Building for bees: the effect of plant arrangement on pollinator communities in managed habitats

Commodities

• Additional Plants: native plants
• Animals: bees

Practices

• Crop Production: pollination, pollinator habitat, pollinator health
• Natural Resources/Environment: biodiversity, habitat enhancement, hedgerows, other, wildlife
• Sustainable Communities: sustainability measures

Pollinator populations are declining in urban, agricultural, and natural areas. A major driver of these declines is a reduction in the abundance and diversity of flowering plants which pollinators depend on for food. In agricultural landscapes, the establishment of diverse floral meadows (“pollinator strips”) can attract pollinators and contribute to sustainable agriculture. By determining optimized planting arrangements within pollinator strips for pollinator, plant, and community health, we can minimize the inputs and maximize benefits to farmers. In our proposed project, we will examine the effects of planting arrangement (i.e., whether plant species are in groups or intermingled in a strip) on 1) pollinator visitation rates and patterns, 2) predicted pollinator community resilience, and 3) seed set of plants within the strip. We will establish 12 plots (6 in groups, 6 intermingled) of 6 species of plants at Penn State’s Larson Agricultural Center. The plots will be sampled for pollinators, and floral traits will be measured. The background pollinator and floral communities will also be monitored. Subsequently, collected insects will be identified to species, plant species seed set will be measured, and plant-pollinator network interactions will be modeled. These studies will determine if grouped plantings (which are easier to produce and manage) are equally beneficial for the plants and their pollinators, as well as if certain plant and pollinator species are more resilient to variation in planting design. In addition to supporting growers, these studies can inform pollinator habitat design in urban and natural landscapes.

To determine whether planting arrangement (i.e., planted in groups or intermingled) impacts the abundance, diversity and resilience of the pollinator community, and the outcomes for plant pollination, we will create 12 plots (6 with grouped planting arrangement, 6 intermingled , Figure 1) at the Larson Agricultural Research Center in Rock Springs, PA. Within each of these plots, we will:

1. Evaluate the effect of arrangement on insect visitor species richness and abundance to each plant species.

Expected Outcome: We expect more competition among pollinator species when plants are in conspecific groups, resulting in higher abundance of more competitive species (e.g., Bombus spp., as in [19]) and less overall pollinator species diversity in these plots when compared to intermingled plots. We expect species to be more competitive when they are social and have a recruitment foraging strategy (as in Bombus spp.). We expect that plant species with high floral area on an individual per-plant basis (e.g., Euthamia graminifolia, Pycnanthemum virginianum) to vary less across these two treatment groups, while plant species with lower floral area per plant (e.g., Rudbeckia fulgida, Echinacea purpurea) will vary more. 

2. Evaluate the effect of arrangement on the resilience of the overall pollinator community and the plant-pollinator network

Expected Outcome: Higher rates of competition in grouped plots are expected to reduce pollinator diversity for each plant (Objective 1), and thus reduce overall pollinator species diversity within each plot. Additionally, reduced species diversity on each plant species will result in a less connected network within a plot (i.e., fewer pollinator species will be visiting each plant). This will lead to a pollinator community that is at higher risk of decline given reduced connection [23], and therefore the arrangement is less self-sustaining in the long-term.

3. Measure the effect of arrangement on seed set (percent viable seeds per inflorescence) for selected plants.

Expected Outcome: Plants in grouped plots will experience higher competition among conspecifics, which will reduce the efficiency of pollination and reduce seed set. (This outcome was previously demonstrated in [19] with planting density but was only found on one plant species and has not been studied through planting arrangement). Plants with high floral area will vary less across these two treatment groups, while plant species with lower floral area will vary more (as in Objective 1).

Alternative Expected Outcome: Plants in grouped plots will experience more conspecific pollen transfer, resulting in more effective pollination and higher seed set [24].