Sampling bias exaggerates a textbook example of a trophic cascade

Understanding how ecosystems respond to the loss and recovery of large predators is a major challenge because these free-living systems are difficult to sample properly. We show how an accepted practice of nonrandom sampling has confounded this understanding in a textbook system (Yellowstone National Park) where carnivore [Canis lupus (wolf)] recovery is often associated with a trophic cascade involving changes in herbivore [Cervus canadensis (elk)] 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 overstory aspen (Populus tremuloides) by a factor of 3-8 compared to random sampling. Sampling only the tallest young plants favored plants taller than the preferred browsing height of elk and overlooked non-regenerating aspen stands. Our results demonstrate how seemingly minor departures from principled sampling can generate substantial misunderstandings about the strength of trophic cascades in response to large predator recovery.