Biodiversity Protects Plant-Pollinator Phenological Synchrony from Climate Change

by Lia Metzger

Biodiversity has been linked to the protection and sustenance of ecosystems against the loss of individual species. Studies have found that climate change, a contributor to the loss of species, has caused significant changes in phenology, mostly in species active in the spring. The biodiversity insurance hypothesis has never been expanded to include phenological synchrony as a possible buffer against the loss of individual species due to climate change. Bartomeus et al. (2013) investigated the phenological changes of wild bee species and of commercial apple crops over 46 years to find if bees and apples had phenological synchrony and if this was related to the richness of pollinator species. Using a contemporary data set, the authors picked pollinators that most frequently visited apple and tested for their phenological complimentarity. Bee and apple data were compared over time to find phenological mismatch and the rate of phenological change for different species with respect to apple bloom. Phenological synchrony was then tested against wild bee biodiversity. Phenological synchrony was found to increase with increasing biodiversity of the bee species and stabilize over time even though the rate of phenological shifting differed between species.

In order to test for the effects of climate change on the phenological synchrony of pollinators and apple, Bartomeus et al. compared independent data of commercial apple and wild bee species from New York, USA over 46 years. Apple was used because it blooms early in the spring and plants with early blooms have been found to have the most phenological shifts due to climate change’s increasing temperatures. Twenty-six wild bee species were chosen to study by sampling the pollinators visiting commercial apple crops in New York between 2009 and 2011. Rare species and managed species were eliminated from the study to assure that all species are significant contributors and that phenological shifts were not due to managed reproduction. Geographical limits on the longitude and corrections on the effect of latitude were conducted prior to data analysis to account for geographical differences between apple orchards.

Statistical analysis was performed to find the phenological complimentarity of pollinators and plants from the present-day data. To measure the rate of phenological change for bee species and apple, the slope of the peak bloom was compared against the year. Phenological asynchrony was determined by computing the difference between the date of bee specimen collection and the date of peak apple bloom for that year. Any bees that could not have interacted with apple because their active time fell outside of the peak blooming period for apple were left out of the analysis. Additionally, the authors created a simulation analysis to investigate the effects of pollinator species richness on plant-pollinator phenological asynchrony and the stability of the asynchrony over time (46 years). The 26 bee species were randomly sampled to created communities with different species richness levels and for each richness level, a regression analysis of phenological asynchrony of the species against the year was conducted. Slopes closest to zero from the regression analysis indicated stability of pollinator and apple bloom asynchrony over time.

Bartomeus et al. discovered that there was phenological complimentarity between bee species from 2009 to 2011, so the data for different species could be compiled to compare phenological changes between wild bee species and apple. Both apple and wild bee species advanced the bloom peak and active periods, and mean April temperature also increased. There was no change in the degree of asynchrony over time, which suggests that the bees and apple had a stable level of phenological synchrony. While the mean phenological synchrony was very high (slopes were close to zero), different bee species had different phenological drifts over time.

Overall, however, the differential rates of phenological drifts evened out to stabilize the phenological synchrony. From the simulation of richness of bee species, the authors found that the higher the level of richness of bee species, the higher the baseline phenological synchrony and the more stable phenological asynchrony. These results indicate that even with the effects of climate change on the phenologies of apple and bee species, the plant-pollinator phenological synchrony for activity remains stabile. Additionally, increased biodiversity of pollinators has the effect of maintaining the pollinator-plant relationships by increasing phenological synchrony and stabilizing it overtime.

Bartomeus, I., Park, M., Gibbs, J., Danforth, B., Lakso, A., Winfree, R., 2013. Biodiversity ensures plant–pollinator phenological synchrony against climate change. Ecology Letters 16, 1331−1338.

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