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  1. Abstract

    Quantifying ecosystem resilience to disturbance is important for understanding the effects of disturbances on ecosystems, especially in an era of rapid global change. However, there are few studies that have used standardized experimental disturbances to compare resilience patterns across abiotic gradients in real‐world ecosystems. Theoretical studies have suggested that increased return times are associated with increasing variance during recovery from disturbance. However, this notion has rarely been explicitly tested in field, in part due to the challenges involved in obtaining long‐term experimental data. In this study, we examined resilience to disturbance of 12 coastal marsh sites (five low‐salinity and seven polyhaline [=salt] marshes) along a salinity gradient in Georgia, USA. We found that recovery times after experimental disturbance ranged from 7 to >127 months, and differed among response variables (vegetation height, cover and composition). Recovery rates decreased along the stress gradient of increasing salinity, presumably due to stress reducing plant vigor, but only when low‐salinity and polyhaline sites were analyzed separately, indicating a strong role for traits of dominant plant species. The coefficient of variation of vegetation cover and height in control plots did not vary with salinity. In disturbed plots, however, the coefficient of variation (CV) was consistently elevated during the recovery period and increased with salinity. Moreover, higher CV values during recovery were correlated with slower recovery rates. Our results deepen our understanding of resilience to disturbance in natural ecosystems, and point to novel ways that variance can be used either to infer recent disturbance, or, if measured in areas with a known disturbance history, to predict recovery patterns.

     
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  2. Abstract

    Consumers often deplete local resources and aggregate along edges of remaining resources, forming “consumer fronts.” We examined the factors that promoteSesarma reticulatumcrab aggregations at saltmarsh creek heads to explain the directional but slow movement of these fronts. We also created artificial creek heads to test the hypothesis that hydrological conditions at creek heads create superior habitat for crabs. Soil temperatures were ˜11–12% cooler, hydrogen sulfide concentrations lower (0.0 vs. ˜0.58 mg/L), and dissolved oxygen concentrations twofold higher at the creek head versus the marsh platform. In the artificial creek‐head experiment, altering hydrological conditions led to lower dissolved sulfide levels, higher dissolved oxygen levels, and increased densities of crab burrows andSesarmacrabs. Moreover, the elevation of the soil surface declined rapidly at artificial creek heads versus controls, suggesting that crabs were increasing erosion. Our results suggest that abiotic conditions for crabs are better at the leading edge of the creek head than the trailing edge, explaining the directional movement of the front. Moreover, the speed at which the front propagates appears to be limited by the rate at which the creekhead erodes, rather than by crab mobility. The directional and slow movement ofSesarmafronts compared to consumer fronts of other invertebrates appears to result from the inextricable link betweenSesarmaand marsh geomorphology, whereas other consumer fronts are associated mostly with food resources.

     
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