Showing papers by "University of Göttingen published in 2003"

Sea-level fluctuations during the last glacial cycle

Abstract: The last glacial cycle was characterized by substantial millennial-scale climate fluctuations1,2,3,4,5, but the extent of any associated changes in global sea level (or, equivalently, ice volume) remains elusive. Highstands of sea level can be reconstructed from dated fossil coral reef terraces6,7, and these data are complemented by a compilation of global sea-level estimates based on deep-sea oxygen isotope ratios at millennial-scale resolution8 or higher1. Records based on oxygen isotopes, however, contain uncertainties in the range of ±30 m, or ±1 °C in deep sea temperature9,10. Here we analyse oxygen isotope records from Red Sea sediment cores to reconstruct the history of water residence times in the Red Sea. We then use a hydraulic model of the water exchange between the Red Sea and the world ocean to derive the sill depth—and hence global sea level—over the past 470,000 years (470 kyr). Our reconstruction is accurate to within ±12 m, and gives a centennial-scale resolution from 70 to 25 kyr before present. We find that sea-level changes of up to 35 m, at rates of up to 2 cm yr-1, occurred, coincident with abrupt changes in climate.

Gene prediction with a hidden Markov model and a new intron submodel.

Abstract: Motivation: The problem of finding the genes in eukaryotic DNA sequences by computational methods is still not satisfactorily solved. Gene finding programs have achieved relatively high accuracy on short genomic sequences but do not perform well on longer sequences with an unknown number of genes in them. Here existing programs tend to predict many false exons. Results: We have developed a new program, AUGUSTUS, for the ab initio prediction of protein coding genes in eukaryotic genomes. The program is based on a Hidden Markov Model and integrates a number of known methods and submodels. It employs a new way of modeling intron lengths. We use a new donor splice site model, a new model for a short region directly upstream of the donor splice site model that takes the reading frame into account and apply a method that allows better GC-content dependent parameter estimation. AUGUSTUS predicts on longer sequences far more human and drosophila genes accurately than the ab initio gene prediction programs we compared it with, while at the same time being more specific. Availability: Aw eb interface for AUGUSTUS and the executable program are located at http://augustus.gobics. de. Supplementary Information: The datasets used for testing and training are available at http://augustus.gobics.de/

Pharmacological Modulation of Cortical Excitability Shifts Induced by Transcranial Direct Current Stimulation in Humans

Abstract: Transcranial direct current stimulation (tDCS) of the human motor cortex results in polarity-specific shifts of cortical excitability during and after stimulation. Anodal tDCS enhances and cathodal stimulation reduces excitability. Animal experiments have demonstrated that the effect of anodal tDCS is caused by neuronal depolarisation, while cathodal tDCS hyperpolarises cortical neurones. However, not much is known about the ion channels and receptors involved in these effects. Thus, the impact of the sodium channel blocker carbamazepine, the calcium channel blocker flunarizine and the NMDA receptor antagonist dextromethorphane on tDCS-elicited motor cortical excitability changes of healthy human subjects were tested. tDCS-protocols inducing excitability alterations (1) only during tDCS and (2) eliciting long-lasting after-effects were applied after drug administration. Carbamazepine selectively eliminated the excitability enhancement induced by anodal stimulation during and after tDCS. Flunarizine resulted in similar changes. Antagonising NMDA receptors did not alter current-generated excitability changes during a short stimulation, which elicits no after-effects, but prevented the induction of long-lasting after-effects independent of their direction. These results suggest that, like in other animals, cortical excitability shifts induced during tDCS in humans also depend on membrane polarisation, thus modulating the conductance of sodium and calcium channels. Moreover, they suggest that the after-effects may be NMDA receptor dependent. Since NMDA receptors are involved in neuroplastic changes, the results suggest a possible application of tDCS in the modulation or induction of these processes in a clinical setting. The selective elimination of tDCS-driven excitability enhancements by carbamazepine proposes a role for this drug in focussing the effects of cathodal tDCS, which may have important future clinical applications.

