Sampling bias exaggerates a textbook example of a trophic cascade

Understanding how wildland ecosystems respond to the loss and recovery of large predators is vital to decipher the forces that structure food webs and to guide the practice of ecosystem conservation, restoration, and rewilding. This is a major scientific challenge, however, because these large-scale, uncontrolled systems are difficult (or impossible) to sample properly. We show how a tradition of nonrandom sampling has confounded this understanding in a textbook system (Yellowstone National Park) where carnivore [wolf (Canis lupus)] recovery is often associated with a trophic cascade involving changes in herbivore [elk (Cervus canadensis)] behavior and density that promote plant regeneration. Long-term data indicate that a customary practice of sampling only the tallest young plants overestimated regeneration of aspen (Populus tremuloides) by a factor of 3-44 compared to random sampling. Our results demonstrate how seemingly minor departures from principled sampling can generate substantial misunderstandings about the strength of trophic cascades caused by large terrestrial predators.