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Hunter Roose, PhD Candidate and Elizabeth Jahn, PhD Student

Department of Biological Sciences, Michigan Technological Universty

Roose's Abstract:

Individuals within populations can differ markedly in their use of resources, generating variation in trophic niches that influence population-level niche structure. Interspecific competition is often thought to reduce a population’s total niche width (TNW), yet individuals may broaden their resource use to buffer against resource limitation. However not all individuals respond to competition in the same way; some increase their trophic niche breadth in response to resource scarcity, whereas others maintain narrow trophic niches or overlap extensively with competitors. Such variation in individual responses generates measurable differences in niche position and raises questions about which individuals occupy positions outside the population’s core realized niche. Recently, the concept of a keystone niche individual has been proposed to describe individuals whose resource use exerts a disproportionate effect on a population’s TNW and identifies those exploiting resources beyond the population’s typical niche space. Yet it remains unclear whether keystone niche individuals consistently emerge under competitive conditions, or whether negative interactions instead constrain individual niche divergence, limiting the expression of distinct trophic niches. Here, we explore the keystone niche individual framework in a comparative field study contrasting individual niche contributions in a sympatric population of native brook trout (Salvelinus fontinalis) and non-native rainbow trout (Oncorhynchus mykiss) with an allopatric brook trout population. We use fatty acid profiles of blood from repeatedly sampled brook trout to quantify individual trophic niches and evaluate the presence and characteristics of keystone niche individuals. We found that interspecific interactions constrained individual niche divergence, such that disproportionate individual contributions to TNW were absent in our sympatric population. By comparing individual niche contributions under contrasting competitive scenarios, we find that keystone foraging phenotypes may be diminished or absent under competitive conditions, potentially homogenizing resource use within populations.

Jahn's Abstract:

The New York Bight (NYB) is a temperate shelf ecosystem influenced by the convergence of the Mid-Atlantic cold-pool, Gulf Stream, and outflow from the Hudson River estuary. The NYB is highly productive, particularly during the summer months from the late spring to early fall, which facilitates an increased abundance of small forage species into the system. These species may serve as prey resources for several populations of large megafauna, including migratory shark assemblages that are present in the NYB during the summer months. As many predators may depend on small forage species, such as Atlantic menhaden (Brevoortia tyrannus), recent work has suggested that the NYB may follow wasp-waist ecosystem dynamics during the productive summer months. In wasp-waist ecosystems, one or a few pelagic forage species dominate biomass and control energy flow within an ecosystem by exerting top-down control on zooplankton and bottom-up control on both meso-predators and higher-order predators. With decreased complexity in intermediate trophic levels, wasp-waist systems are characterized by low ecosystem stability, as interactions across trophic levels are limited to a few strong interactions across critical prey species. We hypothesize that the migratory shark assemblage in the NYB will rely heavily on abundant pelagic forage prey species during the summer to fall months, which may imply wasp-waist dynamics are occurring during this time. Here, we present a conceptual model to quantify interaction strengths between migratory shark species and their prey resources. This framework will enable us to characterize food web structure, evaluate the potential for wasp-waist dynamics, and ultimately infer the stability of the food web during the summer months. To quantify interaction strength, we will use a combination of data streams: Bayesian stable isotope mixing models using carbon, nitrogen, and sulfur values from ten migratory shark species will be used to estimate their resource use, and biomass data from nearshore trawls will measure available prey biomass. Initial model outputs indicate a strong reliance on pelagic forage-dominant resources across shark species, indicating the abundance of forage species is critical for maintaining predator biomass in this seasonally dynamic system. Quantifying interaction strengths and characterizing food web structure will improve our understanding of system stability and help assess its resilience to perturbations under ongoing environmental change, including rising sea surface temperatures and shifting species distributions.