I am pleased to announce that research on bison diet @nachusa led by @Blackburn_RC, along w @barberecology and me in @SERestoration journal, Restoration Ecology is out on early view! onlinelibrary.wiley.com/doi/10.1111/re…
@Nachusa @Blackburn_RC @BarberEcology @SERestoration Ryan worked hard on this project as part of his MSc thesis in my lab. For it, we really wanted to know what bison were eating since they were the first herd to be studied that had been reintroduced into restored prairie, rather than on remnant (never plowed prairie). 
Of course, we thought they’d eat grass (duh). That’s why managers wanted to reintroduce them! @Nachusa there is a lot of burning to maintain prairie, but that gives competitive advantage to grasses over forbs.
So we see lower plant diversity in older plantings; ire exacerbates this (stay tuned for another paper by Ryan, which looks at plants specifically, accepted in @naturalareas). Bison are supposed to eat those grasses and maintain diversity, even in older, frequently-burned sites.
But other research showed bison rely more heavily on N-fixing forbs, at least out in the west. nature.com/articles/srep1… We wanted to know if Nachusa’s bison were eating more N-fixing forbs, or grasses or?? And we wanted to know if their diet changed with age, sex, and season.
So, we asked managers to pluck tail hairs for us during the annual round up, when the bison get tags and health checks. Then, we segmented the hairs and overlaid hair from one year and the following year to approximate tail hair growth rate.
Then we segmented the hairs (10cm each segment) and ran them through the mass spec to get their d15N and d13C stable isotope values.
Stable isotopes are an analytical version of the idiom “you are what you eat” bc you are what you eat, isotopically. Your hair reflects what you’ve eaten - I could pull a hair from you and tell you if you’re vegetarian, how much corn your diet relies on, etc.
Same is true of bison. Because the plants they likely eat separate out in isotope space like is shown, we can use a Bayesian mixing model to predict the proportion of their diet made up from each plant group.
Shown are the bison hairs, by season, by sex, and the plant groups with CIs.
We found bison eat similarly with age, but that cows relied on slightly higher N-fixing forbs than bulls and bulls relied on higher amounts of C4 grasses (the iconic tall grasses of the prairie here in IL - things like Big Bluestem).
We also saw seasonal shifts - bison rely highly on C4 grasses in summer but shift to C3, wetland, and N-fixing forbs towards fall. This makes sense - the C4 grasses senesce but wetland and C3 grasses stay green a bit longer, as do N-fixing forbs.
N-fixing and other forbs overall make up 31% of bison diet, which was unexpected! This was mostly driven by grazing outside summer. C4 grasses only make up around 30% of bison diet in fall, for example. And there is a spike in reliance on N-fixing and other forbs in fall, spring.
Nachusa is known for its high forb diversity - are bison just eating largely what’s available or are they preferentially foraging these forbs?
We aren’t sure, but we do know that these forbs have similar C:N as the C4 grasses, so maybe they’re selective? Maybe the high protein content of N-fixers is what they want before a long hard winter?
We do see bison selecting forbs in spring after burning sometimes - the plants are just shooting up and it’s impossible for them to be accidentally eaten when they go for grass at these times.
So maybe they just like forbs more than we thought and in other places bison are, there aren’t enough of them to be selected. These are things our research can’t answer but are interesting.
The fact that we saw bison not focusing solely on grasses means the hypothesized increases in plant diversity may not materialize or may take longer than thought, as bison graze forbs and may then increase habitat for C3 grasses to establish.
So, if you’re a manager and you want to reintroduce bison, we’d recommend thinking about how much wetland habitat is available and expecting some impacts around wetlands since bison love grazing them.
It also seems bison can do quite well in a highly diverse prairie with grasses, and a fair amount of forbs, too. They may even prefer the high diversity of restorations, but more work is needed to understand if it's preference or just what's available. /END
*1cm each segment, not 10!!!
For some reason, the thread got cut off...here is the rest:
