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Native pollinators and, particularly bees, are a critical component of agricultural systems. Unfortunately, many factors are leading to their declines, including habitat loss. Consequently, approaches have emerged that aim to restore pollinator habitat in managed landscapes. A widely adopted technique in Europe and North America is the planting of flowering shrubs and forbs along field edges. These habitats usually include a variety of species, chosen because they are attractive to pollinators and because they flower continuously over those pollinators' flight seasons. Because there are many potential plant species with different flowering times and pollinator preferences, selecting a subset is challenging. Here, we develop a tool that identifies a plant mix that optimizes some assessment criteria (e.g., pollinator visitation, richness, or phenology). We test our tool by showing that it identifies mixes that better satisfy these criteria than ones found using conventional expert-driven methods, when applied to a plant–pollinator dataset.

We recently introduced a model that predicts the degree to which a plant's lifetime seed production may be constrained by unpredictable shortfalls of pollen receipt ("pollen limitation"). Burd's comment in this issue criticized our analysis, first by arguing that the empiricalliteraturedocumentsmuchhigherlevelsofpollenlimitation than our model predicts and then suggesting that the apparent dis- crepancy stemmed from our (1) underestimating the costs of securing a fertilized ovule and (2) assuming too little unpredictability in whole- plant pollen receipt. We reply as follows. First, the empirical literature must be consulted carefully. Burd relies on pollen supplementation experiments performed on parts of plants or on whole plants but dur- ing only one reproductive season for polycarpic perennials; in both cases, resource reallocation often leads to gross overestimates of pol- lenlimitation.Wecomprehensivelyreviewpollenlimitationestimates that are free of these estimation problems and find strong agreement with our model predictions. Second, although cost estimates for dif- ferent components of seed production are imprecise, errors are likely to be small relative to the 11,000-fold differences observed across plant species, the primary focus of our article. Finally, contrary to Burd's argument, pollen receipt by entire plants is much more pre- dictable than that by individual flowers because the flower-to-flower variation "averages out" when summed across many flowers. Our model usesparametervalues that arein broad agreement with theem- pirical record of modest plant-to-plant variation in pollen receipt and thuspredictsthegenerallymodestpollen limitationthatisobservedin nature.

Many, many more pollinators needed Numerous studies have shown that biodiversity is necessary for ecosystem function. The majority of these, however, have taken place at relatively small experimental scales. Winfree et al. looked across more than 3000 square kilometers for relationships between biodiversity and crop pollination (see the Perspective by Kremen). The number of wild bee species required for successful pollination rapidly increased with spatial scale, largely owing to variation in the species present across sites and the degree to which the most abundant species played a role. In the end, more than an order of magnitude more species than predicted by smaller-scale experiments were required for full ecosystem functioning. Science , this issue p. 791 ; see also p. 741

North America has more than 4000 bee species, yet we have little information on the health, distribution, and population trends of most of these species. In the United States, what information is available is distributed across multiple institutions, and efforts to track bee populations are largely uncoordinated on a national scale. An overarching framework for monitoring U.S. native bees could provide a system that is responsive to national needs, resources, and capacities. Five major action areas and priorities for structuring a coordinated effort include: (1) Defining the scope, aims, and cost of a national native bee monitoring program; (2) Improving the national capacity in bee taxonomy and systematics; (3) Gathering and cataloging data that are standardized, accessible, and sustainable; (4) Identifying survey methods and prioritizing taxa to monitor; and (5) Prioritizing geographic areas to be monitored. Here, we detail the needs, challenges, and opportunities associated with developing a multi-layered U.S. national plan for native bee monitoring.

Most of the world's crops depend on pollinators, so declines in both managed and wild bees raise concerns about food security. However, the degree to which insect pollination is actually limiting current crop production is poorly understood, as is the role of wild species (as opposed to managed honeybees) in pollinating crops, particularly in intensive production areas. We established a nationwide study to assess the extent of pollinator limitation in seven crops at 131 locations situated across major crop-producing areas of the USA. We found that five out of seven crops showed evidence of pollinator limitation. Wild bees and honeybees provided comparable amounts of pollination for most crops, even in agriculturally intensive regions. We estimated the nationwide annual production value of wild pollinators to the seven crops we studied at over $1.5 billion; the value of wild bee pollination of all pollinator-dependent crops would be much greater. Our findings show that pollinator declines could translate directly into decreased yields or production for most of the crops studied, and that wild species contribute substantially to pollination of most study crops in major crop-producing regions.

Current pesticide risk assessment practices use the honey bee, Apis mellifera L., as a surrogate to characterize the likelihood of chemical exposure of a candidate pesticide for all bee species. Bees make up a diverse insect group that provides critical pollination services to both managed and wild ecosystems. Accordingly, they display a diversity of behaviors and vary greatly in their lifestyles and phenologies, such as their timing of emergence, degree of sociality and foraging and nesting behaviors. Some of these factors may lead to disparate or variable routes of exposure when compared to honey bees. For those that possess life histories that are distinct from A. mellifera, further risk assessments may be warranted. In January 2017, forty bee researchers, representative of regulatory agencies, academia and agrochemical industries, gathered to discuss the current state of science on pesticide exposure to non-Apis bees and to determine how well honey bee exposure estimates, implemented by different regulatory agencies, may be protective for non-Apis bees. Workshop participants determined that although current risk assessment procedures for honey bees are largely conservative, several routes of exposure are unique to non-Apis bees and warranted further investigation. In this forum article, we discuss these key routes of exposure relevant to non-Apis bees and identify important research gaps that can help inform future bee risk assessment decisions.

The relationship between pesticides and pollinators, while attracting no shortage of attention from scientists, regulators, and the public, has proven resistant to scientific synthesis and fractious in matters of policy and public opinion. This is in part because the issue has been approached in a compartmentalized and intradisciplinary way, such that evaluations of organismal pesticide effects remain largely disjoint from their upstream drivers and downstream consequences. Here, we present a socioecological framework designed to synthesize the pesticide-pollinator system and inform future scholarship and action. Our framework consists of three interlocking domains-pesticide use, pesticide exposure, and pesticide effects-each consisting of causally linked patterns, processes, and states. We elaborate each of these domains and their linkages, reviewing relevant literature and providing empirical case studies. We then propose guidelines for future pesticide-pollinator scholarship and action agenda aimed at strengthening knowledge in neglected domains and integrating knowledge across domains to provide decision support for stakeholders and policymakers. Specifically, we emphasize (1) stakeholder engagement, (2) mechanistic study of pesticide exposure, (3) understanding the propagation of pesticide effects across levels of organization, and (4) full-cost accounting of the externalities of pesticide use and regulation. Addressing these items will require transdisciplinary collaborations within and beyond the scientific community, including the expertise of farmers, agrochemical developers, and policymakers in an extended peer community.

