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PART 2 STRUCTURE

CHAPTER 5 Biogeography and Origins

      5.1 Tropical Biogeography

      A latitudinal diversity gradient in the marine biosphere has long been recognised, with an increase in species richness with decreasing latitude from the poles to the tropics (Ekman 1953; Briggs 1974). The circumtropical belt of high diversity is not uniform within this gradient, as some fauna, such as in mangrove‐lined estuaries, show very variable diversity. However, this gradient holds true for many vertebrates and invertebrates in inshore and shelf ecosystems and has been attributed to high water temperatures and maximum solar irradiation in proximity to the equator (Jablonski et al. 2017; Crame 2020). Within the tropics, there is also much longitudinal variation within faunal groups.

      Although recognized much earlier, it was Ekman (1953) who popularised the idea of there being a distinct ‘warm‐water’ fauna. This fauna was split into two provinces: ‘The Indo‐West Pacific’ (IWP) and ‘The Atlanto‐East‐Pacific’ with the former encompassing the islands of the Central Pacific, the Indo‐Malayan region, Hawaii, subtropical and tropical Australia, the Indian Ocean, and subtropical Japan and the latter encompassing ‘Subtropical and Tropical America’ and ‘Subtropical and Tropical West Africa.’ This idea was further refined in 1974 by Briggs who categorised the tropical ocean into four regions: ‘The IWP,’ ‘The Eastern Pacific,’ ‘The Western Atlantic,’ and ‘The Eastern Atlantic.’ It has long been understood that the richest and most diverse fauna is found in the shallow (<200 m) waters of the tropics. The tropics was defined by Briggs (1974) by the 20 °C isotherm for the coldest month of the year, and longitudinally, it was recognized that barriers exist that are effective in separating one region from another with a high degree of endemism.

      Today, the separation of tropical faunas is more complex due to advances in our knowledge of species distributions, fossil evidence and evidence that has been provided by major advances in genetics (Jablonski et al. 2017). Briggs and Bowen (2013) and Veron et al. (2015) have, respectively, summarized the tropical faunal provinces based on the distribution patterns of fish and corals.

An improved knowledge base has advanced our understanding of the distribution and evolution of the Atlantic fauna (Briggs and Bowen 2013), which is now identified as consisting of four origins of Atlantic genera: (i) relict (Tethys Sea) origins prior to the collision of Africa and Europe about 12–20 million years ago (Ma); (ii) origins in the New World (West Atlantic‐East Pacific) prior to the closure of the Isthmus of Panama about 3.1 Ma; (iii) radiations within the Atlantic; and (iv) invasions from the Indo‐Pacific via southern Africa (Figure 5.1).

      The relationship among the provinces highlights the geographic origin of species and the effect of both soft and hard biogeographic barriers. For example, one soft barrier is the freshwater discharge of the Amazon River which separates the boundaries between the Caribbean (CA) and Brazilian (BR) provinces. This boundary was identified by the fact that 348 reef fish species are shared between the CA and BR provinces which represent about 42% of the species diversity of the CA and 74% of the BR. The fauna is much more diverse in the CA (Luiz et al. 2012). Another soft barrier is the open‐water expanse of the mid‐Atlantic; the CA shares 105 reef fish species with the Tropical Eastern Atlantic (TEA). These transatlantic species account for about 27% of the shallow TEA fish fauna. The BR shares a similar fauna with the TEA. About 112 fish species are shared between them and may be considered as transatlantic.

The open sea to the east and west of the mid‐Atlantic ridge provinces of Ascension and St. Helena is the next most permeable barrier within the Atlantic. Both islands share 64 and 71 fish species with the eastern and western Atlantic, respectively. Most of these species are transatlantic in character, having relatively large latitudinal ranges; they are known to be associated with floating debris in the open ocean, which is most likely the main conduit of connectivity.

FIGURE 5.1 Map of the Atlantic Ocean showing warm‐temperate biogeographic provinces (orange), tropical biogeographic provinces (lime green), and the biogeographic pathways that contribute to biodiversity in these provinces (blue arrows). Parallel arrow sizes indicate relative size of migratory flows. Acronym: TEA: Tropical Eastern Atlantic.