| Abstract |
The order Isopoda is a species-rich and morphologically diverse taxon of peracarid crustaceans. They not only occur in marine environments but have also successfully conquered freshwaters and the terrestrial realm. This thesis examines the global diversity and biogeographic patterns of isopods within all three environments. After summarising what is known about their natural history, distribution, and ecological and economic importance, attention is drawn to the taxonomic diversity of isopods and their rate of description. A global list of accepted species names, including their authorities and the year of first description, was gathered from the World Register of Marine Species (WoRMS). Since the first formal description of an isopod species by Linnaeus in 1758, 10,687 isopod species in 1,557 genera and 141 families have been described by a cohort of 755 first authors. Although the number of authors has increased manyfold over time, the average number of species described per first author has declined, and the description rate has slowed down. Shifting the attention to global biodiversity patterns within isopods, the latitudinal diversity gradient (LDG) in species richness is first examined, followed by bioregionalisations within aquatic and terrestrial environments. To do so, a dataset of worldwide occurrence records was compiled from the Ocean Biogeographic Information System (OBIS) and the Global Biodiversity Information Facility (GBIF). Isopods exhibit a bimodal LDG with higher species richness in the southern hemisphere for marine isopods and in the northern hemisphere for non-marine isopods. Sampling bias in the data does not significantly affect the overall shape of the LDG. However, when accounted for, the location of diversity peaks tends to move towards tropical latitudes compared to the observed species richness, which is highest in mid-latitudes. A clustering algorithm was employed to identify distinct biogeographic regions based on their species composition. It delineated 33 marine, 28 terrestrial, and 23 freshwater bioregions – all with high endemicity – many of which agree with other biogeographical frameworks in the respective environments. The analysis also revealed substantial geographical gaps in the data, especially in Africa, Asia, Indonesia, South America, and offshore marine areas, which future research should aim to close.
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