Long-term ecosystem responses to livestock removal in the Mojave Desert

Background/Question/Methods

Disturbances associated with large mammal herbivory can have dramatic ecosystem effects, in some cases leading to a degraded ecosystem state. Although removal of disturbance factors may help effect recovery to pre-disturbance conditions, in some cases disturbance removal may have unexpected or even negative effects. For example, if disturbance is so intense that an ecosystem crosses a threshold of degradation, conditions following disturbance removal may favor introduced over native plant species and some changes may be effectively irreversible. At a site in the Mojave Desert, we asked how plant communities and soil characteristics responded to removal of livestock grazing, and how these responses varied according to historic grazing intensity. We investigated properties of soils and plant communities immediately following and seven years post-livestock removal in previously grazed desert scrub of the Mojave National Preserve (MNP). Data were collected along grazing gradients associated with former livestock water points to better understand how the net effects of livestock removal may vary according to historic grazing intensity.

Results/Conclusions

Preliminary examination of our data suggests that, in the absence of active restoration efforts, ecosystem recovery following livestock removal occurs very slowly. Seven years following livestock removal, heavily impacted areas (100 m from water points) were still easily distinguished from lower-impact areas (1600 m from water points) according to variables such as shrub density (lower at 100 m) and gap sizes between perennial species (larger at 100 m). Soils also were still more compacted in high impact areas after seven years, though appeared to have become less compacted at 100 m. Overall, the effects of historic livestock grazing are still apparent for soils and native perennial vegetation seven years after livestock removal. The results of this study lend insight into how time and grazing history affect recovery of a desert ecosystem characterized by slow plant growth. The impetus for livestock removal from MNP was conservation of habitat for the federally threatened desert tortoise (Gopherus agassizii), and our results suggest that further active restoration efforts (beyond livestock removal) may be necessary to achieve ecosystem recovery at timescales required for conservation.