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Increasing atmospheric CO2 levels change the elemental composition in plants, altering their nutritional quality and affecting consumers and ecosystems. Ecological stoichiometry provides a framework for investigating how CO2-driven nutrient dilution in pollen affects bees by linking changes in pollen chemical element proportions to the nutritional needs of bees. We investigated the consequences of five years of Free Air CO2 Enrichment (FACE) in a mature oak-dominated temperate forest on the elemental composition of English oak (Quercus robur) pollen. We measured the concentrations and proportions of 12 elements (C, N, P, S, K, Na, Ca, Mg, Cu, Zn, Fe, and Mn) in Q. robur pollen-bearing flowers collected from the Birmingham Institute for Forest Research (BIFoR) FACE facility. An elevated CO2 (eCO2) level of 150 ppm above ambient significantly reduced the S, K, and Fe levels and altered the multi-element ratio, with different elements behaving differently. This shift in pollen multi-element composition may have subsequent cascading effects on higher trophic levels. To assess the impact on bees, we calculated the stoichiometric mismatch (a measure of the discrepancy between consumer needs and food quality) for two bee species, Osmia bicornis (red mason bee) and Apis mellifera (honey bee), that consume oak pollen in nature. We observed stoichiometric mismatches for P and S, in pollen under eCO2, which could negatively affect bees. We highlight the need for a comprehensive understanding of the changes in pollen multi-element stoichiometry under eCO2, which leads to nutrient limitations under climate change with consequences for bees. [ABSTRACT FROM AUTHOR]

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 CO 2 increases leading to nutrient dilution in plant tissues, aggravating nutritional imbalances in consumers and challenging Earth's herbivore populations. CO 2 -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. • Biogeochemical processes affect our biological conservation efforts. • Ecological stoichiometry and ionomics may be used in conservation biology. • This brings research closer to nature and offers tools for asking important questions. • We present examples of pollinator conservation and CO 2 driven nutrient dilution. [ABSTRACT FROM AUTHOR]

Simple Summary: Pollen comprises many organic substances (sugars, lipids, proteins, amino acids, vitamins, etc.), all of which are built from elements such as carbon, hydrogen, oxygen, nitrogen, phosphorus, sodium, potassium, zinc, and approximately twenty others. These special nutritional elements compose the cells, tissues, and bodies of all the life forms on our planet and are needed by bee larvae for healthy growth. However, not all plants produce pollen containing these elements in proportions needed specifically by bees, meaning that not all pollens are nutritionally balanced for bees. Moreover, the decrease in plant diversity is thought to be among the main causes of the dwindling numbers of pollinators worldwide. Currently, governments and societies are attempting to combat this pollinator decline by providing nutritionally balanced and diverse food plants to pollinators. Knowing which nutritional elements are crucial for the bee diet and understanding why are prerequisites for tailoring conservation efforts for this group of insects, which are substantially important for human nutrition and ecosystem functioning. Basic information obtained from feeding experiments is important for synergistically understanding how plant diversity within certain species that produce pollens with rich or scarce amounts of certain nutritional elements influences bee health and prosperity. 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. [ABSTRACT FROM AUTHOR]