Читать книгу Tropical Marine Ecology - Daniel M. Alongi - Страница 46

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.

Source: Briggs and Bowen (2013), figure 1, p. 1024. © John Wiley & Sons.

Another soft barrier is the relatively cool Benguela Current that separates the Benguela (Atlantic Ocean) from the Agulhas (Indian Ocean) provinces. At least 47 fish species have colonised the Atlantic from the Indian Ocean with at least 38 of these species also found in the TEA where they account for about 10% of the total number of species (Floeter et al. 2008). How they move from the Indian to the Atlantic is a matter of some speculation, but two perspectives have arisen (Rocha et al. 2005a; Reese et al. 2010). As indicated by phylogeographic and paleontological studies on the distributions of some molluscs and fish, there may be a colonisation route through the Agulhas Province west to Brazil. The second hypothesis is that several individual events that may be attributed to the cessation of upwelling at the end of the glacial cycles, or to warm cyclonic eddies that periodically cross the Benguela Current, fostered multiple pulses of colonisation into the Atlantic. Ocean circulation between the Atlantic and Indian Oceans is thought to be minor compared with the connectivity between the Indian and Pacific Oceans, but the minor circulation appears to be sufficient over time to result in some transfer of fauna.

West to east dispersal has been resolved based on phylogeny as the transatlantic distribution of 112 fish species shows a clear connection across the Atlantic (Bowen et al. 2006) or possibly recent colonisation between Atlantic provinces (Rocha et al. 2005b). Between the CA and BR, species flow is mainly from north to south across the soft Amazon barrier, although there has been some dispersal in the opposite direction (Rocha et al. 2008). The CA has 150 fish genera with 24 endemic and the BR has 117 genera with only 3 endemic, which suggests that most of the BR fish fauna originated in the CA.

The warm‐temperate provinces of the Atlantic have been called the ‘impoverished outposts of adjacent tropical areas’ (Floeter et al. 2008). The Lusitania Province that borders the TEA shows that most fish species originated from the adjacent tropical province. Similarly, most of the fauna of the warm‐temperate Carolina Province originates from the Caribbean. The Gulf Stream plays an important role as a mode of transport as shown by the islands of Bermuda which are clearly populated from the Caribbean. The CA has been functioning as the centre of origin for the warm provinces of the Atlantic as indicated by its great diversity and the dispersal of fauna to north, east, and south. The greatest diversity of coastal invertebrate species within the CA occurs from Cuba through the Antilles to Colombia and Venezuela (Miloslavich et al. 2010).

The tropical Atlantic continues to gain species from other sources. Evolutionary separation is stimulated by the three soft barriers of the Amazon, mid‐Atlantic, and the Benguela. A few species have colonised north from the BR to add to the species richness of the CA (Rocha et al. 2008). Parapatric speciation (two subpopulations of a species evolve reproductive isolation from one another while continuing to exchange genes) may predominate due to the softness of these barriers while allopatric speciation (speciation that happens when two populations of the same species become isolated from each other due to geographic changes) may predominate across wide stretches of the open ocean between the West, Central, and East Atlantic (Briggs and Bowen 2013).

The Atlantic contains numerous genera that were once part of a ‘general New World (Western Atlantic and Eastern Pacific) fauna’ prior to the rise of the isthmus of Panama about 3.1 Ma; at least three tropical Atlantic fish species are thought to be relicts from the Tethys Sea (Briggs and Bowen 2013). Trans‐Pacific migrations may have been the key to the fact that about 20 genera had apparently reached the New World. Caribbean reef fish are more closely related to those of the eastern Pacific than to those of the eastern Atlantic as many New World genera do not occur in the eastern Atlantic. The Caribbean Province may be the centre of ‘evolutionary innovation’ (Briggs and Bowen 2013) given that it has 24 endemic genera and 272 endemic species.

