Functional biodiversity of New Zealand's marine fishes versus depth and latitude : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Statistics at Massey University, Auckland, New Zealand
Understanding patterns and processes governing biodiversity along broad-scale environmental gradients requires an assessment of not only taxonomic richness, but also morphological and functional traits of organisms. The deep sea is the largest habitat on earth and provides many important ecosystem services. Decreases in light, temperature, and trophic resources, along with increases in pressure that occur with greater depth, renders the deep sea one of the most constraining environments for supporting life. However, little is known about how biodiversity, and especially functional biodiversity, changes along the depth gradient. This thesis aimed to fill this gap by using a combination of traits associated with food acquisition and locomotion to quantify and characterise patterns of functional diversity across large-scale depth and latitude gradients and to investigate potential mechanisms driving biodiversity.
First, to identify the major selective forces acting on morphology, I documented patterns of variation in the traits of fishes at broad spatial scales. I found that with increasing depth, fishes, on average, became larger and more elongate, and had a larger oral gape and eye size. With increasing depth, fish morphology shifted towards body shapes that enable energy-efficient undulatory swimming styles and an increased jaw-length versus mouth width to aid opportunistic feeding.
Second, I investigated the role of environmental filters versus biotic interactions in shaping the functional space of communities along depth and latitude gradients by measuring the intra- and inter-specific richness, dispersion and regularity in functional trait space. I found that functional alpha diversity was unexpectedly high in deep-sea communities, but decreased with increasing latitude, and that competition within and among species shaped the multi-dimensional functional space for fishes at the local alpha diversity level.
Third, I described spatial patterns in functional beta diversity for New Zealand marine fishes versus depth and latitude, and delineated functional bioregions. The functional turnover in fish communities was greater across depth than latitude, and latitudinal functional turnover decreased with increasing depth. I surmise that environmental filtering may be the primary driver of broad-scale patterns of beta diversity in the deep sea.
Overall, this thesis contributes new knowledge regarding broad-scale functional biodiversity patterns across depth and latitude via the morphological and functional traits of New Zealand’s marine fishes. Through the measurement of individual trait variation, and the quantification of functional alpha and beta diversity, this thesis characterised variation in the traits of fishes over large spatial scales, determined the spatial turnover of functional traits, and described the relative importance of environmental versus biotic drivers in shaping the functional space of deep-sea communities. These contributions provide foundational understanding for future research on the functional diversity of marine fishes, biodiversity patterns across the depth gradient, and the monitoring of biodiversity change across New Zealand’s latitudinal and depth gradients.