Mangrove Floristics and Biogeography Revisited: Further Deductions from Biodiversity Hot Spots, Ancestral Discontinuities, and Common Evolutionary Processes

Abstract

This treatment provides an updated account of mangrove biodiversity patterns and evolution based on ancestral biogeography and extant floristics. Mangrove plants occur where they do in the world because past and current factors influence their dispersal and establishment. Key limiting factors include: land barriers, dispersal across water, climatic conditions, and availability of biotic material. Over time, each of these factors has morphed and changed as each driver varied during the evolution of the 80 species and hybrids from at least 18 family lineages. In this treatment, the factors influencing current distributional patterns have been looked at closely with the specific distributions of each genotype considering prior historical influences. Distributional patterns, like diversity hot spots spanning large geographic areas, species gradients, and discontinuities are considered tangible evidence of the appearance and evolution of each mangrove entity.

2.1 Introduction

Mangroves are dominant coastal shoreline habitats of tropical and subtropical regions of the world (Tomlinson 2016; Spalding et al. 2010). They comprise chiefly flowering trees and shrubs uniquely adapted to tidal conditions and a special combination of factors that influence coastal and estuarine shorelines, like seawater, periodic inundation and exposure, waves and wind, strong currents and runoff, and fine sediments (Duke et al. 1998). In such settings, mangroves take the form of distinctly vegetated and often densely structured habitats of verdant closed canopies that clad coastal margins and estuaries of the tropics and subtropics.

In tropical waters, mangrove stands are often positioned between two of the world’s iconic ecosystems—coral reefs and tropical rainforests. These are each, biota-structured ecosystems of tropical shorelines, intimately linked by integrated roles in coastal and estuarine ecosystem processes. However, such links are threatened by human development and habitats like mangroves are in serious decline worldwide (Duke et al. 2007). These habitats are also further dependent on mobile biota uniquely adapted to the habitats’ unusual and often dramatic physicochemical gradients. Developed over millennia, these linked and mutual relationships have become vital to the survival of each species (e.g., Mumby et al. 2004).

Mangroves are recognized further for their unique morphological and physiological adaptations for coping with salt, exposed conditions, saturated soils, and regular tidal inundation along this unique estuarine and coastal marine habitat. Some of these distinctive attributes include: exposed breathing roots above ground; extra stem support structures; salt-excreting leaves; low water potentials and high intracellular salt concentrations for maintaining favorable water relations in saline environments; and their unusual viviparous, water-dispersed propagules. Such a unique combination of attributes has enabled mangroves to dominate tropical shorelines worldwide. In this chapter, the floristics and biogeography of these amazing plants and habitat has been re-evaluated; this includes taking a fresh look at some of the key factors responsible for their unusual diversity and distribution worldwide.

2.2 Factors Influencing Mangrove Distributions

The genetic composition of todays’ mangrove flora, while clearly subject to present-day climatic and geographical conditions (Duke 1995; Duke et al. 1998; Triest 2008), appears to be largely relict. Species are where they are today because of past events and circumstances. As such, todays’ distributional patterns cannot always be explained by current-day deterministic factors alone. Moreover, each mangrove plant type is influenced by a combination of attributes including: individual physiologies, ecology, dispersal ability, propagule buoyancy and longevity, geological circumstances, evolutionary rates, and the genesis of each taxon.

The key criteria are summarized in the following ten generalized factors (ordered in 3 groupings) as the ones mostly influencing the biogeography and evolution of mangroves. These criteria are based on the observations of a number of authors, including Tomlinson (2016), Duke et al. (1998), and Triest (2008), and modified further here.

2.2.1 Floristics and Biogeography

• 1. Diversity of Plant Types Makeup Mangrove Habitat. The species numbers of 80 taxa are not necessarily high, but mangrove taxa are present in a broad cross-section of plant family lineages (Tomlinson 2016). They chiefly comprise a small but diverse selection of shrub and tree species from 18 plant families with around 69 species and 11 hybrid intermediates (Table 2.1). There are 32 genera represented with all but one being a flowering plant (Table 2.2).

• 2. Variable Ranges for Different Species. The distributional ranges of mangrove species vary considerably (Spalding et al. 2010; Duke 2011, 2013, 2014a). While some extant mangrove taxa occur worldwide, others have more local distributional ranges. The overall high concentration of mangrove species in the Indo-Malesian area today defines the chief diversity hot spot for mangroves, comparable with other shallow water, tropical marine habitats, like seagrass and reefal coral species (Hoeksema 2007). And likewise, a further secondary hot spot for mangroves occurs in the Caribbean Central American area. A general feature of mangrove taxa is their high incidence of hybrid species (as mostly infertile, intermediate forms), along with sibling species (species that are unusually similar in morphology and genetic makeup).

• 3. Phylogenetic Relationships within Key Mangrove Genera. It is important to have detailed information about the relationships amongst species of polyspecific mangrove genera (like Avicennia, Bruguiera, Rhizophora, Sonneratia). This is essential for identifying and understanding the genetic lineages and relationships amongst closely related taxa (Duke 1992, 1995, 2010; Duke et al. 2002; Duke and Ge 2011; Lo et al. 2014). Such knowledge is further useful when ranking species by ancestral age, and in constructing phylogenies. There is also the idea, pursued in this treatment, of possible common patterns existing between genera where each entity may have evolved, diversified, and speciated in response to common overwhelming geophysical circumstances and drivers.