The temporal distribution of resources is an important aspect of habitat quality that can substantially impact population success. Although it is widely accepted that floral resources directly influence wild bee population sizes, we lack experimental data evaluating how resource availability affects colony growth via demographic mechanisms. To achieve this, we tracked marked individuals in bumble bee (Bombus vosnesenskii) colonies to evaluate whether worker survival and reproduction responded to experimentally elevated forage early in colony development. Specifically, we assessed the effect of early resource environment on worker and sexual offspring production, and the survival and body size of individual workers. We also assessed whether responses of colonies differed when exposed to higher or lower resource environments at a relatively smaller (~ 10 workers) or larger (~ 20 workers) size. Resource supplementation always resulted in greater total offspring and male production; however, the influence of supplementation on worker production and quality depended on colony size at the start of supplementation. Among colonies that were initially smaller, colonies that were supplemented produced fewer but larger bodied and longer lived workers compared to control counterparts. Among colonies that were initially larger, colonies that were supplemented produced more workers than corresponding controls, but without changes to worker quality. Collectively, these results provide clear experimental evidence that greater resource availability early in colony development increases overall productivity, and indicate that colonies may pursue different allocation strategies in response to the resource environment, investing in more or better workers.

Bumble bee (Bombus) species are ecologically and economically important pollinators, and many species are in decline. In this article, we develop a mechanistic model to analyse growth trajectories of Bombus vosnesenskii colonies in relation to floral resources and land use. Queen production increased with floral resources and was higher in semi-natural areas than on conventional farms. However, the most important parameter for queen production was the colony growth rate per flower, as opposed to the average number of available flowers. This result indicates the importance of understanding mechanisms of colony growth, in order to predict queen production and enhance bumble bee population viability. Our work highlights the importance of interpreting bumble bee conservation efforts in the context of overall population dynamics and provides a framework for doing so.

Wild bees are highly valuable pollinators. Along with managed honey bees, they provide a critical ecosystem service by ensuring stable pollination to agriculture and wild plant communities. Increasing concern about the welfare of both wild and managed pollinators, however, has prompted recent calls for national evaluation and action. Here, for the first time to our knowledge, we assess the status and trends of wild bees and their potential impacts on pollination services across the coterminous United States. We use a spatial habitat model, national land-cover data, and carefully quantified expert knowledge to estimate wild bee abundance and associated uncertainty. Between 2008 and 2013, modeled bee abundance declined across 23% of US land area. This decline was generally associated with conversion of natural habitats to row crops. We identify 139 counties where low bee abundances correspond to large areas of pollinator-dependent crops. These areas of mismatch between supply (wild bee abundance) and demand (cultivated area) for pollination comprise 39% of the pollinator-dependent crop area in the United States. Further, we find that the crops most highly dependent on pollinators tend to experience more severe mismatches between declining supply and increasing demand. These trends, should they continue, may increase costs for US farmers and may even destabilize crop production over time. National assessments such as this can help focus both scientific and political efforts to understand and sustain wild bees. As new information becomes available, repeated assessments can update findings, revise priorities, and track progress toward sustainable management of our nation's pollinators.

Global trends in pollinator-dependent crops have raised awareness of the need to support managed and wild bee populations to ensure sustainable crop production. Provision of sufficient forage resources is a key element for promoting bee populations within human impacted landscapes, particularly those in agricultural lands where demand for pollination service is high and land use and management practices have reduced available flowering resources. Recent government incentives in North America and Europe support the planting of wildflowers to benefit pollinators; surprisingly, in North America there has been almost no rigorous testing of the performance of wildflower mixes, or their ability to support wild bee abundance and diversity. We tested different wildflower mixes in a spatially replicated, multiyear study in three regions of North America where production of pollinator-dependent crops is high: Florida, Michigan, and California. In each region, we quantified flowering among wildflower mixes composed of annual and perennial species, and with high and low relative diversity. We measured the abundance and species richness of wild bees, honey bees, and syrphid flies at each mix over two seasons. In each region, some but not all wildflower mixes provided significantly greater floral display area than unmanaged weedy control plots. Mixes also attracted greater abundance and richness of wild bees, although the identity of best mixes varied among regions. By partitioning floral display size from mix identity we show the importance of display size for attracting abundant and diverse wild bees. Season-long monitoring also revealed that designing mixes to provide continuous bloom throughout the growing season is critical to supporting the greatest pollinator species richness. Contrary to expectation, perennials bloomed in their first season, and complementarity in attraction of pollinators among annuals and perennials suggests that inclusion of functionally diverse species may provide the greatest benefit. Wildflower mixes may be particularly important for providing resources for some taxa, such as bumble bees, which are known to be in decline in several regions of North America. No mix consistently attained the full diversity that was planted. Further study is needed on how to achieve the desired floral display and diversity from seed mixes.

Lifetime seed production can be constrained by shortfalls of pollen receipt ('pollen limitation'). The ovule oversupply hypothesis states that, in response to unpredictable pollen availability, plants evolve to produce more ovules than they expect to be fertilized, and that this results in pollen limitation of seed production. Here, we present a cartoon model and a model of optimal plant reproductive allocations under stochastic pollen receipt to evaluate the hypothesis that an oversupply of ovules leads to increased pollen limitation. We show that an oversupply of ovules has two opposing influences on pollen limitation of whole-plant seed production. First, ovule oversupply increases the likelihood that pollen receipt limits the number of ovules that can be fertilized ('prezygotic pollen limitation'). Second, ovule oversupply increases the proportion of pollen grains received that are used to fertilize ovules ('pollen use efficiency'). As a result of these opposing influences, ovule oversupply has only a modest effect on the degree to which lifetime seed production is constrained by pollen receipt, producing a small decrease in the incidence of pollen limitation. Ovule oversupply is not the cause of the pollen limitation problem, but rather is part of the evolutionary solution to that problem.

Network approaches have become a popular tool for understanding ecological complexity in a changing world. Many network descriptors relate directly or indirectly to specialization, which is a central concept in ecology and measured in different ways. Unfortunately, quantification of specialization and network structure using field data can suffer from sampling effects. Previous studies evaluating such sampling effects either used field data where the true network structure is unknown, or they simulated sampling based on completely generalized interactions. Here, we used a quantitative niche model to generate bipartite networks representing a wide range of specialization and evaluated potential sampling biases for a large set of specialization and network metrics for different network sizes. We show that with sample sizes realistic for species-rich networks, all metrics are biased towards overestimating specialization (and underestimating generalization and niche overlap). Importantly, this sampling bias depends on the true degree of specialization and is strongest for generalized networks. As a result, null models simulating generalized interactions may misrepresent sampling bias. We show that other methods used for empirical data may be biased in the opposite direction and strongly overestimate sampling completeness. Some network metrics are barely related between small and large sub-samples of the same network and thus may often not be meaningful. Small samples also overestimate interspecific variation of specialization within generalized networks. While new approaches to deal with these challenges have to be developed, we also identify metrics that are relatively unbiased and fairly consistent across sampling intensities and we identify a provisional rule of thumb for the number of observations required for accurate estimates. Our quantitative niche model can help understand variation in network structure capturing both sampling effects and biological meaning. This is needed to connect network science to fundamental ecological theory and to give robust quantitative answers for applied ecological problems.