The greatest richness of invertebrate and vertebrate species lies in the IWP. This has been acknowledged since at least the time of Ekman (1953) and the reasons for this richness continue to be a source of rich debate (Bellwood et al. 2012; Veron et al. 2015). The IWP spans an immense area, and this is reflected in the unique distribution patterns of fish (Allen 2008; Mundy et al. 2010). Allen (2008) calculated an average range for reef fish of 9 357 070 km², and Mundy et al. (2010) found that of the fish fauna of the US Phoenix and Line Islands nearly 70% ranged from the Indian Ocean to their study area at the eastern edge of the SW Pacific. Obviously, a high level of connectivity is maintained across vast expanses of ocean (Eble et al. 2011) including the barriers between the eastern and western Indian Ocean and across the far eastern Pacific. In contrast, there is considerable flow of fauna between the western Pacific and the eastern Indian Ocean that are connected via the Indonesian Throughflow (ITF). Considerable research has focused on this connectivity (Williams et al. 2002; Gaither et al. 2010). The bridge between both oceans was not nearly so open as it is now; during the last glaciation about 18 000 years ago, sea levels were considerably lower (about 130 m) than at present, although there was still a narrow connection. Nevertheless, during the Pliocene–Pleistocene glaciations, the ITF was reduced, lessening the chance for ecological connectivity. Gaither et al. (2010) have provided some evidence of genetic distinctions within 15 of 18 species of fish, crustaceans, and echinoderms between both oceans, although these genetic breaks are no larger than found elsewhere in the sea (e.g., Horne et al. 2008). On the other hand, Veron (1995) considered the coral fauna of the eastern Indian Ocean to be nearly identical to that of the western Pacific underscoring the fact that there have been, and continues to be, connections between the faunas of the Indian and Pacific Oceans.

There are also connections between the western Pacific with the Hawaiian archipelago and the central Pacific with the eastern Pacific. However, the eastern Indian and western Pacific may eventually be distinguished. Briggs and Bowen (2013) point out two points that must be considered in this regard: (i) the unique possibility of overlap by distinct fauna by the fact that the Indo‐Pacific barrier is different from other barriers in that it switches on and off in 100 000 year oscillations (Rocha et al. 2005b) and (ii) the Pacific fauna is expanding westwards due to the presence of Indian and Pacific taxa hybridising in the Indian Ocean (Hobbs et al. 2009). The barrier between the Indian and Pacific Oceans thus appears to be diffuse in the current day and ephemeral over evolutionary time, being spread across 25° longitude from the Sunda shelf to the Cocos/Keeling Island group. Briggs and Bowen (2013) concluded that the distinction between the western Pacific and the eastern Indian Oceans has been blurred due to repeated invasions of species, although the eastern Indian Ocean at the height of the last glaciation 18 000 years ago may have been a distinct biogeographic province.

Mangrove species diversity is higher in the IWP than in the East Pacific and Atlantic, with a global total of 80 species currently. Mangroves first appeared on the shores of the Tethys Sea, having diverged from terrestrial forbearers during the mid‐Cretaceous (Duke 2017). Mangrove dispersal of the 32 genera in 18 families began during the early‐ to mid‐Eocene (Duke 2017). Their evolution has been closely related to sea‐level changes throughout geological time (Srivastava and Prasad 2019). Two biogeographic subregions for mangroves are recognised: the Indo‐Malaysia from the Indian Ocean to Southeast Asia and Australasia from New Guinea and Australia to the islands of the West Pacific, fostering the notion that mangroves originated in Southeast Asia and expanded across the Pacific to the west coast of the Americas and westward to East Africa and then to the east and west coasts of the Atlantic. A further secondary hotspot for mangroves occurs in the Caribbean Central American area (Duke 2017). The floras of these two relatively rich subregions constitute a centre of diversity at the convergence of the Indo‐Pacific encompassing the seas around Sumatra and the southern half of peninsular Malaysia to the easternmost point of New Guinea.

Seagrasses are also composed of relatively few species: currently 58 species in 12 genera. They appear to have evolved more than once and have an evolutionary history that is still the subject of debate. Seagrasses are divided into 5 families and 12 genera: Hydrocharitaceae (Halophila, Enhalus, Thalassia), Ruppiaceae (Ruppia), Zosteraceae (Zostera, Phyllospadix), Posidoniaceae (Posidonia), and Cymodoceaceae (Amphibolis, Cymodocea, Halodule, Syringodium, Thalassodendron). Globally, seagrass distribution is divided into six regional floras: temperate North Atlantic, tropical Atlantic, Mediterranean, temperate North Pacific, tropical Indo‐Pacific, and temperate Southern Ocean. Species richness is positively correlated with decreasing latitude with the greatest richness occurring in Southeast Asia (Hogarth 2015).