Table 2.1 Mangrove species of the world: Families and genera shaded are exclusively mangrove; Genera marked with an asterisk have been classified comprising their own family, named, Diospyraceae, Barringtoniaceae, Aegicerataceae, Nypaceae, Aegialitidaceae, and Pellicieraceae, respectively; Species underlined refer those of the Atlantic East Pacific region. Species in bold occur naturally in both regions; Reported hybrid taxa are shown with an “X” before the species name

Table 2.2 Distribution of family, genera, and species in the two subregion hot spots of the world as the Indo West Pacific (IWP) and the Atlantic East Pacific (AEP); Summarized counts from Table 2.1

2.2.2 Extant Influencing Factors

• 4. Dispersal by Sea. Mangroves have mostly, water-buoyant propagules dispersed by water currents across seas and between estuaries, driven by wind, waves, tides, and ocean circulation (Rabinowitz 1978; Lo et al. 2014). Over half the mangrove species have unusually, well-developed viviparous propagules instead of seeds. By contrast, vivipary is rare in other plant ecotypes. But, one unanswered question remains: how does vivipary influence dispersal? It is clear that long distance dispersal (compare with Nathan et al. 2008) in mangroves is fundamentally important and well utilized, but there appear to be finite limits to both seed and viviparous propagule viability and longevity in transit (e.g., Drexler 2001). This is most evident however in the absence of native mangrove species in key locations, like the central Pacific Hawaiian Islands, prior to their introduction by people (Allen 1998).

• 5. Topographic Elevation Range. Mangroves are naturally restricted to a very narrow elevation range between mean sea level and the highest water levels of spring tides (Duke 1992; Duke et al. 1998). At lower limits of the range, they appear constrained by tidal inundation frequencies mostly less than 50% of the time, dependent on possible limitations during submergence with gas exchange of exposed roots. At higher intertidal margins, species appear constrained by desiccation as well as competition for light from supratidal upland specialists . To cope with conditions in the intertidal zone, mangrove plants have well-recognized specialized abilities to deal with salt and saturated airless soils, by their distinctive exposed air breathing root surfaces and lenticels.

• 6. Ecological and Climate Conditions. Mangroves are constrained worldwide by extant climatic conditions. Generally, mangroves are confined to sheltered tropical coastlines where mean monthly seawater temperatures drop only to around 20 °C (Duke et al. 1998). Based on observed different latitudinal limits (e.g., Duke 2006), there appear to be species–specific differences in temperature tolerances. However, a recent assessment has challenged this view, reporting no differences between widely distributed genera, Avicennia and Rhizophora (Quisthoudt et al. 2012). This view is however at odds with present understandings, and it may be limited by not accounting for observed variations in temperature tolerances amongst and between species; as an example, a notable cold-tolerant variant is known for Avicennia marina, var. australasica in south eastern Australia, and there are a number of other examples (Duke 1992, 1995, 2006). These observations emphasize the great importance of knowing the ecophysiological tolerances of each genotype, especially those at distributional endpoints. Furthermore, mangrove taxa are often restricted further to areas of higher rainfall, as well as estuaries of larger riverine catchments (Duke et al. 1998). Mangroves can be more abundant and diverse in areas of unconsolidated sediments, like the mouths of larger estuaries, while also growing on hard substrates of rock and coral.

2.2.3 Dispersal Pathways and Evolutionary Processes

• 7. Disjunctions of Distribution. There are several major genetic discontinuities amongst present-day mangrove distributions worldwide (Duke 1995; Triest 2008; Takayama et al., 2013). Such disjunctions are marked by extant genetic anomalies where no current dispersal barriers exist. Disjunctions may be characterized by: the presence or absence of taxa, or the presence of sibling taxa on either side of an anomaly. It is also likely that relationships amongst taxa may be proportional to the magnitude and age of the disjunction, as well as to the specific characters of each plant involved—especially in their dispersal ability, propagule buoyancy, and longevity; also cs. Nathan et al. (2008). For these reasons, an ancient primary disjunction is recognized separating the Atlantic East Pacific (AEP) and Indo West Pacific (IWP) , with distinct species, common genera, and separate families; while “younger” disjunctions , like that between SE Asia and Australia, is marked by sibling species, separate intra-specific forms, with notable common genera and families.

• 8. Distributional Fluctuations. Where climate conditions and sea levels change, the distributional extent of individual mangrove species both expand and contract (Duke 1995; Duke et al. 1998). This indicates that while mangroves are able to disperse and colonize unoccupied shorelines, they also at times have become extinct along other shorelines.

• 9. Appreciable Geological Age. Mangroves are known to exist from ancient fossil records of macrofossils and pollen dating back to the mid Cretaceous era, possibly up to around 100 million years ago (Churchill 1973; Plaziat et al. 2001). This is the case in varying degrees for extant common genera today. Where recognizable fossil taxa match existing mangrove plants, this reveals a genetically conservative situation where the presence of persistent genotypes as common species is indicative of relatively slow rates of genetic mutation and diversification over tens of millions of years. As such, species numbers today are indicative of the realised speciation rates for each lineage.

• 10. Continental Drift Theory and Mangrove Evolution. The dispersal and speciation of key mangrove angiosperm lineages took place at the same time as the breakup of the massive supercontinent of Gondwana (Duke et al. 2002; Triest 2008). Over the last 55 million years at least, large continental fragments have moved around the globe ferrying plants and animals with them. This explains why plants with both low and high dispersal abilities may have comparable distributional ranges. Over time, oceans and seaways that once existed have closed, while land barriers have both emerged and disappeared. Understanding the occurrence of these barriers and their effectiveness, helps explain how water-borne mangrove plants might have dispersed to their current day distributional endpoints. Furthermore, in the Indo West Pacific (IWP), former Gondwanan southern land mass fragments, including Africa, India and Australia, have migrated dramatically northward to separately join with Asia. These massive movements must surely have seriously influenced the diversity and distribution of mangroves and other species throughout the region.