Biodiversity-ecosystem functioning experiments have established that species richness and composition are both important determinants of ecosystem function in an experimental context. Determining whether this result holds for real-world ecosystem services has remained elusive, however, largely due to the lack of analytical methods appropriate for large-scale, associational data. Here, we use a novel analytical approach, the Price equation, to partition the contribution to ecosystem services made by species richness, composition and abundance in four large-scale data sets on crop pollination by native bees. We found that abundance fluctuations of dominant species drove ecosystem service delivery, whereas richness changes were relatively unimportant because they primarily involved rare species that contributed little to function. Thus, the mechanism behind our results was the skewed species-abundance distribution. Our finding that a few common species, not species richness, drive ecosystem service delivery could have broad generality given the ubiquity of skewed species-abundance distributions in nature.

PREMISE OF THE STUDY Wildfire changes the demography, morphology, and behavior of plants, and may alter the pollinator community. Such trait changes may drastically alter the outcome of pollination mutualisms on plants; however, the direct role of fire on these mutualisms is poorly known. METHODS Following a pair of fires in the northern California coast range chaparral, we censused floral visitor communities of Trichostema laxum (Lamiaceae), quantified visiting bee behavior, and estimated outcrossing rates using a widespread Mendelian recessive floral polymorphism across a matrix of populations in burned and unburned sites. We also compared pre- and postfire floral visitation in two populations. RESULTS Outcrossing rates were significantly lower in burned areas; however, our data suggest that the much larger size of plants in burned areas, not burn status itself, drove this pattern. Large-bodied bees dominated floral visitor communities after fire, likely recruiting to the abundant postfire floral resources. These bees visited more flowers per plant than did the smaller bees prevalent before fire and in unburned areas, likely increasing selfing through geitonogamy (within-plant pollination), an effect made possible by the far larger size of plants in burned areas. CONCLUSIONS Outcrossing rates dropped substantially after wildfires because of changes in the pollinators, plant display size, and their interactions. Reductions in outcrossing following fire may have important implications for population resilience and evolution in a changing climate with more frequent fires.

© 2017 by the Ecological Society of America The relationship between biodiversity and the stability of ecosystem function is a fundamental question in community ecology, and hundreds of experiments have shown a positive relationship between species richness and the stability of ecosystem function. However, these experiments have rarely accounted for common ecological patterns, most notably skewed species abundance distributions and non-random extinction risks, making it difficult to know whether experimental results can be scaled up to larger, less manipulated systems. In contrast with the prolific body of experimental research, few studies have examined how species richness affects the stability of ecosystem services at more realistic, landscape scales. The paucity of these studies is due in part to a lack of analytical methods that are suitable for the correlative structure of ecological data. A recently developed method, based on the Price equation from evolutionary biology, helps resolve this knowledge gap by partitioning the effect of biodiversity into three components: richness, composition, and abundance. Here, we build on previous work and present the first derivation of the Price equation suitable for analyzing temporal variance of ecosystem services. We applied our new derivation to understand the temporal variance of crop pollination services in two study systems (watermelon and blueberry) in the mid-Atlantic United States. In both systems, but especially in the watermelon system, the stronger driver of temporal variance of ecosystem services was fluctuations in the abundance of common bee species, which were present at nearly all sites regardless of species richness. In contrast, temporal variance of ecosystem services was less affected by differences in species richness, because lost and gained species were rare. Thus, the findings from our more realistic landscapes differ qualitatively from the findings of biodiversity–stability experiments.

Native forbs have become a more central component of restoration programs, especially because of their role in supporting crop pollinators. This study evaluates the success of different native forb mixes and seeding rates using shared goals of restoration practitioners and agroecologists, namely percent native species cover, floral resources, native diversity, and cost-effectiveness. At 6 sites with hedgerows adjacent to agricultural lands in California’s Central Valley, we planted 3 native forb seed mixes at 3 seeding rates and monitored germination, percent cover, and floral resources for 2 to 3 years. We also evaluated the cost of the mixes based on seeding rates and original seed prices. More than mix type, relative seeding rate strongly affected germination, cover, and floral resource success. The relative benefits of seeding with more species diminished at higher seeding rates, especially when cost was considered. Cover increased significantly over the years but diversity declined sharply after the first year. Increased cover of target species was mainly due to the effect of 1 dominant species Grindelia camporum, common gumplant. We summarize data from a similar forb restoration study showing that the species that dominated in our mix-and-rate experimental sites also attracted the greatest diversity and abundance of pollinators. These findings highlight trade-offs and balance-points within restoration and pollination services goals. We offer suggestions on how to weigh those trade-offs, given particular priorities and how native forb plantings can support combined goals of pollination services and restoration.

Many organisms exhibit parental optimism, producing more of the initial stages of offspring (e.g., eggs, embryos) than they can usually mature. For plants, parental optimism may be linked to the risk of seed production being limited by pollen receipt (pollen limitation). Here we elaborate a stochastic model of pollen limitation developed by Haig and Westoby (1988) and Burd (2008) and link it with published data on the magnitudes of prepollination costs versus postpollination costs of seed production in 80 plant species. We demonstrate that parental optimism should be expected when prepollination costs of seed production are small relative to postpollination costs. This was observed for most (62 of 80) of the plant taxa surveyed. Under parental optimism, plants overinvest in securing fertilized ovules, and consequently pollen limitation is predicted to be uncommon. However, for a sizable minority of plant species (18 of 80), prepollination costs approach or exceed postpollination costs. For these s...

Habitat loss from urban development threatens native plant populations in many regions of the world. In addition to direct plant mortality, urban intensification potentially impacts pollinator communities and in turn disrupts the pollination mutualisms that are critical to the viability of native plant populations. We placed standardized flowering plant arrays into woodlands along a gradient of increasing urban land use to simultaneously quantify landscape-scale and local-scale effects on pollinators and on reproduction of two spring ephemeral wildflowers (Claytonia virginica and Polemonium reptans) in woodland fragments in the Mid-Atlantic Region of North America. Greater pollinator abundance and associated diversity significantly reduced the degree of pollen limitation, demonstrating that pollinator populations are critical to successful pollination of these plant populations. However, landscape-scale habitat loss did not reduce pollinator abundance or diversity. Habitat loss at the landscape scale therefore does not appear to drive changes in pollination in this woodland system. Rather, local-scale habitat characteristics were more important, with pollinators being more abundant in brighter woodland patches for one plant species, and in larger patches for the other species. Because we found abundant pollinators and adequate pollination even in isolated, urban woodland fragments, our results are encouraging for the conservation of both plants and pollinators in urban landscapes.

