Latitudinal variation in ecological opportunity and intraspecific competition indicates differences in niche variability and diet specialization of Arctic marine predators.

David J Yurkowski,Steve Ferguson,Derek C G Muir,Christina A D Semeniuk,Lisa L Loseto,Aaron T Fisk,Tanya M Brown,Emily S Choy

doi:10.1002/ece3.1980

Abstract

Individual specialization (IS), where individuals within populations irrespective of age, sex, and body size are either specialized or generalized in terms of resource use, has implications on ecological niches and food web structure. Niche size and degree of IS of near‐top trophic‐level marine predators have been little studied in polar regions or with latitude. We quantified the large‐scale latitudinal variation of population‐ and individual‐level niche size and IS in ringed seals (Pusa hispida) and beluga whales (Delphinapterus leucas) using stable carbon and nitrogen isotope analysis on 379 paired ringed seal liver and muscle samples and 124 paired beluga skin and muscle samples from eight locations ranging from the low to high Arctic. We characterized both within‐ and between‐individual variation in predator niche size at each location as well as accounting for spatial differences in the isotopic ranges of potential prey. Total isotopic niche width (TINW) for populations of ringed seals and beluga decreased with increasing latitude. Higher TINW values were associated with greater ecological opportunity (i.e., prey diversity) in the prey fish community which mainly consists of Capelin (Mallotus villosus) and Sand lance (Ammodytes sp.) at lower latitudes and Arctic cod (Boreogadus saida) at high latitudes. In beluga, their dietary consistency between tissues also known as the within‐individual component (WIC) increased in a near 1:1 ratio with TINW (slope = 0.84), suggesting dietary generalization, whereas the slope (0.18) of WIC relative to TINW in ringed seals indicated a high degree of individual specialization in ringed seal populations with higher TINWs. Our findings highlight the differences in TINW and level of IS for ringed seals and beluga relative to latitude as a likely response to large‐scale spatial variation in ecological opportunity, suggesting species‐specific variation in dietary plasticity to spatial differences in prey resources and environmental conditions in a rapidly changing ecosystem.

Highlights

Food web models are typically studied at the species level where trait variation among individuals is often not incorporated (Miller and Rudolf 2011)
Ecology and Evolution published by John Wiley & Sons Ltd
The slope between within-individual component (WIC) and Total isotopic niche width (TINW) for ringed seals was significantly lower than beluga, not significantly different from 0, and similar to relationships observed in “individual specialist” sea otters (Enhydra lutris; slope = 0.23; Newsome et al 2015), implying a high degree of dietary individuality in populations of ringed seals which have a larger TINW possibly driven by ecological opportunity and being omnivorous

Summary

Introduction

Food web models are typically studied at the species level where trait variation among individuals is often not incorporated (Miller and Rudolf 2011). It is widely accepted in the ecological literature that substantial dietary variation exists among individuals of a given species or population (Rudolf and Lafferty 2011). Diet specialization with latitude that differs across individuals (Bolnick et al 2003) As such, these individual specialists may have different ecological roles in terms of their habitat use and feeding relationships within an ecosystem. Individual specialists may be more susceptible to ecosystem perturbations such as changing prey diversity and abundance, than generalist ones (Miller and Rudolf 2011)

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Conclusion