The relative importance and influence of each criterion explained here (1–10) is entirely species-dependent. And, for this treatment, criteria 1, 2, and 3 rely on best available taxonomic discrimination along with the most up to date records of worldwide distributions. Criterion 4 concerns dispersal by propagules, which can be over-rated for long-distance dispersal if data on propagule buoyancy and longevity for individual taxa are unavailable. And, while vivipary in mangroves may enhance long-distance dispersal, its key function and value lies in its enhanced success for essential local re-establishment, rejuvenation, and habitat turnover. In all cases , dispersal ability must always be considered finite and limited. Criteria 4, 5, and 6, describe the predominant and major physiological constraints that particularly influence within-site species distributions, based chiefly on temperature, moisture, and tides. Criteria 7, 8, 9, and 10 all relate to biological evolutionary processes in response to historical changes in geological and climatic conditions, where these influence changes to genetic makeup with mutations and occasional speciation.

This treatment considers all these relevant, prominent features of each species entity by taking a closer look at individual occurrence records. Summary information is presented for each mangrove genus, listing all species and hybrids (Appendix 1). Detailed descriptions that accompany these observations include diagnostic characters along with current distribution maps of all mangrove taxa, as displayed in the e-book field guide, World Mangrove iD (Duke 2013, 2014a). Other extant influencing factors have been explained further and summarized by Duke et al. (1998).

2.3 Floristics and Distribution

Mangrove plants collectively form an ecological type rather than a genetic entity with a singular genetic makeup for coping with intertidal life. Amongst the 32 genera with mangrove species, each entity has separately refined its own obligate specialized attributes for the tidal wetland habitat. These features are manifest in entities with often distinctive anatomy, physiology, and functionality. Each genus has varying numbers of representatives for each of the 18 plant families (see Table 2.1; Duke 2011, 2014a). And, all but one are flowering plants; the odd one is Acrostichum, the fern family Pteridaceae. Their collective occupation of the upper tidal zone is considered a relatively recent geological occurrence, mostly post Cretaceous. Otherwise, mangrove habitat is comparable to rainforests, with all families present in mangroves being present also in tropical rainforests.

Of these 32 plant genera, 17 are exclusive to mangrove habitat, including: Acrostichum, Aegialitis, Aegiceras, Avicennia, Bruguiera, Camptostemon, Ceriops, Conocarpus, Kandelia, Laguncularia, Lumnitzera, Nypa, Osbornia, Pelliciera, Rhizophora, Scyphiphora, and Sonneratia. The other 15 genera are less specialized, with both mangrove and non-mangrove entities present, including, Acanthus, Barringtonia, Brownlowia, Crenea, Cynometra, Diospyros, Dolichandrone, Excoecaria, Heritiera, Mora, Muellera, Pavonia, Pemphis, Tabebuia, and Xylocarpus.

As an example, Pemphis has a widespread IWP species, and a sole upland species located inland as an isolated population on the island of Madagascar (Tomlinson 2016). While others, like Brownlowia, Diospyros, and Tabebuia, have single mangrove species and a greater number of upland species (Duke 1992). In addition, unusually widespread and distinct hybrid intermediates are reported in five genera including, Bruguiera, Lumnitzera, Sonneratia, Rhizophora, and Avicennia (Huang et al. 2014; note that the recently reported Avicennia hybrid lacks morphological description). Overall, the total number of mangrove species in each genus is relatively low, being one or two. For relatively larger genera, the number of mangrove species plus hybrids worldwide does not exceed 12.

These relatively low levels of diversity are believed to be the result of the harsh and saline growth conditions present in intertidal habitats. The extreme conditions favor high levels of optimized efficiency for the survival and evolution of mangrove inhabitants (cs. Provine 2004). Such broadly defined factors support the underlying tenet in this treatment, which states that the resilience and survival of each species is largely dependent on its individual distribution, dispersal capabilities, levels of taxonomic divergence, and phylogeny . In this way, evolving local and regional environmental/ecological factors are expected to have a key role in defining the genetic characteristics that ultimately define all mangrove taxa known today.

All mangrove species in the world today are considered in this review, along with their current distribution maps, showing records of living, introduced, and fossil records (Duke 2013, 2014a; see Appendix 1). These maps provide the latest information available on the distribution of each entity. But, despite such outcomes, the description and characterization of mangrove plant types remains incomplete. Hence, there is an ongoing need to revise and update each entity, especially since occurrences are likely to change (Duke et al. 2007). There might also be new undescribed taxa. For example, with Pelliciera, while currently described as one species (Tomlinson 2016), it appears this taxon has subspecific forms within its limited Central American range (Castillo-Cardenas et al. 2005; plus personal observations). There are also notable taxonomic discrepancies and gaps in established larger genera, like Rhizophora in northern South America and West Africa (Ceron-Souza et al. 2010; Triest 2008).

