Rising atmospheric CO₂is reducing the protein concentration of a floral pollen source essential for North American bees

Abstract

At present, there is substantive evidence that the nutritional content of agriculturally important food crops will decrease in response to rising levels of atmospheric carbon dioxide, C a . However, whether C a -induced declines in nutritional quality are also occurring for pollinator food sources is unknown. Flowering late in the season, goldenrod (Solidago spp.) pollen is a widely available autumnal food source commonly acknowledged by apiarists to be essential to native bee (e.g. Bombus spp.) and honeybee (Apis mellifera) health and winter survival. Using floral collections obtained from the Smithsonian Natural History Museum, we quantified C a -induced temporal changes in pollen protein concentration of Canada goldenrod (Solidago canadensis), the most widespread Solidago taxon, from hundreds of samples collected throughout the USA and southern Canada over the period 1842-2014 (i.e. a C a from approx. 280 to 398 ppm). In addition, we conducted a 2 year in situ trial of S. canadensis populations grown along a continuous C a gradient from approximately 280 to 500 ppm. The historical data indicated a strong significant correlation between recent increases in C a and reductions in pollen protein concentration (r 2 ¼ 0.81). Experimental data confirmed this decrease in pollen protein concentration, and indicated that it would be ongoing as C a continues to rise in the near term, i.e. to 500 ppm (r 2 ¼ 0.88). While additional data are needed to quantify the subsequent effects of reduced protein concentration for Canada goldenrod on bee health and population stability, these results are the first to indicate that increasing C a can reduce protein content of a floral pollen source widely used by North American bees.

Support for the reproductive ground plan hypothesis in a solitary bee: links between sucrose response and reproductive status

“…Bees are facing global declines owing to myriad factors, including reduced availability of high-quality forage [10][11][12]. Furthermore, modern agricultural practices, such as irrigation and pesticide use, have been demonstrated to alter the nutritional composition of floral nectar and pollen, and subsequently influence bee preferences and visitation rates [54][55][56][57].…”

The unrealized potential of herbaria for global change biology

“…In addition, flowers and leaves of herbarium specimens can contain signatures of interactions, such as the accumulation of defensive compounds induced by insect herbivores (Zangerl and Berenbaum ) and pollen (Ziska et al. ).…”

“…In one of the few herbarium studies to explore how pollution affects species interactions, Ziska et al. () analyzed pollen preserved in herbarium specimens and demonstrated that elevated CO 2 concentrations reduced pollen protein concentrations, altering nutrient availability for pollinators, with possible fitness consequences for both pollinators and plants. In a separate study, Zangerl and Berenbaum () provided support for the natural enemy release hypothesis of plant invasion (Keane and Crawley , Mitchell and Power ) by measuring exotic plant defensive compounds before and after introduction of their co‐evolved herbivores.…”

Examining plant physiological responses to climate change through an evolutionary lens

“…Elevated [CO 2 ] alters the nutritional quality of nectar rewards through direct effects on photosynthesis and sugar production (Watanabe et al, 2014). Increases in [CO 2 ] over the past 170 years also reduced pollen protein concentration in Solidago canadensis (Ziska et al, 2016). Such changes in either nectar or pollen rewards could adversely affect pollinators and the strength of pollinator-mediated selection on plant traits.…”