Big Bluestem (\(Andropogon gerardii\)) is a perennial warm-season grass that once dominated the tallgrass prairies of North America, earning it the nickname “monarch of the prairie.” It is one of the “Big Four” grasses that characterize this ecosystem, growing to impressive heights of up to eight feet. The plant’s extensive, deep fibrous root system can extend down ten feet or more, anchoring the soil and making it highly drought-resistant. As a primary producer, Big Bluestem provides a massive source of biomass, which supports a complex food web, ranging from large grazing mammals to specialized invertebrates.
Large Grazing Mammals
The most significant consumers of Big Bluestem are large herbivores, which historically shaped the prairie landscape through their grazing patterns. Native grazers such as Bison and White-tailed Deer consume the vegetative parts of the grass, particularly in the spring and early summer. They target the young, tender shoots when the grass is rapidly growing and its nutritional content is at its peak.
Big Bluestem is highly palatable and nutritious during this period, often referred to as an “ice-cream grass” because livestock and wildlife prefer it over many other species. The crude protein content can be high, maintaining between 14-18% from May through August, making it valuable summer forage. Domesticated livestock, primarily cattle, also graze it extensively in managed pastures due to this high nutritional value.
While the grass is resilient and can withstand substantial defoliation, the pattern of consumption is important for its survival. Continual grazing that reduces the plant below six to eight inches can weaken the stand, allowing less desirable species to outcompete it. Forage quality drops significantly as the plant matures into the fall and winter, with protein levels dipping below 6%, making the older stalks less appealing to most large grazers.
Small Herbivores and Seed Eaters
Consumption of Big Bluestem is not limited to large-scale defoliation; smaller animals also utilize the grass in specialized ways. Rodents, such as the meadow vole, feed on the leaves and young shoots during the summer months. As the season progresses, these small herbivores shift their focus to the seeds, which provide a vital food source to sustain them through the winter.
The seeds of Big Bluestem are a dietary component for a variety of birds, especially songbirds and upland game species. Prairie chickens and sparrows, including the Grasshopper Sparrow, are known to consume the seeds. Small mammals, such as rabbits, graze on the leaves and stems, while the grass’s tall, dense structure provides them with cover and nesting habitat.
Seed consumption impacts the grass’s reproductive success, while the grazing on leaves affects its overall biomass.
Insect Consumers and Stem Borers
Invertebrates represent a diverse group of consumers, often engaging in specialized feeding that affects different parts of the grass. Defoliating insects, such as various grasshopper species, consume the leaf blades of Big Bluestem. The larval stages of several butterflies, specifically skippers like the Delaware Skipper and Dusted Skipper, are herbivores that use the leaves as a food source.
Other insect interactions involve targeting the plant’s internal structure or reproductive output. The bluestem seed midge (\(Contarinia wattsi\)) specializes in the plant’s seeds, with its larvae capable of reducing seed yields by over 50%. Stem-boring insects, like the larvae of certain flies in the genus \(Thrypticus\), tunnel into the culm, or stem, where they feed and can overwinter at the base. Sap-sucking insects, such as aphids, draw nutrients directly from the plant’s vascular system, impacting the grass’s vigor.