Landscape diversity moderates the effects of bee visitation frequency to flowers on crop production.
Abstract
Reductions in natural habitat are implicated in declining honeybee Apis mellifera L. and wild bee populations, thereby threatening crop production. This concern has stimulated interest in identifying landscape-level impacts on bee-mediated pollination services, but previous studies have only inferred connections between landscape, bees and yield through generalized linear regressions. We examined landscape impacts on bee-mediated crop yield using both a traditional linear regression approach and conditional process modelling, which combined landscape features, bee visits to crop flowers, and the interactions between landscape and bee visits to flowers into a single model predicting crop yield. We used the pumpkin Cucurbita pepo L. system in New York State and recorded bees visiting pumpkin flowers in 2011 and 2012. Landscape diversity and percentage of semi-natural grassland around each pumpkin field were calculated. Results from the traditional approach indicated that landscape diversity, percentage of grassland in the landscape, bumblebee Bombus impatiens Cresson and honeybee visitation frequency each positively predicted yield. A common conclusion from these results is that pumpkins grown in highly diverse or high grassland coverage landscapes would have greater yields via bumblebee and honeybee visits to flowers. However, this inference does not preclude the possibility that landscape features may be associated with crop yield, independent of bee visits to flowers. Results from conditional process modelling indicated that only pumpkins grown in highly diverse landscapes were predicted to have greater yields as a consequence of more bumblebee visits to pumpkin flowers. None of the landscape features predicted greater fruit yields as a consequence of more honeybee visits to pumpkin flowers. This novel analysis indicated that traditional approaches may be misinterpreting the relationships between these variables. Synthesis and applications. Bumblebees benefited from a diverse landscape, and their visits to flowers positively impacted pumpkin production. Conservation of a diverse landscape should be promoted to support improved pumpkin production. Growers can use this information to decide where to plant pumpkins to improve the potential for high yields, to identify scenarios where landscape diversity could be increased, and where supplementation with bees might be beneficial. Bumblebees benefited from a diverse landscape, and their visits to flowers positively impacted pumpkin production. Conservation of a diverse landscape should be promoted to support improved pumpkin production. Growers can use this information to decide where to plant pumpkins to improve the potential for high yields, to identify scenarios where landscape diversity could be increased, and where supplementation with bees might be beneficial.