2.4 Dispersal Pathways and Evolutionary Processes

Prior assessment of mangrove global biogeography (including, Duke 1992, 2011, 2014b; Tomlinson 2016; Triest 2008) shows a general consensus for two hot spots disproportionately spanning sub-regions of the Indo West Pacific (IWP) and Atlantic East Pacific (AEP) . These are shown in Fig. 2.1 along with all distribution information summarized from the 70 species maps (excluding hybrids). While the IWP hot spot, also referred to as the Indo Australian Archipelago biodiversity hot spot (Cowman and Bellwood 2013), is about three times more diverse, there are significant species concentrations in the AEP spanning the Caribbean American Isthmus. Both hot spots form gradients in species richness driven by estuarine diversity along respective tropical shorelines. In this way, estuarine systems influenced by individual catchment runoff (Duke et al. 1998), act as diversity nodes to form species enclaves and gradients connecting intertidal habitats. At its center, the IWP hot spot hosts 54 mangrove species, along with around 500 species of coral and 5000 species of fish (Hoeksema 2007). By contrast, the AEP hot spot has 17 mangrove species and proportionately fewer associated shallow water marine species.

Fig. 2.1

The current world map of the distribution of mangroves (blue shading) showing diversity as numbers of specific taxa (species and nominal hybrids). Also identified are two outlying areas where mangrove taxa have been introduced in recent decades. Overall, there is notable separation in eastern and western global bioregions with two disproportionate diversity hot spots : the dominant Indo Australian Archipelago area north of Australia; and a lesser one, the Caribbean American Isthmus area north-west of South America

The genesis of mangrove hot spots seems almost certainly due to past changes in continental drift acting in concurrence with speciation and dispersal events driving vicariance and re-association (cs. Duke 1995; Duke et al. 2002). To have such a profound influence, these massive changes in global geography have coincided with the evolution of early angiosperms to derive the dominant mangrove entities known today. Over time, the global displacement of continental fragments has changed dramatically to form today’s distribution maps with biota occurring for the most part constrained according to current day circumstances. But, there are notable unexplained patterns sparking debate and speculation about their relevance to the evolution of mangroves.

The approach in this article has been to further review current evidence, identifying each vital clue to understanding how each mangrove entity might have evolved. The evidence (also see Fig. 2.1) includes: (1) areas of species occurrence and richness; (2) hot spots associated with clines in species diversity; and (3) unexplained discontinuities across hot spot gradients in the absence of apparent current-day influences on gene flow.

2.5 Dispersal Barriers

Extant mangrove distributions are collectively defined by a number of tangible barriers blocking water-borne gene dispersal (Duke et al. 1998; Duke et al. 2002; Triest 2008), including: (a) cold water (<20 °C in summer) mostly in higher latitudes; (b) land mass, mostly longitudinally applied; and (c) broad water expanses, also mostly longitudinal. The current locations of these three barrier types are displayed in Fig. 2.2. The effectiveness of land barriers and adverse cold temperatures are each relatively self-evident. Mangroves are restricted to mostly tropical latitudes except in deviations created by respective oceanic circulation patterns affecting ocean shoreline occurrences (arrows show major oceanic current gyres).

Fig. 2.2

Dispersal barriers of temperature, land, and water compared with genetic discontinuities restricting gene flow around coastal margins of the Atlantic East Pacific and the Indo West Pacific regions (Adapted from Triest 2008): The extant distributional range of mangroves with areas of outlying introduced populations are shown as blue shaded areas; The circumstances surrounding the four barriers (1–4) and three discontinuites (5–7) marked are discussed in the text

Ocean water distances can be barriers to buoyant propagule dispersal where widespread mangrove taxa have viviparous, living propagules with distinctly limited survival times at sea (e.g., see Steinke 1986; Drexler 2001). While it is evident that seed-distributing mangrove types are less widespread (no families in common between IWP and AEP regions), it is also curious why species with greatly different survival abilities have comparable global ranges (both IWP and AEP), like Rhizophora and Avicennia. Their viviparous propagules can last up to 150 days, or less than 15 days, respectively. In the absence of more information on dispersal abilities for each mangrove entity, it suffices to say that water distance can be a barrier if propagules have limited capacity to stay afloat and remain viable. In support of this contention, there are other important clues displayed in two examples of species introductions, where modern day ranges have been artificially extended to isolated previously unoccupied mangrove-suitable habitat. These are evidence of active natural barriers.

In one case (Fig. 2.2, site 1), Rhizophora mangle was artificially established during the 1920s in the northern Pacific Hawaiian islands, accompanied by a second introduced species Bruguiera sexangula (Allen 1998). Their establishment and subsequent spread among these islands clearly demonstrated the suitability of previously unoccupied habitat. Hence, the only thing preventing their introduction to these islands before this was the ocean water surrounding them—for at least around 5000 years. This is tangible evidence of an existing water distance barrier for mangroves crossing the Pacific Ocean. Only two mangrove species, Acrostichum aureum and Rhizophora samoensis, and three genera (Avicennia, Rhizophora, Acrostichum) occur naturally on both sides of the barrier. And, there are significantly greater lower taxonomic level differences within respective subregions (see Table 2.2). The oddity in this, has been the westward dispersal of R. samoensis, which appears only possible naturally if facilitated by an ancient chain of volcanic islands (Schlanger et al. 1981; Schlanger and Premoli-Silva 1981). And, this suggestion has some credibility where there are corresponding distributional records of a small number of shallow water reef fishes across the southwestern Pacific (Cowman and Bellwood 2013). In the Atlantic however, water distances appear much less effective as a barrier. While this ocean has large ocean water distances, there appears to be qualified gene flow between eastern and western shorelines (Takayama et al. 2013). In support of this contention, all seven native species on the African west coast occur also in the Americas (Table 2.3). And, while four genera on the American east coast are not recorded on the African west coast, five common genera occur naturally on either side of the ocean. However, the depauperate diversity of mangrove assemblages in West Africa remains a mystery. It is proposed that this low diversity might reflect the relative stability of this area over the last 40–50 my.

