Seabird species‐ and assemblage‐level isotopic niche shifts associated with changing prey availability during breeding in coastal Newfoundland

Abstract

Shifting prey availability can lead to altered species interactions, indicated by variation in the dietary niche breadth and position of species within an assemblage. On the Newfoundland coast, annual inshore spawning migration of the dominant forage fish, Capelin Mallotus villosus, provides an excellent opportunity to investigate the influence of varying prey availability on dietary niche breadth and position among species. During June–August 2017, we investigated species‐ and assemblage‐level dietary responses to shifting Capelin availability of three Capelin‐eating, sympatrically breeding auk species, the Atlantic Puffin Fratercula arctica, Razorbill Alca torda and Common Murre Uria aalge. The diet of Leach's Storm Petrels Oceanodroma leucorhoa, which breed alongside the three auk species but are not known to rely on Capelin, was also examined to determine dietary shifts throughout breeding that were unrelated to Capelin availability. We quantified stable isotope ratios (δ15N, δ13C) in seabird blood components (plasma, cellular component) collected both before and after spawning Capelin arrived in the study area and compared isotopic niche breadth within a Bayesian framework. At the species level, auk trophic position increased and isotopic niche breadth narrowed after Capelin arrived, suggesting a more Capelin‐based diet. Simultaneously, trophic diversity of the auk assemblage, reflecting the extent of spacing among niches of species, decreased after spawning Capelin arrived inshore. Contrastingly, increased trophic position but broader isotopic niche breadth during higher relative to lower Capelin availability for Leach's Storm Petrel confirm that this species is probably not affected by the inshore arrival of Capelin, but instead that isotopic changes may be more related to a shift in breeding stage to chick‐rearing. Overall, our findings reiterate the importance of Capelin as a prey resource for breeding auks in coastal Newfoundland, but that the degree of reliance on Capelin varies among species, possibly allowing coexistence of these ecologically similar species. The findings highlight potential changing species interactions, such as increased competition, under declines in Capelin biomass.

Moult chronology and strategies of sympatric Great (Ardenna gravis) and Sooty (A. grisea) Shearwaters based on stable isotope analysis

“…Additionally, Capelin collected in the study area in both years had similar d 13 C (À20.2 to À20.4&) and d 15 N (12.0-12.2&; Carvalho & Davoren 2020) to our shearwater feathersalthough these have not been corrected with a discrimination factor. The isotopic niche breadth of both shearwater species in the study area has been shown to narrow throughout the summer seasonalso suggesting convergence on a similar prey type/ diet (Gulka et al 2017, which has been found for other marine predator species in coastal Newfoundland (Gulka et al 2017, Jenkins & Davoren 2021. Additionally, these two species also show high spatial overlap at sea (Carvalho & Davoren 2019.…”

High trophic niche overlap in mixed‐species colonies using artificial nests

“…2017b) may allow species to contract their dietary niche to focus on temporarily super‐abundant prey, increasing their overlap without necessarily increasing interspecific competition (Forero et al . 2004, Jenkins & Davoren 2020). The lack of differences in productivity between European Rollers breeding solitarily and in mixed‐species colonies, or between Lesser Kestrels in small or large colonies (Supporting Information Table S3), suggests that prey depletion around colonies in our study area is unlikely and that food availability is high enough to allow species with similar niches to coexist.…”

“…Alternatively, high niche overlap among sympatric species may reflect high availability of resources (Pianka 1981, Charter et al 2018, Jenkins & Davoren 2020. In the Castro Verde Special Protection Area, traditional farming practices associated with agro-steppes are maintained through specific agri-environmental policy schemes, ensuring high-quality foraging habitats for many farmland species (Catry et al 2013, 2017a, Silva et al 2018.…”

Amino Acid δ15N Can Detect Diet Effects on Pollution Risks for Yellow-Legged Gulls Overlooked by Trophic Position

“…Estimations of their trophic position (TP) are key to determine ecosystem properties as food-chain length (Vander Zanden and Fetzer, 2007;Reum et al, 2015) or the biomagnification of pollutants (Storr-Hansen et al, 1995;Dietz et al, 2000;Kelly et al, 2008). For instance, changes in TP are indicative of alterations in food web structure (Jenkins and Davoren, 2020) or in nutrient inputs (Ruiz-Cooley et al, 2017). Particularly, seabirds have been extensively used as biomonitors of persistent organic and trace metal contaminants (e.g., OSPAR, 1999), with biomagnification effects critically dependent on their TP (Ramos et al, 2013;Gatt et al, 2020).…”