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Research Project: Understanding Water-Driven Ecohydrologic and Erosion Processes in the Semiarid Southwest to Improve Watershed Management

Location: Southwest Watershed Research Center

Title: Native lagomorphs suppress grass establishment in a shrub-encroached, semi arid grassland

Author
item Abercrombie, S.t. - University Of Arizona
item Koprowski, J.l. - University Of Arizona
item Nichols, Mary
item Fehmi, J.s. - University Of Arizona

Submitted to: Ecology and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2018
Publication Date: 11/1/2018
Citation: Abercrombie, S., Koprowski, J., Nichols, M.H., Fehmi, J. 2018. Native lagomorphs suppress grass establishment in a shrub-encroached, semi arid grassland. Ecology and Evolution. 2018:1-11.
DOI: https://doi.org/10.1002/ece3.4730

Interpretive Summary: Overgrazing by livestock has been thought to be a primary driver of shrub encroachment in the southwestern United States, but shrublands have continued to persist even after livestock removal and grassland restoration efforts. We hypothesized that small mammals, such as rodents and rabbits, may continue to suppress grass establishment even after livestock removal. We conducted an experiment in southeastern Arizona to exclude herbivores from enclosed areas where we placed lawn sod. Five different types of enclosures were used to allow entry by different animals depending on body size. We measured grass use before and during the rainy monsoon season, and used motion activated cameras to document animal visitations. Desert cottontails were detected more frequently than any of the other 11 herbivore species observed. Our results show that desert cottontails are significant consumers of herbaceous vegetation in shrub-encroached arid grasslands. Herbivory pressure from desert cottontails and other native herbivores may act as a biotic feedback contributing to the competitive advantage and persistence of shrubs. Grassland restoration and shrub reduction efforts should consider rabbit herbivory pressure and encourage populations of their natural predators, such as coyotes.

Technical Abstract: Over the last two centuries, shrub encroachment into arid grasslands has been associated with reduced grass abundance, increased soil erosion, and local declines in biodiversity. Overgrazing by livestock and the associated reduction of fine fuels has been thought to be a primary driver of shrub encroachment in the southwestern United States, but shrublands have continued to persist despite livestock removal and grassland restoration efforts. We hypothesized that the effects of herbivory feedbacks from native mammals may continue to suppress grass establishment even after livestock removal. An herbivore exclusion experiment in southeastern Arizona included five treatment levels allowed access to native mammals based on relative body size and separated the effects of rodents, lagomorphs, and mule deer. We included two control treatments and replicated each treatment 10 times (n = 50). We introduced uniform divisions of lawn sod (Cynodon dactylon) into each exclosure for 24-hour periods prior to (n = 2) and following (n = 2) the monsoon rains and used motion-activated cameras to document herbivore visitations. In the pre-monsoon trials, treatments that allowed lagomorph access had less sod biomass relative to other treatments (p < 0.001), averaging only 44% (± 36%) and 29% (± 45%) remaining biomass after the 24-hour trial periods. Following the onset of monsoons, differences in remaining biomass among treatments disappeared. Two lagomorph species occurred at our site: black-tailed jackrabbits (Lepus californicus) and desert cottontails (Sylvilagus audubonii). Desert cottontails were detected more frequently than any of the other 11 herbivore species present at the site, accounting for 83% of detections during the pre-monsoon trials. Significantly more (p < 0.001) desert cottontails were detected during the pre-monsoon trials (2,077) compared to the post-monsoon trials (174), which coincided with biomass removal from lagomorph accessible treatments. Synthesis and applications. Desert cottontails are significant consumers of herbaceous vegetation in shrub-encroached arid grasslands. Herbivory pressure from desert cottontails and other native herbivores may act as a biotic feedback contributing to the competitive advantage and persistence of shrubs. Grassland restoration and shrub reduction efforts should consider lagomorph herbivory pressure and encourage populations of their natural predators.