Seaside Sparrows reveal contrasting food web responses to large‐scale stressors in coastal Louisiana saltmarshes

Abstract

Turner. 2017. Seaside Sparrows reveal contrasting food web responses to large-scale stressors in coastal Louisiana saltmarshes. Ecosphere 8(7):e01878. 10.1002/ecs2.1878Abstract. Large-scale ecosystem disturbances can alter the flow of energy through food webs, but such processes are not well defined for Gulf of Mexico saltmarsh ecosystems vulnerable to multiple interacting stressors. The 2010 Deepwater Horizon (DWH) oil spill significantly affected the composition of terrestrial saltmarsh communities in Louisiana, and thus had the potential to alter energy pathways through terrestrial and aquatic food webs, with direct consequences for higher trophic-level species restricted to these habitats. The Seaside Sparrow (Ammodramus maritimus) is endemic to saltmarshes and relies completely on the habitat and resources they provide; thus, the sparrows can serve as indicators of ecological change in response to disturbances. We analyzed food web pathways for birds residing in oiled and unoiled saltmarshes for the four years following the oil spill by quantifying the bulk carbon and nitrogen stable isotopes and fatty acid profiles of liver tissues, in addition to primary producers (e.g., marsh grasses) and invertebrate consumers representing the major energy resources in these systems. The stable isotope values of primary producers and most invertebrate consumers did not differ between oiled and unoiled sites, suggesting that the energy pathways within the food web were stable in spite of observed declines in these populations following the spill. The tracer profiles of the Seaside Sparrows confirmed that there was a nominal effect of oil on resource use or trophic position (TP). However, we detected significant interannual variation in resource use by these birds; the sparrows occupied a lower TP and exhibited greater assimilation of resources derived from benthic-aquatic relative to terrestrial pathways in 2013 compared to other years. This distinction is likely attributable to the effects of Hurricane Isaac in 2012, whose significant storm surge extensively inundated the saltmarsh landscape. Despite widespread concern for the saltmarsh ecosystem after the DWH event, the significant effects noted at the population level translated into only subtle differences to the flow of energy through this food web. These results demonstrate varying responses to different degrees of landscape-level disturbance, such as oil and hurricanes, and establish the need to better understand food web dynamics in these saltmarsh ecosystems.

A multi‐biomarker approach supports the use of compound‐specific stable isotope analysis of amino acids to quantify basal carbon source use in a salt marsh consumer

“…Seaside sparrows ( n = 17) were flushed into a continuous line of four to five, 12‐m mist nets oriented perpendicular to the marsh edge in June 2014 in Barataria Bay near Port Sulphur, LA, USA (see Olin et al) . Netted individuals were euthanized via thoracic compression and dissected for liver tissue collection in the field.…”

“…For complete FAA sample preparation, laboratory analyses, and analytical precision, see Olin et al Briefly, FA profiles were quantified using a three‐step procedure: (1) triplicate extractions of freeze‐dried liver tissue (sparrows), muscle tissue (crabs), or whole insects in a 2:1 chloroform/methanol solution for gravimetric determination of the total lipid; (2) derivatization of FA methyl esters (FAMEs) using sulfuric acid in methanol (1:100 mixture); and (3) identification and quantification of FAMEs using a model 7890N gas chromatograph (Agilent Technologies, Santa Clara, CA, USA) coupled to a flame ionization detector. Seaside sparrow and invertebrate samples were analyzed alongside Supelco (Sigma‐Aldrich, St Louis, MO, USA) and Nu‐Chek (Elysian, MN, USA) FA standard mixes and an internal standard (α‐cholestane; Sigma‐Aldrich 170 no.…”

“…Tissue samples from seaside sparrows and invertebrates that remained following FAA extractions were used for bulk tissue SIA. For complete SIA sample preparation, laboratory analyses, and analytical precision, see Olin et al. Briefly, the relative abundances of carbon ( 13 C/ 12 C) and nitrogen ( 15 N/ 14 N) stable isotopes were determined using ca 0.5–1.0 mg lipid‐free sub‐samples using a ThermoFinnigan Delta Plus isotope ratio mass spectrometer (ThermoFinnigan, San Jose, CA, USA) coupled to an elemental analyzer (Costech, Valencia, CA, USA).…”

“…SIA and FAA) where trophic modification is imprecisely understood. We focused on the seaside sparrow as a model organism in the salt marsh ecosystem, because it is resident in these habitats across its life history and forages both amongst emergent vegetation and along the marsh sediment surface . We chose primary consumers as proxies for distinct basal carbon sources available to sparrows.…”

“…We focused on the seaside sparrow as a model organism in the salt marsh ecosystem, because it is resident in these habitats across its life history and forages both amongst emergent vegetation and along the marsh sediment surface. [33][34][35][36] We chose primary consumers as proxies for distinct basal carbon sources available to sparrows. Specifically, leafhoppers (Prokelisia spp., family Delphacidae) and leaf bugs (Trigonotylus sp., family Miridae) represent emergent vegetation-derived (i.e.…”