Agricultural intensification is a leading cause of global biodiversity loss, which can reduce the provisioning of ecosystem services in managed ecosystems. Organic farming and plant diversification are farm management schemes that may mitigate potential ecological harm by increasing species richness and boosting related ecosystem services to agroecosystems. What remains unclear is the extent to which farm management schemes affect biodiversity components other than species richness, and whether impacts differ across spatial scales and landscape contexts. Using a global metadataset, we quantified the effects of organic farming and plant diversification on abundance, local diversity (communities within fields), and regional diversity (communities across fields) of arthropod pollinators, predators, herbivores, and detritivores. Both organic farming and higher in‐field plant diversity enhanced arthropod abundance, particularly for rare taxa. This resulted in increased richness but decreased evenness. While these responses were stronger at local relative to regional scales, richness and abundance increased at both scales, and richness on farms embedded in complex relative to simple landscapes. Overall, both organic farming and in‐field plant diversification exerted the strongest effects on pollinators and predators, suggesting these management schemes can facilitate ecosystem service providers without augmenting herbivore (pest) populations. Our results suggest that organic farming and plant diversification promote diverse arthropod metacommunities that may provide temporal and spatial stability of ecosystem service provisioning. Conserving diverse plant and arthropod communities in farming systems therefore requires sustainable practices that operate both within fields and across landscapes., EML and DC were supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2014‐51106‐22096. BMF was supported by the National Council for Scientific and Technological Development‐CNPq, Brasília, Brazil #305062/2007‐7. SGP and MO were supported by the Felix Trust and STEP Project (EC FP7 244090).

Geographical variation in pollinators visiting a plant can produce plant populations adapted to local pollinator environments. We documented two markedly different pollinator climates for the spring ephemeral wildflower Claytonia virginica: in more northern populations, the pollen-specialist bee Andrena erigeniae dominated, but in more southern populations, A. erigeniae visited rarely and the bee-fly Bombylius major dominated. Plants in the northern populations experienced faster pollen depletion than plants in southern populations. We also measured divergent pollen-related plant traits; plants in northern populations produced relatively more pollen per flower and anther dehiscence was more staggered than plants in southern populations. These plant traits might function to increase pollen dispersal via the different pollen vectors.

Nectar mediates interactions between plants and pollinators in natural and agricultural systems. Specialized microorganisms are common nectar inhabitants, and potentially important mediators of plant-pollinator interactions. However, their diversity and role in mediating pollination services in agricultural systems are poorly characterized. Moreover, agrochemicals are commonly applied to minimize crop damage, but may present ecological consequences for non-target organisms. Assessment of ecological risk has tended to focus on beneficial macroorganisms such as pollinators, with less attention paid to microorganisms. Here, using culture-independent methods, we assess the impact of two widely-used fungicides on nectar microbial community structure in the mass-flowering crop almond (Prunus dulcis). We predicted that fungicide application would reduce fungal richness and diversity, whereas competing bacterial richness would increase, benefitting from negative effects on fungi. We found that fungicides reduced fungal richness and diversity in exposed flowers, but did not significantly affect bacterial richness, diversity, or community composition. The relative abundance of Metschnikowia OTUs, nectar specialists that can impact pollination, was reduced by both fungicides. Given growing recognition of the importance of nectar microorganisms as mediators of plant-pollinator mutualisms, future research should consider the impact of management practices on plant-associated microorganisms and consequences for pollination services in agricultural landscapes.

SummarySpecies-rich tropical communities are expected to be more specialized than their temperate counterparts [1–3]. Several studies have reported increasing biotic specialization toward the tropics [4–7], whereas others have not found latitudinal trends once accounting for sampling bias [8, 9] or differences in plant diversity [10, 11]. Thus, the direction of the latitudinal specialization gradient remains contentious. With an unprecedented global data set, we investigated how biotic specialization between plants and animal pollinators or seed dispersers is associated with latitude, past and contemporary climate, and plant diversity. We show that in contrast to expectation, biotic specialization of mutualistic networks is significantly lower at tropical than at temperate latitudes. Specialization was more closely related to contemporary climate than to past climate stability, suggesting that current conditions have a stronger effect on biotic specialization than historical community stability. Biotic specialization decreased with increasing local and regional plant diversity. This suggests that high specialization of mutualistic interactions is a response of pollinators and seed dispersers to low plant diversity. This could explain why the latitudinal specialization gradient is reversed relative to the latitudinal diversity gradient. Low mutualistic network specialization in the tropics suggests higher tolerance against extinctions in tropical than in temperate communities.

Variation in the availability of food resources over space and time is a likely driver of how landscape structure and composition affect animal populations. Few studies, however, have directly assessed the spatiotemporal variation in resource availability that arises from landscape pattern, or its effect on populations and population dynamic parameters. We tested the effect of floral resource availability at the landscape scale on the numbers of worker, male, and queen offspring produced by bumble bee, Bombus vosnesenśkii, colonies experimentally placed within complex agricultural-natural landscapes. We quantified flower densities in all land use types at different times of the season and then used these data to calculate spatially explicit estimates of floral resources surrounding each colony. Floral availability strongly correlated with landscape structure, and different regions of the landscape showed distinct seasonal patterns of floral availability. The floral resources available in the landscape surrounding a colony positively affected the number of workers and males it produced. Production was more sensitive to early- than to later-season resources. Floral resources did not significantly affect queen production despite a strong correlation between worker number and queen number among colonies. No landscape produced high floral resources during both the early and late season, and seasonal consistency is likely required for greater queen production. Floral resources are important determinants of colony growth and likely affect the pollination services provided by bumble bees at a landscape scale. Spatiotemporal variation in floral resources across the landscape precludes a simple relationship between resources and reproductive success as measured by queens, but nonetheless likely influences the total abundance of bumble bees in our study region.

This review by a multidisciplinary team maps key components and emerging connections within the intellectual landscape of agroecology. We attempt to extend and preview agroecology as a discipline in which agriculture can be conceptualized within the context of global change and studied as a coupled system involving a wide range of social and natural processes. This intrinsic coupling, combined with powerful emerging drivers of change, presents challenges for the practice of agroecology and agriculture itself, as well as providing the framework for some of the most innovative research areas and the greatest potential for innovation for a sustainable future in agriculture. The objective of this review is to identify forward-looking scientific questions to enhance the relevance of agroecology for the key challenges of mitigating environmental impacts of agriculture while dramatically increasing global food production, improving livelihoods, and thereby reducing chronic hunger and malnutrition over the coming decades.

