Sampling bias exaggerates a textbook example of a trophic cascade ...

• Main Authors: Brice, Elaine, Larsen, Eric, MacNulty, Daniel
• Format: Dataset
• Language: English
• Published: Dryad 2021
• Subjects: FOS Natural sciences; Aspen; Populus tremuloides; elk; Cervus canadensis; wolf; Canis lupus; carnivore; predator indirect effects; nonrandom sampling; sampling bias; preferred browsing height; Ungulate
• Online Access: https://dx.doi.org/10.5061/dryad.2z34tmpnj; https://datadryad.org/stash/dataset/doi:10.5061/dryad.2z34tmpnj

Understanding trophic cascades in terrestrial wildlife communities is a major challenge because these systems are difficult to sample properly. We show how a tradition of nonrandom sampling has confounded this understanding in a textbook system (Yellowstone National Park) where carnivore [Canis lupus (wolf)] recovery is associated with a trophic cascade involving changes in herbivore [Cervus canadensis (elk)] behavior and density that promote plant regeneration. Long-term data indicate a 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 because it favored plants taller than the preferred browsing height of elk and overlooked non-regenerating aspen stands. Random sampling described a trophic cascade, but it was weaker than the one that nonrandom sampling described. Our findings highlight the critical importance of basic sampling principles (e.g., randomization) for achieving an accurate ... : We measured browsing and height of young aspen (≥ 1 year-old) in 113 plots distributed randomly across the study area (Fig. 1). Each plot was a 1 × 20 m belt transect located randomly within an aspen stand that was itself randomly selected from an inventory of stands with respect to high and low wolf-use areas (Ripple et al. 2001). The inventory was a list of 992 grid cells (240 × 360 m) that contained at least one stand (Appendix S1). A “stand” was a group of tree-size aspen (>10 cm diameter at breast height) in which each tree was ≤ 30 m from every other tree. One hundred and thirteen grid cells were randomly selected from the inventory (~11% of 992 cells), one stand was randomly selected from each cell, and one plot was randomly established in each stand. Each plot likely represented a genetically-independent sample (Appendix S1). We measured aspen at the end of the growing season (late July to September), focusing on plants ≤ 600 cm tall, which we termed “young aspen.” For each stand, we measured ...