Landscape drivers of pollination services in urban gardens
While much scientific literature documents declines in bee populations, relatively little information focuses on the impact of landscape level factors on bee populations and communities. This is extremely important, as changes in land-use and landscape structure impact pollinators, their target plants, and plant-pollinator interactions. Indeed, theory suggests that landscape fragmentation and degradation may lead to declines in pollinator richness, abundance, and function, particularly in agroecosystems. Most research on pollinators in urban agroecosystems, in particular, focus on factors driving bee richness and abundance. Few look at pollination services in urban gardens.
We examined how landscape-level habitat features determine pollination services in urban gardens. We worked in 8 urban gardens in Santa Cruz County California, characterized by three distinct landscape surroundings: forests, agriculture, or developed land. We measured pollination for sunflowers placed in gardens for limited time periods, and otherwise grown in greenhouse conditions. We measured seed set, total seed mass, and average seed mass for each experimental treatment: flowers open to pollinators, flowers excluded from all pollination, flowers excluded from bee-pollination but hand self-pollinated, and flowers excluded from bee-pollination but hand cross-pollinated. We compared pollination metrics among pollination treatments and landscapes with ANOVA followed by paired t-tests.
As expected, pollination treatment affected the average seed mass per seed. Sunflowers that were not hand pollinated had a significantly lower seed set than plants open to pollinators (p=0.028), and the average seed mass was lower for hand self-pollinated flowers than in openly pollinated flowers (p=0.03), which confirms the that these controls were appropriate for measuring optimal seed set. Further, seed mass did not differ between hand cross-pollinated flowers and open flowers, suggesting that cross-pollinating results in a seed set most akin to leaving a flower open to pollinators.
Seed set and seed mass did not differ in gardens with different landscape surrounds (developed vs. forest vs. agriculture), but did vary with individual landscape variables. Relationships between effect sizes (open vs. closed-pollinated flowers) of seed measurements and landscape characteristics were not significant, likely due to low site replication. However, seed set of all open pollinated flowers did decline with increases in urban, developed land cover within 1km (=0.147, p=0.0399). Seed count also increased with increasing average bee abundance at each garden (=0.2171, p=0.01081). These results suggest that habitat factors that affect bee abundance, including increasing urbanization, may impact pollination services within urban gardens.