Facilitation of Implicit Motor Learning by Weak Transcranial Direct Current Stimulation of the Primary Motor Cortex in the Human

Abstract: Transcranially applied weak direct currents are capable of modulating motor cortical excitability in the human. Anodal stimulation enhances excitability, cathodal stimulation diminishes it. Cortical excitability changes accompany motor learning. Here we show that weak direct currents are capable of improving implicit motor learning in the human. During performance of a serial reaction time task, the primary motor cortex, premotor, or prefrontal cortices were stimulated contralaterally to the performing hand. Anodal stimulation of the primary motor cortex resulted in increased performance, whereas stimulation of the remaining cortices had no effect. We conclude that the primary motor cortex is involved in the acquisition and early consolidation phase of implicit motor learning.

Rapid Induction of Distinct Stress Responses after the Release of Singlet Oxygen in Arabidopsis

Abstract: The conditional fluorescent (flu) mutant of Arabidopsis accumulates the photosensitizer protochlorophyllide in the dark. After a dark-to-light shift, the generation of singlet oxygen, a nonradical reactive oxygen species, starts within the first minute of illumination and was shown to be confined to plastids. Immediately after the shift, plants stopped growing and developed necrotic lesions. These early stress responses of the flu mutant do not seem to result merely from physicochemical damage. Peroxidation of chloroplast membrane lipids in these plants started rapidly and led to the transient and selective accumulation of a stereospecific and regiospecific isomer of hydroxyoctadecatrieonic acid, free (13S)-HOTE, that could be attributed almost exclusively to the enzymatic oxidation of linolenic acid. Within the first 15 min of reillumination, distinct sets of genes were activated that were different from those induced by superoxide/hydrogen peroxide. Collectively, these results demonstrate that singlet oxygen does not act primarily as a toxin but rather as a signal that activates several stress-response pathways. Its biological activity in Arabidopsis exhibits a high degree of specificity that seems to be derived from the chemical identity of this reactive oxygen species and/or the intracellular location at which it is generated.

Fruit set of highland coffee increases with the diversity of pollinating bees.

Abstract: The worldwide decline of pollinators may negatively affect the fruit set of wild and cultivated plants Here, we show that fruit set of the self-fertilizing highland coffee (Coffea arabica) is highly variable and related to bee pollination In a comparison of 24 agroforestry systems in Indonesia, the fruit set of coffee could be predicted by the number of flower-visiting bee species, and it ranged from ca 60% (three species) to 90% (20 species) Diversity, not abundance, explained variation in fruit set, so the collective role of a species-rich bee community was important for pollination success Additional experiments showed that single flower visits from rare solitary species led to higher fruit set than with abundant social species Pollinator diversity was affected by two habitat parameters indicating guild-specific nesting requirements: the diversity of social bees decreased with forest distance, whereas the diversity of solitary bees increased with light intensity of the agroforestry systems These results give empirical evidence for a positive relationship between ecosystem functions such as pollination and biodiversity Conservation of rainforest adjacent to adequately managed agroforestry systems could improve the yields of farmers

α-Neurexins couple Ca 2+ channels to synaptic vesicle exocytosis

Abstract: Synapses are specialized intercellular junctions in which cell adhesion molecules connect the presynaptic machinery for neurotransmitter release to the postsynaptic machinery for receptor signalling. Neurotransmitter release requires the presynaptic co-assembly of Ca2+ channels with the secretory apparatus, but little is known about how synaptic components are organized. Alpha-neurexins, a family of >1,000 presynaptic cell-surface proteins encoded by three genes, link the pre- and postsynaptic compartments of synapses by binding extracellularly to postsynaptic cell adhesion molecules and intracellularly to presynaptic PDZ domain proteins. Using triple-knockout mice, we show that alpha-neurexins are not required for synapse formation, but are essential for Ca2+-triggered neurotransmitter release. Neurotransmitter release is impaired because synaptic Ca2+ channel function is markedly reduced, although the number of cell-surface Ca2+ channels appears normal. These data suggest that alpha-neurexins organize presynaptic terminals by functionally coupling Ca2+ channels to the presynaptic machinery.

Mass flowering crops enhance pollinator densities at a landscape scale

Abstract: To counteract the decline of pollinators in Europe, conservation strategies traditionally focus on enhancing the local availability of semi-natural habitats, as supported by the European Union’s Common Agriculture Policy. In contrast, we show that densities of bumblebees, an important pollinator group in agroecosystems, were not determined by the proportion of semi-natural habitats in agricultural landscapes. Instead, bumblebee densities were positively related to the availability of highly rewarding mass flowering crops (i.e. oilseed rape) in the landscape. In addition, mass flowering crops were only effective determinants of bumblebee densities when grown extensively at the landscape scale, but not at smaller local scales. Therefore, future conservation measures should consider the importance of mass flowering crops and the need for management schemes at landscape level to sustain vital pollination services in agroecosystems.

