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Context: Wild insects provide essential ecosystem services, including pollination, in both wild and managed landscapes. Over the past century, agricultural intensification and habitat loss have affected the amount and temporal availability of floral resources in the landscape—resources that all pollinating insects depend on. A reduction in the abundance and temporal continuity (i.e., gaps/bottlenecks in resources) of resources, for example, is associated with decreased occurrence of several bumble bee species within agricultural landscapes in Wisconsin. This has the potential to decrease the supply of pollination services to a variety of economically important crops. Objective: We inventoried the supply and demand of pollinators and pollinator dependent crops in a major fruit and vegetable production area in Wisconsin. Method: We applied a model to predict the occurrence of wild bumble bees as a function of landscape-scale resource abundance and continuity as an index of pollinator "supply" and combined this with spatially-explicit data on pollinator-dependent crop production to identify areas of high pollination "demand". Results: In an important fruit producing area of central Wisconsin, we found a clear spatial mismatch between pollinator supply and pollination demand, with nearly 70% of landscapes with 15 or more hectares of pollinator dependent crops (e.g., cranberries, squash) exhibiting a lower bumble bee supply index relative to the intensity of crop demand. We found the source of this mismatch was largely due inadequate floral resource conditions for bumble bees, particularly due to high levels of resource discontinuity observed in the most agriculturally intensive landscapes (which also had the greatest pollination demands). Conclusions: Our results suggest that measures to increase crop diversity, reduce the size of fields, and focus on conserving and improving semi-natural habitat in the landscapes surrounding crop fields may support improved floral resource conditions. Such changes could ultimately bolster bumble bee populations, help stabilize the supply of pollination services, and improve the sustainability and economic stability of Wisconsin's agricultural landscapes. [ABSTRACT FROM AUTHOR]

Context: Agricultural intensification is contributing to a global species decline. Underlying mechanisms include toxic effects of pesticides on non-target organisms and reductions in habitat and food availability. However, the effects of agricultural intensification on body condition, particularly of ecosystem service-providing arthropods, are poorly understood. Objectives: Here, we investigated whether variations in the body condition of common lady beetle species (Coleoptera: Coccinellidae) can be explained by the composition and configuration of the surrounding landscape. Assuming strong seasonal variation in food availability in intensively farmed regions, we included the entire period of lady beetle activity in our study. Methods: Lady beetles were collected from April to September 2011 in 30 landscapes in southern Wisconsin, USA. We examined how body size, body density, and lipid content of the beetles responded to the percentage of intensive cropland, habitat diversity, and edge density in the surrounding landscape. Results: The strongest predictor of body condition was the percentage of intensive cropland. For every 10% increase in cropland, body density decreased by about 3.9% and lipid content by 6.4%. Landscape diversity and edge density correlated with body condition of individual species. Conclusions: In agriculturally intensified landscapes, lady beetles with reduced body condition may produce fewer offspring, have lower survival rates, and exert less effective pest control. Thus, our results suggest a mechanistic link between landscape patterns and observed declines in lady beetle populations. Our results also show that the expansion of monocultures affects even common cropland-associated species such as Harmonia axyridis, suggesting a long-term decline in biocontrol services in simplified agricultural landscapes. [ABSTRACT FROM AUTHOR]

Context: Landscape studies often focus on determining how the landscape around a discrete set of field sites affects an abiotic or biotic response measured at those sites. To run this type of analysis, a decision must be made about the spatial extent over which the landscape affects the response. Many authors have acknowledged the limitations of common approaches to estimating this spatial extent of a landscape effect and some have proposed alternatives. However, many alternative methods have thus far been difficult to implement. Objectives: The R package, 'scalescape' which we introduce in this paper, builds on and improves the usability of these alternative methods for estimating the spatial scale of a landscape effect on an abiotic or biotic response. Methods and results: The package uses established approaches that weight landscape variables based on their distance from where a response is measured. It integrates well-used functions for performing regression in R, such as lm(), glm(), lmer(), glmer(), and gls(), with landscape weightings of spatial predictor variables. Here, we provide an introduction to 'scalescape' including a user guide detailing its functionality and step-by-step instructions. We also conduct simulations to illustrate and validate the 'scalescape' approach. Conclusions: We conclude that 'scalescape' builds on previously proposed methods for landscape analyses by making it easier to implement distance-weighted approaches and by improving the statistical validity of these analyses through the use of GLS models and bootstrapping. [ABSTRACT FROM AUTHOR]

