(Beschta & Ripple 2008). These types of changes are less readily observed but are still a cause of concern for the cascading effects of apex predator removal. Abiotic Effects of Removal
The second major effect of a trophic cascade triggered by the eradica-
tion of the gray wolf (Canis lupus) is the multi-dimensional change occurring in the landscape and surrounding physical environment caused by the removal of the apex predator. In the Beschta and Ripple (2008) study, after wolf removal led to over-browsed riparian zones, the dynamics of rivers within the Olympic Peninsula National park began to change. In general, riparian plants provide root structure which anchors and prevents erosion of river channels. In the absence of these roots and with increased sediment load, the channel widens, migrates more often, as well as increases braiding—an increase in indirect channel pathways and instability. The channel widening and bank erosion in turn creates a feedback in which there is less optimal habitat for riparian plants, like cottonwoods, to root. Beschta and Ripple ’s (2008) study outlines the multi-dimensional impacts of wolf removal and calls attention to large-scale physical changes that can occur in the presence of trophic disruption.
Physical landscape changes also took place in the Johnston et al.
(2007) study in which competitive exclusion of the beaver (Castor canadensis) population led to lower water levels. Beavers were pushed out by elk over-grazing and the subsequent decrease in willow densities. These beavers abandoned their dams which degraded over time, releasing blocked water and lowering water levels of areas up-stream from the structures. The drop in stream level left willows far above the water line and produced the previously discussed stress. Similarly to the Beschta & Ripple (2008) study, streams were also widened by the absence of anchoring roots.
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