The ecological context of the past century explains the significance of returning apex carnivores to Yellowstone National Park, replete with myriad environmental changes in the past 20 years. How many of these changes are a result of carnivore recovery is a fascinating and pertinent question. It might have been anticipated that woody plants might respond in a trophic cascade, but only modest reductions in elk density were expected, and behavioral mediation was not postulated. There was no expectation of an increase in bison, at least in the context of wolf reintroduction. A substantial reduction in elk density did in fact occur in the late 1990s and early 2000s, along with suggestive evidence of behavior change in elk. Within a few years after wolf reintroduction, in a few places, there was improved height growth, but much heterogeneity, in aspen and willow stems. There has not been a uniform plant response (i.e., trophic cascade) evident across the landscape. Willow, aspen, and cottonwood have each contributed to different perspectives on indirect effects of carnivores on woody plants.
There appears to be general agreement that the Yellowstone trophic cascade (1) was not by wolves alone, as grizzly bears, cougars, and the hunting public were all involved in elk numerical reduction; (2) does not extend to willow as much as aspen; and (3) has only just begun to unfold. Unsettled issues abound, including the spatial pattern of plant recovery, role of bison as the now-primary large herbivore, role of prey behavioral change, causes of heterogeneity in plant response, and how to best characterize the extent of trophic cascade. The future trajectories of all species of large carnivores, ungulate prey, and forage plants (both woody and grassland species) cannot presently be predicted, but need to be considered in research planning. Such planning needs to be long-term, dare I say decadal in extent. While the system will not be experimentally manipulated, experiments involving system components should be encouraged. Ultimately, research involving apex carnivores in other systems will establish the generality of findings from Yellowstone. Recovery of large carnivores in Yellowstone National Park, including the gray wolf, grizzly bear, and cougar, was an unparalleled conservation victory, and contributed to ecological complexity that will be a research challenge to decipher. Returning the wolf to Yellowstone brought this national park much closer to the much-vaunted management goal of “natural regulation”, and a vigorous science program is needed now more than ever.