Balancing agricultural production with other ecosystem services is a vexing challenge. The Yahara River watershed in southern Wisconsin is a place where tensions among farmers, policymakers, and citizens at-large run high because nutrient loss from the agricultural practices of a few drive the impairment of surface waters for many. Reducing manure and fertilizer application, as well as increasing perennial grass cover have been proposed as potential solutions. Using the Agro-IBIS agroecosystem model, we examined 48 scenarios of future land management and climate for the Yahara River watershed to the year 2070. Scenarios included combinations of reduced livestock and increased perennial grassland under alternative climate trajectories. Results suggested that business as usual will lead to further environmental degradation with phosphorus-loading to waterways increasing 13, 7, and 23% under baseline, warmer and drier, and warmer and wetter climates, respectively. Watershed-wide phosphorous yield and nitrate leaching could be reduced by 50%, but only when nutrient application was reduced 50% and grassland cover was increased 50%. Furthermore, water quality improvements only materialized 50 years after modified land management practices were implemented under the most likely future climate. Our findings highlight that improving water quality under a changing climate will require long-term investment and transformative changes to current agricultural land use and land cover. Agricultural management solutions exist but are unlikely to be implemented without policies that incentivize transformative agricultural change. [ABSTRACT FROM AUTHOR]

Irruptive forest insects such as bark beetles undergo intermittent outbreaks that cause landscape-scale tree mortality. Despite their enormous economic and ecological impacts, we still have only limited understanding of the dynamics by which populations transition from normally stable endemic to irruptive densities. We investigated density-dependent changes in mountain pine beetle reliance on stressed hosts, host selection, spatial configuration of attacks, and the interaction of host selection and spatial configuration by performing a complete census of lodgepole pine across six stands and 6 years. In addition, we compared the dynamics of mountain pine beetle with those of other bark beetles. We found that as population size increased, reliance on stressed trees decreased and new attacks shifted to larger trees with thicker phloem and higher growth rates that can support higher offspring production. Moreover, the spatial configuration of beetle-attacked trees shifted from random to spatially aggregated. Further, we found evidence that beetle utilization of larger trees was related to aggregation behavior as the size of tree attacked was positively correlated at 10–25 m, within the effective distance of pheromone-mediated signaling. In contrast, non-irruptive bark beetle species did not exhibit such density-dependent spatial aggregation at the stand scale or switches in host selection behavior. These results identify how density-dependent linkages between spatial configuration and host utilization can converge to drive population transitions from endemic to irruptive phases. Specifically, a combination of stand-level spatial aggregation, behavioral shifts, and higher quality of attainable hosts defines a critical threshold beyond which continual population growth becomes self-driving. [ABSTRACT FROM AUTHOR]

Context: It is estimated that over one billion milkweed stems need to be restored to sustain the eastern North American migratory population of monarch butterflies; where and in what context the stems should be placed on the landscape is key to addressing habitat deficits. Objectives: We assessed how the amount of appropriate habitat surrounding a particular patch of monarch habitat affects monarch presence and reproduction. To ensure that habitat restoration efforts are targeted towards areas that maximize monarch population growth, it is important to understand the effects of landscape heterogeneity on monarch occurrence in habitat patches (i.e. grasslands with milkweeds) across the landscape. Methods: Over two summers (2018–2019), we surveyed monarch adults, larvae, and eggs at sixty grassland sites in Wisconsin that varied in patch size and landscape context (proportion grassland, forest edge density, and road density). We also estimated milkweed density and floral richness to characterize local patch quality. Results: Adult monarch abundance was highest at patches with the lowest proportion of surrounding grassland and lowest road density, and was heavily influenced by patch quality variables. Egg and larva density in a patch increased with milkweed density and floral richness within a patch. Patch size was unrelated to monarch abundance. Conclusions: These results suggest that optimal sites for monarch habitat restoration are within landscapes which contain little habitat and that high milkweed density and floral richness and abundance should be conservation goals. [ABSTRACT FROM AUTHOR]

