PS 17-245 - Pollination services in community orchards: Suburban bees carry more pollen than those collected in urban and exurban orchards

Abstract: Wild pollinators are important drivers of plant fertilization in community orchards. Many of these orchards serve low socioeconomic populations by providing access to healthy fruits and vegetables which are otherwise not affordable or obtainable. However, the surrounding landscape may influence pollinator behavior and resulting pollinator services. For instance, in urbanized landscapes, flower density is often lower and flower patches farther apart, favoring more generalized foraging strategies, which could result in greater heterospecific pollen transfer. To determine whether pollination services are influenced by landscape features, we selected ten community orchards along an urbanization gradient in the metropolitan region of St. Louis, Missouri. Pollinators were collected during 5-minutes observations of each individual tree of spring-flowering fruit trees in the Rosaceae family (e.g., apples, pears, etc.), sacrificed, and identified to species. Pollen was removed from the insect using ethyl acetate, stained with Calberla’s Fluid, and examined under a microscope. We identified using a pollen library of all flowering plant species from all sample sites and counted pollen grains in two categories: heterospecific pollen or conspecific pollen. We quantified focal pollen load, pollen purity (percent focal pollen carried), and non-focal pollen richness for each insect, and tested for the effect of urbanization on focal pollen load, pollen purity and non-focal pollen diversity.

In support of our hypothesis, total pollen carried by visiting insects was lowest in urban orchards. However, insects collected from suburban orchards carried the most pollen, which is contrary to our predictions. Richness of heterospecific pollen was also higher for insects collected in suburban orchards relative to exurban orchards. These results may reflect a higher density of spring flowering trees (e.g., Cercis canadensis, Cornus florida) in suburban landscapes relative to urban landscapes where tree density overall is lower and exurban landscapes where stands of naturalized forest contain fewer flowering trees. Pollen fidelity tended to decrease with increasing urbanization, resulting in insects collected in exurban orchards having the highest pollen fidelity. These results suggest that pollination services may be lower in urban orchards relative to exurban and suburban orchards. Whether these observed differences in pollinator behavior affect pollination success and, ultimately, fruit yield is yet to be determined. However, these data provide evidence in support of a gradient in pollination services that aligns with urbanization in this region, potentially exacerbating limited access to fresh and affordable foods.