Modulation of cortical excitability by weak direct current stimulation--technical, safety and functional aspects.

Abstract: Publisher Summary Achieving short- or even long-term neuroplastic functional modifications of cortical networks through the modulation of activity and excitability of neuronal ensembles has been the focus of many research activities in the last decades. The application of weak direct currents has been shown to elicit cortical excitability and activity shifts during, and after the end of stimulation in animals and humans, and thus could evolve as a promising technique in this field of research. In animals, intracortical or epidural electrodes have been used for direct current (DC) stimulation. Weak direct currents can be applied to humans non-invasively, transcranially and painlessly to induce focal, prolonged, but yet reversible shifts of cortical excitability, the duration and direction of which depend on stimulation duration and polarity. This chapter provides an overview of the basic and functional effects of weak direct current stimulation in animals and in humans. The chapter discusses the technical considerations and summarizes the available safety criteria that are expected to prevent harmful or unwanted effects of the stimulation.

The Generally covariant locality principle: A New paradigm for local quantum field theory

Abstract: A new approach to the model-independent description of quantum field theories will be introduced in the present work. The main feature of this new approach is to incorporate in a local sense the principle of general covariance of general relativity, thus giving rise to the concept of a locally covariant quantum field theory. Such locally covariant quantum field theories will be described mathematically in terms of covariant functors between the categories, on one side, of globally hyperbolic spacetimes with isometric embeddings as morphisms and, on the other side, of *-algebras with unital injective *-monomorphisms as morphisms. Moreover, locally covariant quantum fields can be described in this framework as natural transformations between certain functors. The usual Haag-Kastler framework of nets of operator-algebras over a fixed spacetime background-manifold, together with covariant automorphic actions of the isometry-group of the background spacetime, can be re-gained from this new approach as a special case. Examples of this new approach are also outlined. In case that a locally covariant quantum field theory obeys the time-slice axiom, one can naturally associate to it certain automorphic actions, called ``relative Cauchy-evolutions'', which describe the dynamical reaction of the quantum field theory to a local change of spacetime background metrics. The functional derivative of a relative Cauchy-evolution with respect to the spacetime metric is found to be a divergence-free quantity which has, as will be demonstrated in an example, the significance of an energy-momentum tensor (up to addition of scalar functions) for the locally covariant quantum field theory. Furthermore, we discuss the functorial properties of state spaces of locally covariant quantum field theories that entail the validity of the principle of local definiteness.

The δC Isoform of CaMKII Is Activated in Cardiac Hypertrophy and Induces Dilated Cardiomyopathy and Heart Failure

Abstract: Recent studies have demonstrated that transgenic (TG) expression of either Ca 2+ /calmodulin-dependent protein kinase IV (CaMKIV) or CaMKIIδ B , both of which localize to the nucleus, induces cardiac hypertrophy. However, CaMKIV is not present in heart, and cardiomyocytes express not only the nuclear CaMKIIδ B but also a cytoplasmic isoform, CaMKIIδ C . In the present study, we demonstrate that expression of the δ C isoform of CaMKII is selectively increased and its phosphorylation elevated as early as 2 days and continuously for up to 7 days after pressure overload. To determine whether enhanced activity of this cytoplasmic δ C isoform of CaMKII can lead to phosphorylation of Ca 2+ regulatory proteins and induce hypertrophy, we generated TG mice that expressed the δ C isoform of CaMKII. Immunocytochemical staining demonstrated that the expressed transgene is confined to the cytoplasm of cardiomyocytes isolated from these mice. These mice develop a dilated cardiomyopathy with up to a 65% decrease in fractional shortening and die prematurely. Isolated myocytes are enlarged and exhibit reduced contractility and altered Ca 2+ handling. Phosphorylation of the ryanodine receptor (RyR) at a CaMKII site is increased even before development of heart failure, and CaMKII is found associated with the RyR in immunoprecipitates from the CaMKII TG mice. Phosphorylation of phospholamban is also increased specifically at the CaMKII but not at the PKA phosphorylation site. These findings are the first to demonstrate that CaMKIIδ C can mediate phosphorylation of Ca 2+ regulatory proteins in vivo and provide evidence for the involvement of CaMKIIδ C activation in the pathogenesis of dilated cardiomyopathy and heart failure.