Pollinators are undergoing a global decline. Although vital to pollinator conservation and ecological research, species-level identification is expensive, time consuming, and requires specialized taxonomic training. However, deep learning and computer vision are providing ways to open this methodological bottleneck through automated identification from images. Focusing on bumble bees, we compare four convolutional neural network classification models to evaluate prediction speed, accuracy, and the potential of this technology for automated bee identification. We gathered over 89,000 images of bumble bees, representing 36 species in North America, to train the ResNet, Wide ResNet, InceptionV3, and MnasNet models. Among these models, InceptionV3 presented a good balance of accuracy (91.6%) and average speed (3.34 ms). Species-level error rates were generally smaller for species represented by more training images. However, error rates also depended on the level of morphological variability among individuals within a species and similarity to other species. Continued development of this technology for automatic species identification and monitoring has the potential to be transformative for the fields of ecology and conservation. To this end, we present BeeMachine, a web application that allows anyone to use our classification model to identify bumble bees in their own images. [ABSTRACT FROM AUTHOR]

Context: Resource movements across ecosystem boundaries are important determinants of the diversity and abundance of organisms in the donor and recipient ecosystem. However the effects of cross-ecosystem movements of materials at broader spatial extents than a typical field study are not well understood. Objectives: We tested the hypotheses that (1) variation in abundance of 57 forest songbird species within four foraging guilds is explained by modeled emergent aquatic insect biomass inputs from adjacent lakes and streams and (2) the degree of association varies across foraging guilds and species within guilds. We also sought to determine the importance of emergent aquatic insects while accounting for variation in local forest cover and edge. Methods: We spatially modeled the degree to which distribution and abundance of songbirds in different foraging guilds was explained by modeled emergent aquatic insect biomass. We used multilevel models to simultaneously estimate the responses of species in four different insectivorous guilds. Bird abundance was summarized from point counts conducted over 24 years at 317 points. Results: Aerial insectivores were more abundant in areas with high estimated emergent insect biomass inputs to land (regression coefficient 0.30, P < 0.05) but the overall abundance of gleaners, bark-probers, and ground-foragers was not explained by estimated emergent insect abundance. The coursing aerial insectivores had the strongest association with emergent insects followed by willow flycatcher, olive-sided flycatcher, and alder flycatcher. Conclusions: Modeling cross-ecosystem movements of materials at broad spatial extents can effectively characterize the importance of this ecological process for aerial insectivorous songbirds. [ABSTRACT FROM AUTHOR]

Tundra ecosystems are often unproductive, and peaks in resource availability are highly variable in space and time. Therefore, the success of organisms inhabiting these biomes likely depends on their ability to efficiently exploit heterogeneously distributed resources. We assessed how orb-weaving spider (Larinioides patagiatus) aggregations near Lake Mývatn, Iceland, respond to large midge (Diptera: Chironomidae) emergences that subsidize ecological communities in the surrounding landscape. The emergences occur for only a few weeks each summer, and the subsidy declines with distance from the lakeshore, producing large spatiotemporal variation in prey availability that might drive orb-weaver foraging behavior. We conducted three surveys during different phases of the summer emergence along a distance gradient to quantify variation in spider aggregation, web size, and habitat use in response to prey abundance. We found that aggregation size increased with the abundance of aerial prey, with the highest orb-weaver densities occurring at peak midge emergence. In contrast, changes in web size did not vary with midge abundance, but rather were determined by physical habitat structure. Our results illustrate how orb-weaver aggregations and web building can respond to spatiotemporal variation in resource subsidies across ecosystem boundaries. [ABSTRACT FROM AUTHOR]

Context: Anthropogenic landscape simplification and natural habitat loss can negatively affect wild bees. Alternatively, anthropogenic land-use change may diversify landscapes, creating complementary habitats that maintain overall resource continuity and diversity. Objectives: We examined the effects of landscape composition, including land-cover diversity and percent semi-natural habitat, on wild bee abundance and species richness within apples, a pollinator-dependent crop. We also explored whether different habitats within diverse landscapes can provide complementary floral resources for bees across space and time. Methods: We sampled bees during apple bloom over 2 years within 35 orchards varying in surrounding landscape diversity and percent woodland (the dominant semi-natural habitat) at 1 km radii. To assess habitat complementarity in resource diversity and temporal continuity, we sampled flowers and bees within four unique habitats, including orchards, woodlands, semi-natural grasslands, and annual croplands, over three periods from April-June. Results: Surrounding landscape diversity positively affected both wild bee abundance and richness within orchards during bloom. Habitats in diverse landscapes had different flower communities with varying phenologies; flowers were most abundant within orchards and woodlands in mid-spring, but then declined over time, while flowers within grasslands marginally increased throughout spring. Furthermore, bee communities were significantly different between the closed-canopy habitats, orchards and woodlands, and the open habitats, grasslands and annual croplands. Conclusions: Our results suggest that diverse landscapes, such as ones with both open (grassland) and closed (woodland) semi-natural habitats, support spring wild bees by providing flowers throughout the entire foraging period and diverse niches to meet different species' requirements. [ABSTRACT FROM AUTHOR]

