Title:  Avian response to structural changes in willow (Salix spp.) dominated habitats in Yellowstone’s Northern Range: top-down versus bottom-up controls of ecosystem function

Contact: Lisa Baril

Funding: Yellowstone National Park

Project Abstract           

         Yellowstone National Park’s Northern Range has experienced a loss of deciduous woody vegetation since the turn of the 20^th century.  The loss and low stature of deciduous woody vegetation as been attributed to factors including elk (Cervus elaphus) herbivory, loss of beaver (Castor canadensis), fire, and/or climate change.   However expansion of this habitat type, in particular willows (Salix spp.), has been observed in recent years.  Although willow communities are a rare habitat type in YNP they may be critically important to the persistence of several bird species associated with willows in the region.

            This “release” of woody vegetation coincides with the reintroduction of wolves (Canis lupus) in 1995 and 1996.  It has been suggested that the expansion of deciduous woodlands is the result of a “trophic cascade” where by predation by wolves has altered the density and foraging habits of elk resulting in reduced herbivory and growth of deciduous woody plants.  This hypothesis has stimulated considerable interest and research in how top level predators may drive “top-down” controls on ecosystem function and biodiversity and has renewed interest in reintroducing carnivores to locations where they were extirpated by humans to restore ecosystems.  An alternative hypothesis is that willow growth is due to warmer growing season temperatures.  The release of deciduous vegetation also coincides with years of above average annual temperatures and relatively long growing seasons suggesting bottom-up controls on ecosystem function. 

            The objectives of this study are to quantify change in the aerial extent of willow across the YNP Northern Range, determine how bird species abundances and community diversity vary with aerial extent and height of willow and test hypotheses on controls of willow by determining spatial and temporal patterns of association between willow expansion and wolf distribution and growing season temperature.  Results of this study will lead to a better understanding of avian communities associated with riparian willow habitats and factors responsible in driving willow release.

 

 

 

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Last modified  May 9, 2006