No abstract

Studies of animal movement and migration over large geospatial scales have long relied on natural continental-scale hydrogen isotope (δ2H) gradients in precipitation, yet the physiological processes that govern incorporation of δ2H from precipitation into plant and then herbivore tissues remain poorly understood, especially at the molecular level. Establishing a biochemical framework for the propagation of δ2H through food webs would enable us to resolve more complicated regional-scale animal movements and potentially unlock new applications for δ2H data in animal ecology and eco-physiology. Amino acid δ2H analysis offers a promising new avenue by which to establish this framework. We report bulk tissue δ2H, δ13C, and δ15N data as well as amino acid δ2H and δ13C data from three Pipevine swallowtail (Battus philenor) tissues—caterpillars, butterfly bodies, and wings—as well as their obligate plant source: pipevine leaves (Aristolochia macrophylla). Insects are often dominant herbivores in terrestrial food webs and a major food source for many higher-level consumers, so it is particularly important to understand the mechanisms that influence insect tissue δ2H values. Our data reveal extensive δ2H variation within and among individuals of a relatively simple plant-herbivore system that cannot be explained by temporal or geospatial gradients of precipitation δ2H or dietary differences. Variations in essential amino acid δ2H and δ13C indicate that B. philenor acquire these compounds from an additional source that is isotopically distinct from pipevine leaves, potentially gut microbes. We also found multiple isotopic carryover effects associated with metamorphosis. This study emphasizes the strong influence of physiology on consumer-diet δ2H discrimination in a local population of pipevines and swallowtails and provides a template that can be broadly applied to Lepidoptera—the second most diverse insect order—and other holometabolous insects. Understanding these physiological mechanisms is critical to interpreting the large degree of δ2H variation in consumer tissues often observed at a single collection site, which has implications for using δ2H isoscapes to study animal movement. Further investigation into amino acid δ2H holds promise to elucidate how subsets of amino acids may be best utilized to address specific ecological and physiological questions for which bulk tissue δ2H is insufficient.

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Stable isotope analyses of archival specimens have revealed trophic declines over the past 150 yr in a growing number of predatory pelagic seabirds that breed in the Hawaiian Islands. However, they have not examined whether isotopic shifts occurred primarily during a specific phase of the annual cycle, which could allow us to better identify the causes of trophic declines and potential consequences for affected populations. We evaluated seasonal (breeding versus nonbreeding season) foraging habits of 2 ecologically distinct species, Newell’s shearwater Puffinus newelli and Laysan albatross Phoebastria immutabilis, and extended this analysis back 50 and 100 yr, respectively. Our assessment relied on amino acid δ15N proxies for nutrient regime use (δ15NPhe) and trophic position (Δδ15NGlu-Phe) from 2 tissues (feather and bone collagen) reflecting different time scales. Both study species exhibited season-specific isotopic shifts resulting in more pronounced seasonality in modern populations. We also identified inter-species differences in nutrient regime use, regardless of season. Laysan albatross experienced a trophic decline exclusive to the breeding season, while their nonbreeding season foraging ecology has remained constant over the past century. In contrast, the nutrient regime at the base of the food chain for Newell’s shearwaters during the nonbreeding season underwent a shift within the last 50 yr, and the trophic decline they experienced was heavily weighted toward the nonbreeding season. Efforts to mitigate potential fitness consequences of future trophic declines might benefit from focusing on fisheries management near the Hawaiian Islands, where susceptible seabirds forage during winter/spring, rather than the entire North Pacific.