Climate change is altering important interactions among component species (Tylianakis et al. 2008). In ecological communities, many wildflowers rely on arthropod pollinators for reproduction often producing food rewards (e.g., nectar) to facilitate this interaction (National Research Councel 2007). Environmental stress brought on by climate change impacts resource allocation in plants, reducing the quality of food rewards (Memmott et al. 2007). Additionally, pollinators may be pushed outside their physiological limits resulting in range shifts or earlier emergence times. Consequences of global climate change on plant-pollinator interactions are not well understood and disruptions at this interface may have far-reaching repercussions for ecosystems (Tylianakis et al. 2008; Hegland et al. 2009). We ask how climate stress moves through plant rewards to pollinators and back again.
We tested bumblbee thermoregulation under heat stress in two species: B. impatiens and B. vosnesenskii. Bees consumed sugar solutions ranging from 0% sugar (water) to 75% sugar before being subjected to heat stress. Thoracic and abdominal temperatures were compared. Next, we grew Brassica rapa plants under 9 climate regimes in growth chambers and measured floral rewards. We offer these plants to bees in an array, determine specific foraging preferences, then take these same plants through to seed germination.
Floral rewards, and thus flower attractiveness to pollinators is influenced by climate. We show how this initial effect cascades through pollinators, impacting thermoregulation and visitation rates, thereby limiting plant fecundity in the next generation.
B. rapa nectar reward was negatively influenced by increased temperature and decreased water availability (X2=78.57, p < 0.001; X2=13.78, p=0.001). Nectar volume decreased as temperature increased (X2=83.53, p<0.001), with no effect of water treatment (X2=3.38, p=0.18) with no TxW interactions.
In lab trials, heat transfer rate between thorax and abdomen (Td) increased with sugar concentration for both species (t=-3.45 and -2.36, p=0.006 and <0.001 respectively). Heat transfer to the abdomen was ~0.0045 oC per percent increase in sugar concentration.
On average, bees spent more time at B. rapa plants grown under 25°C, and limited access to water (TxW, F(2,120)=4.00, p=0.02) and returned to these plants more often, even after sampling others (TxW, F(2,120)=4.67. The number of seeds each plant later produced correlated not only to the number of visits (R2=0.33, p=<0.001) but also the length of visits (R2=0.24, p=<0.001).