Natural habitats can affect the population dynamics of mobile insects, and the spatial and temporal effects on agricultural pest species may be especially relevant for tailoring management strategies to the farm context. Drosophila suzukii Matsumura (Diptera: Drosophilidae) is suspected to utilize woodland habitat for wild hosts and overwintering with possible adverse effects of woods on adjacent fruit crops. A two-year study in the Upper Midwest, USA examined if the amount of woodland in the landscape affects early season activity and relative abundance of D. suzukii in raspberry fields. Thirty-five farms were selected to span a gradient of low-to-high woodland area at the 1.5 km scale. The first capture of D. suzukii occurred earlier at farms in high woodland landscapes. However, woodland area was not correlated with metrics of D. suzukii abundance in raspberry (growth rate, peak, fall, or total fly catch) suggesting similar crop infestation risk across landscapes. However, woodland area was negatively correlated with fall fly catch in the adjacent woods and significantly more flies were captured in the woods than raspberry. This study suggests woodland landscapes affect early season crop risk and the high numbers of D. suzukii in the woods have implications for understanding overwintering. [ABSTRACT FROM AUTHOR]

Pesticides are implicated in current bee declines. Wild bees that nest or forage within agroecosystems may be exposed to numerous pesticides applied throughout their life cycles, with potential additive or synergistic effects. In pollinator-dependent crops, pesticides may reduce bee populations, creating trade-offs between pest management and crop pollination. In this three-year study, we examined the effects of pesticides on the abundance and species richness of wild bees within apple orchards of southern Wisconsin. We additionally deployed colonies of Bombus impatiens, a native and common species, in order to relate colony performance to orchard pesticide use. Utilizing grower spray records, we developed 'toxicity scores' as a continuous index of pesticide use for each orchard, a measure that incorporated each pesticide's relative toxicity to bees, its residual activity, and its application rate. While there was no relationship between total wild bee abundance and species richness with toxicity scores, there was a significant, negative effect on sweat bees, Lasioglossum spp. Many of these sweat bees are small-bodied, have short foraging ranges, are social, and have long foraging periods, all traits that could increase bee exposure or sensitivity to orchard pesticides. In addition, sentinel bumble bee colonies at orchards with high toxicity scores produced fewer, and smaller, workers. Bumble bees may also have a greater sensitivity and exposure to orchard pesticides due to their sociality and long foraging periods. Our results demonstrate that certain bee taxa may have a higher exposure or sensitivity to on-farm pesticide applications, and could therefore be vulnerable in agroecosystems. [ABSTRACT FROM AUTHOR]

High stocking densities on grazed pastures may promote nitrous oxide (NO) loss from soil to the atmosphere. However, studies of NO fluxes in cool-season pastures of North America are lacking. We performed two experiments in which measured NO fluxes were bootstrapped with re-sampling (n = 100, with 10,000 iterations), which allowed us to generate an empirical distribution of mean fluxes to understand how pasture management strategies might affect NO emissions. In Experiment 1, NO fluxes were estimated in southern Wisconsin pastures under rotational grazing, continuous grazing, haymaking, and no agronomic production. Nitrous oxide fluxes were significantly positive under rotational grazing at our research farm [21.6 (se = 10.3) μg m h], but not significantly different than zero under the other three treatments or rotationally grazed paddocks across eight working farms. In Experiment 2, we measured NO fluxes in eastern Nebraska before, during, and after two rotational grazing events under two N-input treatments-inorganic N fertilizer and supplemented dried distillers grains-and an unfertilized control. Nitrous oxide fluxes were positive (20-100 μg m h) in periods following rain, but otherwise not significantly different than zero. Post-grazing, NO emissions were lower from the control than fertilized or supplemented treatments. These experiments show cool-season pastures can be a source of NO to the atmosphere, but primarily following grazing events that coincide with significant precipitation. However, even though on-farm paddocks are in varying states of recovery from defoliation, farm scale emissions, although episodic, are likely to be positive in years with above average precipitation. [ABSTRACT FROM AUTHOR]

