Impacts and interactions of multiple human perturbations in a California salt marsh

Abstract

Multiple disturbances to ecosystems can influence community structure by modifying resistance to and recovery from invasion by non-native species. Predicting how invasibility responds to multiple anthropogenic impacts is particularly challenging due to the variety of potential stressors and complex responses. Using manipulative field experiments, we examined the relative impact of perturbations that primarily change abiotic or biotic factors to promote invasion in coastal salt marsh plant communities. Specifically we test the hypotheses that nitrogen enrichment and human trampling facilitate invasion of upland weeds into salt marsh, and that the ability of salt marsh communities to resist and/or recover from invasion is modified by hydrological conditions. Nitrogen enrichment affected invasion of non-native upland plants at only one of six sites, and increased aboveground native marsh biomass at only two sites. Percent cover of native marsh plants declined with trampling at all sites, but recovered earlier at tidally flushed sites than at tidally restricted sites. Synergistic interactions between trampling and restricting tidal flow resulted in significantly higher cover of non-native upland plants in trampled plots at tidally restricted sites. Percent cover of non-native plants recovered to pre-trampling levels in fully tidal sites, but remained higher in tidally restricted sites after 22 months. Thus, perturbations that reduce biotic resistance interact with perturbations that alter abiotic conditions to promote invasion. This suggests that to effectively conserve or restore native biodiversity in altered systems, one must consider impacts of multiple human disturbances, and the interactions between them.

Salt Marsh as a Coastal Filter for the Oceans: Changes in Function with Experimental Increases in Nitrogen Loading and Sea-Level Rise

“…In terms of N application rates and loads, our study is closely matched to the extra-high fertilization treatment in Great Sippewissett Salt Marsh and exceeds the ∼70 µM NO 3 − treatment by four orders of magnitude. Other studies of nutrient enrichment in U.S. Pacific Coast, pickleweed-dominated salt marsh have shown that urea addition increases salt marsh productivity, alters community structure [37], [52] and increases susceptibility to species invasions [53]. Organic forms of N, such as urea, depend on microbial mineralization for plant availability and are therefore considered “slow-release” applications [54].…”

Physical disturbance determines effects from nitrogen addition on ground vegetation in boreal coniferous forests

“…; Strengbom et al. ; Goldman Martone & Wasson ; Houseman et al. ) and that variability in plant community responses to nutrient addition is largely context‐dependent (Pennings et al.…”

“…Moreover, N enrichment may, in combination with physical disturbance, increase the risk of establishment of invasive plant species (e.g. Hobbs & Huenneke ; Burke & Grime ; Leishman & Thomson ; Goldman Martone & Wasson ). According to the ‘ fluctuating resource hypothesis ’ (Davis et al.…”

Multiple Stressors Influence Salt Marsh Recovery after a Spring Fire at Mugu Lagoon, CA

“…While examples from the Gulf and East coast ecosystems can provide some guidance as to how long-term recovery of ecosystem processes like accretion and carbon storage may progress after fire, the timing of the recovery in S. pacifica-dominated systems may result in vastly different outcomes. Multiple disturbances to salt marsh ecosystems have proven complex for ecosystem health and management (Martone and Wasson 2008;Zedler 2010). The Green Meadows and Camarillo Springs Fires highlight the need for ecosystem level management in the face of multiple environmental stressors, particularly in the face of accelerated SLR.…”