Floral diversity rescues pollination services in constructed prairie patches on a reclaimed mine site

Background/Question/Methods

Constructed prairies can improve plant diversity and habitat quality for native species on degraded mine sites.  Pollinators, essential for reproduction of many prairie forbs, may be limiting in sites far from high-quality pollinator habitats.  Increased floral diversity, however, could rescue pollination services in isolated prairies by attracting diverse insect pollinators.  We investigated the influence of floral species richness and morphological diversity on the pollination and seed production of the annual herb Iberis umbellata in 33 20 m diameter constructed prairie plots on a reclaimed mine site.  The plots were located 18 – 620 m from a hardwood forest remnant.  We created three seed mixtures standardized for plant abundance: a base mix of grasses and forbs with shallow flowers, a mix of shallowed-flowered forb species not included in the base mix, and a high diversity mix including forbs with deep corollas.  Each plot was seeded with one standardized bag of base mix and an additional bag of one of the three mixes to create low species diversity, high species diversity, and high morphological diversity treatments. We expected that seed production of Iberis would decline with distance from the forest, but that floral diversity would ameliorate the distance effects on pollination. 

Results/Conclusions

Iberis umbellata produced 0 – 2 seeds per flower.  We found a weak negative association between mean number of seeds per flower and distance from the forest edge (R²=0.12,  p = 0.05) for all plots combined.  Distance interacted significantly with seeding treatment such that strong negative effects of distance from the remnant forest on seed production were only observed in low diversity plots (R²=0.67, p < 0.001).  In low diversity plots, Iberis seed production declined by approximately 40% from plots adjacent to the forest to plots located at 500 m from forest. We did not find a significant difference between increasing floral species diversity with similar floral morphologies or different floral morphologies. These results indicate that proximity to high quality habitat, even over relatively small spatial scales, can influence pollination services to patches of restored prairie vegetation. Importantly, floral diversity can rescue pollinator services in small prairie patches up to 600 m from high quality habitat.  Greater floral species diversity, rather than greater morphological diversity, appeared to have the most important influence on pollination dynamics of Iberis.