Date of Award

3-2-2021

Publication Type

Master Thesis

Degree Name

M.Sc.

Department

Biological Sciences

Keywords

Elasmobranch Liver Tissue, Food Web Stability, Intraguild Predation, Species Interactions, Stable Isotope Analysis, Stomach Content Analysis

Supervisor

Nigel E. Hussey

Rights

info:eu-repo/semantics/embargoedAccess

Abstract

Animal interactions structure food webs with stability being contingent on the presence and strength of multi-species interactions. Intraguild predation (IGP) is a complex interaction that can impact species at the individual, population and community levels, ultimately determining the strength, direction and linearity of trophic cascades and species abundance across trophic levels. IGP occurs among a minimum of three species; a predator (IGpredator) that kills and consumes a prey (IGprey) with which it competes for a common resource. Through a systematic literature search, I determined traditional to modern approaches to measure the occurrence and effect of IGP and then identified the research effort afforded to the different implication levels and IGP effects characterized by Polis et al. (1989). I highlighted IGP effects that require focused attention and provided recommendations on methods that could be used to address knowledge gaps. To understand the role of IGP in higher order predators, I focused on the large shark assemblage given their largely unknown role in top down control and limited IGP studies to date. The large shark assemblage exhibits high phenotypic plasticity that results in varied functional roles (e.g. secondary vs. tertiary piscivores) suggesting complex IGP interactions occur. Stable isotope analysis (SIA) provides an approach to reconstruct consumer diet to examine IGP, however, a detailed understanding of tissue preparation techniques is first required to ensure accurate interpretation of results. Elasmobranch liver is a useful high turnover tissue for IGP studies, but it contains high lipid levels and is expected to retain urea and TMAO for osmotic balance which can bias isotopic values. I found that deionized water washing for urea and TMAO removal was not required as δ15N values were not modified following treatment. Residual lipid within lipid extracted liver samples, however, required the development of C:N thresholds to derive ecologically relevant liver isotopic values. A preliminary comparison between muscle and liver tissue highlighted the value of liver for understanding short vs. long term movements and its application for IGP studies. The occurrence, class and consistency of IGP among large sharks was examined using published stomach content data and prey contributions from stable isotope mixing models. IGP was present among all sharks with the strength and class varying by species, ontogeny and over time (i.e. daily vs. annually). Understanding shark functional roles within marine food webs can improve management practices through the lens of multi-species interactions; targeted conservation on shark species involved in moderate levels of IGP with high connectance among species may enhance food web stability.

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