Linking a cougar decline, trophic cascade, and catastrophic regime shift in Zion National Park

The strength of top-down forces in terrestrial food webs is highly debated as there are few examples illustrating the role of large mammalian carnivores in structuring biotic and abiotic systems. Based on the results of this study we hypothesize that an increase in human visitation within Zion Canyon of Zion National Park ultimately resulted in a catastrophic regime shift through pathways involving trophic cascades and abiotic environmental changes. Increases in human visitors in Zion Canyon apparently reduced cougar (Puma concolor) densities, which subsequently led to higher mule deer (Odocoileus hemionus) densities, higher browsing intensities and reduced recruitment of riparian cottonwood trees (Populus fremontii), increased bank erosion, and reductions in both terrestrial and aquatic species abundance. These results may have broad implications with regard to our understanding of alternative ecosystem states where large carnivores have been removed or are being recovered. (c) 2006 Elsevier Ltd. All rights reserved.