Alien plants form important interactions with flower visitors across many systems and are especially likely to dominate these interactions in disturbed habitats where native plants are rare. Most studies of alien plant–pollinator interactions have focused on the effects of alien plants on native plant reproduction; however, these alien plants may also be important food resources for native bees especially if they dominate the “floral market”. Dominance of alien species in the diet of bees could occur due to high relative density and abundance of alien plants and flowers, and/or to bee preference for alien plants when they are present. We investigated bees’ use of and preference for alien plants using two data sets from distinct regions of North America, central California and southern New Jersey. In each system, we sampled bees and flowering plants in multiple habitat types that differ in level of disturbance and in the relative abundance of alien plant species. Alien plants as a group dominated interactions with bee communities in more disturbed habitats. Importance, however, varied among plant species such that a subset of highly used plant species drove the overall pattern of use within the community. Despite higher use of alien plants in more disturbed sites, alien plants as a group were not more preferred. Rather, bees’ use of individual alien plants correlated with plant abundance in the community. Consistent with the interpretation that bees use, but do not prefer, alien plants, we found no effect of alien plant abundance or richness on bee abundance or richness.

Evaluating the success of restoration projects requires well-designed studies. Among the decisions that need to be made are what taxonomic groups to study and when to conduct the monitoring. To explore how these decisions can influence assessments of restoration success, we examined species richness and composition data collected over several years on different terrestrial fauna (landbirds, rodents, bees, and beetles) at Sacramento River restoration and remnant riparian sites. Our selection of study organisms enabled us to ask whether variability in species richness among restoration sites is less for vagile taxa than for sedentary taxa, and if invertebrates display greater variability among sites than vertebrates. Our results demonstrate that responses to restoration can vary depending upon the season when it is assessed, and the taxa that are studied. For all taxa except bees, there was considerable variability in the relative performance of taxa at restoration sites from one sampling date to the next, such that the relative ranking of the sites often changed dramatically. Comparisons of β -diversity (variability in species richness across sites) revealed that certain taxonomic groups were more spatially variable in their response to restoration than others. Among vertebrates, sedentary taxa (rodents) had significantly higher variability in species richness across sites than highly vagile taxa (birds); however, no such pattern was observed for invertebrates. Overall, vertebrates had lower variability than invertebrates, suggesting that evaluations of restoration success based on a few better-known taxonomic groups (e.g., birds, rodents) may be inadequate to represent the biodiversity response of other groups (e.g., insects).

The ability to predict the responses of ecological communities and individual species to human-induced environmental change remains a key issue for ecologists and conservation managers alike. Responses are often variable among species within groups making general predictions difficult. One option is to include ecological trait information that might help to disentangle patterns of response and also provide greater understanding of how particular traits link whole clades to their environment. Although this “trait-guild” approach has been used for single disturbances, the importance of particular traits on general responses to multiple disturbances has not been explored. We used a mixed model analysis of 19 data sets from throughout the world to test the effect of ecological and life-history traits on the responses of bee species to different types of anthropogenic environmental change. These changes included habitat loss, fragmentation, agricultural intensification, pesticides and fire. Individual traits significantly affected bee species responses to different disturbances and several traits were broadly predictive among multiple disturbances. The location of nests – above vs. below ground – significantly affected response to habitat loss, agricultural intensification, tillage regime (within agriculture) and fire. Species that nested above ground were on average more negatively affected by isolation from natural habitat and intensive agricultural land use than were species nesting below ground. In contrast below-ground-nesting species were more negatively affected by tilling than were above-ground nesters. The response of different nesting guilds to fire depended on the time since the burn. Social bee species were more strongly affected by isolation from natural habitat and pesticides than were solitary bee species. Surprisingly, body size did not consistently affect species responses, despite its importance in determining many aspects of individuals’ interaction with their environment. Although synergistic interactions among traits remain to be explored, individual traits can be useful in predicting and understanding responses of related species to global change.

Nontarget species such as pollinators may be of great importance to the restoration process and the long-term functioning of restored habitats, but little is known about how such groups respond to habitat restoration. I surveyed bee communities at five equal-aged restored sites, paired with five reference sites (riparian remnants) along the Sacramento River, California, United States. Flower availability and bee visitation patterns were also measured to examine the restoration of pollination function. Restoration of structural vegetation allowed diverse and abundant native bee communities to establish at the restoration sites; however, the composition of these important pollinator communities was distinct from that in the remnant riparian sites. Differences did not arise primarily from differences in the composition of the flowering-plant community; rather there must be other physical characteristics of the restored sites or differences in nesting site availability that led to the different pollinator communities. Because sites were spatially paired, the differences are unlikely to be driven by landscape context. Bee life-history and other biological traits may partially explain the differences between bee communities at restored and remnant sites. Patterns of visitation to native plant species suggest that pollination function is restored along with pollinator abundance and richness; however, function may be less robust in restored habitats. An examination of interaction networks between bees and plant species found at both restored and remnant riparian sites showed less redundancy of pollinators visiting some plants at restored habitats.

We recently introduced a model that predicts the degree to which a plant's lifetime seed production may be constrained by unpredictable shortfalls of pollen receipt ("pollen limitation"). Burd's comment in this issue criticized our analysis, first by arguing that the empirical literature documents much higher levels of pollen limitation than our model predicts and then suggesting that the apparent discrepancy stemmed from our (1) underestimating the costs of securing a fertilized ovule and (2) assuming too little unpredictability in whole-plant pollen receipt. We reply as follows. First, the empirical literature must be consulted carefully. Burd relies on pollen supplementation experiments performed on parts of plants or on whole plants but during only one reproductive season for polycarpic perennials; in both cases, resource reallocation often leads to gross overestimates of pollen limitation. We comprehensively review pollen limitation estimates that are free of these estimation problems and find strong agreement with our model predictions. Second, although cost estimates for different components of seed production are imprecise, errors are likely to be small relative to the1,000-fold differences observed across plant species, the primary focus of our article. Finally, contrary to Burd's argument, pollen receipt by entire plants is much more predictable than that by individual flowers because the flower-to-flower variation "averages out" when summed across many flowers. Our model uses parameter values that are in broad agreement with the empirical record of modest plant-to-plant variation in pollen receipt and thus predicts the generally modest pollen limitation that is observed in nature.

There is compelling evidence that more diverse ecosystems deliver greater benefits to people, and these ecosystem services have become a key argument for biodiversity conservation. However, it is unclear how much biodiversity is needed to deliver ecosystem services in a cost-effective way. Here we show that, while the contribution of wild bees to crop production is significant, service delivery is restricted to a limited subset of all known bee species. Across crops, years and biogeographical regions, crop-visiting wild bee communities are dominated by a small number of common species, and threatened species are rarely observed on crops. Dominant crop pollinators persist under agricultural expansion and many are easily enhanced by simple conservation measures, suggesting that cost-effective management strategies to promote crop pollination should target a different set of species than management strategies to promote threatened bees. Conserving the biological diversity of bees therefore requires more than just ecosystem-service-based arguments.

