Bite by bite: why North American herbivores confront such a variable diet

As you travel across North America, grasslands are everywhere, from roadside strips to boundless open prairie. It is easy think of acres of grass and forbs (flowering herbs) as just mouthfuls of forage for local herbivores. Give me a moment to present the more beautiful truth.

In a recent paper in Ecology, Dr. Ellen Welti, Dr. Kirsten de Beurs and I analyzed data we gathered visiting 54 grasslands in the summer of 2016. In each, we cut 0.1m2 strips of vegetation—clip plots—to study food web nutrition. As always, we are grateful for funding support from the National Science Foundation. You can download the paper for your personal use from the publication page of this website, here.

Now, if you want to get *realllly* fundamental you can think of the vegetation from these grasslands following the same recipe of 25 chemical elements. We asked if a grasshopper or bison munched away in each grassland, how nutritious would they be? Why and how are some grasslands more yummy?

When elements are classified by their need by plants and animals (top), just animals (middle) and whose function is still unknown (bottom), we see evidence of concentrations above soil availability (elements lie above the dashed line) for elements like N, P, and K (the main ingredients in Miracle Gro and other fertilizers) but also Boron and Na. This is true for both grasses and forbs.
A grasshopper doing what it does best.

Above, we plot average element ppm (parts per million) in the plants against the avg in the soils; elements above the dashed line are accumulated by plants. The standbys NPK accumulate, also B(oron) and Mo(lybdenum). Plants tend to *avoid* animal essential nutrients except Na.

The Supply Side Hypothesis: increasing precipitation or soil availability of an element R increases access by the plant. All of these graphics for our hypotheses created by Deborah Kaspari.

But then we looked for patterns *across* North America grasslands and found the same element could vary 1000-fold in ppm (amount per bite). The Supply Side hypothesis reasonably predicts that this is caused by soils—where a soil is rich in element R, it is often so in its plants.

Baptisia alba, an early blooming Nitrogen fixer on Konza prairie that is taking advantage of a spring burn.

This is where the first difference between grasses and forbs emerged. Forbs tended to be pickier, harvesting more universal nutrients where available. Grasses, on the other hand, were more indiscriminate, harvesting even non-essential nutrients like Cd and Sr when available.

The NP Catalyst hypothesis, that predicts when the “heavy hitters” of the ionome, Nitrogen and Phosphorus increase in a plant’s tissue, demand for all sorts of other elements useful to metabolism also increase.

A second hypothesis, championed by Dr. Puni Jeyasingh up the road at Oklahoma State suggests that the availability of macronutrients like N and P—that build our metabolic machinery—drives the need for the other nutrients. This too, won some support: grasslands rich in N and P increased uptake of catalysts like Zn and Cu.

The Grazing Hypothesis suggests that consumers are primary sources of nutrients via their urine and feces, often depositing elements that are essential to the bacteria of their microbiome.

A third idea: nutrients are recycled by herbivores in the form of poop and pee. Sadly (in retrospect) we called this the ‘Grazing Hypothesis’, not ‘PoopPee’. Grazing by cattle increased plant ppm in elements like Fe, Cu, and Cr: straight outta the colon and its own microbiome.

The Nutrient Dilution hypothesis suggests that any factor promoting biomass growth without adding extra nutrients to the soil will decrease nutrient quality, bite for bite.

Finally, we looked for Nutrient Dilution: how increases in plant biomass dilute its nutrition per bite. This is where the second diff between forbs and grasses emerged. Forbs, richer in nutrients in the first place, tend to decline in ppm when they grow more. Less so, grasses.

Woodcut by Dr. Ellen Welti.

The upshot? Plant nutrient density—not just biomass—is key to herbivore health. We found the 2 plant groups followed different rules. Since forbs are more prone to Nutrient Dilution, increases in CO2, temperature, and precipitation likely target forb-feeders more.

More generally we provide a framework of four hypotheses to explore the geography of nutrition for Earth’s consumers, and show that plants don’t slavishly track the nutrients in the soil, but create their tissues by integrating across the entire abiotic and biotic environment.

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