Historically, where forest habitats are deemed as the pristine landscape state, anthropogenic habitats such as managed grasslands or open spaces are often perceived to be antagonistic and of secondary conservation priority. Traditionally, studies on biodiversity responses to ecological variation, i.e. edge effect, have mostly focused on forest habitats. Yet recently there has been increased attention on communities beyond the forest edge in an effort to better understand how interactions between forests and adjacent habitats may potentially affect regional biodiversity. However, in Europe and the Mediterranean basin (a biodiversity hotspot), areas with high landscape heterogeneity and high edge density, there is a paucity of studies analysing the community responses across forest and 'beyond edge' habitats across ecotones. In a protected area of central Italy, we investigated the responses of ground-dwelling arthropods [Araneae (spiders), Chilopoda (centipedes) and Carabidae (ground beetles)], which were differentiated into habitat-specific guilds (forest, edge and grassland species) across a forest-grassland ecotone. We investigated the extent to which a habitat edge influenced communities of arthropods associated with either the forest or grassland, and how far from the edge this effect penetrated into each habitat. Twelve 150 m-transects perpendicular to a forest-grassland edge were established and arthropods were sampled at nine progressive distances across the ecotone. An indicator species analysis was used to detect species significantly associated with forest, edge-belt or grassland habitats, which were assumed representative of the respective communities. Logistic models of indicator species richness and abundances were used to describe responses of grassland and forest communities across the ecological boundaries. We found that grassland and edge habitats had habitat specialists and higher species richness compared to the forest habitat. Moreover, the occurrence of grassland-specific species was influenced by the presence of an edge up to 15 m from the habitat border. In contrast forest-associated indicator species were not affected by proximity to the habitat edge, rather individuals typical of forest habitats tended to 'spill over' into grassland habitats. These findings support the hypothesis that in a forest-grassland mosaic, forest species are less sensitive to an edge and influence the community beyond the forest edge and into the grassland more than the reverse, i.e. the effect was asymmetric. From these data, we estimated that a minimum grassland habitat width of 600 m is necessary for grassland species to maintain a core area that is relatively unaffected by the spillover of species from adjacent forest habitats. Incorporating the directional influences of adjacent communities on each other allows for an empirical assessment of habitat vulnerability that doesn't a priori value the conservation of one habitat over another. [ABSTRACT FROM AUTHOR]

Emergent aquatic insects can provide inputs to terrestrial ecosystems near lentic and lotic waterbodies, producing ecosystem linkages at the aquatic-terrestrial interface. Although aquatic insect emergence has been examined for individual sites, the magnitude and spatial distribution of this phenomenon has not been examined at regional spatial scales. Here, we characterize this cross-habitat linkage for the state of Wisconsin, USA (169,639 km). We combined GIS hydrological data with empirical data and predictive models of aquatic insect production to estimate annual aquatic emergence for the state of Wisconsin. Total emergence (lentic + lotic) was estimated to be about 6,800 metric tons of C y. Lentic systems comprised 79% of total estimated insect emergence, primarily due to the large amount of lake surface area relative to streams. This is due to both basic ecosystem geometry and the overall abundance of lakes in Wisconsin. Spatial variation was high: insect emergence in southwestern Wisconsin was dominated by streams, whereas for most of the rest of the state insect emergence was dominated by lakes. Lentic inputs to land were highly concentrated (relative to lotic inputs) because lakes have a high ratio of surface area to buffer area. Although less concentrated, the spatial extent of lotic influence was greater: statewide, four times more land area fell within the 100 m buffer zones of streams compared to lakes. Large waterbodies (almost all of which were lakes) were hotspots of insect emergence and input to land. Aquatic insect inputs exceed estimated terrestrial secondary production in 13% of buffer area, and by a factor of 100 or more adjacent to large lakes (>50,000 ha). The model sensitivity analysis showed that the simplifying assumptions and sources of potential error in the input variables had a minor impact on the overall results. [ABSTRACT FROM AUTHOR]

