Trophic Consequences of Avian Rewilding
Several apex predators including grizzly bears, cougars, sea otters and wolves were decimated from the North American landscape but in recent decades, thanks to increased protection from hunting and persecution, have been ‘rewilding’ vast areas across North America. As apex predators, these species exert strong top-down effects (direct and indirect) that can greatly alter ecosystem dynamics and structure. The rewildling of apex predators is a rich source of ‘natural experiments’ and has been the focus of several studies evaluating the ecological consequences of these recovering trophic cascades.
The Zuckerberg lab, in collaboration with the U.S. Geographical Survey and the National Park Service, is investigating the direct and indirect consequences of bald eagle recovery within Voyageurs National Park. Recovering eagle populations are predicted to directly affect mesopredators (i.e. osprey) through increased competition for nesting territories; and prey (i.e. great blue herons, common loons and double-crested cormorants) through increased predation pressure of breeding birds, chicks and/or eggs. Indirectly, recovering eagle populations are predicted to impact breeding success of colonial nesters (i.e. double-crested cormorants) by facilitating alternative mesopredators (i.e. gulls) to prey on nests after flushing away adults; and to also alter competition dynamics between prey. The team is also investigating how these interactions are mediated by anthropogenic effect including climate. Results from this project will address key knowledge gaps regarding the ecological effects of recovering apex predators and will provide recommendations on current avian monitoring and management.
Masting and Avian Irruptions
Resources in ecosystems are an important component of biotic interactions, fostering connections within and among trophic levels through exchange of energy and nutrients. Fruiting vegetation plays a critical functional role for both the producers and consumers of the seeding cycle, and the natural ebb and flow of these resources into the food web can both directly and indirectly create or perturb demographic waves in populations.
Masting is a unique phenomenon where plants produce seed at volumes well above the average annual output. It has been documented across many terrestrial taxa that range from graminoids to trees, with some species adhering to predictable cycles while others infrequently and erratically boom and bust. Spatial and temporal synchrony is a unique aspect of this phenomenon that amplifies the impact to ecosystems at both local and geographic scales.
White spruce is an ideal species to investigate drivers of masting events due to the irregular annual cycle of cone volumes. The inconsistent crops also offer the opportunity to examine how this pulsed resource may impact or even influence the movement patterns of irruptive seed-eating birds. Pine siskins (Spinus pinus) and red crossbills (Loxia curvirostra) may experience abrupt changes in their demography during masting years when adults can more successfully produce multiple clutches, ultimately leading to a mass departure of young birds the following year when there is collapse in seed resources across extensive spatial scales. This type of interaction can provide insight into the dynamics of boreal ecosystems, seeking a mechanistic understanding of how constituent species with highly variable resources function in the face of a rapidly changing climate.
Recent related publications
Strong, C., B. Zuckerberg, J. L. Betancourt, and W. D. Koenig. 2015. Climatic dipoles drive two principal modes of North American boreal bird irruption. Proceedings of the National Academy of Sciences, 112(21): E2795-E2802. Link