Costello, MJBowen, MChaudhary, Chhaya2019-02-272019http://hdl.handle.net/2292/45576This thesis describes the latitudinal, longitudinal and depth gradients in marine species diversity using several diversity measures and spatial scales. The analysis provide new insights into the generality of the gradients in species diversity based on distribution data for over 50,000 species obtained from the Ocean Biogeographic Information System (OBIS). I found patterns that contrast with pre-existing hypotheses on the spatial gradients in species diversity with latitude and depth. In contrast to the prevailing paradigm that the latitudinal gradient in species richness was unimodal and peaked at the equator, I found that species richness was bimodal. There was a dip in richness at the equator and peaks on either side of it. This was the case for alpha (local) and gamma (regional) diversity, calculated in 5° latitudinal bands and equal area hexagons. Moreover, species richness of pelagic and benthic, vertebrate and invertebrate taxa, declined or levelled off between 20°-25°C. This levelling off spatially coincided with where temperature was maximal, indicating that temperature is the most likely cause of the bimodality. On comparing the latitudinal gradients in species richness between 1955-1984 and 1985- 2015, I found higher richness in the subtropics, a northward shift in richness in the northern hemisphere, and a lower dip at the equator after 1985. The shift in the gradients after 1985, along with the temperature rise over time, suggests that the tropics are already too hot for some species. It thus seems clear that some species are expanding their ranges into higher latitudes, with a decrease in richness at the equator. In contrast to species richness, I found species turnover peaked at the equator, and was positively correlated to ocean and land area. I suggest that greater ocean connectivity reduces turnover at higher latitudes. However, tropical latitudes are separated into different oceans. In addition, high biogenic habitat heterogeneity, and perhaps spatial heterogeneity due to thermal stress, may explain the increasing turnover in the tropics towards the equator. It seems likely that species richness will continue to decline and turnover will increase in the tropics as a response to climate warming. I found that species richness exponentially declined with depth using all measures of diversity. Moreover, the coastal regions were more heterogeneous than the deep sea in terms of species composition. This could be because of the high productivity in the shallower waters. It is also possible that depth gradients at local scales, and for selected taxa, may vary from the general gradient found here.Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. Previously published items are made available in accordance with the copyright policy of the publisher.https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htmThesisCopyright: The authorhttp://purl.org/eprint/accessRights/OpenAccessQ112947999