If the world gets warmer, what happens to grazers?
Not an easy question. There are many grassland climate change experiments, but these are of limited utility here. Grazed and ungrazed grasslands are starkly different such that the consequences of warming for ungrazed grasslands are unlikely to apply to grazed grasslands.
If experiments don't help, then we need to look at how grazers respond to short-term variability in climate and compare that with geographic patterns that might represent long-term patterns.
When I've looked at how bison respond to inter-annual variation in climate, hot years don't affect them.
Yet, when we look across temperature gradients, hot places have small bison. Across 22 herds and a quarter million weights of bison, it's clear that herds in hotter places have smaller animals. Sometimes up to 500 lbs lighter.
Why the difference between short- and long-term patterns?
This is where experiments come in handy. When exposed to elevated temperatures short-term, grasslands begin to lose nitrogen. Over the long-term, these losses accumulate which drives down the quality of grass for grazers to eat.
One hot year, no problem. Many hot years and grazers don't grow as big.
The differences among bison are dramatic.
Back of the envelope calculation shows that just a 1°C increase in temperature across the US could cost the cattle industry $1 billion. Considering projections are for multiple degree C increases, those costs would accumulate.
[Regarding details, I'm about to submit this paper. We'll see how it's met.]