Agricultural landscapes worldwide are under increased pressure to provide food, feed, fiber, and fuel for a growing human population. These demands are leading to changes in agricultural landscapes and subsequent declines in biodiversity. We used citizen science data from the North American Butterfly Association and remotely-sensed land cover data from the US Department of Agriculture to study relationships between agricultural landscape composition and butterfly community structure in the Midwestern US. Landscape-level butterfly species richness (based on rarefaction estimates) was highest in agricultural landscapes with relatively low amounts of cropland, relatively high amounts of woodland, and intermediate amounts of grassland and wetland. Rarefied richness generally declined with the dominance of any of these land cover types. Unlike other land cover types, urban development had a consistent negative effect on rarefied richness. Butterfly community structure (based on relative abundance) was also significantly related to the amount of cropland, woodland, grassland, and wetland in the landscape. The rarest butterfly species were associated with woodland-, grassland-, and wetland-dominated landscapes, likely due to their association with plant species occurring in savannahs, prairies, and marshes, respectively. Assuming that variation across space reflects changes over time, our results support conclusions from previous studies that removal of natural and seminatural habitats alters butterfly community structure and decreases species diversity in agricultural landscapes. [ABSTRACT FROM AUTHOR]

Multichannel omnivory by generalist predators, especially the use of both grazing and epigeic prey, has the potential to increase predator abundance and decrease herbivore populations. However, predator use of the epigeic web (soil surface detritus/microbe/algae consumers) varies considerably for reasons that are poorly understood. We therefore used a stable isotope approach to determine whether prey availability and predator hunting style (active hunting vs. passive web-building) impacted the degree of multichannel omnivory by the two most abundant predators on an intertidal salt marsh, both spiders. We found that carbon isotopic values of herbivores remained constant during the growing season, while values for epigeic feeders became dramatically more enriched such that values for the two webs converged in August. Carbon isotopic values for both spider species remained midway between the two webs as values for epigeic feeders shifted, indicating substantial use of prey from both food webs by both spider species. As the season progressed, prey abundance in the grazing food web increased while prey abundance in the epigeic web remained constant or declined. In response, prey consumption by the web-building spider shifted toward the grazing web to a much greater extent than did consumption by the hunting spider, possibly because passive web-capture is more responsive to changes in prey availability. Although both generalist predator species engaged in multichannel omnivory, hunting mode influenced the extent to which these predators used prey from the grazing and epigeic food webs, and could thereby influence the strength of trophic cascades in both food webs. [ABSTRACT FROM AUTHOR]

Cross-ecosystem movement of nutrients and biomass can have important effects on recipient systems. Emerging aquatic insects are subsidies to terrestrial ecosystems and can influence foodweb interactions in riparian systems. In a 2-year field experiment, we simulated aquatic insect deposition by adding adult midge carcasses (150 g dry mass m year) to 1-m heathland plots at a site with low natural midge deposition. We established four levels of midge-addition treatments and measured stable isotopes (δC and δN) in plants and arthropods within each treatment. We used a multiple-source isotope Bayesian mixing model to estimate the terrestrial versus aquatic contribution to the diets of arthropods. Aquatic resources were incorporated into plant, detritivore, and predator biomass. Detritivorous Collembola showed the greatest difference in isotope values (+3 ‰ δN and +4 ‰ δC) between midge-addition and reference treatments. Isotope values of small spiders followed the same trend of enrichment as Collembola while other arthropods (mites and large spiders) were only enriched after 2 years of midge addition. Although predator diets did not change, they became isotopically enriched via their likely prey (Collembola). Plants also had elevated δN (+1 ‰) in midge-addition treatments. The time required and amount of midge-derived C and N detected varied and depended on trophic position. Midge-derived nutrients were no longer present in arthropod biomass in the year following midge addition. Aquatic insect carcasses can be rapidly incorporated into terrestrial food webs in nearshore habitats, and repeated inputs can be detected at multiple trophic levels, thus highlighting the importance of the detrital pathway for aquatic to terrestrial cross-ecosystem subsidies. [ABSTRACT FROM AUTHOR]

