Predators eat and scare their prey. By doing so, they reduce the temporal and spatial intensity of herbivory and indirectly benefit many plant communities. This process by which predators can structure ecosystems via their top-down effects on prey and indirectly on vegetation is termed a “trophic cascade”. It has become one of the major conservation paradigms of the past decade. Yet, there remain controversies and disagreements about the role that trophic cascades play in ecosystem structuring. Do the top-down effects of top carnivores really have primacy in driving food-web dynamics and ecosystem structuring? If so, should we restore top predators to degraded ecosystems, and how can we predict the effect of doing so? And do the patterns observed in wild places hold for human-dominated landscapes?
These are the broad questions addressed by Prof Mark Boyce in the Zoology Departmental Seminar today. Mark is a population modeller and conservation biologist who for the past 40 years has worked on understanding the dynamics of arguably the most well studied attempt to use trophic cascade theory to restore ecosystem integrity: the reintroduction of wolves to Yellowstone National Park after a 70 year absence. In today’s seminar, Mark described how the predictions of an early population dynamic model he developed guided the reintroduction program, and showed that wolves would drive down elk numbers but not cause native ungulates to perish, and that hunting would remain viable (outside the park). A decade later, in the context of adaptive management (a systematic approach to improve resource management by learning from past management outcomes), Mark updated the model to reflect new knowledge on the differences between how wolves and hunters select elk to kill. The new model performed well in predicting observed declines elk numbers.
Following this Mark and his students undertook studies that showed there was increased growth of willow following wolf reintroduction, providing evidence consistent with a trophic cascade operating. However, Mark noted that the trophic cascade story is far more complex: there is extreme variability between sites and species (e.g. Aspen, cottonwood, did not show such clear patterns), and strong seasonal effects. In addition, studies they undertook with radio-collared elk suggested that both trait-mediated (the so-called “landscape of fear”) and density-mediated mechanisms were driving this trophic cascade. Using the results of work he has undertaken in human dominated ecosystem Mark then showed that the negative impacts of humans overwhelm trophic cascades outside the park.
Mark’s illuminating narrative of four decades of research at Yellowstone highlights that predator-prey systems and trophic cascades are complex and inherently dynamic, but that simple tractable models can provide clarity on key processes, and that while top predators can restore ecosystem resilience, we should not necessarily expect similar patterns in more disturbed ecosystems.
Dr Shelly Lachish is a Daphne Jackson postdoctoral Fellow and population ecologist. Her current research examines the consequences of wildlife diseases at a broader scale: on communities of species within ecosystems. Follow Shelly on @shellylachish.