Hydrologic Regime and Herbivory Stabilize an Alternative State in Yellowstone National Park

Abstract

Abstract. A decline in the stature and abundance of willows during the 20th century occurred throughout the northern range of Yellowstone National Park, where riparian woody-plant communities are key components in multiple-trophic-level interactions. The potential causes of willow decline include climate change, increased elk browsing coincident with the loss of an apex predator, the gray wolf, and an absence of habitat engineering by beavers. The goal of this study was to determine the spatial and temporal patterns of willow establishment through the 20th century and to identify causal processes. Sampled willows established from 1917 to 1999 and contained far fewer young individuals than was predicted from a modeled stable willow population, indicating reduced establishment during recent decades. Two hydrologically distinct willow establishment environments were identified: finegrained beaver pond sediments and coarse-grained alluvium. Willows established on beaver pond sediment earlier in time, higher on floodplain surfaces, and farther from the current stream channel than did willows on alluvial sediment. Significant linear declines from the 1940s to the 1990s in alluvial willow establishment elevation and lateral distance from the stream channel resulted in a much reduced area of alluvial willow establishment. Willow establishment was not well correlated with climate-driven hydrologic variables, but the trends were consistent with the effects of stream channel incision initiated in ca. 1950, 20-30 years after beaver dam abandonment. Radiocarbon dates and floodplain stratigraphy indicate that stream incision of the present magnitude may be unprecedented in the past two millennia. We propose that hydrologic changes, stemming from competitive exclusion of beaver by elk overbrowsing, caused the landscape to transition from a historical beaver-pond and willowmosaic state to its current alternative stable state where active beaver dams and many willow stands are absent. Because of hydrologic changes in streams, a rapid return to the historical state may not occur by reduction of elk browsing alone. Management intervention to restore the historical hydrologic regime may be necessary to recover willows and beavers across the landscape.

Where does biodiversity go from here? A grim business-as-usual forecast and a hopeful portfolio of partial solutions

“…Returning megafaunal species to what remains of their historical ranges (97) can yield a number of overlapping benefits: the return of these charismatic species undoes population extinctions, makes habitats more interesting and exciting, and can restore ecological interactions with appealing system-wide consequences. The repatriation of wolves to Yellowstone National Park in 1995 not only titillated tourists but also revived a multispecies trophic interaction involving elk, beavers, and trees, which has rejuvenated the region's riparian ecosystems (98,99).…”

Restoration as experiment

“…Unexpected events or interactions in ecological communities may create thresholds that shift succession in quite different directions than those envisioned at the start. An instructive example of unintended, in this case benefi cial, consequences is re-introduction of wolves (Canis lupus) to the Rocky Mountains in North America (Ripple and Beschta, 2004;Wolf et al, 2007). Willows (Salix spp.)…”

Herbivore control in connected seascapes: habitat determines when population regulation occurs in the life history of a key herbivore

“…Nearshore macrophyte ecosystems are particularly prone to overgrazing, shifting to heavily overgrazed barrens when herbivore populations expand without control (Wolf et al 2007, Ling et al 2015. When ecological roles are disproportionately distributed across the species assemblage, the health of the ecosystem is linked intimately with the population ecology of a few key species.…”