Abstract
Shrub encroachment into arid grasslands has been associated with reduced grass abundance, increased soil erosion, and local declines in biodiversity. Livestock overgrazing and the associated reduction of fine fuels has been a primary driver of shrub encroachment in the southwestern United States, but shrublands continue to persist despite livestock removal and grassland restoration efforts. We hypothesized that an herbivory feedback from native mammals may contribute to continued suppression of grasses after the removal of livestock. Our herbivore exclusion experiment in southeastern Arizona included five treatment levels and allowed access to native mammals based on their relative body size, separating 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‐hr 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 44% (SD 36%) and 29% (SD 45%) remaining biomass after the 24‐hr trial periods. Following the onset of monsoons, differences in remaining biomass among treatments disappeared. Desert cottontails (Sylvilagus audubonii) 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. We conclude that desert cottontails are significant consumers of herbaceous vegetation in shrub‐encroached arid grasslands and they, along with other native herbivores, may act as a biotic feedback contributing to the competitive advantage and persistence of shrubs.
Highlights
Worldwide, the dynamics of arid and semiarid grasslands have shifted over the past 150 years to favor the spread and dominance of shrubs over forbs and grasses (Van Auken, 2009)
Based on their high detection frequency in pilot studies, we expected that mule deer (Odocoileus hemionus) and lagomorphs would be the primary consumers of herbaceous vegetation
For the amount of remaining biomass in the sod trials, we found significant differences for both the treatments (F4,45 = 14.38, p < 0.001) and the trial periods (F3,121 = 31.11, p < 0.001), as well as a significant interaction between treatment and trial period (F12,121 = 6.65, p < 0.001; Figure 4). In both of the pre‐monsoon trials (June and early July), we found a reduction in biomass in the MEDIUM and the TOTAL ACCESS treatments (F4,40 = 24.54, p < 0.001), but we did not detect a difference in treatment re‐ sponses across the two pre‐monsoon trial periods (F1,45 = 1.1, p = 0.298)
Summary
The dynamics of arid and semiarid grasslands have shifted over the past 150 years to favor the spread and dominance of shrubs over forbs and grasses (Van Auken, 2009). We expected that the degree of herbivory would vary by herbivore guild due to differences in dietary preferences, metabolic demands, and relative abundance, and that those effects could be differentiated among ex‐ closure treatments based on relative body size. Based on their high detection frequency in pilot studies, we expected that mule deer (Odocoileus hemionus) and lagomorphs (desert cottontails [Sylvilagus audubonii] and black‐tailed jackrabbits [Lepus californicus]) would be the primary consumers of herbaceous vegetation. We antici‐ pated that the seasonality of forage availability would influence the degree of herbivore pressure, increasing forage demand in the dry period before the primary growing season
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