Melatonin: a hormone, a tissue factor, an autocoid, a paracoid, and an antioxidant vitamin

Abstract: Melatonin, a derivative of an essential amino acid, tryptophan, was first identified in bovine pineal tissue and subsequently it has been portrayed exclusively as a hormone. Recently accumulated evidence has challenged this concept. Melatonin is present in the earliest life forms and is found in all organisms including bacteria, algae, fungi, plants, insects, and vertebrates including humans. Several characteristics of melatonin distinguish it from a classic hormone such as its direct, non-receptor-mediated free radical scavenging activity. As melatonin is also ingested in foodstuffs such as vegetables, fruits, rice, wheat and herbal medicines, from the nutritional point of view, melatonin can also be classified as a vitamin. It seems likely that melatonin initially evolved as an antioxidant, becoming a vitamin in the food chain, and in multicellular organisms, where it is produced, it has acquired autocoid, paracoid and hormonal properties.

Chelating agents in root canal treatment: mode of action and indications for their use.

Abstract: Chelating agents were introduced into endodontics as an aid for the preparation of narrow and calcified root canals in 1957 by Nygaard-Ostby. A liquid solution of ethylenediaminetetraacetic acid (EDTA) was thought to chemically soften the root canal dentine and dissolve the smear layer, as well as to increase dentine permeability. Although the efficacy of EDTA preparations in softening root dentine has been debated, chelator preparations have regained popularity recently. Almost all manufacturers of nickel-titanium instruments recommend their use as a lubricant during rotary root canal preparation. Additionally, a final irrigation of the root canal with 15-17% EDTA solutions to dissolve the smear layer is recommended in many textbooks. This paper reviews the relevant literature on chelating agents, presents an overview of the chemical and pharmacological properties of EDTA preparations and makes recommendations for their clinical use.

The Effectiveness of Psychodynamic Therapy and Cognitive Behavior Therapy in the Treatment of Personality Disorders: A Meta-Analysis

Abstract: OBJECTIVE: The authors conducted a meta-analysis to address the effectiveness of psychodynamic therapy and cognitive behavior therapy in the treatment of personality disorders. METHOD: Studies of psychodynamic therapy and cognitive behavior therapy that were published between 1974 and 2001 were collected. Only studies that 1) used standardized methods to diagnose personality disorders, 2) applied reliable and valid instruments for the assessment of outcome, and 3) reported data that allowed calculation of within-group effect sizes or assessment of personality disorder recovery rates were included. Fourteen studies of psychodynamic therapy and 11 studies of cognitive behavior therapy were included. RESULTS: Psychodynamic therapy yielded a large overall effect size (1.46), with effect sizes of 1.08 found for self-report measures and 1.79 for observer-rated measures. For cognitive behavior therapy, the corresponding values were 1.00, 1.20, and 0.87. For more specific measures of personality disorder patholog...

Effects of landscape context on herbivory and parasitism at different spatial scales

Abstract: Local community structure and interactions have been shown to depend partly on landscape context. In this paper we tested the hypothesis that the spatial scale experienced by an organism depends on its trophic level. We analyzed plant-herbivore and herbivore-parasitoid interactions in 15 agricultural landscapes differing in structural complexity using the rape pollen beetle (Meligethes aeneus), an important pest on oilseed rape (Brassica napus), and its parasitoids. In the very center of each landscape a patch of potted rape plants was placed in a grassy field margin strip for standardized measurement. Percent non-crop area of landscapes was negatively related to plant damage caused by herbivory and positively to the herbivores’ larval mortality resulting from parasitism. In a geographic scale analysis, we quantified the structure of the 15 landscapes for eight circular sectors ranging from 0.5 to 6 km diameter. Correlations between parasitism and non-crop areas as well as between herbivory and non-crop area were strongest at a scale of 1.5 km, thereby not supporting the view that higher trophic levels experience the world at a larger spatial scale. However, the predictive power of non-crop area changed only slightly for herbivory, but greatly with respect to parasitism as scales from 0.5 to 1.5 km and from 1.5 to 6 km diameter increased. Furthermore, the effect of non-crop area tended to be stronger in parasitism than herbivory suggesting a greater effect of changes in landscape context on parasitoids. This is in support of the general idea that higher trophic levels should be more susceptible to disturbance.

