We found no evidence that the biomass of predators was related to the abundance of herbivorous arthropods in a field; or that biomass of herbivores was correlated to change in plant biomass between the two types of mesocosms. We did, however, find that in fields with low predator abundance there was greater herbivory on grasses (nutritious host), but that in high predator fields goldenrod was increasingly impacted (safe host), as is predicted by past work in old‐field ecosystems. The findings support the generalizability of landscape moderated biodiversity to non‐agricultural systems and suggests that observed shifts in food webs have implications for community and ecosystem dynamics.     

Using faecal metabarcoding to examine consumption of crop pests and beneficial arthropods in communities of generalist avian insectivores

Generalist insectivorous birds can provide ecosystem services in agricultural landscapes by consuming arthropod pests, or they can provide disservices when they consume beneficial arthropods. To examine bird impacts on arthropod communities, including pest control services, we need to know which arthropods birds commonly consume. Faecal metabarcoding is an emerging technique that can be used to identify prey from faecal samples, often to the species level. We used faecal metabarcoding to study diets of birds inhabiting the ecotone between soybean fields and adjacent grasslands in a largely agricultural landscape in Illinois, USA, during the summer of 2017. Whereas previous studies have used faecal metabarcoding to compare bird diets among species or among capture sites, we analysed samples from multiple species within a community at replicate sites. We collected and sequenced DNA from 132 faecal samples from 25 bird species captured at six sites. We found that birds consumed an extremely large and varied diet that differed among both species and sites, suggesting that birds were consuming prey opportunistically as available at each site. Of the nine most commonly detected prey species, three are known pests of soybeans. Bird diets also contained significantly more species of herbivorous prey than natural enemies. Finally, we discovered that American Goldfinches Spinus tristis, a highly granivorous species, may consume arthropods more frequently than expected and thus may provide ecosystem services in agricultural landscapes. Our study demonstrates that birds within this system consume a large variety of prey, suggesting that they may be able to respond quickly to pest outbreaks and contribute to agricultural resiliency.     

Role of arthropod communities in bioenergy crop litter decomposition†

The extensive land use conversion expected to occur to meet demands for bioenergy feedstock production will likely have widespread impacts on agroecosystem biodiversity and ecosystem services, including carbon sequestration. Although arthropod detritivores are known to contribute to litter decomposition and thus energy flow and nutrient cycling in many plant communities, their importance in bioenergy feedstock communities has not yet been assessed. We undertook an experimental study quantifying rates of litter mass loss and nutrient cycling in the presence and absence of these organisms in three bioenergy feedstock crops—miscanthus (Miscanthus x giganteus), switchgrass (Panicum virgatum), and a planted prairie community. Overall arthropod abundance and litter decomposition rates were similar in all three communities. Despite effective reduction of arthropods in experimental plots via insecticide application, litter decomposition rates, inorganic nitrogen leaching, and carbon–nitrogen ratios did not differ significantly between control (with arthropods) and treatment (without arthropods) plots in any of the three community types. Our findings suggest that changes in arthropod faunal composition associated with widespread adoption of bioenergy feedstock crops may not be associated with profoundly altered arthropod‐mediated litter decomposition and nutrient release.     

The perennial biogas crops cup plant (Silphium perfoliatum L.) and field grass pose better autumn and overwintering habitats for arthropods than silage maize (Zea mays L.)

Perennial energy crops (PECs) can reduce the negative impacts of intensive silage maize cultivation on agroecosystems in Central Europe. Furthermore, the remaining vegetation of PECs after harvest may provide suitable habitat and more beneficial overwintering conditions for arthropods than maize. It was hypothesized that after harvest and in winter, arthropod abundance and biomass are higher in PECs than in silage maize. In a field experiment arranged in a factorial split-plot design of eight main plots (plot size: 240 m²), the two PECs cup plant (Silphium perfoliatum L.) and field grass were compared with silage maize (Zea mays L.) regarding their suitability as autumn (post-harvest) and overwintering habitats for arthropods. Soil temperature, moisture as well as biomass and abundance of autumn-active and overwintering arthropods were analyzed for these three crops. Suction sampling was used during autumn and emerging arthropods were sampled with emergence trap sets in spring. In PEC plots, soils were moister and less exposed to cold temperatures than in silage maize. Compared with silage maize, total arthropod abundance and biomass were higher in PEC plots for both sampling periods. Results were similar for most examined arthropod taxa. The results of this study demonstrate that, compared with silage maize, PECs provide suitable post-harvest habitats and constitute more suitable overwintering habitats for arthropods. Differences are likely to be based on lack of disturbance and the provision of vegetation structures after harvest that function as overwintering habitats for arthropods. It can be concluded that the positive effects of PECs on ground arthropods are not limited to their growing time but continue to a certain extend after harvest and during winter.     

