Heatwaves are an increasingly common extreme weather event across the globe and are projected to surge in frequency and severity in the coming decades. Plant‐pollinator mutualisms are vulnerable due ...to interacting effects of extreme heat on insect pollinator foraging behaviour and their forage plants.
We designed an experiment to parse the impact of extreme heat on bumblebee foraging mediated directly through air temperature and indirectly through changes in plant rewards.
Temperatures simulating a moderate heatwave negatively impacted foraging bumblebees reducing the proportion of successful foraging bouts, foraging bout duration and plant and flower visitation and indirect stress through reduced nectar production that limited foraging bout duration.
Our experimental results provide a mechanistic link between climate, plants and pollinators and suggest in situ conditions from heatwaves could have profound negative consequences for bumblebee colony persistence and maintenance of pollination services.
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Bumble bees are a critical group of flower-visiting insects that provide pollination services to a diversity of wild and commercial plants. Recently, a number of formerly common species of North ...American bumble bees have shown evidence of precipitous declines in range and relative abundance, adding ongoing concern regarding the stability of pollinator populations writ large. While a number of factors have been implicated in bumble bee declines, the loss of habitat as a result of expansion and intensification of agricultural lands is of utmost concern. As agricultural lands have supplanted flower-rich natural habitat such as prairies and wetlands, the location, amount, and timing of floral resource availability has been dramatically altered. Thus far, our understanding of how the combination of where flowers are in space and time is poorly understood, particularly in the context of how changes have impacted the bumble bees dependent on floral resources. As such, the objective of this dissertation is to enhance our understanding of how changes in spatiotemporal resource abundance impact bumble bee behavior, colony growth and reproduction, and population trends over the course of agricultural change in US midwestern landscapes - with a particular focus on Wisconsin agroecosystems using the common eastern bumble bee (Bombus impatiens Cresson) as a model species in laboratory and field experiments. Throughout this work, I have revealed that the temporal availability of floral resources in agricultural landscapes impacts bumble bee foraging behavior and colony growth and reproduction. Specifically, foraging bumble bees utilize the resource pulses that are commonplace in modern agricultural landscapes - foraging for less time, and more frequently to exploit resources when they are available, regardless of the context of the surrounding landscape. I also found that colonies of B. impatiens grow faster, and often gain more mass when food resources are continuously available, as contrasted with when resources are only available as short, but comparatively large pulses. However, the number of reproductive individuals produced by colonies is identical regardless of the temporal presentation of food. This result was consistent in both laboratory queenless microcolonies and free-foraging, queen-right field colonies. In addition, I explored the methods commonly used to estimate floral resource abundance in foraging landscapes, and compared them in their ability to both accurately describe actual floral abundance, as well as their capacity to predict bumble bee abundance from a commonly used survey method. I found that the ability of land cover, expert opinion, and empirical methods for floral resource estimation to predict bumble bee abundance was largely similar. However, estimates of floral abundance varied greatly - especially between expert opinion and empirical estimates. Lastly, I examined trends in historical agricultural intensification metrics and their relationship to changes in bumble bee relative abundance and community composition. Here, I found that metrics of increasing agricultural intensity are clearly associated with the decline of a 40% of bumble bee species in the US Midwest. Additionally, I document several species increasing in relative abundance that positively associate with agricultural intensity, suggesting our agricultural practices have inadvertently selected winners and losers in the bumble bee community over the last century. The research herein enhances our understanding of how the availability of floral resources over time influence bumble bee behavior, colony growth and reproduction, abundance, and historical population trends. While focusing largely on Bombus impatiens, the range of organismal scales covered in this dissertation, from behavior to populations and communities, provides a framework to evaluate the response of less common species to spatiotemporal resource availability. Applications of this work range from understanding how landscape changes in floral abundance might affect bumble bee species persistence, to the large-scale changes in agricultural practices needed to stem further loss of bumble bee species.
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.
1. Resource pulses, narrow periods of high resource availability, can elicit strong behavioural responses across diverse taxa. Mass‐flowering agricultural crops are an example of a resource pulse ...that insect pollinators exploit. However, the underlying mechanism behind changes in pollinator behaviour associated with mass‐flowering crops is still relatively unexplored.2. The present study quantified the behavioural response of bumble bees, an important wild pollinator, to commercial cranberry bloom, an important mass‐flowering crop in Wisconsin, U.S.A. Over a 2‐year period, foraging trip duration was measured using radio frequency identification at 14 farms situated across landscape contexts, ranging from high to low natural area (woodland amount). Using transect surveys, floral resource abundance at a landscape scale was estimated.3. It was found that bumble bees were highly sensitive to temporal changes in landscape‐level resource abundance associated with the onset of cranberry bloom, during which they decreased foraging trip duration by 22% and increased the number of foraging trips during bloom by 24% on average relative to the period before and after bloom. This phenomenon was consistent across colonies, individual bees, and landscape contexts, despite a higher abundance of flowers in low woodland landscapes. Bumble bee colonies growing in low‐ and high‐woodland landscapes exhibited a similar performance.4. As mass‐flowering crops are probably a factor influencing bumble bee foraging behaviour in agricultural regions, investigations should continue into how variable resource landscapes, particularly those offering resource pulses, affect wild pollinators and the pollination services they provide.
