Ecological Thresholds for Salt Marsh Nekton and Vegetation Communities
Salt marsh vegetation and nekton respond to stressors in estuarine systems, providing ideal indicators of change. Here we characterize the structure of these communities along a gradient of anthropogenic stress (e.g., human population size of watersheds, degree of hydrological alteration of marshes), identify ecological thresholds that are linked to community condition, and provide potential decision thresholds for land managers based on an evaluation of community condition. Salt marsh nekton and vegetation community data (species composition and abundance) were compiled from over 180 discrete data sets from marshes along the Atlantic coast from Maine to Virginia. Using multivariate techniques (e.g., Principal Component Analysis, Canonical Correspondence Analysis, Analysis of Similarities), patterns of community change along a gradient of hydrologic impact and degree of watershed development were elucidated. Several levels of community complexity, individual species’ abundances, and relative abundances of life history-based groups were used to identify potential metrics for ecological thresholds. The nekton community displayed shifts in community structure along a gradient of human population size (e.g., anthropogenic stress gradient) in surrounding watersheds, from resident fish-dominated communities at marshes in watersheds with low human populations to shrimp-dominated communities (Palaemonidae species) in watersheds with high human populations. Vegetation communities from reference (relatively hydrologically undisturbed) marshes were dominated by obligate halophytes (e.g., salt meadow grasses) with low proportions of invasive plants. Impacted (e.g., tidally restricted) marshes had fewer halophytic obligate wetland plants, more facultative wetland plant species, and higher proportions of invasive species. Shifts in characteristics of nekton and vegetation communities toward the impacted state can be used as ecological thresholds upon which decision thresholds for land managers can be based. For example, detection of declining proportions of killifish and transient fish and increasing proportions of Palaemonidae shrimp (the ecological threshold metrics), would indicate a community changing from moderate to poor condition (the decision threshold), thereby triggering management actions. Ecological thresholds are presented for both nekton and vegetation communities and are presented in the context of a desired resource goal (good, moderate, poor condition) that can be used by resource managers to evaluate responses to restoration activities or the overall condition of the marsh community. Findings from this study are most directly relevant to northeastern US salt marshes, but the methods used to derive the ecological threshold metrics can be applied to other regions.
KeywordsGrass shrimp Palaemonidae shrimp Killifish Fundulidae species Salt marsh community condition Salt marsh nekton Salt marsh vegetation Anthropogenic stress
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