Table 2.3 Tentative ranking of mangrove genera (for the relevant species present in mangroves) by age (oldest to youngest, 1–32) based on earliest fossil evidence coupled with distributional extent across global regions and subregions (IWP = Indo West Pacific; AEP = Atlantic East Pacific; E Africa = East Africa; Indo Mal = Indo Malesia; Austral Asia = southern Asia to Australia; W Amer = West America; E Amer = East America; W Africa = West Africa), shown as numbers of species excluding hybrid intermediates. Dated (mya) fossil evidence were described by Plaziat et al. (2001), others (*) were deduced from Appendix 2

In a second example (Fig. 2.2, site 2), a mangrove outlier population on the North American west coast was established with the introduction of Avicennia marina to Mission Bay, around 30–50 years ago (Kay 2007). There were no mangroves extant in this area beforehand . The local native mangrove populations of Avicennia germinans are currently limited to a few 100 km further south in northern Mexico, presumably constrained by the temperature limited growth of that species. Of course, this needs to be assessed further, but the south east Australian source stock of introduced A. marina has greater cold tolerance than the local Avicennia species; this concurs with the unusual natural occurrence of A. marina at exceptionally high latitude sites in Australia. This shows notable limitations with the conclusion by Quisthoudt et al. (2012) that high latitude limits might be usefully re-evaluated considering temperature tolerances of individual species and their local genotypes. In any case, as shown for Hawaii, in Mission Bay, artificial and recent occupation of vacant habitat has proven to be suitable for mangroves, previously not occupied by natural means. Could this be evidence of different temperature tolerances of different genotypes, and the current effectiveness of a temperature barrier on native mangrove species along the American Northwest coast?

The effectiveness of land barriers is aptly demonstrated in one prime example (Fig. 2.2, site 3; Table 2.3)—the African land mass. This long-established land barrier works together with the Pacific Ocean water barrier to effectively divide the AEP and IWP global regions. For the African land barrier, only one species, Acrostichum aureum, occurs in common between east and west coasts. And, at the genus level, while there are three in common, two other Atlantic genera are unknown in the Indian Ocean, and seven Indian Ocean genera are unknown in the Atlantic.

In another example of a land barrier (Fig. 2.2, site 4), the Central American Isthmus (CAI) has been much less effective in blocking gene flow. While eight genera are shared, there are just two genera not shared across the CAI. The diminished effectiveness of this barrier however corresponds with its recent age, with closure of the isthmus land bridge between the Pacific and Atlantic Oceans, 3–4 mya (Coates et al. 1992; Schmidt 2007). This clearly demonstrates why extant barriers must be assessed in their historical context when evaluating their longer-term effectiveness.

In summary, the map in Fig. 2.2, shows the locations of key extant barriers along with those examples of introduced taxa mentioned earlier. The figure also shows the location of a number of potentially ancient barriers (black bars). These appear no longer to function as currently situated, but each is recognized for its otherwise odd and localized genetic discontinuity at differing taxonomic levels.

2.6 Discontinuities and Deductions Surrounding Relict Barriers

There are notable genetic discontinuities within and amongst present-day barriers (Fig. 2.2, sites 4–7 for example). Our knowledge of these instances is based on prior biogeographical accounts (e.g., Tomlinson 2016; Triest 2008; Spalding et al. 2010) along with other detailed assessments of gene flow amongst specific taxa in particular areas (Duke et al. 2002; Takayama et al. 2013; Lo et al. 2014). Such discontinuities are manifest as unexplained distributional patterns where the range of a taxon might end abruptly, or where sibling (closely related) genotypes exist either in separation or coexisting in overlapping distributional ranges. These instances stand out because they are largely unexplained in the context of the present-day barriers described previously. But, where discontinuities might be artifacts of past geological events over 10 mya, these are better considered as valuable evidence from which we might identify taxa at particular dates and circumstances.

One significant discontinuity exists between Indo-Malesian and Australasian subregions (Fig. 2.2, site 5). The boundary of contact is well-recognized as Wallace’s Line, marking the collision juncture of Australian and Asian biotas around 20 mya (e.g., Duke et al. 2002). For mangrove plants, this is marked by 21 genera in common, while 2 are restricted to Asia, and one to the south. But, while 35 species (Appendix 1) are shared between subregions , 7 species are partially or wholly restricted to the south including Aegialitis annulata, Avicennia integra, Bruguiera exaristata, Camptostemon schultzii, Ceriops australis, Ceriops pseudodecandra, and Diospyros littorea. And, 11 others are restricted to the north, Aegialitis rotundifolia, Aegiceras floridum, Brownlowia tersa, Camptostemon philippinense, Ceriops decandra, Ceriops zippeliana, Heritiera fomes, Kandelia candel, Kandelia obovata, Sonneratia apetala, and Sonneratia griffithii.

Another recognized discontinuity is that between north and south coasts of New Guinea (Fig. 2.2, site 6). This is part of the same collision event that formed Wallace’s Line (Duke 1995). While this discontinuity is much “weaker” (=lower taxonomic level differences), it is marked by sibling taxa replacing each other on either coast like Avicennia alba and A. marina, Rhizophora apiculata (Asian form) and R. apiculata (Australian form), and Sonneratia caseolaris and S. lanceolata.