Despite a recent emphasis on understanding cross-habitat interactions, few studies have examined the ecological linkages between lakes and surrounding terrestrial habitats. The current paradigm of land–lake interactions is typically unidirectional: the view is that nutrients and matter are transported downslope from the surrounding watershed to their ultimate lacustrine destination. Emergent aquatic insects, which spend their larval stages in lake sediments and emerge as adults to mate over land, can act as vectors of material, energy and nutrients from aquatic to terrestrial habitats. In this study, we document a gradient of midge (Diptera: Chironomidae) infall rates into terrestrial habitats (measured as g dw midges m−2 d−1) surrounding eight lakes in Northern Iceland (≈66°N latitude). Lakes ranged from having virtually no midge infall (for example, Helluvaðstjörn, 0.03 g m−2 d−1) to extreme levels (for example, Mývatn, 19 g m−2 d−1) with abundances of midges decreasing logarithmically with distance from shore. Annual midge input rates are estimated as high as 1200–2500 kg midges ha−1 y−1. As midges are approximately 9.2% total N, this can result in a significant fertilization effect of terrestrial habitats with consequences for plant quality and community structure. In addition, we used naturally-occurring δ13C and δ15N isotopes to examine food web structure and diet sources of terrestrial arthropod consumers surrounding lakes with differing amounts of midge input. Terrestrial arthropods showed increased utilization of aquatic-derived (that is, midge) C relative to terrestrial sources as midge infall increased. This pattern was particularly pronounced for predators, such as spiders and opiliones, and some detritivores (Collembola). These findings suggest that, despite being largely ignored, aquatic-to-terrestrial linkages can be large and midges can fuel terrestrial communities by directly serving as resources for predators and decomposers. [ABSTRACT FROM AUTHOR]

Traditionally, researchers have used measurements of carbon stable isotopes to infer the composition of consumers’ diets. However, since consumer’s tissues may process carbon isotopes differently, particularly following a diet shift, it is possible to use measurements of carbon isotopes in multiple tissues to determine not only the composition of an individual’s diet, but also the temporal dynamics thereof. This study examined how stable isotopes of carbon (13C/12C, expressed as δ 13C) changed in different adult tissues of two predacious beetles, Harmonia axyridis and Coccinella septempunctata (Coleoptera: Coccinellidae). In the laboratory, we switched ladybeetles from a C3-based diet (soybean aphids, Aphis glycines) to a C4-based one (corn leaf aphids, Rhopalosiphum maidis). The δ 13C of metabolically active tissues such as the body fat and reproductive organs changed rapidly (≤5 days) following the diet shift. Tissues expected to be more metabolically inert, such as wings, changed more slowly over the same period. Although these general patterns were largely similar between males and females, females had more rapid changes in δ 13C in fat and reproductive tissues. However, females showed a significant depletion in δ 13C after 10 days, while males’ δ 13C continued to increase. Given the results of this experiment, it is now possible to distinguish between ladybeetles eating a mixed diet (beetles with multiple tissues at similar, intermediate, equilibrial δ 13C signatures) from those that have shifted diets (beetles with different tissues at distinctly different δ 13C values). Thus, this approach can be used broadly to infer not only what constitutes the diet of a consumer, but also the temporal history of dietary intake. [ABSTRACT FROM AUTHOR]

Although many studies now examine how multiple factors influence the dynamics of herbivore populations, few studies explicitly attempt to document where and when each is important and how they vary and interact. In fact, how temporal variation in top-down (natural enemies) and bottom-up (host plant resources) factors affect herbivore dynamics has been suggested as a particularly important yet poorly understood feature of terrestrial food webs. In this study we examined how temporal changes in predator density (wolf spiders, sheet-web builders, and mirid egg predators) and host-plant resources (plant quality and structural complexity) influence the population dynamics of the dominant phytophagous insects on Atlantic-coast salt marshes, namely Prokelisia planthoppers (Homoptera: Delphacidae). We designed a factorial experiment in meadows of Spartina alterniflora to mimic natural variation in vegetation quality and structure by establishing two levels of plant nutrition (leaf nitrogen content) by fertilization, and two levels of habitat complexity by adding leaf litter (thatch). We then assessed seasonal changes in the strength of bottom-up (plant quality) and top-down (predator) impacts on planthopper populations. Planthopper populations responded positively to increased plant quality treatments in late summer. Despite the greater number of planthopper adults colonizing fertilized Spartina plots compared to unfertilized controls, the offspring of these colonists were much less abundant at the end of the season in fertilized plots, particularly those with thatch. The initial colonization effect was later erased because arthropod predators selectively accumulated in fertilized plots where they inflicted significant mortality on all stages of planthoppers. Predators rapidly colonized fertilized plots and reached high densities well in advance of planthopper colonization, a response we attribute to their rapid aggregation in complex-structured habitats with readily available alternative prey. Our results suggest that plant resources not only mediate the strength of predator impacts on herbivore populations, but they also promote the coupling of predator and prey populations and thus influence when enemy impacts are realized. [ABSTRACT FROM AUTHOR]