Date of Award

10-30-2020

Publication Type

Master Thesis

Degree Name

M.Sc.

Department

Biological Sciences

Keywords

biogeography, Elton, Grinnell, Hutchinson, MacArthur, Rapoport's rule

Supervisor

Nigel E. Hussey

Rights

info:eu-repo/semantics/embargoedAccess

Abstract

A widely recognized pattern in ecology is the latitudinal diversity gradient: increasing biodiversity with decreasing latitude. The latitude niche-breadth hypothesis states that the stable climate of the tropics allows for increased specialization (smaller niche), promoting greater biodiversity in the available niche space. The highly dynamic climate of the poles drives the evolution of generalists (larger niche), limiting biodiversity. While the fundamental question of “what drives species richness?” on land remains debated, it is even less understood in the marine environment. Elasmobranchs (sharks, skates, and rays) are a data-rich, globally distributed group that occupy an array of functional roles, inhabiting coastal to open ocean habitats from the poles to the tropics. In this thesis, I use a global-scale stomach contents dataset to calculate standardized Levin’s niche breadth for 237 populations of 85 elasmobranch species in order to examine spatial patterns in niche breadth. I find that niche breadth varies widely across all functional, taxonomic, and regional groups, highlighting the diversity and potential resiliency of this clade. Niche breadth of elasmobranchs does not follow a latitudinal gradient. Instead, niche varies with depth, with niche breadth generally increasing with increasing depth. This depth gradient is strongest in bottom-dwelling elasmobranchs with smaller range sizes and weakest in wide-ranging pelagic elasmobranchs. This pattern suggests that for species with limited mobility, specialization may mediate coexistence in highly biodiverse areas with elevated competition. Why this pattern applies in a depth, but not latitudinal, gradient remains unclear.

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