In recent years, a discontinuity between east and west coasts of the Malay Peninsula (Fig. 2.2, site 7) has been identified in a number of detailed genetic studies (Triest 2008; Guo et al. 2016), but its effectiveness depends on the relevant mangrove’s dispersal abilities. For dispersal specialists, like Rhizophora, local gene exchanges are governed predominantly by current flows (Lo et al. 2014; Wee et al. 2014), while for species with more limited dispersal abilities, like Ceriops and Lumnitzera there is a notable ancient land barrier effect (Tan et al. 2004; Liao et al. 2007; Su et al. 2006). The circumstances are explained by the partial submergence of an ancient much larger Sund Peninsula whose remnant outcrops now form the Indonesian Archipelago. The submergence event was the result of the massive collision between Australian and Asian land masses as these continental fragments continue to relocate and adjust. The separation of biotas in the Indian Ocean and the South China Sea therefore appear to have much greater isolation than expected. At least nine species (Appendix 1) affected include: Aegialtis rotundiolia, Ceriops decandra, Ceriops zippeliana, Ceriops tagal, Heritiera fomes, Kandelia candel, Kandelia obovata, Sonneratia apetala, and Sonneratia griffithii. Much of the genetic differences concern variations within individual species.

2.7 Localized Extinction Events and Recovery

Mangrove distributions have varied considerably over more than 50 million years with both notable range extensions and instances of localized extinction (Duke 1995; Plaziat et al. 2001; Triest 2008). Events most likely to have caused such extreme outcomes have almost certainly been associated with changes in climate, reduced rainfall, and cooling temperatures. A key driver for these events could be associated with shifts in continental fragments as they drifted from one climate zone to another. Or, it could be changes to climate zones from altered oceanic circulation patterns and/or atmospheric conditions.

For north–south orientated sections of coastline in particular, temperature increases have driven poleward shifts, while temperature decreases have driven retreat and extinction from higher latitudes. This is shown in global fossil records (Plaziat et al. 2001) with species also becoming extinct from entire regions of warmer temperatures when rainfall decreased dramatically, like the Middle East and the Caribbean–northern South America areas 20–30 mya. The following case studies illustrate some key processes at play.

Consider the widespread IWP taxon, the mangrove palm, Nypa fruticans (Appendix 1). It is restricted naturally today to wet, warm areas within the IWP region. However, fossil evidence shows its past distribution included not only both IWP and AEP regions, but that it also sometimes was in sites of notably higher latitude >40° north and south around 50–55 mya. It is significant that in recent times, the species has been reintroduced into the AEP on multiple occasions (Duke 2013, 2014a), demonstrating the current suitability of such locations and massive changes in local climates. Today, the nearest comparable restrictive climate conditions would be those currently present in the Middle East, west of India. In these locations, Nypa is excluded despite its proximity to viable, abundant populations further east. And, the dispersal capacity of Nypa is apparently reasonably effective with notable scattered populations spread across islands of the North-West Pacific, and further afield.

A number of mangrove species and genera have shown latitudinal range contractions from past fossil distributions (Appendix 1), including, Acrostichum, Aegiceras, Avicennia, Bruguiera, Ceriops, Heritiera, Pelliciera, and Rhizophora. However, only Pelliciera has shown partly comparable reductions in longitudinal range. Curiously, this relates to similar areas of the AEP in each case. Unlike Nypa, Pelliciera was not quite extinguished in the AEP, but its broad ancestral range was notably reduced to its western limit (Plaziat et al. 2001). Since then, there has been limited recovery east. As noted for Nypa, there are few current-day restrictions in climate, so such climate factors apparently appear not to be limiting the species today. However, Pelliciera does seem to have serious limitations for long-distance dispersal because of its poorly buoyant propagules. Furthermore, these are heavily consumed by small crabs (Dangremond 2015). These observations demonstrate the diverse factors to be considered along with the species-specific eco-physiological attributes of each species entity.

2.8 Time Line of Geological Events , Dispersal, and Speciation

Instances of speciation, extinction, and dispersal are driven by internal and external factors where a concurrence of events offers preliminary evidence. As such, ages estimated by carbon dating or genetic dating from rates of mutation and divergence might correspond with dates from fossil records and relevant geological events. In this way, three key instances of diversification seem relevant to the dominant genera of mangrove plants namely Rhizophora and Avicennia: (1) the diversification and dispersal to both AEP and IWP regions from a more or less common point of origin; (2) secondary diversification separately of each lineage in AEP and IWP occurrences; and (3) subsequent further secondary diversification.

The key global geological changes of continental drift that took place during the last 118 million years are shown in Fig. 2.3. Over this period, there were a series of dramatic and progressive global events with each having profound influences on the plants and animals living during these times. The four most significant circumstances (Fig. 2.3) include, Event 1: the separation of Africa and South America (~100 mya) and progressive opening of the South Atlantic Ocean; Event 2: the closure of the Tethys Sea between Africa and Eurasia 25–35 mya; Event 3: the separation of India (70–75 mya) and Australia (~50 mya) with their subsequent collisions with Asia (~40 mya and ~15 mya, respectively); and Event 4: the opening of the North Atlantic (from ~60 mya onwards).

Fig. 2.3

Maps of continental drift in four stages between 118 mya and the present day. Notable events shown with arrows, include: (1) widening of the Southern Atlantic; (2) closure of the Tethys Sea (between Africa and Eurasia); (3) narrowing of the Pacific with northern drifts of India and Australia across the Indian Ocean; and (4) opening of the North Atlantic Ocean

Fossil evidence (Table 2.3) links mangrove plants with at least the last three circumstances from 55 mya onwards. However, for the first point, it is reasonable to speculate that precursor mangrove flowering plants would almost certainly have benefitted from the creation and progressive widening of the South Atlantic Ocean. The first angiosperm flowering plants appeared nearby around this time. Prior to the Tethys closure, around 40–55 mya, recognized mangrove genera, Nypa, Acrostichum, Rhizophora, Avicennia, Pelliciera, Sonneratia, Bruguiera, Ceriops, Heritiera, and Aegiceras were present around its shoreline, as well as further afield. It seems likely that these populations were for the most part contiguous. Species like Nypa were notably widespread, suggestive of widespread, moist, warm climatic conditions. Then with the closure of the Tethys, widely spread taxa would have been divided as precursors of AEP and IWP regional differences. This was likely to be compounded by populations separated along the northern and southern shorelines of the ancient Tethys Ocean. The subsequent diversification of species was largely dependent on individual circumstances as each genus further diversified and dispersed.

