Wolf, elk, and aspen food web relationships: Context and complexity

Eisenberg, Cristina; Seager, S; Hibbs, David

Aspen (Populus tremuloides) forests are inﬂuenced by a synergy of bottom-up (resources-driven) and top-down (predator-driven) processes. Since the 1920s, ecologists have observed the decline of many aspen communities throughout the Intermountain West. The extent and possible drivers of this decline are topics of much recent scientiﬁc study. In addition to bottom-up effects,which include drought, ﬁre suppression, and disease, ungulate herbivory is a contributing factor. Trophic cascades are ecological relationships in which an apex predator produces strong top-down, direct effectson its prey and indirect changes in faunal and ﬂoral communities at lower trophic levels. Apex predators,such as the gray wolf (Canis lupus), have been linked to aspen vigor and recruitment, via trophic cascades mechanisms. Scientists have hypothesized that returning wolves to the landscape enables aspen torecruit into the forest overstory, via the density-mediated and behaviorally-mediated effects of wolves on their ungulate prey, primarily elk (Cervus elaphus). A synthesis of scientiﬁc ﬁndings on this topic is presented, identifying trends in the ecological impacts of wolves in aspen communities in a variety of ecosystems, and suggesting areas for further investigation. Knowledge gaps include the interaction of top-down (e.g.,predators) and bottom-up (e.g., drought, ﬁre, hydrology, logging) effects, and how the ecological context of the interaction affects the outcome. Future horizons involve exploring these food web relationships asa complex of inter-level interactions in a more integrated, empirical manner. We suggest adopting a new standard for the aspen/wolf ecology literature by shifting its emphasis from trophic cascades to food web studies.

Wolf, elk, and aspen food web relationships: Context and complexity

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