Understanding of bee nutritional ecology to optimize conservation strategies for bees

The application of ionomics and ecological stoichiometry benefits conservation biology with necessary ecological and evolutionary relevance, allowing unresolved problems to be addressed. The use of ionomics and ecological stoichiometry enables consideration that changes in the environmental nutritional supply affect the ecophysiology, behavior, health and fitness of individuals, influencing their ecological interactions and population functioning. The resulting knowledge can help promote better conservation and restoration strategies. Ultimately, ionomics and ecological stoichiometry facilitate improved forecasting and mitigation of the negative effects of current global change. Here, we present the theoretical background followed by the application of ionomics and ecological stoichiometry in biological conservation. We also propose avenues for future research. For example, larval and adult pollinating insects belong to different feeding guilds, and larvae rely on various stoichiometrically (im)balanced foods (showing herbivory, pollinivory, detritivory or even carnivory). Therefore, the ecology and diversity of pollinators may be shaped by the nutritional quality of larval food, which is required for physiological development into fully functional adults. Although a stoichiometric balance during larval development is crucial for pollinator health and fitness, pollinator conservation is focused on the nutritional needs of adults. Another example is atmospheric CO2 increases leading to nutrient dilution in plant tissues, aggravating nutritional imbalances in consumers and challenging Earth's herbivore populations. CO2-driven nutrient dilution may affect food webs, ecosystems and human wellbeing. However, our understanding of this phenomenon is minimal. These and other unresolved conservation biology problems may be studied and solved using ionomics and ecological stoichiometry.

To predict the quantity and quality of food available to pollinators in various landscapes over time, it is necessary to collect detailed data on the pollen, nectar, and sugar production per unit area and the flowering phenology of plants. Similar data are needed to estimate the contribution of plants to the functioning of food webs via the flow of energy and nutrients through the soil–plant-nectar/pollen-consumer pathway. Current knowledge on this topic is fragmented. This database is the first compilation of data on the various food resources produced by 1612 plant species belonging to 755 genera and 133 families, including crop plants and wild plants, annuals and perennials, animal- and wind-pollinated plants, and weeds and trees growing in different ecosystems under various environmental conditions. The dataset consists of 103 parameters related to the traits of plant species and geographical and environmental factors, allowing for precise calculations of the amounts of nectar, pollen and energy provided by plants and available to consumers in the considered flora or ecosystem on a daily basis throughout the year. These parameters, gathered by us and extracted from the available literature, describe pollen, nectar, and sugar production (where applicable, in mass, volume and concentration units), honey yield, the timing and duration of flowering, flower longevity, numbers of plants and flowers per unit area, weather conditions (temperature and precipitation), geographical location, landscape, and syntaxonomy. The data were obtained from various, mostly European, pedoclimatic zones, and the majority of the data were available for plant species and communities present in Central Europe, especially in Poland, where research on floral resources has a long tradition. These data are representative of the whole continent and may be used as a reference for plant communities occurring on continents other than Europe since the database allows the consideration of differences in the production of resources by a single plant species growing in different communities. This dataset provides a unique opportunity to test hypotheses related to the functioning of food webs, nutrient cycling, plant ecology, and pollinator ecology and conservation. The data are released under a CC-BY-NC-SA license, and this paper must be properly cited when using the database.

We investigated nutrition as a potential mechanism underlying the link between floral diversity/composition and wild bee performance. The health, resilience, and fitness of bees may be limited by a lack of nutritionally balanced larval food (pollen), influencing the entire population, even if adults are not limited nutritionally by the availability and quality of their food (mainly nectar). We hypothesized that the nutritional quality of bee larval food is indirectly connected to the species diversity of pollen provisions and is directly driven by the pollen species composition. Therefore, the accessibility of specific, nutritionally desirable key plant species for larvae might promote bee populations. Using a fully controlled feeding experiment, we simulated different pollen resources that could be available to bees in various environments, reflecting potential changes in floral species diversity and composition that could be caused by landscape changes. Suboptimal concentrations of certain nutrients in pollen produced by specific plant species resulted in reduced bee fitness. The negative effects were alleviated when scarce nutrients were added to these pollen diets. The scarcity of specific nutrients was associated with certain plant species but not with plant diversity. Thus, one of the mechanisms underlying the decreased fitness of wild bees in homogenous landscapes may be nutritional imbalance, i.e., the scarcity of specific nutrients associated with the presence of certain plant species and not with species diversity in pollen provisions eaten by larvae. Accordingly, we provide a conceptual representation of how the floral species composition and diversity can impact bee populations by affecting fitness-related life history traits. Additionally, we suggest that mixes of 'bee-friendly' plants used to improve the nutritional base for wild bees should be composed considering the local flora to supplement bees with vital nutrients that are scarce in the considered environment.