Based on extant and fossil records of Rhizophora, a schematic for the phylogeny of genotypes of this genus (Fig. 2.4; Duke et al. 2002) shows likely speciation pathways portrayed as coincident with the geological circumstances described earlier (Fig. 2.3). In consideration of such matters in concordance with known physiological constraints (cs. Figure 2.2), the dispersal and distribution of evolving taxa are described in the following scenario. While AEP and IWP taxa were being separated by the Tethys closure, each was becoming more influenced by region specific events. To better understand these events and circumstances, it is useful to follow likely dispersal paths with known positions of land masses and oceans during periods identified by available fossil evidence (Fig. 2.5; e.g., Plaziat et al. 2001).

Fig. 2.4

A proposed phylogeny for Rhizophora taxa between present day and 118 mya, assuming a single ancestral progenitor possibly created with the emerging Atlantic Oceans (Duke et al. 2002): Seen as a “slice in time,” two major radiations with subsequent divergences in extant vicariant populations from global subregions (AEP and IWP) to ocean areas (green lineages). Hybrid intermediates (yellow lineages) are known between species at each instance of co-existence

Fig. 2.5

Proposed dispersal pathways taking account of all mangrove species with the help of fossil records; continental locations; land, sea, and temperature barriers; and, overall species affinities

In the AEP, Rhizophora populations with blunt obtuse leaf tips would have spread along shorelines of the newly forming Atlantic oceans. One group (Fig. 2.5, path #2), as proposed R. racemosa progenitors, progressed south from Africa to South America. While the other (Fig. 2.5, path #4), as possible R. mangle progenitors may have spread west along the North American coastline before migrating south as conditions progressively cooled during the Eocene. The early progenitor lineages had separated and developed over millions of years, remaining isolated until they were reunited as different species when North and South American land masses merged and formed (Fig. 2.5, point C). The further separation of R. mangle and its sibling, R. samoensis, is likely to have arisen during this time as populations became isolated on opposing sides of North America which was finally completely isolated after closure of the Caribbean American Isthmus (Fig. 2.5, path #3). During this time, R. mangle and R. racemosa more or less matched ranges on both sides of the Atlantic.

In the IWP, Rhizophora populations with spiked mucronate leaf tips would have spread east along shorelines of the ancestral Indian Ocean as the far western shores of a vast Pacific Ocean. One group (Fig. 2.5, path #10), as proposed R. apiculata progenitors, may have progressed eastward to Southeast Asia where founding populations possibly became isolated on either side of the Sund Peninsula. Meanwhile, the other group (Fig. 2.5, path #6), as R. mucronata progenitors, spread south along the East African shoreline where some plants spread to the island of India as it drifted north. The Indian land mass carried populations of R. mucronata north to join the Asian shoreline (Fig. 2.5, path #7). Other plants would have spread to the Australian shoreline, and changed slightly to become the R. mucronata sibling, R. stylosa (Fig. 2.5, path #8). Then, after Australia collided with the Sund Peninsula and Southeast Asia, all diverging IWP species would have been united into the overlapping distributions seen today (Fig. 2.5, point H & I). It is of some interest, that the dispersal and speciation processes were somewhat similar in respective hemispheres where divergent populations appear to be reunited (equalized) after long periods of separation and divergence.

It is of additional great significance, that two distinct and different genera, Rhizophora and Avicennia, in particular, have comparable global distributions; despite acknowledged differences in key functional characteristics, including dispersal ability (cs. Steinke 1986; Drexler 2001). This firmly suggests there might be common overall circumstances influencing dispersal and distribution beyond individual plant differences. So, why do these different genera have similar global distributions? It has been explained earlier that this is despite dispersal being limited by extant land and sea distance barriers. Could it be that the influence of continental drift dominates the evolution of mangrove plants rather than their functional differences? The answer may be explored further by investigating whether common patterns exist in Avicennia and other genera, when substituting each genus in the previous scenario for Rhizophora. In doing so, some consideration can be made for local influences, particularly, any reported differences in dispersal ability, along with respective levels of taxonomic differentiation.

In the scenario for Avicennia, the corresponding AEP species might be as follows: A. germinans for R. racemosa; A. schaueriana for R. mangle; and A. bicolor for R. samoensis. And, matching IWP species would be: A. marina for R. stylosa; A. officinalis for R. apiculata; A. alba for R. mucronata; and two other—A. integra appears to a sibling derivative of A. officinalis while A. rumphiana is a sibling derivative of A. marina.

Then, while Sonneratia is only recorded in the IWP, the corresponding species for that comparison include S. alba for R. stylosa; S. apetala for R. apiculata; S. caseolaris for R. mucronata; and three others—S. lanceolata is a sibling derivative of S. caseolaris; but S. ovata and S. griffithii appear to be outlier derivatives of S. alba.

