Unravelling macroecological patterns in extant planktonic Foraminifera

Abstract

Present-day ecological communities and the deep-time fossil record both inform us about the processes that give rise to, and maintain, diversity of life on Earth. However, these two domains differ in temporal, spatial and taxonomic scales. Integrating these scales remains a major challenge in biodiversity research,
mainly because the fossil record gives us an incomplete picture of the extinct communities. Planktonic Foraminifera provide an excellent model system to integrate present and past changes in biodiversity. They are single-celled marine zooplankton that produce calcite shells, yielding a remarkably complete fossil record across millions of years, and are alive today enabling genetic and ecological studies. Their fossil record has been widely used in the fields of stratigraphy and palaeoclimate. However, we have limited knowledge about their ecology, preventing us from fully understanding the evolutionary processes that shaped their diversity through time. The primary objective of this thesis is to improve our understanding of community ecology of extant planktonic Foraminifera species, to enable us to more comprehensively study their fossil record. I created a large image dataset of over 16,000 individuals from a historical museum collection (Chapter 2) and assessed its potential biases (Chapter 3). Using the data gathered from the collection, I investigated the extent to which individuals of the same species vary in shell size (Chapter 4). Size relates to many physiological and ecological characteristics of an organism, thus understanding how it varies within species and across space gives us insights about the function of the species in the ecosystem. Planktonic Foraminifera species greatly differ in how much size variation is explained by environmental (temperature and productivity) and/or ecological (local relative abundance) conditions, suggesting that the known pattern of large size at favourable conditions is not widespread in the group. Next, I explored how planktonic Foraminifera species interact with each other in ecological communities (Chapter 5). Their fossil record suggests that competition among species is an important ecological interaction limiting the number of species that can emerge within the group. I tested whether species are competing today in the oceans, and found no evidence for negative interactions. This result suggests that either the ecological processes acting on communities today are different than the ones driving planktonic Foraminifera evolution, or that competition among species did not shape the patterns we observe in their fossil record. Together, these discoveries extend our current understanding of planktonic Foraminifera biology and highlight the complexity of ecological dynamics. Future work using the planktonic Foraminifera fossil record to understand marine biodiversity changes will require scientific research across different scales as well as considering other interacting plankton groups.

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ORCID for Thomas Ezard: orcid.org/0000-0001-8305-6605

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