Regional and intraseasonal variation in diet of wintering and staging Atlantic brant

Abstract

Regional and intraseasonal patterns of food use influence populations through impacts on breeding success, survival, and distribution of individuals. We used both traditional foregut content analysis and stable carbon and nitrogen isotopes in liver and leg muscle to determine intraseasonal patterns in the diet of Atlantic brant geese (Branta bernicla hrota) from early winter through spring staging (1 Dec-31 May 2007-2008) along the mid-Atlantic coast of the United States. Overall, brant diet consisted of macroalgae (52%), salt marsh cordgrass (22%), eelgrass (18%), and terrestrial grass and clover (8%). Mean d 13 C and d 15 N values differed among these 4 food sources. Therefore, we used an isotope mixing-model (SIAR) to estimate the relative contributions of each source to brant diet among regions and months. Wintering brant in northern and southern regions ate mostly macroalgae throughout the wintering period and ate more salt marsh and terrestrial grasses in spring. Brant in central regions had a more stable diet from December to May. Regional and intraseasonal patterns in brant diet are likely affected by several factors including variation in food source availability and quality due to synergistic effects of long-term annual and intraseasonal changes in abundance of submerged aquatic vegetation. Our estimates of diet combined with information on brant daily energy requirements and forage quality can be used to more accurately determine carrying capacity of wintering brant geese given established population objectives. Ó 2014 The Wildlife Society.

Isotopic Differences between Forage Consumed by a Large Herbivore in Open, Closed, and Coastal Habitats: New Evidence from a Boreal Study System

“…The isotopic difference between open and closed habitats observed in our study (3.0‰ on average) was slightly larger than in other studies conducted in boreal forests (0–2.2‰: [ 22 ], reviewed in [ 23 ] and [ 43 ]), and this holds for cuts (2.5‰) and peatlands (4.4‰), but not for canopy openings (1.9‰). The isotopic differences between terrestrial vegetation (open and closed habitats) and seaweeds (littoral habitats) were similar to the ranges reported in the literature (our study: δ 13 C = 14.8‰ and δ 15 N = 5.9‰; other studies: δ 13 C = [9.3‰,19‰] and δ 15 N = [6.4‰,9‰], [ 26 – 28 ]). The difference in δ 15 N between inland vegetation and coastal plants was 2.6‰, which is smaller than what has been observed for inland and coastal terrestrial plants in Africa (5–10‰; [ 29 ]), but closer to what has been observed for inland and coastal terrestrial plants in northern California (~3‰;[ 29 ]).…”

“…In our study, the isotopic values of forage from open, closed and supralittoral habitats were relatively similar compared to the isotopic values frequently used in diet reconstruction studies (e.g. the contrasting isotopic values of marine and terrestrial autotrophs; [ 25 , 26 , 28 ]). In addition, there was a high degree of overlap between isotopic values from closed, open and supralittoral habitats.…”

“…SIA may represent an additional tool to quantify the habitat-related foraging behaviour of forest dwelling herbivores inhabiting coastal areas, both at the scale of individuals and over large temporal scales. Indeed, carbon (δ 13 C) and/or nitrogen (δ 15 N) isotopic signatures of forage can vary between open and closed forest habitats [ 21 – 24 ], between plants and seaweed [ 25 – 28 ], and between inland and coastal plants (e.g. [ 29 ]).…”

“…Hence, our first objective was to determine whether we could detect isotopic differences between open, closed, supralittoral, and littoral habitats using the isotopic signatures of carbon (δ 13 C) and nitrogen (δ 15 N) of forage taxa consumed by white-tailed deer ( Odocoileus virginianus; hereafter “deer” ). We predicted lower δ 13 C in closed than in open habitats [ 21 – 24 ], and lower δ 13 C and δ 15 N in terrestrial (open and closed) than in supralittoral [ 29 ] and littoral habitats [ 26 – 28 ]. Because different vegetation communities can occupy open and closed habitats, respectively, and isotopic signatures can vary between vegetation taxa, our second objective was to verify whether we could detect isotopic differences between open and closed forest habitats for a single species; we retained Cornus canadensis as the species of interest because this forage species was found in both open and closed habitats and is heavily used by deer in our study system [ 31 , 32 ].…”

An Integrated Population Model for Harvest Management of Atlantic Brant

“…A wintering ground productivity survey provides an annual index of the proportion of juveniles in the population (i.e., age ratio; Roberts and Padding 2019). Despite these data, available models do not formally link between a given year's age ratio and future population metrics, thus ignoring an important process for long‐lived species with delayed breeding like geese (Koons et al 2006, 2014). Integrating MWS counts with these additional data would allow managers to leverage the relative strengths and weaknesses of each data source to improve harvest management decisions.…”