Natural Product Hybrids as New Leads for Drug Discovery

Abstract: Natural products play an important role in the development of drugs, especially for the treatment of infections and cancer, as well as immunosuppressive compounds. However, the number of natural products is limited, whereas millions of hybrids as combinations of parts of different natural products can be prepared. This new approach seems to be very promising in the development of leads for both medicinal and agrochemical applications, as the biological activity of several new hybrids exceeds that of the parent compounds. The advantage of this concept over a combinatorial chemistry approach is the high diversity and the inherent biological activity of the hybrids.

Spectral and photometric evolution of young stellar populations: The impact of gaseous emission at various metallicities

Abstract: We include gaseous continuum and line emission into our galev models for the spectral and photometric evolution of Simple Stellar Populations (SSPs) for various metallicities in the range 0:02 Z=Z 2:5. This allows to extend them to significantly younger ages than before. They now cover the age range from 4 Myr all through 14 Gyr. We point out the very important contributions of gaseous emission to broad band fluxes and their strong metallicity dependence during very early evolutionary stages of star clusters, galaxies or subgalactic fragments with vigorous ongoing star formation. Emission- line contributions are commonly seen in these actively star-forming regions. Models without gaseous emission cannot explain their observed colors at all, or lead to wrong age estimates. We use up-to-date Lyman continuum (=Lyc) emission rates and decided to use recent empirical determinations of emission line ratios relative to H for subsolar metallicities. We justify this approach for all situations where no or not enough spectral information is available to determine all the parameters required by photoionization models. The eects of gaseous line and continuum emission on broad band fluxes are shown for dierent metallicities and as a function of age. In addition to the many filter systems already included in our earlier models, we here also include the HST NICMOS and Advanced Camera for Surveys (=ACS) filter systems.

Loss and recovery of wings in stick insects

Abstract: The evolution of wings was the central adaptation allowing insects to escape predators, exploit scattered resources, and disperse into new niches, resulting in radiations into vast numbers of species1. Despite the presumed evolutionary advantages associated with full-sized wings (macroptery), nearly all pterygote (winged) orders have many partially winged (brachypterous) or wingless (apterous) lineages, and some entire orders are secondarily wingless (for example, fleas, lice, grylloblattids and mantophasmatids), with about 5% of extant pterygote species being flightless2,3. Thousands of independent transitions from a winged form to winglessness have occurred during the course of insect evolution; however, an evolutionary reversal from a flightless to a volant form has never been demonstrated clearly for any pterygote lineage. Such a reversal is considered highly unlikely because complex interactions between nerves, muscles, sclerites and wing foils are required to accommodate flight4. Here we show that stick insects (order Phasmatodea) diversified as wingless insects and that wings were derived secondarily, perhaps on many occasions. These results suggest that wing developmental pathways are conserved in wingless phasmids, and that ‘re-evolution’ of wings has had an unrecognized role in insect diversification.

Early diagnosis and follow-up of aortitis with [(18)F]FDG PET and MRI.

Abstract: The aim of this prospective study was to compare fluorine-18 fluorodeoxyglucose ([(18)F]FDG) positron emission tomography (PET) with magnetic resonance imaging (MRI) in patients with early aortitis, at the time of initial diagnosis and during immunosuppressive therapy. The study population consisted of 15 patients (nine females and six males; median age 62 years, range 26-76 years) who presented with fever of unknown origin or an elevated erythrocyte sedimentation rate or elevated C-reactive protein and who showed pathological aortic [(18)F]FDG uptake. Fourteen of these patients had features of early giant cell arteritis (GCA), while one had features of early Takayasu arteritis. During follow-up, seven PET scans were performed in six patients with GCA 4-30 months (median 19 months) after starting immunosuppressive medication. The results of [(18)F]FDG imaging were compared with the results of MRI at initial evaluation and during follow-up and with the clinical findings. At baseline, abnormal [(18)F]FDG uptake was present in 59/104 (56%) of the vascular regions studied in 15 patients. Seven follow-up PET studies were performed in six patients. Of 30 regions with initial pathological uptake in these patients, 24 (80%) showed normalisation of uptake during follow-up. Normalisation of [(18)F]FDG uptake correlated with clinical improvement and with normalisation of the laboratory findings. All except one of the patients with positive aortic [(18)F]FDG uptake were investigated with MRI and MRA. Thirteen of these 14 patients showed inflammation in at least one vascular region. Of 76 vascular regions studied, 41 (53%) showed vasculitis on MRI. Of 76 vascular regions studied with both PET and MRI, 47 were concordantly positive or negative on both modalities, 11 were positive on MRI only and 18 were positive on PET only. MRI was performed during follow-up in six patients: of 17 regions with inflammatory changes, 15 regions remained unchanged and two showed improvement. Whole-body [(18)F]FDG PET is valuable in the primary diagnosis of early aortitis. The results of [(18)F]FDG PET and MRI in the diagnosis of aortitis in this study were comparable, but FDG imaging identified more vascular regions involved in the inflammatory process than did MRI. In a limited number of patients [(18)F]FDG PET was more reliable than MRI in monitoring disease activity during immunosuppressive therapy.