Abstract A rapidly warming climate is driving changes in biodiversity worldwide, and its impact on insect communities is critical given their outsized role in ecosystem function and services. We use ...a long‐term dataset of North American bumble bee species occurrences to determine whether the community temperature index (CTI), a measure of the balance of warm‐ and cool‐adapted species in a community, has increased given warming temperatures. CTI has increased by an average of 0.99°C in strong association with warming maximum summer temperatures over the last 30 years with the areas exhibiting the largest increases including mid‐ to high latitudes as well as low and high elevations—areas relatively shielded from other intensive global changes. CTI shifts have been driven by the decline of cold‐adapted species and increases in warm‐adapted species within bumble bee communities. Our results show the pervasive impacts and ecological implications warming temperatures pose to insects.
Agricultural intensification is a key suspect among putative drivers of recent insect declines, but an explicit link between historical change in agricultural land cover and insect occurrence is ...lacking. Determining whether agriculture impacts beneficial insects (e.g. pollinators), is crucial to enhancing agricultural sustainability. Here, we combine large spatiotemporal sets of historical bumble bee and agricultural records to show that increasing cropland extent and decreasing crop richness were associated with declines in over 50% of bumble bee species in the agriculturally intensive Midwest, USA. Critically, we found that high crop diversity was associated with a higher occurrence of many species pre‐1950 even in agriculturally dominated areas, but that current agricultural landscapes are devoid of high crop diversity. Our findings suggest that insect conservation and agricultural production may be compatible, with increasing on‐farm and landscape‐level crop diversity predicted to have positive effects on bumble bees.
Agricultural intensification is a key suspect among putative drivers of recent insect declines, but an explicit link between historical change in agricultural land cover and insect occurrence is lacking. Here, we find that historical patterns of agricultural intensification are strongly related to patterns in bumble bee species occurrence, specifically a loss in crop species richness and increased cropland proportion.
Abstract We investigate solvent effects in the hydrodeoxygenation of 4-propylguaiacol (4PG, 166 amu), a key lignin-derived monomer, over Ru/C catalyst by combined operando synchrotron photoelectron ...photoion coincidence (PEPICO) spectroscopy and molecular dynamics simulations. With and without isooctane co-feeding, ring-hydrogenated 2-methoxy-4-propylcyclohexanol (172 amu) is the first product, due to the favorable flat adsorption configuration of 4PG on the catalyst surface. In contrast, tetrahydrofuran (THF)—a polar aprotic solvent that is representative of those used for lignin solubilization and upgrading—strongly coordinates to the catalyst surface at the oxygen atom. This induces a local steric hindrance, blocking the flat adsorption of 4PG more effectively, as it needs more Ru sites than the tilted adsorption configuration revealed by molecular dynamics simulations. Therefore, THF suppresses benzene ring hydrogenation, favoring a demethoxylation route that yields 4-propylphenol (136 amu), followed by dehydroxylation to propylbenzene (120 amu). Solvent selection may provide new avenues for controlling catalytic selectivity.
1. Resource pulses, narrow periods of high resource availability, can elicit strong behavioural responses across diverse taxa. Mass‐flowering agricultural crops are an example of a resource pulse ...that insect pollinators exploit. However, the underlying mechanism behind changes in pollinator behaviour associated with mass‐flowering crops is still relatively unexplored.
2. The present study quantified the behavioural response of bumble bees, an important wild pollinator, to commercial cranberry bloom, an important mass‐flowering crop in Wisconsin, U.S.A. Over a 2‐year period, foraging trip duration was measured using radio frequency identification at 14 farms situated across landscape contexts, ranging from high to low natural area (woodland amount). Using transect surveys, floral resource abundance at a landscape scale was estimated.
3. It was found that bumble bees were highly sensitive to temporal changes in landscape‐level resource abundance associated with the onset of cranberry bloom, during which they decreased foraging trip duration by 22% and increased the number of foraging trips during bloom by 24% on average relative to the period before and after bloom. This phenomenon was consistent across colonies, individual bees, and landscape contexts, despite a higher abundance of flowers in low woodland landscapes. Bumble bee colonies growing in low‐ and high‐woodland landscapes exhibited a similar performance.
4. As mass‐flowering crops are probably a factor influencing bumble bee foraging behaviour in agricultural regions, investigations should continue into how variable resource landscapes, particularly those offering resource pulses, affect wild pollinators and the pollination services they provide.
Centuries of landscape changes associated with agriculture have dramatically reduced the amount and increased the temporal variability of the floral resources that support key pollinating insects ...such as bumble bees. Adapting to these novel resource conditions is important to ensure the persistence of bumble bee species. While several species appear to be in decline in modern agricultural landscapes, others have thrived, suggesting adaptation to exploit highly variable floral resources. Bombus impatiens, the common eastern bumble bee, is a prime example of such a species.
We designed an experiment to compare how free‐foraging colonies of B. impatiens performed adjacent to areas with either temporally continuous or variable (pulsed) patches of purple tansy (Phacelia tanacetifolia) plantings.
We found that colonies in Phacelia landscapes grew faster, had gained more mass, and produced more gynes than did colonies in reference landscapes with no Phacelia.
Comparing colony responses between pulsed and continuous flowering resources showed that total mass gain at the end of the experiment was greater with continuous flowering resources. In contrast, colony growth rate and total gyne production were comparable for colonies adjacent to Phacelia plantings that were continuous versus pulsed.
While low in statistical replication, given the scale of the experimental manipulation, our experiment shows that although B. impatiens colonies can exploit periods of resource discontinuity and gain mass, these continuously available floral resources appear important for colony growth and benefit gyne production.
Landscape supplementation of floral resources reveals that continuously available floral resources provide a greater benefit to bumble bee colony performance (colony mass, gyne production, foraging activity) relative to discontinuous resource availability.
