The Impact of Fire on Nectar Quality and Quantity for Insect Pollinator Communities

Abstract

Grassland insect pollinators have undergone population declines due to habitat loss and degradation. Patch-burning is a management method used to reduce woody plant encroachment and maintain grassland habitats. However, the impacts of fire on the quality and quantity of floral resources (nectar volume, total sugar, and sucrose concentration) available to insect pollinators are understudied. Evaluating how time since last fire and season of fire impact nectar quality and quantity is vital to understanding the overall impact of fire on insect pollinator communities. For this study, 10 flowering forb species (Asclepias viridis, Baptisia alba, Cirsium undulatum, Desmodium canadense, Monarda citriodora, Monarda fistulosa, Oenothera speciosa, Penstemon tubaeflorus, Ruellia humilis, Spiranthes cernua) were selected in The Nature Conservancy's Joseph H. William's Tallgrass Prairie Preserve in an area managed with patch-burning and cattle grazing. Study sites were burned in spring 2018, summer 2018, summer 2019, and spring 2020, with three replicates of each treatment (for 12 total burn units). In each unit, we collected nectar from flowering individuals of each plant species with microcapillary tubes to measure standing crop or the nectar in flowers at any given time that is available to insect pollinators. Total volume of nectar and sucrose concentration were recorded, and total mg of sugar was calculated. Desmodium canadense flowering individuals were only located in units burned the previous spring, whereas P. tubaeflorus flowering individuals were only located in units burned the previous summer. For the eight remaining species, total volume of nectar, total mg of sugar, and sucrose concentration varied with different burn regimens for different species. Cirsium undulatum inflorescences with crab spiders (Thomisidae) had higher volumes of nectar and higher amounts of sugar available. This study highlights how a patch-burn approach with different fire return intervals may benefit insect pollinators at the community level by providing a wide range of nectar qualities and quantities to support insect pollinators with different nutritional needs.