Grass-eating and the Nachusa bison — Do plants fight back?

By Luke D. Fannin
PhD Candidate, Dartmouth College, Hanover, NH

​Grass-eating, or graminivory, as it is called by scientists, is a strange behavior. At the outset, grasses, at least compared to the many plant foods that humans consume regularly, look unappetizing; they are tough to chew, they are full of fibers that make them difficult to digest, and they are often covered in dust and sand from growing close to the soil surface. But for many mammals, ranging in size from tiny voles (20-24 grams) all the way up to gigantic white rhinoceroses (2400 kilograms), grasses are dietary staples, and these so-called “grazers” (grass-eating mammals) are pivotal in Earth ecosystems. Nachusa Grasslands is home to one of North America’s most important grazing mammals, the bison (Bison bison), which eats grasses in most months of the year. But if grasses are such difficult foods to eat, how do bison–let alone any other mammals–eat them?

Bison are North America’s largest wild grazers (‘grass-eaters’) and have profound effects on plant communities across the United States, including Nachusa Grasslands, where they have been present since 2014.

​As it turns out, bison have a few tricks up their proverbial sleeves as it pertains to eating grasses. For starters, bison are ruminant mammals, which means they have highly specialized stomachs that allow them the ability to regurgitate and then re-chew partially digested plant foods. Think of how a domestic cow eats; the cow first swallows a bite of food but then proceeds to regurgitate that bite of food and chew it again, and again, and again… until finally those food particles are small enough to pass through the rest of the digestive system. With each subsequent swallow, foods are bathed in stomach juices teeming with bacteria, which also help to weaken the structural integrity of fibrous foods and liberate nutrients. This digestive trick is incredibly helpful for bison, as it allows them to digest grasses in a way that humans cannot.
 
Secondly, bison are endowed with highly specialized teeth, termed hypsodonty. Unlike the surfaces of our own teeth, which fully poke out of our gums when they erupt, bison’s teeth emerge in a piecemeal fashion over the course of their lives, meaning that they usually have more teeth hiding within their skulls at any given point in time. A good analogy for how bison teeth work is the lead found in the tip of a mechanical pencil: while there is always lead at the pencil tip exposed for writing, there is far more lead stored within the pencil that emerges to eventually replace the used-up lead after writing is finished. Hypsodonty is thus a useful trait for Nachusa bison because most grass plants are also abrasive (see below), meaning they remove tooth surface, or enamel, during chewing. Hypsodonty allows bison to continuously provide new replacement tooth surface over the course of their lives, allowing them to keep eating grass plants without having to worry about completely losing their teeth in the process (although they are not actively growing teeth, and this excess amount of tooth eventually wears out in old age).

Bison teeth are hypsodont, meaning that the dental crowns are tall and most of this length is stored within the bones of the face and jaw. This trait provides a hidden reservoir of tooth material that can be pushed out when the surfaces of the teeth wear, allowing for tooth function to be preserved in the face of extremely abrasive conditions. In addition, when bison teeth wear, the enamel forms elaborate cutting surfaces with a harder underlying tooth material (referred to as dentin, which is the brown material contrasted against the pearly-white enamel on the tooth surfaces). These elaborate cutting surfaces help bison mince plant foods.

But with all this talk of bison traits to eating grass, it is easy to view the grasses themselves as passive players in this story of herbivory at Nachusa. But can grasses fight back? My own work at Nachusa Grasslands seeks to answer this question with one plant trait that may act as a defense against herbivory . . . silica levels. While we often think of silica as being a component of our modern technology (e.g., in computer chips), silica is also one of the most abundant minerals in Earth’s soils. Grasses, in turn, uptake silica into their own tissues during growth, where it usually gets deposited into silica bodies called phytoliths. The function of silica accumulation in grasses is debated, but one thought is that it provides a defense against defoliation (i.e., the removal of leaves). As suggested previously, grass plants are abrasive, and it is the silica in grass leaves that hypothetically works to remove enamel. As such, high levels of silica in grass leaves may deter mammalian consumers from eating them — for risk of damaging their teeth — helping to prevent defoliation. Silica also works to stiffen plant leaves, making them more difficult to chew and digest, while also further inhibiting microbial digestion in the digestive tract. Thus, silica be an inducible defense for grasses, wherein a re-growing grass plant responds to previous herbivory by increasing silica uptake to prevent severe defoliation in the future.
 
But silica has other important functions in grass plants that are unrelated to herbivory. For example, increased silica uptake in grasses is also important for relieving environment stresses related to both high temperatures and low water availability. Therefore, it remains challenging to determine whether the high silica levels found in some prairie grass plants are primarily a response to intense herbivory pressure (i.e., from roaming bison) or are instead a plastic response related to changes in local environmental conditions that may increase growing stresses.

This wooly panic grass (Dicanthelium villosissimum) may also store silica from the soil in another unusual place — its hairs. How these hairs deter herbivory by bison remains to be studied, but such hairs may contribute to the silica uptake by this current cool-season grass species.

Bison at Nachusa can exhibit immense herbivory pressure on the local grassland community and repeat herbivory at certain locations can create what are termed “grazing lawns”, where grass plants are maintained in a short-statured state as compared to surrounding vegetation. It remains unknown whether the grass species within these “grazing lawns” at Nachusa possess higher silica levels in their tissues than similar species within exclusion plots, but previous work on the African Serengeti has suggested that grasses in such features are more silica-rich than similar species under less extreme herbivore pressure.

My current work at Nachusa Grasslands seeks to shed light on this mystery. By taking advantage of bison exclusion plots set up across the lands of the reserve, I can compare the silica levels of different grass species (and other related traits, such as photosynthetic pathways, leaf toughness, and nutrient levels) in locations where they have experienced almost a decade of bison herbivory (2014 – 2021) to those same species in exclusion areas where bison herbivory has been much less intense or non-existent. My hope for my continuing work at Nachusa is to disentangle the various factors that may influence silica levels in various grass tissues of prairie plants and, by extension, in the diets of Nachusa bison. My overarching goal is to figure out if re-introduced bison are helping to change the functional traits of grasses growing within the Nachusa reserve, and if so, what this might mean for plant community structure and resilience moving forward.

Exclosure or herbivore exclusion plots are key to testing hypotheses regarding inducible plant defenses and bison herbivory at Nachusa Grasslands. The bison have access to the plants on the left side of the photo, but in the right side of the photo begins the exclosure, a space that is completely isolated by fencing and the bison cannot enter. Notice the taller vegetation in the exclosure. Numerous exclosures are found throughout the bison unit.

The author (L.D. Fannin) in his mobile lab processing collected grass plants in the field at Nachusa in June 2021. To prevent molding, grass plants need to be dehydrated before transport, and the food dehydrator at the far right allows for the dehydration of plant remains to occur quickly before plants are then ground and stored with desiccant. The author is currently analyzing these collected plant parts for their silica content at Dartmouth.

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​Luke D. Fannin is a PhD candidate at Dartmouth College, Hanover, NH within the Ecology, Evolution, Environment, and Society graduate program. He is broadly interested in plant-herbivore co-evolution and how the behavior of animals and the characteristics of their diets structure patterns of morphological evolution, with a focus on the dentition. He received his Bachelor of Science in Zoology from The Ohio State University in 2018.

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Luke Fannin was a 2021 Scientific Research Grant recipient​ from the Friends of Nachusa Grasslands. Interested in supporting Nachusa's science? Just designate your Donation to "Scientific Research Grants."