Wild bee populations are declining due to human activities, such as land use, which strongly affect the composition and diversity of available plants and food sources. The chemical composition of food (i.e. nutrition), in turn, determines health, resilience and fitness of bees. However, for pollinators, the term health is recent and subject to debate as is the interaction between nutrition and wild bee health. We define bee health as a multidimensional concept in a novel integrative framework linking bee biological traits (physiology, stoichiometry and disease) and environmental factors (floral diversity, nutritional landscapes). Linking information on tolerated nutritional niches and health in different bee species will allow us to better predict their distribution and responses to environmental change and thus support wild pollinator conservation.

1. Honey bees require minerals for a complete diet. However, minerals from flowers can be inadequate in concentration and composition. Therefore, honey bees may drink ‘dirty water’ from natural sources such as puddles. Some research has attempted to simulate this through honey bee bioassays, but to date, these have tested minerals individually, not as mixtures as would occur in nature. Here, for the first time, we use honey bees in bioassays in which a range of mineral mixtures are presented together in choice experiments. 2. Six minerals (NaCl, KCl, CaCl2, MgCl2, NH4Cl, and KH2PO4) were used in mixtures to simulate different mineral stoichiometries, which may occur in ‘dirty water’, such as puddles, from which honey bees often drink. Based on the honey bee mineral tolerance ranges from the literature, these mixtures were offered in aqueous solutions at low, medium, high, and mixed molar concentrations. Deionised water and sucrose were neutral and positive controls, respectively. Petri dishes were set up in containers in a laboratory. Twenty worker honey bees (Apis mellifera L.) were placed into each container and observed for drinking behaviour for 1 h. 3. Honey bees preferred the mixed molar treatment comprising a high Na:K ratio, a medium molarity of NaCl and a low molarity of the other minerals. This novel finding suggests that mixed mineral ‘dirty water’ should be investigated on a larger scale with multiple hives in the field and highlights the importance of stoichiometrically balanced honey bee diets.

Local and global changes affect which pollen varieties are available to bees in the environment. Therefore, wild bees cannot always access the optimally balanced diet required for their survival. Our feeding experiment showed that the nutritional quality of the pollen diet eaten by bee larvae is shaped not by pollen diversity but by a specific pollen species composition that results in specific nutrients being scarce or sufficient; this species composition influences bee survivability, development and mass. We proposed that the functioning of bee populations and communities may depend on the floral diversity of the local habitat, which determines whether a nutritionally balanced pollen diet obtained from specific species can be provided to bee larvae. Holistically considering wild bee fitness and health and the different characteristics of the food base at both the ecosystem and bee biology levels can provide new, important knowledge for conserving bees and their critical ecological roles.

Life histories of species may be shaped by nutritional limitations posed on populations. Yet, populations contain individuals that differ according to sex and life stage, each of which having different nutritional demands and experiencing specific limitations. We studied patterns of resource assimilation, allocation and excretion during the growth of the solitary bee Osmia bicornis (two sexes) under natural conditions. Adopting an ecological perspective, we assert that organisms ingest mutable organic molecules that are transformed during physiological processes and that the immutable atoms of the chemical elements composing these molecules may be allocated to specific functions, thereby influencing organismal fitness and life history. Therefore, using the framework of ecological stoichiometry, we investigated the multielemental (C, N, S, P, K, Na, Ca, Mg, Fe, Zn, Mn, Cu) compositions of six components of the bee elemental budget: food (pollen), eggs, pupae, adults, cocoons and excreta. The sexes differed fundamentally in the assimilation and allocation of acquired atoms, elemental phenotypes, and stoichiometric niches for all six components. Phosphorus, which supports larval growth, was allocated mainly (55-75%) to the cocoon after larval development was complete. Additionally, the majority (60-99%) of the Mn, Ca, Mg and Zn acquired during larval development was allocated to the cocoon, probably influencing bee fitness by conferring protection. We conclude that for holometabolous insects, considering only the chemical composition of the adult body within the context of nutritional ecology does not provide a complete picture. Low ratios of C to other nutrients, low N:P and high Na concentrations in excreta and cocoons may be important for local-scale nutrient cycling. Limited access to specific nutritional elements may hinder bee development in a sex-dependent manner, and N and P limitations, commonly considered elsewhere, may not play important roles in O. bicornis. Sexual dimorphism in nutritional limitations due to nutrient scarcity during the larval stage may influence bee population function and should be considered in bee conservation efforts.

