Diversity dynamics of New Zealand's temperate shallow-marine ecosystems through the Cenozoic
The fossil record provides the only evidence of how ecosystems have evolved over long timescales (multi-centennial to millennial) and how they have responded to changes in geological and environmental processes. Quantifying the impact of these geological and environmental processes on biodiversity is fundamental to understanding the paradigm of modern biodiversity loss and the ecological reverberations of anthropogenic climate change. Crucially, biodiversity is spatially dependent, and our perception of biodiversity may differ depending on the spatial scale of observation. As a result, understanding the diversity dynamics of the fossil record is both a question of time and space. Recent evidence suggests that studies at the regional spatial scale are optimal for understanding macroevolutionary and macroecological processes. Despite this, regional-scale studies using data from the fossil record are limited, particularly in the southern hemisphere.
This thesis focuses on the temporal and spatial impacts of geological, climatic, and oceanographic processes on New Zealand’s regional shallow-marine molluscan ecosystems through the Cenozoic Era (the last 66 Myrs). New Zealand hosts remarkably complete Cenozoic stratigraphic and fossil records; the latter is regarded as the most complete shallow-marine fossil record in the southern hemisphere. As a result, this record provides an ideal case study to examine macroevolutionary and macroecological processes in the shallow marine realm at the regional scale. In addition, New Zealand’s modern and Cenozoic shallow-marine ecosystems are temperate. Temperate marine ecosystems are vastly understudied compared to their tropical counterparts in both modern ecosystems and the fossil record, and their response to climate change is poorly understood, particularly in the southern hemisphere.
This thesis comprises four related research papers that form the main chapters. These are introduced by detailed background, data analysis, and methods chapters. The overarching aim of these papers is to: (1) quantify the impact of known confounding effects in the fossil record, particularly the Pull of the Recent; (2) evaluate the spatial dependence of biodiversity in the fossil record, particularly on temporal patterns of beta diversity; (3) understand the impact of climate change on taxonomic and functional biogeographic patterns of shallow-marine biodiversity; (4) employ the methods developed in the previous chapters to test the importance and role of several important geological and climatic factors on the spatial structuring New Zealand’s shallow-marine biodiversity.
First, I find that the Pull of the Recent effects <2 % of genera and <4 % of species when considering taxa missing from the youngest ~5 Myrs of the New Zealand shallow-marine molluscan fossil record. Furthermore, the taxonomic composition of molluscs missing from the youngest part of the fossil record cannot easily be explained by effects related to shell mineralogical composition, body size, habitat, or taxonomic class. Lithification has minimal effect on the impact of the Pull of the Recent but does have a notable effect on apparent range-through diversity in the Pleistocene. Secondly, I show that beta diversity is spatially dependent at local to regional spatial scales and that uneven spatial sampling can influence recovered temporal trends in beta diversity. We argue that newly developed methods that allow for multisite beta diversity, partitioned into total dissimilarity, spatial turnover and nestedness, are the most suitable for elucidating patterns of beta diversity in the fossil record. Thirdly, I find that the interrelationship between two biogeographic patterns, the species-area relationship and functional diversity-area relationship, shows a strong, positive correlation to regional oceanic temperature, suggesting a long-lived and persistent association in the structuring of biodiversity, and temperature-dependence of functional redundancy, over the last ~45 Ma. Lastly, I find that oceanic warming seemingly has both a long-term positive effect on regional taxonomic diversity and rates of origination, but a negative correlation with regional functional diversity and its spatial distribution. This negative long-term relationship between regional functional diversity and oceanic temperature is likely driven by functional resilience to extinction during cool intervals rather than a negative response to warming per se.
In summary, this thesis confirms the high quality of the New Zealand shallow-marine fossil record and identifies oceanic temperature as a major positive correlate of regional taxonomic diversity, origination, and functional redundancy. This correlation is not consistent across spatial scales, a reflection of other abiotic and biotic processes that are likely important. These findings are relevant to our understanding of macroevolutionary and macroecological processes, particularly how temperate shallow-marine ecosystems may respond to future warming. For New Zealand, this research suggests, as a baseline, that natural warming may increase equilibrium taxonomic diversity and functional redundancy in New Zealand. However, anthropogenic climate change is associated with increased loss of biodiversity and habitat destruction in the geological short term. This is likely to alter the future trajectory of New Zealand’s biodiversity anticipated from natural warming, which may have far-reaching impacts for the short- and long-term future of New Zealand’s unique and highly endemic marine fauna. Furthermore, the results discussed of this thesis also suggest the unfolding sixth mass extinction may be qualitatively different (i.e., higher levels of functional extinction) from previous mass extinction events and have correspondingly unforeseeable consequences. Lastly, and importantly, this thesis highlights the need for further study in shallow-marine temperate ecosystems, particularly at finer time scales, to better our understanding of diversity dynamics, and extinction risk.