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Woody plant encroachment is a main driver of landscape change in drylands globally. In the Chihuahuan Desert, past livestock overgrazing interacted with prolonged drought to convert vast expanses of black grama (Bouteloua eriopoda) grasslands to honey mesquite (Prosopis glandulosa) shrublands. Such ecosystem state transitions have greatly reduced habitat for grassland wildlife species, increased soil erosion, and inhibited the delivery of ecosystem services to local communities. The potential for wild herbivores to trigger or reinforce shrubland states may be underappreciated, however, and few studies compare herbivory effects across multiple consumer taxa. Here, I address the roles of multiple mammalian herbivores in driving or reinforcing landscape change in the Chihuahuan Desert by examining their effects on plant communities over multiple spatial and temporal scales, as well as across plant life stages. Moreover, I studied these herbivore effects in the context of precipitation pulses, long-term climate influences, competitive interactions, and habitat structure. I used two long-term studies that hierarchically excluded herbivores by body size over 25 years (Herbivore Exclosure Study) and 21 years (Ecotone Study), and a perennial grass seedling herbivory experiment. Native rodents and lagomorphs were especially important in determining grass cover and plant community composition in wet periods and affected perennial grass persistence over multiple life stages. Conversely, during drought, climate drove declines in perennial grass cover, promoting shrub expansion across the landscape. In that shrub-encroached state, native small mammals reinforced grass loss in part because habitat structure provided cover from predators. This research advances our understanding of an underappreciated component of ecosystem change in drylands – small mammal herbivory – and highlights the need to incorporate positive feedbacks from native small mammals into conceptual models of grassland-shrubland transitions.
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Modifying connectivity to promote state change reversal: the importance of geomorphic context and plant–soil feedbacks
Abstract Alternative states maintained by feedbacks are notoriously difficult, if not impossible, to reverse. Although positive interactions that modify soil conditions may have the greatest potential to alter self‐reinforcing feedbacks, the conditions leading to these state change reversals have not been resolved. In a 9‐yr study, we modified horizontal connectivity of resources by wind or water on different geomorphic surfaces in an attempt to alter plant–soil feedbacks and shift woody‐plant‐dominated states back toward perennial grass dominance. Modifying connectivity resulted in an increase in litter cover regardless of the vector of transport (wind, water) followed by an increase in perennial grass cover 2 yr later. Modifying connectivity was most effective on sandy soils where wind is the dominant vector, and least effective on gravelly soils on stable surfaces with low sediment movement by water. We found that grass cover was related to precipitation in the first 5 yr of our study, and plant–soil feedbacks developed following 6 yr of modified connectivity to overwhelm effects of precipitation on sandy, wind‐blown soils. These feedbacks persisted through time under variable annual rainfall. On alluvial soils, either plant–soil feedbacks developed after 7 yr that were not persistent (active soils) or did not develop (stable soils). This novel approach has application to drylands globally where desertified lands have suffered losses in ecosystem services, and to other ecosystems where connectivity‐mediated feedbacks modified at fine scales can be expected to impact plant recovery and state change reversals at larger scales, in particular for wind‐impacted sites.
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- Award ID(s):
- 1832194
- PAR ID:
- 10453960
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Ecology
- Volume:
- 101
- Issue:
- 9
- ISSN:
- 0012-9658
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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