One of the values of biodiversity is that it may provide 'biological insurance' for services currently rendered by domesticated species or technology. We used crop pollination as a model system, and investigated whether the loss of a domesticated pollinator (the honey bee) could be compensated for by native, wild bee species. We measured pollination provided to watermelon crops at 23 farms in New Jersey and Pennsylvania, USA, and used a simulation model to separate the pollen provided by honey bees and native bees. Simulation results predict that native bees alone provide sufficient pollination at > 90% of the farms studied. Furthermore, empirical total pollen deposition at flowers was strongly, significantly correlated with native bee visitation but not with honey bee visitation. The honey bee is currently undergoing extensive die-offs because of Colony Collapse Disorder. We predict that in our region native bees will buffer potential declines in agricultural production because of honey bee losses.

Summary 1 Concern about a global decline in wild pollinators has increased interest in how pollinators are affected by human land use, and how this, in turn, affects crop pollination. 2 We measured wild bee visitation to four summer vegetable crops, and investigated associations between flower visitation rates and land-use intensity at local and landscape scales. We studied 29 farms in New Jersey and Pennsylvania, USA. Over 2 years we recorded >7400 bee visits to crop flowers and identified 54 species of wild bees visiting crops. 3 Wild bees were the dominant flower visitors at three of the four crops studied; domesticated honeybees, Apis mellifera L., provided the remainder of visits. 4 Ordination of the two best studied crops showed that the wild bee species visiting tomato, Solanum lycopersicum L., were distinct from those visiting watermelon, Citrullus lanatus (Thunb.) Matsum. & Nakai. 5 Crop visitation by wild bees was not associated with organic farming, nor with natural habitat cover at either local or landscape scale. 6 Synthesis and applications. The extent of crop visitation by wild bees observed in this study is among the highest recorded. In contrast to previous studies of crop visitation by wild bees, we did not find negative effects of conventional farming or natural habitat loss. In our study system, organic and conventional farms differ little in field size, crop diversity and weedy flower diversity, unlike some systems where organic farms have smaller fields with greater crop and weed diversity. Such variables may be more important than organic vs. conventional farming practices. Second, small patches of natural habitat are dispersed throughout our entire study system, and this habitat heterogeneity may support high bee abundance even in landscapes with a low proportion of natural habitat overall. Our findings suggest that agri-environment schemes and similar programmes that work through local habitat restoration should target farms in intensively agricultural (homogeneous) landscapes to gain maximum conservation benefits.

The strength of interactions among species in a network tends to be highly asymmetric. We evaluate thehypothesis that this asymmetry results from the distribution of abundance among species, so that speciesinteractions occur randomly among individuals. We used a database on mutualistic and antagonistic bipartitequantitative interaction networks. We show that across all types of networks asymmetry was correlated withabundance, so that rare species were asymmetrically affected by their abundant partners, while pairs ofinteracting abundant species tended to exhibit more symmetric, reciprocally strong effects. A null model showsthat abundance provides a sufficient explanation of the asymmetry structure in some networks, but suggests therole of additional factors in others. Although not universal, our hypothesis holds for a substantial fraction ofnetworks analyzed here, and should be considered as a null model in all studies aimed at evaluating theecological and evolutionary consequences of species interactions. Fil: Vazquez, Diego P.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Provincia de Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Universidad Nacional de Cuyo. Instituto Argentino de Investigaciones de las Zonas Áridas; Argentina Fil: Melián, Carlos J.. University of California; Estados Unidos Fil: Neal, M. Williams. Bryn Mawr College; Estados Unidos Fil: Blüthgen, Nico. Universität Würzburg; Alemania Fil: Krasnov, Boris R.. Ben Gurion University of the Negev; Israel Fil: Poulin, Robert. University of Otago; Nueva Zelanda

Within mosaic landscapes, many organisms depend on attributes of the environment that operate over scales ranging from a single habitat patch to the entire landscape. One such attribute is resource distribution. Organisms' reliance on resources from within a local patch vs. those found among habitats throughout the landscape will depend on local habitat quality, patch quality, and landscape composition. The ability of individuals to move among complementary habitat types to obtain various resources may be a critical mechanism underlying the dynamics of animal populations and ultimately the level of biodiversity at different spatial scales. We examined the effects that local habitat type and landscape composition had on offspring production and survival of the solitary bee Osmia lignaria in an agri-natural landscape in California (U.S.A.). Female bees were placed on farms that did not use pesticides (organic farms), on farms that did use pesticides (conventional farms), or in seminatural riparian habitats. We identified pollens collected by bees nesting in different habitat types and matched these to pollens of flowering plants from throughout the landscape. These data enabled us to determine the importance of different plant species and habitat types in providing food for offspring, and how this importance changed with landscape and local nesting-site characteristics. We found that increasing isolation from natural habitat significantly decreased offspring production and survival for bees nesting at conventional farms, had weaker effects on bees in patches of seminatural habitat, and had little impact on those at organic farm sites. Pollen sampled from nests showed that females nesting in both farm and seminatural habitats relied on pollen from principally native plant species growing in seminatural habitat. Thus connectivity among habitats was critical for offspring production. Females nesting on organic farms were buffered to isolation effects by switching to floral resources growing at the farm site when seminatural areas were too distant. Overall local habitat conditions (farm management practices) can help bolster pollinators, but maintaining functional connectivity among habitats will likely be critical for persistence of pollinator populations as natural habitats are increasingly fragmented by human activities.

Many ecosystem services are delivered by organisms that depend on habitats that are segregated spatially or temporally from the location where services are provided. Management of mobile organisms contributing to ecosystem services requires consideration not only of the local scale where services are delivered, but also the distribution of resources at the landscape scale, and the foraging ranges and dispersal movements of the mobile agents. We develop a conceptual model for exploring how one such mobile-agent-based ecosystem service (MABES), pollination, is affected by land-use change, and then generalize the model to other MABES. The model includes interactions and feedbacks among policies affecting land use, market forces and the biology of the organisms involved. Animal-mediated pollination contributes to the production of goods of value to humans such as crops; it also bolsters reproduction of wild plants on which other services or service-providing organisms depend. About one-third of crop production depends on animal pollinators, while 60-90% of plant species require an animal pollinator. The sensitivity of mobile organisms to ecological factors that operate across spatial scales makes the services provided by a given community of mobile agents highly contextual. Services vary, depending on the spatial and temporal distribution of resources surrounding the site, and on biotic interactions occurring locally, such as competition among pollinators for resources, and among plants for pollinators. The value of the resulting goods or services may feed back via market-based forces to influence land-use policies, which in turn influence land management practices that alter local habitat conditions and landscape structure. Developing conceptual models for MABES aids in identifying knowledge gaps, determining research priorities, and targeting interventions that can be applied in an adaptive management context.