Nonnative plant invasion increases urban vegetation structure and influences arthropod communities

Aim

Ecological theory and empirical evidence indicate that greater structural complexity and diversity in plant communities increases arthropod abundance and diversity. Nonnative plants are typically associated with low arthropod abundance and diversity due to lack of evolutionary history. However, nonnative plants increase the structural complexity of forests, as is common in urban forests. Therefore, urban forests are ideal ecosystems to determine whether structural complexity associated with nonnative plants will increase abundance and diversity of arthropods, as predicted by complexity literature, or whether structural complexity associated with nonnative plants will be depauperate of arthropods, as predicted by nonnative plant literature.

Location

We sampled 24 urban temperate deciduous and mixed forests in two cites, Raleigh, North Carolina and Newark, Delaware, in the eastern United States.

Methods

We quantified ground cover vegetation and shrub layer vegetation in each forest and created structural complexity metrics to represent total, nonnative and native understory vegetation structural complexity. We vacuum sampled arthropods from vegetation and quantified the abundance, biomass, richness and diversity of spiders and non-spider arthropods.

Results

Nonnative plants increase understory vegetation complexity in urban forests. In Raleigh and Newark, we found support for the hypotheses that dense vegetation will increase arthropod abundance and biomass, and against the hypothesis that nonnative vegetation will decrease arthropods. Urban forest arthropod abundance and biomass, but not diversity, increased with greater nonnative and native structural complexity.

Main Conclusions

Invaded urban forests may provide adequate food in the form of arthropod biomass to transfer energy to the next trophic level, but likely fail to provide ecological services and functions offered by diverse species, like forest specialists. Urban land managers should survey urban forests for nonnative and native plant communities and prioritize replacing dense nonnative plants with native species when allocating vegetation maintenance resources.     

论短期农作物生境中节肢动物群落的重建I.群落重建的概念及特性

农田生态系统是一个不稳定的生态系统,包括作物生境和非作物生境两个组成部分。非作物生境对作物生境中的害虫及其天敌可产生正的或负的影响。在短期农作物生境中,周期性的种植和收割使得其中的节肢动物群落亦周期性地呈现出群落重新形成、群落发展和群落瓦解３个阶段。这一群落的发展和自然群落的富替是两种不同的发展方式。因此,将短期农作生境内节肢动物群落的重新形成过程定义为群落的重建,而非作物生境内的节肢物群落称之为     

Aboveground impacts of a belowground invader: how invasive earthworms alter aboveground arthropod communities in a northern North American forest

Declining arthropod communities have recently gained a lot of attention, with climate and land-use change among the most frequently discussed drivers. Here, we focus on a seemingly underrepresented driver of arthropod community decline: biological invasions. For approximately 12 000 years, earthworms have been absent from wide parts of northern North America, but they have been re-introduced with dramatic consequences. Most studies investigating earthworm-invasion impacts focus on the belowground world, resulting in limited knowledge on aboveground-community changes. We present observational data on earthworm, plant and aboveground arthropod communities in 60 plots, distributed across areas with increasing invasion status (low, medium and high) in a Canadian forest. We analysed how earthworm-invasion status and biomass impact aboveground arthropod community abundance, biomass and species richness, and how earthworm impacts cascade across trophic levels. We sampled approximately 13 000 arthropods, dominated by Hemiptera, Diptera, Araneae, Thysanoptera and Hymenoptera. Total arthropod abundance, biomass and species richness declined significantly from areas of low to those with high invasion status, with reductions of 61, 27 and 18%, respectively. Structural equation models suggest that earthworms directly and indirectly impact arthropods across trophic levels. We show that earthworm invasion can alter aboveground multi-trophic arthropod communities and suggest that belowground invasions might be underappreciated drivers of aboveground arthropod decline.     

Comparison of pollinators and natural enemies: a meta‐analysis of landscape and local effects on abundance and richness in crops

