What term describes the severe resource limitation mammoths faced when the Mammoth Steppe transitioned to forests?

Caloric bottleneck

The environmental shift at the end of the Pleistocene posed a significant threat to the mammoths due to their extreme dietary specialization. As the dense, productive grasslands contracted and were replaced by landscapes dominated by sparser woody browse, the availability of high-volume, easily processed grass declined severely. If a mammoth required 400 pounds of specific, dense grass daily, a landscape offering alternative, harder-to-process vegetation, even if abundant, could not meet that specific volume and nutrient requirement for survival. This inability to acquire the required daily caloric load from the new vegetation types created a severe caloric bottleneck, making the maintenance of their enormous body size unsustainable.