Pollination in crops, as in native ecosystems, is a stepwise process that can be disrupted at any stage. Healthy pollinator populations are critical for adequate visitation, but pollination still might fail if crop management interferes with the attraction and retention of pollinators. Farmers must balance the direct benefits of applying insecticide and managing irrigation rates against their potential to indirectly interfere with the pollination process. We investigated these issues in hybrid onion seed production, where previous research has shown that high insecticide use reduces pollinator attraction. We conducted field surveys of soil moisture, nectar production, pollinator visitation, pollen-stigma interactions, and seed set at multiple commercial fields across 2 yr. We then examined how management actions, such as irrigation rate (approximated by soil moisture), or insecticide use could affect the pollination process. Onions produced maximum nectar at intermediate soil moisture, and high nectar production attracted more pollinators. Insecticide use weakly affected pollinator visitation, but when applied close to bloom reduced pollen germination and pollen tube growth. Ultimately, neither soil moisture nor insecticide use directly affected seed set, but the high correlation between pollinator visitation and seed set suggests that crop management will ultimately affect yields via indirect effects on the pollination process.

Entomophilous exotic plant species not only compete directly for space and light with other plants, but also offer resource opportunities for the native pollinator community. Most research on this topic to date has taken the plant perspective, focusing on how successful plant invaders integrate into the native plant-pollinator interaction networks. However, species specific responses of pollinators to the addition of exotic plants are rarely taken into account. We show here that while some bumble bees and other trophic generalist bees can benefit from exotic plant invasions, other species can be negatively affected. Behavioral flexibility may be the key to persist in a changing world.

Agricultural conversion is one of the most prevalent anthropogenic uses on the terrestrial earth. Persistence of organisms in such landscapes is thought to be related to species-specific characteristics such as life history traits and dispersal distance. In an agricultural landscape in California, we examined local (farm-level) and landscape variables associated with nesting preferences of native ground-nesting bees. Compared to the known ground-nesting visitors to crops, bee community nesting on farms was depauperate. Further, more abundant and diverse communities of bees were found nesting at farms with patches of natural habitat near by than farms that were far away from natural habitat. Species responded differently to soil conditions created by farming practices, but the variability in nesting bee abundance was lower in farms near natural habitat than farms far from natural habitat. These findings suggest that most bee species are affected adversely but to varying degrees by agricultural int...

Urbanization within the Tucson Basin of Arizona during the past 50+ years has fragmented the original desert scrub into patches of different sizes and ages. These remnant patches and the surrounding desert are dominated by Larrea tridentata (creosote bush), a long-lived shrub whose flowers are visited by > 120 native bee species across its range. Twenty-one of these bee species restrict their pollen foraging to L. tridentata. To evaluate the response of this bee fauna to fragmentation, we compared species incidence and abundance patterns for the bee guild visiting L. tridentata at 59 habitat fragments of known size (0.002-5 ha) and age (up to 70 years), and in adjacent desert. The 62 bee species caught during this study responded to fragmentation heterogeneously and not in direct relation to their abundance or incidence in undisturbed desert. Few species found outside the city were entirely absent from urban fragments. Species of ground-nesting L. tridentata specialists were underrepresented in smaller fragments and less abundant in the smaller and older fragments. In contrast, cavity-nesting bees (including one L. tridentata specialist) were overrepresented in the habitat fragments, probably due to enhanced nesting opportunities available in the urban matrix. Small-bodied bee species were no more likely than larger bodied species to be absent from the smaller fragments. The introduced European honey bee, Apis mellifera, was a minor faunal element at > 90% of the fragments and exerted little if any influence on the response of native bee species to fragmentation. Overall, bee response to urban habitat fragmentation was best predicted by ecological traits associated with nesting and dietary breadth. Had species been treated as individual units in the analyses, or pooled together into one analysis, these response patterns may not have been apparent. Pollination interactions with this floral host are probably not adversely affected in this system because of its longevity and ability to attract diverse pollinators but will demand careful further study to understand.

By causing extinctions and altering community structure, anthropogenic disturbances can disrupt processes that maintain ecosystem integrity. However, the relationship between community structure and ecosystem functioning in natural systems is poorly understood. Here we show that habitat loss appeared to disrupt ecosystem functioning by affecting extinction order, species richness and abundance. We studied pollination by bees in a mosaic of agricultural and natural habitats in California and dung burial by dung beetles on recently created islands in Venezuela. We found that large-bodied bee and beetle species tended to be both most extinction-prone and most functionally efficient, contributing to rapid functional loss. Simulations confirmed that extinction order led to greater disruption of function than predicted by random species loss. Total abundance declined with richness and also appeared to contribute to loss of function. We demonstrate conceptually and empirically how the non-random response of communities to disturbance can have unexpectedly large functional consequences.

Managing ecosystem services is critical to human survival, yet we do not know how large natural areas must be to support these services. We investigated how crop pollination services provided by native, unmanaged, bee communities varied on organic and conventional farms situated along a gradient of isolation from natural habitat. Pollination services from native bees were significantly, positively related to the proportion of upland natural habitat in the vicinity of farm sites, but not to any other factor studied, including farm type, insecticide usage, field size and honeybee abundance. The scale of this relationship matched bee foraging ranges. Stability and predictability of pollination services also increased with increasing natural habitat area. This strong relationship between natural habitat area and pollination services was robust over space and time, allowing prediction of the area needed to produce a given level of pollination services by wild bees within this landscape.

Female bees are usually confronted with a choice among several flower species that differ in their location and abundance within the community, and in the efficiency with which their pollen and nectar can be harvested. We investigated the effects of distance and flower density of two flower species on pollen collection by providing nest locations for the mason bee Osmia lignaria in natural settings. Distance weakly affected pollen use; on average, bees nesting near a flower species tended to collect more of its pollen than did bees nesting at a greater distance. Flower density did not predictably impact pollen use, and use did not track changes in density during the season. Bees consistently mixed pollen from more distant species, despite substantial added foraging costs, and also mixed when one species was an order of magnitude less abundant than the other. Bees require nectar as well as pollen to feed their offspring, and our preliminary data suggest that the efficiencies of pollen and nectar collection are inversely related between the two flower species, which would favor visitation to both species. Bees appear to collect some pollen from the low-pollen, high-nectar plant while visiting it for nectar. Thus, a nectar-collecting constraint may favor collecting pollen from mixtures of species. Key words: central place foraging, complementary nutrients, nectar, Osmia, pollen, resource abundance, solitary bee, spatial distribution. [Behav Ecol 14:141–149 (2003)]

Author(s): Forrest, JRK; Thorp, RW; Kremen, C; Williams, NM | Abstract: Land-use change frequently reduces local species diversity. Species losses will often result in loss of trait diversity, with likely consequences for community functioning. However, the converse need not be generally true: management approaches that succeed in retaining species richness could nevertheless fail to maintain trait diversity. We evaluated this possibility using bee communities in a California agroecosystem. We examined among site patterns in bee species diversity and functional-trait diversity in a landscape composed of a mosaic of semi-natural habitat, organic farms and conventional farms. We sampled bees from all three habitat types and compiled a data base of life-history ('functional') traits for each species. Although species richness was higher on organic farms than conventional farms, functional diversity was lower in both farm types than in natural habitat. Nesting location (below-ground vs. above-ground) was the primary trait contributing to differences in functional diversity between farms and natural habitat, reflecting observed differences in availability of nesting substrates among habitat types. Other traits, including phenology and sociality, were also associated with species' occurrences or dominance in particular site types. These patterns suggest that management practices common to all farms act as environmental filters that cause similarly low functional diversity in their bee communities. Synthesis and applications. Although our results support the value of organic farming in maintaining abundant and species-rich bee communities, components of bee functional diversity are not well supported in farmed landscapes, regardless of farming practice. Maintenance of natural habitat, and/or the addition of natural habitat elements to farms, is therefore important for the retention of functionally diverse bee assemblages in agroecosystems. Although our results support the value of organic farming in maintaining abundant and species-rich bee communities, components of bee functional diversity are not well supported in farmed landscapes, regardless of farming practice. Maintenance of natural habitat, and/or the addition of natural habitat elements to farms, is therefore important for the retention of functionally diverse bee assemblages in agroecosystems.

