Stress and subsidy effects of seagrass wrack duration, frequency, and magnitude on salt marsh community structure

Abstract

Environmental perturbations can strongly affect community processes and ecosystem functions by acting primarily as a subsidy that increases productivity, a stress that decreases productivity, or both, with the predominant effect potentially shifting from subsidy to stress as the overall intensity of the perturbation increases. While perturbations are often considered along a single axis of intensity, they consist of multiple components (e.g., magnitude, frequency, and duration) that may not have equivalent stress and/or subsidy effects. Thus, different combinations of perturbation components may elicit community and ecosystem responses that differ in strength and/or direction (i.e., stress or subsidy) even if they reflect a similar overall perturbation intensity. To assess the independent and interactive effects of perturbation components, we experimentally manipulated the magnitude, frequency, and duration of wrack deposition, a common stress-subsidy in a variety of coastal systems. The effects of wrack perturbation on salt marsh community and ecosystem properties were assessed both in the short-term (at the end of a 12-week experimental manipulation) and long-term (6 months after the end of the experiment). In the short-term, plants and associated benthic invertebrates exhibited primarily stress-based responses to wrack perturbation. The extent of these stress effects on density of the dominant plant Spartina alterniflora, total plant percent cover, invertebrate abundance, and sediment oxygen availability were largely determined by perturbation duration. Yet, higher nitrogen content of Spartina, which indicates a subsidy effect of wrack, was influenced primarily by perturbation magnitude in the short-term. In the longer term, perturbation magnitude determined the extent of both stress and subsidy effects of wrack perturbation, with lower subordinate plant percent cover and snail density, and higher Spartina nitrogen content in high wrack biomass treatments. However, stress effects on the marsh community were generally less pronounced 6 months after the wrack perturbation, indicating capacity for recovery. Our results demonstrate that individual perturbation components can determine the degree to which its effects on the community elicit primarily stress- and/or subsidy-based responses. Further, the nature and extent of stress-subsidy effects can change over time, depending on species' relative ability to tolerate and/or recover from perturbation.

Effects of oil exposure, plant species composition, and plant genotypic diversity on salt marsh and mangrove assemblages

“…However, the absence of significant oiling effects on S. alterniflora aboveground traits (density, stem height, leaf growth) at the end of the experiment suggests that this species does have some resilience to oil exposure, which is consistent with studies finding that S. alterniflora has a high capacity for disturbance recovery (Hanley et al. ). This aboveground recovery likely occurred due to a translocation of resources from belowground tissues, as evidenced by reduced belowground biomass in the oiled treatments at the end of the experiment.…”

Non‐resource effects of foundation species on meta‐ecosystem stability and function

“…Efforts to understand the functional effects of abiotic connectivity across spatial scales have mainly focused on the trophic assimilation of transported resources (Polis and Hurd 1996, Phoenix et al 2012, Sitters et al 2015 and regional implications of biotic-abiotic interactions mediated by foundation species remain unresolved (Hastings et al 2007). In coastal systems, resource subsidies have been shown to have strong effects on vegetation structure and their accumulation can facilitate disturbances (van de Koppel et al 2005, Deegan et al 2012, Hanley et al 2017. Our field observations provide further evidence for this feedback in mussel bed ecosystems.…”

Complex interactions among stressors evolve over time to drive shifts from short turfs to macroalgae on tropical reefs

“…These local stressors likely commonly occur with a range of magnitudes, frequencies, and durations, yet these characteristics are often not explicitly compared and contrasted in ecological research. Thus, our understanding of the impacts of local stressor temporal regimes is in its infancy (but see Murphy et al 2012, Hanley et al 2017, Fong et al 2018). Here, we provide a novel demonstration that the effects of stressor temporal regime varied and multiple stressors interacted to exhibit complex, emergent interaction effects, demonstrating the need to explicitly contrast stressor temporal regimes under multiple conditions to understand how communities will respond to future challenges.…”

“…Experimental research directly contrasting stressor regimes indicates different temporal stressor regimes have disparate impacts on populations and communities (e.g., Hewitt and Norkko 2007, Molinos and Donohue 2011, Murphy et al 2012, Hanley et al 2017, Fong and Fong 2018). Additionally, there is some indirect evidence that press vs. pulse stressors may differ in their impacts.…”