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1. Advancing the Sea Ice Hypothesis: Trophic Interactions Among Breeding Pygoscelis Penguins With Divergent Population Trends Throughout the Western Antarctic Peninsula

   https://doi.org/10.3389/fmars.2021.526092  

   Gorman, Kristen B. ; Ruck, Kate E. ; Williams, Tony D. ; Fraser, William R. ( September 2021 , Frontiers in Marine Science)

   We evaluated annual and regional variation in the dietary niche of Pygoscelis penguins including the sea ice-obligate Adélie penguin ( Pygoscelis adeliae ), and sea ice-intolerant chinstrap ( Pygoscelis antarcticus ) and gentoo ( Pygoscelis papua ) penguins, three species that nest throughout the western Antarctic Peninsula (AP) to test the sea ice trophic interaction hypothesis , which posits that penguin breeding populations with divergent trends, i.e., declining or increasing, are reliant on differing food webs. Our study relies on values of naturally occurring carbon ( 13 C/ 12 C, δ 13 C) and nitrogen ( 15 N/ 14 N, δ 15 N) stable isotopes as integrated proxies of penguin food webs measured over three years at three different breeding colonies. At Anvers Island in the north, where reductions in sea ice and changes in breeding population trends among sympatric sea ice-obligate (Adélie) and sea ice-intolerant (chinstrap and gentoo) penguins have been most notable, our analyses show that all three species of Pygoscelis penguins became more similar isotopically over the reproductive period. By late chick-rearing at Anvers Island, crèched chicks at 5-weeks-old for all species occupied similar trophic positions. Isotopic mixing models indicated that the proportions of prey provisioned bymore »adult penguins to 5-week-old chicks at Anvers Island were generally similar across species within years, consisting primarily of Antarctic krill ( Euphausia superba ). Crèched Adélie chicks had higher δ 13 C and δ 15 N values at Avian and Charcot Islands, southern breeding colonies where sea ice is more prominent and populations of Adélie penguins have increased or remain stable. Trophic position increased with latitude, while the proportions of prey provisioned by Adélie penguin adults to chicks at southern breeding colonies included species typical of high Antarctic marine food webs, especially crystal krill ( Euphausia crystallorophias ). A Bayesian metric for dietary niche width, standard ellipse area (SEA-B), indicated that Pygoscelis penguins with greater population changes in the north had more variability in dietary niche width than stable populations further south. Our results lend insight on marine food web drivers of Pygoscelis penguin reproduction at the regional scale and question the long-standing paradigm that Antarctic krill are the only food web component critical to penguin reproductive survival in this region of the Southern Ocean.« less
2. Contracting eastern African C4 grasslands during the extinction of Paranthropus boisei

   https://doi.org/10.1038/s41598-021-86642-z  

   Quinn, Rhonda L. ; Lepre, Christopher J. ( March 2021 , Scientific Reports)

   The extinction of theParanthropus boiseiestimated to just before 1 Ma occurred when C₄grasslands dominated landscapes of the Eastern African Rift System (EARS).P. boiseihas been characterized as an herbivorous C₄specialist, and paradoxically, its demise coincided with habitats favorable to its dietary ecology. Here we report new pedogenic carbonate stable carbon (δ¹³C_PC) and oxygen (δ¹⁸O_PC) values (nodules = 53, analyses = 95) from an under-sampled interval (1.4–0.7 Ma) in the Turkana Basin (Kenya), one of the most fossiliferous locales ofP. boisei. We combined our new results with published δ¹³C_PCvalues from the EARS dated to 3–0 Ma, conducted time-series analysis of woody cover (ƒ_WC), and compared the EARS ƒ_WCtrends to regional and global paleo-environmental and -climatic datasets. Our results demonstrate that the long-term rise of C₄grasslands was punctuated by a transient but significant increase in C₃vegetation and warmer temperatures, coincident with the Mid-Pleistocene Transition (1.3–0.7 Ma) and implicating a short-term rise inpCO₂. The contraction of C₄grasslands escalated dietary competition amongst the abundant C₄-feeders, likely influencingP. boisei’s demise.
3. Abyssal deposit feeders are secondary consumers of detritus and rely on nutrition derived from microbial communities in their guts

   https://doi.org/10.1038/s41598-021-91927-4  

   Romero-Romero, Sonia ; Miller, Elizabeth C. ; Black, Jesse A. ; Popp, Brian N. ; Drazen, Jeffrey C. ( December 2021 , Scientific Reports)

