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Abstract Burrowing ecosystem engineers, such as termites, crabs, marmots, and foxes, can profoundly affect the biological structure and ecosystem functions of their environments. However, the relative importance of the effects of burrowing engineers on sediments are challenging to predict and are expected to be influenced by engineer density, engineer functional traits (e.g., burrow morphology), and environmental conditions (e.g., geomorphology, vegetation presence). To develop robust hypotheses predicting the impacts of burrowing ecosystem engineers, we conducted a systematic meta‐analysis evaluating the effects of burrowing crabs on sediment properties, nutrient stocks, and ecosystem functions in soft‐sediment coastal habitats (e.g., salt marshes, mangrove forests, tidal flats). Additionally, we tested the impacts of crab burrow density, burrowing crab superfamily (a proxy for crab burrow morphology and diet), and biotic conditions (i.e., vegetation) on the effects of burrowing crab engineers on coastal sediments. Burrowing crabs rework and oxygenate sediments and accelerate rates of nutrient cycling (i.e., nitrification and CO2 flux). However, the magnitude and direction of burrowing crab effects depend on burrowing crab superfamily, the presence of vegetation, and their interaction. Crab burrow density did not consistently predict burrowing engineer effects on sediments. Future efforts need to focus on implementing rigorous manipulative experiments to assess crab ecosystem engineering effects, since methodological variation has hindered efforts to generalize their effects. Our findings suggest that crab engineering effects are predictable across environmental contexts, and understanding the context dependency of crab engineering effects may promote the management and restoration of the critical ecosystem services that are mediated by crab engineers.

Estuaries, lagoons, and embayments are key components of the coastal ecosystem and they are pervasively exposed to a variety of stressors – the most common of which is eutrophication. In shallow water coastal systems, eutrophication typically manifests as shifts in sediment oxygen horizons and accumulation of toxic reduced compounds in the porewater, which have direct, negative effects on the benthic fauna that live in these sediments. To catalog, prevent, and remediate these stressors, the coastal zone management community has expressed a need to identify targets for biostimulatory compounds and products in the environment that are protective of biological integrity of coastal waters. In response to this need, the present study identified and validated a series of sediment total organic carbon (TOC) and total nitrogen (TN) concentrations associated with changes in macrobenthic community composition across the estuaries and embayments of the Southern California Bight (SCB). Using data from a regional monitoring program, benthic community condition, quantified using the M-AMBI condition assessment index, was modelled as a function of TOC or TN concentration using logistic regression. Then, we evaluated eutrophication thresholds at concentrations corresponding to several relative probabilities (from 0.6 to 0.8), reflecting a range of policy makers’ potential tolerance for risk of failing to meet biointegrity goals. Organic matter thresholds were extracted from the regressions that were predictive of healthy macrobenthic community condition at a 0.8 probability (1.23 mg g−1 TN, 15.51 mg g−1 TOC), 0.7 probability (2.58 mg g-1 TN, 22.9 mg g−1 TOC), or 0.6 probability (3.68 mg g−1 TN, 28.96 mg g−1 TOC) – as an illustration of how these types of stressor response models could be used to set targets for management of these ecosystems. These thresholds were subsequently validated by applying them to data held back from the model creation and by comparing them to taxon-specific inflection points identified from TITAN changepoint analysis of the macrobenthic/TOC/TN calibration data. The different TOC and TN thresholds correctly classified between 67 and 86% of the validation samples, with most of the misclassifications being instances of low organic matter concentration but poor community condition (i.e., false positives). The TOC thresholds identified from our Southern California dataset fell within a similar range (10–35 mg g−1) that has been linked to benthic community impacts from a variety of coastal ocean sites across the Northern Hemisphere. The consistency in thresholds across multiple habitats and different types of biota is suggestive of a general, quantitative threshold for organic matter accumulation in near shore sediment habitats and could be useful for informing the management of coastal ecosystems and setting targets biostimulatory stressors in these systems.

Abstract Burrowing animals profoundly influence plant communities, and changes in the burrower and plant communities together with changing abiotic parameters can shift the influence of burrowers on plants. However, we lack an ability to predict when, where, and how burrowers will influence vegetation. To begin to understand how naturally, varying environmental conditions influence the impacts of burrowers, we need to examine how burrower impacts on marsh plants differ across sites differing in environmental conditions. We manipulated crab presence for multiple years and measured the responses of the dominant plants, Pacific cordgrass (Spartina foliosa) and perennial pickleweed (Sarcocornia pacifica), at three sites in northern California and two sites in southern California. Southern California (Point Conception, CA, to the U.S.–Mexico border) experiences higher air and water temperatures, lower precipitation, and higher porewater salinity levels. Combining data from these field studies with laboratory studies allowed us to generate predictions about burrowing crab effects in salt marshes. Our models included (1) an estimate of grazing pressure on marsh plants by the dominant burrowing crab (Pachygrapsus crassipes) and (2) several soil biogeochemical measurements. Crab effects varied from strongly positive to strongly negative and depended upon estimated crab grazing pressure and edaphic conditions (salinity, ammonium, and nitrate). Relative to crabs at other sites, crabs enhanced cordgrass at sites with intermediate levels of ammonium and extreme salinities. The dependence of crab effects on edaphic conditions suggests that projected interannual variability in temperature, precipitation, and nutrients could lead to more temporally variable impacts of crabs on cordgrass. Understanding the environmental controls on these interactions will help promote cordgrass productivity and stabilize salt marsh ecosystems.