Bee nutrition studies have focused on food quantity rather than quality, and on details of bee biology rather than on the functioning of bees in ecosystems. Ecological stoichiometry has been proposed for studies on bee nutritional ecology as an ecosystem-oriented approach complementary to traditional approaches. It uses atomic ratios of chemical elements in foods and organisms as metrics to ask ecological questions. However, information is needed on the fitness effects of nutritional mismatches between bee demand and the supply of specific elements in food. We performed the first laboratory feeding experiment on the wild bee Osmia bicornis, investigating the impact of Na, K, and Zn scarcity in larval food on fitness-related life history traits (mortality, cocoon development, and imago body mass). We showed that bee fitness is shaped by chemical element availability in larval food; this effect may be sex-specific, where Na might influence female body mass, while Zn influences male mortality and body mass, and the trade-off between K allocation in cocoons and adults may influence cocoon and body development. These results elucidate the nutritional mechanisms underlying the nutritional ecology, behavioral ecology, and population functioning of bees within the context of nutrient cycling in the food web.

1. Bee-friendly plants are defined by the quantity of food they produce and the visitation rates of adult insects foraging for nectar. However, it is pollen nutritional quality that enables proper larval development of bees, affecting their populations. Not all plants produce pollen that satisfies the nutritional requirements of bee larvae, and we lack an understanding of how different plant pollens impact bee nutritional demands. This study examined whether nutritionally desirable key plant species may promote wild bee larval development, which is essential if the population is to thrive. 2. The generalist solitary mason bee Osmia bicornis L. was used as a model species to examine differences between bee larva nutritional demand and host plant nutrient supply; an ecological stoichiometry framework was applied. The stoichiomet-ric ratios of 12 elements were investigated in bee bodies and cocoons (reflecting nutritional demand) and in the pollen supplied by the mother (nutritional supply; N = 15 × 2 sexes). Similarly, the stoichiometry of 62 pollen taxa, including native, alien, and garden plants and crops, was compared with the bee demand based on the literature. 3. Compared to males, females had higher demands for P, Cu and Zn and were supplied with pollen richer in these elements. Therefore, when collecting pollen for their progeny, Osmia provides daughters and sons with different pollen mixtures, reflecting sex-specific nutritional demand. 4. Bees may be limited by the availability of P, Na, Mn, Mg, K, Fe, Ca, Zn and Cu, with high taxonomic variability in their concentrations in pollen. Female fitness may be particularly related to a high proportion of P in the diet. 5. Access to key plant species that allow nutritionally balanced larval diets may be essential for bee development, whether food is gathered intentionally or randomly. Such plant species-and not only those rich in nectar and pollen-should be promoted in wild bee conservation efforts, including planting flower strips and hedgerows. Bee-friendly plants should not be defined and planted solely based on the quantities of food they produce and on the visitation rates of adult insects foraging for energy. K E Y W O R D S bee, conservation, ecology, food, health, nutrition, pollen, wild bee

Floral resource limitation connected with land degradation and habitat loss was identified as potential threat that cause pollinator decline and food resource quality may be the main limiting factor for bee populations. To better understand the nutritional constraints of growing and developing organisms, their colonies and populations, ecological stoichiometry was developed with reference to the elements that, if environmentally scarce, prevent the building of biologically important organic molecules. The least understood aspect of bee nutritional needs concerns stoichiometric balancing and the need for adequate amounts and ratios of nutritional elements in consumed food. I used the framework of ecological stoichiometry to study differences in the demand and supply of nutritional elements and stoichiometric balancing of an important pollinator’s diet: the mason bee Osmia bicornis L. O. bicornis larva is supplied with pollen by its mother. I used a field study to investigate concentrations and stoichiometric ratios of C, N, S, P, K, Na, Ca, Mg, Fe, Zn, Mn, and Cu in the bee production (body and cocoon) of both sexes and their pollen supply, i.e., the only food eaten during larval development. Females had a higher demand for and were supplied with pollen richer in P, Cu and Zn than males. Female fitness may be in particular related to a high P proportion and a low C:P ratio in their diet. Additionally, males had a higher demand for Na and a lower demand for K than females, but these elements were similarly concentrated in the pollen supply for both sexes. Comparison of nutritional demand and supply of the bee suggests that adult females while collecting pollen supply for their progeny, may favor key species that allow for a dietary stoichiometric balance. Moreover, they may provide their daughters and sons with a different mix of pollen that better fulfills sex-specific nutritional demands. Bee production, growth and development may be limited by the availability of P, Na, Mn, Mg, K, Fe Ca, Zn and Cu, i.e., elements that show high taxonomical concentration variabilities in pollen. Therefore, it is likely that the presence of key plant species in the flora, which produce nutritionally balanced pollen for bees, influence bee development and shape bee populations. Changes in bee habitat floral composition shape the available nutritional supply in the environment. In this context, the key plant species must be present in the flora to produce pollen that is nutritionally balanced for bees. Using literature data on the elemental composition of taxonomically different pollen, I suggested pollen species that either promotes or limits bee production, thereby influencing the bee populations. In conclusion, the quality of food sources for bees, not solely the quantity, should be considered in intervention strategies aimed at improving the nutritional base for bees and planting random plant species that offer pollen in large quantities is not a good practice.

