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Contributed Talk

Session: : Communities: Disturbance And Recovery 7

COS 144-4 - Passive recovery of grazing on Santa Cruz Island: 38 years post-eradication of invasive sheep

Wednesday, August 9, 2023
8:45 AM – 9:00 AM PDT
Location: B113
Abstract: Following ecosystem disturbance, allowing passive recovery is a widespread strategy for habitat restoration. Without assisted recovery, ecosystem structure could take multiple decades to return to a system resembling the reference community. Santa Cruz Island, California, where destructive grazing by introduced ungulate herbivores greatly altered vegetation composition, is an important case study for understanding long-term passive vegetation recovery. To investigate how vegetation has responded 38 years after the removal of grazers (1984), we resampled 15 transects on two sides of a fence historically used to separate the grazed and ungrazed portions of the island. The transects were first sampled in 1980, when heavy grazing was ongoing (four years before the eradication of sheep), then 28 years after the cessation of grazing (2012), and lastly 38 years after the cessation of grazing (2022). By comparing temporal trends in vegetation type, as well as current plant species composition between the grazed vs ungrazed sides of the fence, we assessed the progress of passive vegetation recovery and possible legacy effects of grazing on Santa Cruz Island. We found that the woody vegetation, dominated by the island endemic shrub Eriogonum arborescens, followed a similar trajectory of recovery in both grazed and ungrazed sites. Woody vegetation significantly increased in grazed and ungrazed sites from 1980 to 2012, then did not significantly change from 2012 to 2022, in which the woody vegetation cover in ungrazed sites remained significantly higher than in grazed sites during 2012 and 2022. This may suggest that the recovery of native shrubland has slowed or stalled after the period of rapid recovery immediately following the removal of herbivores (1984), in which the ungrazed community has a higher woody recovery due to the earlier removal of grazing pressure compared to the grazed community. We also found that plant community composition significantly differed between the grazed and ungrazed sites in 2022. Specifically, the grazed plant community was more diverse than the ungrazed community, and this difference was driven by native species. The higher diversity in the grazed sites suggests that removal of heavy grazing pressure and subsequent passive recovery can result in higher diversity driven by both native herbaceous and native woody species . Overall, these results indicate that post-herbivore removal vegetation recovery may be non-linear and that decadal-scale studies are important for understanding recovery trajectories.