Insect pollination enhances yield stability in two pollinator-dependent crops

Abstract

One of the most important challenges facing global agriculture is to ensure an adequate, stable food supply while conserving soil, water and biodiversity. The yield stability of pollinator-dependent crops, such as pear and apple, can be negatively affected by variability of the pollination service, which in turn can reduce mean yield. We explored how mean crop yield and yield variability were affected by pollinators and variability in their provision of the pollination service. Over four seasons we conducted a manipulative experiment in six pear and eight apple farms; fruit set (i.e., no. fruits/ no. flowers) was compared between flowers exposed to pollinators and those excluded from pollinators. We also recorded pollinator visitation rate to exposed flowers. We estimated the mean levels and spatial and temporal variability of both pollinator visitation and yield response by calculating the mean values per farm and the spatial (i.e., across trees within farm) and temporal (i.e., across seasons within farm) coefficients of variation (CV) for visitation rate and fruit set. Despite homogeneous irrigation and fertilization, we found strong variability in fruit set in both crops (pear spatial and temporal CV: 0.57 and 0.4, respectively; apple: 0.62 and 0.52). Pollinator exclusion reduced mean fruit set considerably in both crops (pears: a reduction of 50%, apples: 71–92%), and increased spatial and temporal variability (pears: 296% and 197% for spatial and temporal variability, respectively; apples: 385% and 329%). Visitation rates in pears were positively associated with mean fruit set and negatively related to its spatial CV. Also, in this crop we found a positive relation between the spatial CV of visitation rate and fruit set. However, there was no evidence that visitation rate in open-pollinated apple flowers affected either mean fruit set or its spatial or temporal variability. Apple trees received one order of magnitude more visits per flower than pear trees, suggesting that in this system the pollination service meets the pollination demand of the apple crop. Overall, our results highlight the importance of management practices that prioritize pollination service, thus ensuring a high, stable yield.