Riparian ecosystem collapse in Rocky Mountain National Park

Understanding the drivers of ecosystem collapse is critical for resource
management, particularly for protected areas mandated to preserve
biodiversity. In Rocky Mountain National Park, Colorado, tall willows
(Salix spp.) dominated riparian vegetation and a beaver-willow state was
the natural ecosystem type in the Colorado River headwaters. However,
willows comprise a portion of elk diets, and are a preferred food for
recently introduced moose, and the vegetation structure has changed
dramatically since the early 2000s. To assess ecosystem changes we
analyzed time series data on willow height from 1997 to 2021 inside and
outside of three exclosures built to exclude ungulates, area of tall
willows in 1999 and 2019, area of open water from 1953 to 2019, vegetation
composition in 1998 and 2021, ground water depth from 1996 to 2021,
surface water flow from 1953 to 2023, and climate from 1950 to 2023. Tall
willow coverage and open water area declined by >90% from 1999 to
2019. Willow height outside the ungulate exclosures declined by more than
75% since the 1990s, yet within exclosures that were formerly browsed
willow height has increased by up to 500%. Tall willow communities have
largely been replaced by grasslands. Browsing by elk and moose likely
played a pivotal role in triggering a collapse of the beaver-willow state
and formation of an alternative moose-elk-grassland state that appears
stable and may be difficult to reverse without direct human action.
Restoration efforts should include a reduction of herbivory and
reconnection of the river with its floodplain.