To manage agroecosystems for multiple ecosystem services, we need to know whether the management of one service has positive, negative, or no effects on other services. We do not yet have data on the interactions between pollination and pest‐control services. However, we do have data on the distributions of pollinators and natural enemies in agroecosystems. Therefore, we compared these two groups of ecosystem service providers, to see if the management of farms and agricultural landscapes might have similar effects on the abundance and richness of both. In a meta‐analysis, we compared 46 studies that sampled bees, predatory beetles, parasitic wasps, and spiders in fields, orchards, or vineyards of food crops. These studies used the proximity or proportion of non‐crop or natural habitats in the landscapes surrounding these crops (a measure of landscape complexity), or the proximity or diversity of non‐crop plants in the margins of these crops (a measure of local complexity), to explain the abundance or richness of these beneficial arthropods. Compositional complexity at both landscape and local scales had positive effects on both pollinators and natural enemies, but different effects on different taxa. Effects on bees and spiders were significantly positive, but effects on parasitoids and predatory beetles (mostly Carabidae and Staphylinidae) were inconclusive. Landscape complexity had significantly stronger effects on bees than it did on predatory beetles and significantly stronger effects in non‐woody rather than in woody crops. Effects on richness were significantly stronger than effects on abundance, but possibly only for spiders. This abundance‐richness difference might be caused by differences between generalists and specialists, or between arthropods that depend on non‐crop habitats (ecotone species and dispersers) and those that do not (cultural species). We call this the ‘specialist‐generalist’ or ‘cultural difference’ mechanism. If complexity has stronger effects on richness than abundance, it might have stronger effects on the stability than the magnitude of these arthropod‐mediated ecosystem services. We conclude that some pollinators and natural enemies seem to have compatible responses to complexity, and it might be possible to manage agroecosystems for the benefit of both. However, too few studies have compared the two, and so we cannot yet conclude that there are no negative interactions between pollinators and natural enemies, and no trade‐offs between pollination and pest‐control services. Therefore, we suggest a framework for future research to bridge these gaps in our knowledge.     

Habitat functionality for the ecosystem service of pest control: reproduction and feeding sites of pests and natural enemies

• 1 Landscape management for enhanced natural pest control requires knowledge of the ecological function of the habitats present in the landscape mosaic. However, little is known about which habitat types in agricultural landscapes function as reproduction habitats for arthropod pests and predators during different times of the year.
• 2 We studied the arthropod assemblage on six crops and on the seven most abundant native plant species in two landscapes over 1 year in Australia. Densities of immature and adult stages of pests and their predators were assessed using beat sheet sampling.
• 3 The native plants supported a significantly different arthropod assemblage than crops. Native plants had higher predator densities than crops over the course of the year, whereas crops supported higher pest densities than the native plants in two out of four seasonal sampling periods. Crops had higher densities of immature stages of pests than native plants in three of four seasonal sampling periods, implying that crops are more strongly associated with pest reproduction than native plants. Densities of immature predators, excluding spiders, were not different between native plants and crops. Spiders were, however, generally abundant and densities were higher on native plants than on crops but, because some species disperse when immature, there is less certainty in identifying their reproduction habitat.
• 4 Because the predator to pest ratio on native plant species showed little variation, and spatial variation in arthropod assemblages was limited, the predator support function of native vegetation may be a general phenomenon. Incentives that maintain and restore native remnant vegetation can increase the predator to pest ratio at the landscape scale, which could enhance pest suppression in crops.

     

Landscape context of sheetweb spider (Araneae: Linyphiidae) abundance in cereal fields

Aim Sheetweb spiders (Araneae: Linyphiidae, subfamily Linyphiinae) exemplify a highly mobile group of farmland arthropods with very variable abundances in crops within and between years. Despite their dominance in crops during summer, they overwinter predominantly in perennial non‐crop habitats, and their densities in crops during spring should therefore depend on the amount of favourable overwintering habitat in the surrounding landscape. We tested the effect of landscape composition on sheetweb spider abundance with special attention to the range of their aerial dispersal and weather dependence. Location The study was carried out in 18 non‐overlapping landscape sectors of 3 km radius around the city of Göttingen (Germany), forming a gradient from structurally simple, with up to 85% arable land, to structurally complex, with high percentages of grassland and other non‐crop habitats. Methods Sheetweb spider abundances in winter wheat fields were sampled during May and June 2001–2003 with a distance method. They were related to landscape composition at 11 spatial scales between 95 and 3000 m radius around the study sites. Results In 2001 and 2003, spider abundances were enhanced by high percentages of non‐crop habitats in 1–3 km circumference (e.g. from 18 to 130 m^?2 in late May 2001), and multiplied during consecutive sampling periods (e.g. from on average 36 to 131 m^?2 between mid‐May and late June 2001). Spider abundances were constantly low and unrelated to the landscape context in 2002. In that year, immigration appeared to be inhibited by factors connected to exceptionally high amounts of rain during May. Main conclusions Sheetweb spiders responded to landscape composition up to several kilometres away and the effects varied between years, demonstrating the need to consider large space and time‐scales to understand their population dynamics. Semi‐natural habitats should be preserved to enhance these important natural enemies of crop pests in agricultural landscapes.