Original Paper

Biological Invasions

, Volume 12, Issue 2, pp 313-324

First online:

Introduced ungulate herbivore alters soil processes after fire

  • Michelle L. StritarAffiliated withSchool of Forestry and Merriam-Powell Center for Environmental Research, Northern Arizona University
  • , Jennifer A. SchweitzerAffiliated withDepartment of Ecology and Evolutionary Biology, University of Tennessee, Knoxville Email author 
  • , Stephen C. HartAffiliated withSchool of Natural Sciences and Sierra Nevada Research Institute, University of California
  • , Joseph K. BaileyAffiliated withDepartment of Ecology and Evolutionary Biology, University of Tennessee, Knoxville

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Ungulate herbivory can have profound effects on ecosystem processes by altering organic inputs of leaves and roots as well as changing soil physical and chemical properties. These effects may be especially important when the herbivore is an introduced species. Utilizing large mammal exclosures to prevent access by introduced elk at multiple sites along a fire chronosequence, we examined the effects of elk herbivory and fire on soil microbial activity and nutrient availability. Using time since fire as a co-variate and herbivore exclosures, paired with areas outside of the exclosures, we hypothesized that reductions in plant biomass due to herbivory would reduce organic inputs to soils and impact soil microbial activities and nutrient storage. We found three major patterns: (1) when elk were excluded, surface mineral soils had higher soil organic carbon (C), total nitrogen (N), microbial N pools, and increased extra-cellular enzyme activity of a C-acquiring enzyme across a gradient of time since fire. (2) When introduced elk are present, the activity of some extracellular enzymes as well as NO3 availability are enhanced in the soil but the post-fire patterns described above with respect to nutrient accrual over time are delayed. (3) Herbivory by an introduced ungulate upsets the trajectory of ecosystem “recovery” after wildfire and delays soil C and N dynamics by an estimated 14.5–21 years, respectively. These results suggest that introduced, browsing herbivores significantly decelerate ecosystem processes but herbivory by exotics may also result in unpredictability in specific soil responses.


Aspen Decelerated nutrient cycles Elk Fire Introduced herbivores Populus