Greater bee diversity is needed to maintain crop pollination services over time

Background/Question/Methods

The rapid loss of biodiversity worldwide has highlighted the need to better understand how biodiversity affects the functioning of real-world ecosystems. Experiments have found species that diversity is important to ecosystem function, but in natural communities, a few highly abundant species often provide the majority of function at any given time, suggesting that function could be maintained with relatively little biodiversity. However, biodiversity may nonetheless be critical to ecosystem function over longer time scales if different species provide function at different times. We tested this hypothesis using three datasets of blueberry and watermelon pollination by wild bee species at farms in New Jersey, Pennsylvania, and California, USA. We measured bee visits to crop flowers multiple times per year and for multiple years. We also measured per visit pollen deposition by each bee species, which we used to calculate the pollination provided by each species on each date. Finally, we examined how the number of bee species needed to maintain a threshold level of pollination changed with the time period examined, in terms of both the length of the growing season (during which bee species undergo phenological turnover) and the number of years (during which bee species undergo stochastic population fluctuations).

Results/Conclusions

Our datasets included 9,362 specimens from 146 bee species. We found that the importance of bee diversity increased with the time scale examined both within and across years. Two to three times as many bee species were needed to meet a fixed pollination threshold over an entire growing season, as compared to on a single day, and twice as many species were needed over six years compared to during a single year. Comparison to a null model further suggested that this increase was due to genuine turnover in species composition over time, rather than sampling effects. Our results suggest that wild pollinator diversity contributes to the stability of global food supplies by ensuring consistent pollination services over time despite fluctuations in the abundances of individual species. These results also indicate that snapshot studies of biodiversity and ecosystem function may underestimate the number of species needed to maintain consistent ecosystem function over longer time periods.