Global growth and stability of agricultural yield decrease with pollinator dependence

Abstract

Human welfare depends on the amount and stability of agricultural production, as determined by crop yield and cultivated area. Yield increases asymptotically with the resources provided by farmers' inputs and environmentally sensitive ecosystem services. Declining yield growth with increased inputs prompts conversion of more land to cultivation, but at the risk of eroding ecosystem services. To explore the interdependence of agricultural production and its stability on ecosystem services, we present and test a general graphical model, based on Jensen's inequality, of yield-resource relations and consider implications for land conversion. For the case of animal pollination as a resource influencing crop yield, this model predicts that incomplete and variable pollen delivery reduces yield mean and stability (inverse of variability) more for crops with greater dependence on pollinators. Data collected by the Food and Agriculture Organization of the United Nations during 1961-2008 support these predictions. Specifically, crops with greater pollinator dependence had lower mean and stability in relative yield and yield growth, despite global yield increases for most crops. Lower yield growth was compensated by increased land cultivation to enhance production of pollinator-dependent crops. Area stability also decreased with pollinator dependence, as it correlated positively with yield stability among crops. These results reveal that pollen limitation hinders yield growth of pollinator-dependent crops, decreasing temporal stability of global agricultural production, while promoting compensatory land conversion to agriculture. Although we examined crop pollination, our model applies to other ecosystem services for which the benefits to human welfare decelerate as the maximum is approached.

Fig. 1.

General relations of yield (production area⁻¹) to environmental (black lines) and anthropogenic (gray lines) resource availability and the effects of (A) resource variability and (B) altered maximum yield (max).

Fig. 2.

General relations of relative yield (seeds ha⁻¹/maximum yield) to either total pollen receipt by stigmas (A) or pollen receipt caused by animal pollination (Pb: B and C). A and B show two examples of plants differing in pollinator dependence (i.e., percent reduction in seed production in the absence of biotic pollination). In A, the bars denote the total self- and abiotic pollination (Ps) that causes 25% (green) and 65% (blue) pollinator dependence, respectively. B illustrates both higher and more stable yield for a given range of variation in biotic pollination (black bar) if a plant has 25% pollinator dependence (green bar) than if it has 65% dependence (blue bar). C illustrates the variation in relative yield associated with the entire range of biotic pollination for crops in five pollinator-dependence classes.

Fig. 3.

Global trends in (A) mean (±SE) relative yield (detrended yield in year t/detrended maximum yield) and (B) mean annual yield growth (yield ratio for consecutive years: Y_t/Y_t-1) between 1961 and 2008 for 99 crops categorized by pollinator dependence. Dashed lines depict linear regressions based on individual crops.

Fig. 4.

Global trends in mean (±SE) temporal stability of (A) crop yield and (B) yield growth, as measured by the coefficients of annual variation (CV) of detrended data between 1961 and 2008 for 99 crops differing in pollinator dependence. Dashed lines depict linear regressions based on individual crops (note the log₁₀ scaling of both axes).

Fig. 5.

Global temporal trends in mean yield growth between 1961 and 2008 for 99 crops categorized by pollinator dependence.

Fig. 6.

Global trends in the mean (±SE) growth and temporal stability of crop area (A and D, respectively) and production (C and E, respectively) between 1961 and 2008 for 99 crops differing in pollinator dependence and the relation of the correlation between detrended yield and area growth to pollinator dependence during the same period (B). Dashed lines depict linear regressions based on individual crops (in A the six crops for which animal pollination is essential were excluded from the regression analysis). Note the log₁₀ scaling of both axes in D and E.