   Abstract Trophic ecology of detrital-based food webs is still poorly understood. Abyssal plains depend entirely on detritus and are among the most understudied ecosystems, with deposit feeders dominating megafaunal communities. We used compound-specific stable isotope ratios of amino acids (CSIA-AA) to estimate the trophic position of three abundant species of deposit feeders collected from the abyssal plain of the Northeast Pacific (Station M; ~ 4000 m depth), and compared it to the trophic position of their gut contents and the surrounding sediments. Our results suggest that detritus forms the base of the food web and gut contents of deposit feeders have a trophic position consistent with primary consumers and are largely composed of a living biomass of heterotrophic prokaryotes. Subsequently, deposit feeders are a trophic level above their gut contents making them secondary consumers of detritus on the abyssal plain. Based on δ 13 C values of essential amino acids, we found that gut contents of deposit feeders are distinct from the surrounding surface detritus and form a unique food source, which was assimilated by the deposit feeders primarily in periods of low food supply. Overall, our results show that the guts of deposit feeders constitute hotspots of organic matter on the abyssalmore »plain that occupy one trophic level above detritus, increasing the food-chain length in this detritus-based ecosystem.« less
4. Mercury source changes and food web shifts alter contamination signatures of predatory fish from Lake Michigan

   https://doi.org/10.1073/pnas.1907484116  

   Lepak, Ryan F. ; Hoffman, Joel C. ; Janssen, Sarah E. ; Krabbenhoft, David P. ; Ogorek, Jacob M. ; DeWild, John F. ; Tate, Michael T. ; Babiarz, Christopher L. ; Yin, Runsheng ; Murphy, Elizabeth W. ; et al ( November 2019 , Proceedings of the National Academy of Sciences)

   To understand the impact reduced mercury (Hg) loading and invasive species have had on methylmercury bioaccumulation in predator fish of Lake Michigan, we reconstructed bioaccumulation trends from a fish archive (1978 to 2012). By measuring fish Hg stable isotope ratios, we related temporal changes in Hg concentrations to varying Hg sources. Additionally, dietary tracers were necessary to identify food web influences. Through combined Hg, C, and N stable isotopic analyses, we were able to differentiate between a shift in Hg sources to fish and periods when energetic transitions (from dreissenid mussels) led to the assimilation of contrasting Hg pools (2000 to present). In the late 1980s, lake trout δ 202 Hg increased (0.4‰) from regulatory reductions in regional Hg emissions. After 2000, C and N isotopes ratios revealed altered food web pathways, resulting in a benthic energetic shift and changes to Hg bioaccumulation. Continued increases in δ 202 Hg indicate fish are responding to several United States mercury emission mitigation strategies that were initiated circa 1990 and continued through the 2011 promulgation of the Mercury and Air Toxics Standards rule. Unlike archives of sediments, this fish archive tracks Hg sources susceptible to bioaccumulation in Great Lakes fisheries. Analysis reveals thatmore »trends in fish Hg concentrations can be substantially affected by shifts in trophic structure and dietary preferences initiated by invasive species in the Great Lakes. This does not diminish the benefits of declining emissions over this period, as fish Hg concentrations would have been higher without these actions.« less
5. Dietary partitioning promotes the coexistence of planktivorous species on coral reefs.

   Leray, M. ; Alldredge, A. L. ; Yang, J. Y. ; Meyer, C. P. ; Holbrook, S. J. ; Schmitt, R. J. ; Knowlton, N. ; Brooks, A. J. ( January 2019 , Molecular ecology)

   Theories involving niche diversification to explain high levels of tropical diversity propose that species are more likely to co-occur if they partition at least one dimension of their ecological niche space. Yet, numerous species appear to have widely overlapping niches based upon broad categorizations of resource use or functional traits. In particular, the extent to which food partitioning contributes to species coexistence in hyperdiverse tropical ecosystems remains unresolved. Here, we use a molecular approach to investigate inter- and intraspecific dietary partitioning between two species of damselfish (Dascyllus flavicaudus, Chromis viridis) that commonly co-occur in branching corals. Species-level identification of their diverse zooplankton prey revealed significant differences in diet composition between species despite their seemingly similar feeding strategies. Dascyllus exhibited a more diverse diet than Chromis, whereas Chromis tended to select larger prey items. A large calanoid copepod, Labidocera sp., found in low density and higher in the water column during the day, explained more than 19% of the variation in dietary composition between Dascyllus and Chromis. Dascyllus did not significantly shift its diet in the presence of Chromis, which suggests intrinsic differences in feeding behaviour. Finally, prey composition significantly shifted during the ontogeny of both fish species. Our findings showmore »that levels of dietary specialization among coral reef associated species have likely been underestimated, and they underscore the importance of characterizing trophic webs in tropical ecosystems at higher levels of taxonomic resolution. They also suggest that niche redundancy may not be as common as previously thought.« less