As global ecosystems face unprecedented environmental change from increases in temperature, CO2, and nitrogen levels, mutualist relationships such as those between plants and their pollinators have come under particular threat. Each of the three aforementioned variables have been demonstrated to alter healthy . plant/pollinator relationships either through phenological mismatch, reductions in pollinator diversity or the inability of pollinators to locate host plants. Hoover et al. (2012) breaks new ground in the study of environmental factors on pollinator mutualism by looking at the interactive effects of temperature, CO2 levels and N deposition upon the relationship between bumble bees and a species of pumpkin flower in a controlled experiment. The results demonstrated unique interactive impacts on flower morphology, phenology and nectar composition, with bees choosing nectars high in N levels which reduce worker bee longevity rates. Thus, Hoover et al. have demonstrated an important new possible threat to bees and their host plants with regards to climate change.
Hoover, S., Ladley, J., Shchepetkina, A., Tisch, M., Gieseg, S., Tylianakis, J., 2012 Warming, CO2, and nitrogen deposition interactively affect a plant-pollinator mutualism. Ecology Letters, 15. 227-234.
Hoover et al. designed an experiment to study the interactive impact of three major components of climate change on plant/pollinator mutualism. For a host plant, they chose the pumpkin because it depends strongly on bee pollination, are cultivated globally and have large, unisexual flowers capable of producing significant nectar for testing. Researchers grew the pumpkin plants under controlled conditions and created eight treatment combinations with either ambient or elevated levels of temperature, CO2 or N deposition based on average current of future predicted levels. All plants were given the same levels of light and water and the onset of flowering as well as physical attributes were recorded. Next, researchers extracted nectar from the test plants and studied the chemical composition of each sample. To test for bee reaction, Hoover et al. created synthetic nectars by adding sugar and amino acid standards to water and observed bee behavior, recording the amount of nectar consumed after each visit.
The results of the experiment suggest interactivity between temperature increase, CO2 levels and N deposition impact plant/pollinator relationships. Plants which received increased levels of N and temperature produced larger flowers and at a higher frequency than plants exposed to higher CO2 levels rather than temperature, which caused smaller flowers to bloom. N was observed to increase flower diameter while temperature caused a decrease. With regards to phenology, elevated N and temperature caused flowers to bloom earlier by an average of 15.8 days while increased CO2 was observed to delay blooming by about ten days. Interactions between the three factors also changed the sugar levels and chemical compositions of nectar in ways which, individually, they otherwise would not.
With regards to bee preference, researchers found a nonsignificant tendency for the species to prefer the nectar with increased levels of N. Bees also preferred the nectar with high N and elevated CO2 levels but not with high N and high temperature. Furthermore, bees also were drawn to the nectar which represented high levels of all three factors. While bees were drawn overall to nectar with higher levels of N, these nitrogen levels were shown to decrease bee longevity by an average of eight days. Higher CO2 levels also reduced bee longevity by an average of about two days. Finally, higher concentrations of sugar in the nectar brought about by N levels was also demonstrated to negatively impact bee longevity.
The results of this study demonstrate a significant threat to bee and host plant relationships from combinatorial affects of drivers of climate change. Different combinations of elevated temperature, CO2 and N levels were seen to impact the physical structure of plant flowers which determine attractiveness to pollinators. Furthermore, demonstrated phenological changes run the risk of causing a mismatch between bees and their host plants. A threat of phenological mismatch becomes more severe when paired with the fact that bees chose nectars with higher levels of N even though it was shown to reduce longevity. If bee life cycles are shortened, it reduces the window in which they can overlap with their host plants. Understanding how the interaction of climate change drivers impacts ecological relationships is thus very important.