The observed decline in wild bees may be connected to the decreasing diversity of flowering plants. Changes in floral composition shape nutrient availability in inhabited areas, and bee larvae need food rich in body-building nutrients to develop into adults. Adult food, mainly composed of energy-rich nectar, differs from larval food, mainly composed of pollen, and adult bees forage on different plant species for nectar and pollen. Defining bee-friendly plants based on the quantities of food produced, and on the visitation rates of adult pollinating insects leads to the planting of bee habitats with poor-quality food for larvae, which limits their growth and development, and negatively affects the population. Consequently, failing to understand the nutritional needs of wild bees may lead to unintended negative effects of conservation efforts. Ecological stoichiometry was developed to elucidate the nutritional constraints of organisms and their colonies, populations, and communities. Here, I discuss how applying ecological stoichiometry to the study of the nutritional ecology of wild bees would help fill the gaps in our understanding of bee biology. I present questions that should be answered in future studies to improve our knowledge of the nutritional ecology of wild bees, which could result in better conservation strategies.

The least understood aspects of the nutritional needs of bees are the elemental composition of pollen and the bees' need for a stoichiometrically balanced diet containing the required proportions of nutrients. Reduced plant diversity has been proposed as an indirect factor responsible for the pollinator crisis. We suggest stoichiometric mismatch resulting from a nutritionally unbalanced diet as a potential direct factor. The concentrations and stoichio-metric ratios of C, N, S, P, K, Na, Ca, Mg, Fe, Zn, Mn, and Cu were studied in the bodies of honeybees of various castes and sexes and in the nectar and pollen of various plant species. A literature review of the elemental composition of pollen was performed. We identified possible co-limitations of bee growth and development resulting mainly from the scarcity of Na, S, Cu, P and K, and possibly Zn and N, in pollen. Particular castes and sexes face specific limitations. Concentrations of potentially limiting elements in pollen revealed high taxo-nomic diversity. High floral diversity may be necessary to maintain populations of pollen eaters. Single-species crop plantations, even if these species are rich in nectar and pollen, might limit bee growth and development, not allowing for gathering nutrients in adequate proportions. However, particular plant species may play greater roles than others in balancing honeybee diets. Therefore, we suggest specific plant species that may (1) ensure optimal growth and production of individuals by producing pollen that is exceptionally well balanced stoichiometrically (e.g., clover) or (2) prevent growth and development of honey-bees by producing pollen that is extremely unbalanced for bees (e.g., sunflower). Since pollen is generally poor in Na, this element must be supplemented using " dirty water ". Nectar cannot supplement the diet with limiting elements. Stoichiometric mismatch should be considered in intervention strategies aimed at improving the nutritional base for bees.

The energy budget of organisms is a primary factor used to generate hypotheses in ecosystem ecology and evolutionary theory. Therefore, previous studies have focused on the energy costs and benefits of adaptations, the efficiency of energy acquisition and investment, and energy budget limitations. The maintenance of stoichiometric balance is equally important because inconsistency between the chemical composition of the consumer's tissues and that of its food sources strongly affects the major life-history traits of the consumer and may influence the consumer's fitness and shape plant–herbivore interactions. In this short review, the framework of ecological stoichiometry is introduced, focusing on plant–insect interactions in terrestrial ecosystems. The use of the trophic stoichiometric ratio (TSR) index is presented as a useful tool for indicating the chemical elements that are scarce in food and have the potential to limit the growth and development of herbivores, thereby influencing plant – herbivorous insect interactions. As an example , the elemental composition and stoichiometry of a pollen consumer (mason bee Osmia bicornis) and its preferred pollen are compared. The growth and development of O. bicornis may be colimited by the scarcity of K, Na, and N in pollen, whereas the development of the cocoon might be colimited by the scarcity of P, Mg, K, Na, Zn, Ca, and N. A literature review of the elemental composition of pollen shows high taxonomical variability in the concentrations of bee-limiting elements. The optimized collection of pollen species based on the elemental composition may represent a strategy used by bees to overcome stoichiometric mismatches, influencing their interactions with plants. It is concluded that the dependence of life-history traits on food stoichiometry should be considered when discussing life history evolution and plant–herbivore interactions. The TSR index may serve as a convenient and powerful tool in studies investigating plant-insect interactions.