CaV1.3 Channels Are Essential for Development and Presynaptic Activity of Cochlear Inner Hair Cells

Abstract: Cochlear inner hair cells (IHCs) release neurotransmitter onto afferent auditory nerve fibers in response to sound stimulation. During early development, afferent synaptic transmission is triggered by spontaneous Ca2+ spikes of IHCs, which are under efferent cholinergic control. Around the onset of hearing, large-conductance Ca2+-activated K+ channels are acquired, and Ca2+ spikes as well as the cholinergic innervation are lost. Here, we performed patch-clamp measurements in IHCs of mice lacking the CaV1.3 channel (CaV1.3-/-) to investigate the role of this prevailing voltage-gated Ca2+ channel in IHC development and synaptic function. The small Ca2+ current remaining in IHCs from 3-week-old CaV1.3-/- mice was mainly mediated by L-type Ca2+ channels, because it was sensitive to dihydropyridines but resistant to inhibitors of non-L-type Ca2+ channels such as ω-conotoxins GVIA and MVIIC and SNX-482. Depolarization induced only marginal exocytosis in CaV1.3-/- IHC, which was solely mediated by L-type Ca2+ channels, whereas robust exocytic responses were elicited by photolysis of caged Ca2+. Secretion triggered by short depolarizations was reduced proportionally to the Ca2+ current, suggesting that the coupling of the remaining channels to exocytosis was unchanged. CaV1.3-/- IHCs lacked the Ca2+ action potentials and displayed a complex developmental failure. Most strikingly, we observed a continued presence of efferent cholinergic synaptic transmission and a lack of functional large-conductance Ca2+-activated K+ channels up to 4 weeks after birth. We conclude that CaV1.3 channels are essential for normal hair cell development and synaptic transmission.

Honeybee foraging in differentially structured landscapes

Abstract: Honeybees communicate the distance and location of resource patches by bee dances, but this spatial information has rarely been used to study their foraging ecology. We analysed, for the first time to the best of the authors' knowledge, foraging distances and dance activities of honeybees in relation to landscape structure, season and colony using a replicated experimental approach on a landscape scale. We compared three structurally simple landscapes characterized by a high proportion of arable land and large patches, with three complex landscapes with a high proportion of semi-natural perennial habitats and low mean patch size. Four observation hives were placed in the centre of the landscapes and switched at regular intervals between the six landscapes from the beginning of May to the end of July. A total of 1137 bee dances were observed and decoded. Overall mean foraging distance was 1526.1 +/- 37.2 m, the median 1181.5 m and range 62.1-10037.1 m. Mean foraging distances of all bees and foraging distances of nectar-collecting bees did not significantly differ between simple and complex landscapes, but varied between month and colonies. Foraging distances of pollen-collecting bees were significantly larger in simple (1743 +/- 95.6 m) than in complex landscapes (1543.4 +/- 71 m) and highest in June when resources were scarce. Dancing activity, i.e. the number of observed bee dances per unit time, was significantly higher in complex than in simple landscapes, presumably because of larger spatial and temporal variability of resource patches in complex landscapes. The results facilitate an understanding of how human landscape modification may change the evolutionary significance of bee dances and ecological interactions, such as pollination and competition between honeybees and other bee species.