© 2015 Macmillan Publishers Limited. All rights reserved. There is compelling evidence that more diverse ecosystems deliver greater benefits to people, and these ecosystem services have become a key argument for biodiversity conservation. However, it is unclear how much biodiversity is needed to deliver ecosystem services in a cost-effective way. Here we show that, while the contribution of wild bees to crop production is significant, service delivery is restricted to a limited subset of all known bee species. Across crops, years and biogeographical regions, crop-visiting wild bee communities are dominated by a small number of common species, and threatened species are rarely observed on crops. Dominant crop pollinators persist under agricultural expansion and many are easily enhanced by simple conservation measures, suggesting that cost-effective management strategies to promote crop pollination should target a different set of species than management strategies to promote threatened bees. Conserving the biological diversity of bees therefore requires more than just ecosystem-service-based arguments.

Ecosystem services are critical to human survival; in selected cases, maintaining these services provides a powerful argument for conserving biodiversity. Yet, the ecological and economic underpinnings of most services are poorly understood, impeding their conservation and management. For centuries, farmers have imported colonies of European honey bees ( Apis mellifera ) to fields and orchards for pollination services. These colonies are becoming increasingly scarce, however, because of diseases, pesticides, and other impacts. Native bee communities also provide pollination services, but the amount they provide and how this varies with land management practices are unknown. Here, we document the individual species and aggregate community contributions of native bees to crop pollination, on farms that varied both in their proximity to natural habitat and management type (organic versus conventional). On organic farms near natural habitat, we found that native bee communities could provide full pollination services even for a crop with heavy pollination requirements (e.g., watermelon, Citrullus lanatus ), without the intervention of managed honey bees. All other farms, however, experienced greatly reduced diversity and abundance of native bees, resulting in insufficient pollination services from native bees alone. We found that diversity was essential for sustaining the service, because of year-to-year variation in community composition. Continued degradation of the agro-natural landscape will destroy this “free” service, but conservation and restoration of bee habitat are potentially viable economic alternatives for reducing dependence on managed honey bees.

Research on threats to pollination service in agro-ecosystems has focused primarily on the negative impacts of land use change and agricultural practices such as insecticide use on pollinator populations. Insecticide use could also affect the pollination process, through nonlethal impacts on pollinator attraction and postpollination processes such as pollen viability or pollen tube growth. Hybrid onion seed (Allium cepa L., Alliaceae) is an important pollinator-dependent crop that has suffered yield declines in California, concurrent with increased insecticide use. Field studies suggest that insecticide use reduces pollination service in this system. We conducted a field experiment manipulating insecticide use to examine the impacts of insecticides on 1) pollinator attraction, 2) pollen/stigma interactions, and 3) seed set and seed quality. Select insecticides had negative impacts on pollinator attraction and pollen/stigma interactions, with certain products dramatically reducing pollen germination and pollen tube growth. Decreased pollen germination was not associated with reduced seed set; however, reduced pollinator attraction was associated with lower seed set and seed quality, for one of the two female lines examined. Our results highlight the importance of pesticide effects on the pollination process. Overuse may lead to yield reductions through impacts on pollinator behavior and postpollination processes. Overall, in hybrid onion seed production, moderation in insecticide use is advised when controlling onion thrips, Thrips tabaci, on commercial fields.

We combined methods of geometric morphometrics and phylogenetically controlled comparisons to examine the relationship between the shape of mandibles and material used to construct the nest for 5 Osmia Panzer species. We measured leaf- and mud-using species from 2 subgenera: in the subgenus Osmia, O. lignaria Cresson (mud-user), O. cornuta Latreille (mud-user) and O. ribifloris Cockerell (leaf-user), and in the subgenus Centrosmia, O. tanneri Sandhouse (mud-user), and O. bucephela Cresson (leaf-user). Landmark-based geometric morphometrics and multivariate statistics provided a way to compare mandible shapes, distill major shape differences and visualize mandible regions that differed most among bee species. Mandible shape differed significantly among all species based on paired comparisons of Mahalanobis distances. Principal component analysis of the shape variables (relative warps analysis) revealed greater similarity between the 2 leaf-using bees, than among the mud-users. There were however, no consistent differences between these mud- and leaf-using species. Testing with two-way multivariate analysis of variance showed that the differences in shape between the leaf- and mud-using species depended on the subgenus. A broad proximate bite surface characterized leaf-using species. No such trait was common to mud-using species. Mandibles of leaf-using species may vary less because of selection on shape, or because of functional constraints for shearing and macerating leaf material; both constraints are absent in mud-using species. Mandibles of mud-using bees may respond more to selection imposed by other aspects of bee biology.

We analyzed the temporal structure of visitation by bumble bee workers to a single Penstemon strictus plant growing in an array of conspccifics. When tested against a null distribution using a randomization model, the observed pattern of arrivals for the whole group of bees was random, but departures were clustered in time. Certain individuals visited the plant repeatedly and frequently throughout the day. These showed significantly regular arrival and departure schedules, which were likely produced by traplining. We explored whether these more frequent and regular foragers gained a higher reward than random or incidental plant visitors. Using an analytical model, Possingham predicted that a dominating forager that visited a simple, renewing resource in a regular pattern would garner higher and less variable rewards than random visitors. Inspired by these results, but interested in plant-level visitation, we constructed a simulation model of resource dynamics for a multiflowered plant with high visitation. The model incorporates the observed visitation schedules of all bees and independent reward dynamics for each flower on the plant. We calculated the rewards that observed bees would have collected given a range of resource-renewal parameters. More frequent visitors did not return to the plant when whole-plant resource levels were higher, but diese visitors did get greater rewards. Their increased reward resulted from greater foraging efficiency, primarily through selecting (on average) more rewarding flowers than those selected by less frequent, random visitors. Key words: bee, Bombus, foraging, Penstemon strictus, Possingham model, renewing resource, simulation model, trapline, visitation pattern. [Behau Ecol 9:612-621 (1998)]