For Bruguiera in the IWP only, the corresponding species for that comparison include B. exaristata for R. stylosa; B. parviflora for R. apiculata; B. gymnorhiza for R. mucronata; while B. sexangula is a derivative of B. gymnorhiza; but B. cylindrica and B. hainesii are outlier derivatives of B. parviflora.

For Ceriops in the IWP only, the corresponding species for that comparison include C. australis for R. stylosa; C. decandra for R. apiculata; C. tagal for R. mucronata; while C. zippeliana and C. pseudodecandra are outlier derivatives of C. decandra.

While these comparable groupings and patterns require further validation, they do match the hypothesis of common overall dominance of geophysical influences over individual differences in affected taxa. In making this case, it is not suggested that mangrove taxa have common rates of evolution or common taxonomic diversification, nor that we should underestimate the importance of individual functional differences , like dispersal capabilities, propagule longevity, and phenologies. In fact, it is more likely that with greater awareness of such shared influences, this will allow greater understanding of the deterministic role of individual attributes in the evolution of each taxon.

2.9 Shared Evolutionary Processes and Dispersal Pathways

The first step is to review the initial patterns in distributional occurrences with the known dispersal endpoints for each taxa. All mangrove species (excluding recognized hybrid intermediates) have been assigned to 15 spatiotemporal endpoint groupings, summarized in Table 2.4. Five groups occur in the AEP with two on northern coastlines and three on southern coastlines, notably within its ancient Tethyian context. Similarly, 10 groups occur in the IWP with three on northern coastlines and seven on southern coastlines. Groups with repeated names and number, represent likely overlapping dispersal routes associated with similar extant locations.

Table 2.4 Assignment of 70 extant mangrove taxa (species, subspecies, varieties) to 18 dispersal endpoints grouped west (A–E) and east (F–O) of the ancient Tethys Ocean 40–55 mya (see Figs. 2.5 and 2.6). Each is then further assigned to northern or southern hemispheres, as separating increasingly isolated shorelines. These groups correspond with 12 dispersal pathways, discussed later (see Table 2.5). See Appendix 2 for specific group attributions and estimates of taxa numbers and approximate age (mya)

To better understand these groupings, each has been located on an Eocene map (Fig. 2.5, starting from red-shaded continents). On this map, ancient dispersal barriers and pathways of the day (~40 mya) have also been tentatively defined (cp. Fig. 2.2). Dominant barriers include temperature (active across latitudes), sea distances (defined by the dispersal and establishment capability of each species), and land barriers (where they prevent water-borne dispersal). Fossil evidence has helped define the effectiveness and presence of these ancient barriers (cs. Plaziat et al. 2001).

As proposed for the evolution of Rhizophora taxa (Fig. 2.4; Duke et al. 2002), these were widely distributed before closure of the Tethys Sea during the latter Eocene, around 45 mya. Using the Eocene map in Fig. 2.5, there were a number of mangrove genera present along the northern and southern shorelines of the Tethys, of Laurasia, and Gondwana, respectively. With the northward movement of Africa, the Tethys closed cutting off western and eastern distributions. The effect would have been to isolate northern and southern populations that separated over time, becoming more isolated entities in AEP and IWP regions. All these observations, and those that follow, are based on known extant and fossil records, in full consideration of individual attributes of each relevant species entity.

Those in the AEP are likely to have dispersed westward in two more or less separate pathways: a southern path along the coasts of West Africa and South America; and, a northern path along the North American east coast, forced south as conditions cooled, and the Atlantic widened.

For the IWP, dispersal eastward had also started out following two isolated pathways: a northern path along the coast to the Middle East and Southeast Asia; and, a southern path along the coasts of East Africa, India, and Australia. To complicate the dispersal outcomes, the latter two land masses with mangrove inhabitants moved rapidly north at different times to collide with the Middle East and Southeast Asia (respectively) during this period.

In both AEP and IWP, there were, at first, partly isolated pathways that independently later reunited toward their respective dispersal extremes. This has led to the consequential genesis of greater diversity and further radiation that now characterizes each region – and, it has led to the creation of the respective hot spots of extant diversity. The greater complexity of geomorphic changes in the IWP is presumed to be the reason for the greater genetic diversity in that region.

Combining these observations, the proposed common dispersal pathways are depicted in 12 key locations of likely isolation (Table 2.5), leading to genetic differentiation and speciation for respective mangrove genera (Figs. 2.5 and 2.6, Appendix 2). These relate also to the 15 extant location endpoints (Table 2.4). The broad assessment of evidence considered for these depictions include, location and age of fossil records; period maps of continental drift; distribution maps of each species; phylogenetic relationships between and among taxa; extant discontinuities; and key barriers of land, water, and climate.

Table 2.5 Proposed common dispersal pathways are depicted in 12 key locations of likely isolation, leading to genetic differentiation and speciation for the respective mangrove genera (Figs. 2.5 and 2.6, Appendix 2). These relate to 15 extant location endpoints (Table 2.4). The broad assessment of evidence considered for these depictions include the following: location and age of fossil records; period maps of continental drift; distribution maps of each species; phylogenetic relationships between and among taxa; extant discontinuities; and key barriers of land, water, and climate

Fig. 2.6

Proposed timeline of common phylogenies of all mangrove species noting global dispersal points (A–O) and dispersal pathways (1–12). Also see Fig. 2.5

The decision for grouping species in this treatment is based on the idea that instances of genetic divergence and speciation could only have taken place when circumstances resulted in prolonged isolation or during founder events where dispersal from established populations might have reached unoccupied niches suitable for mangrove establishment and diversification. And, that these circumstances would be utilized by all genetic entities present at the time.

2.10 Common Drivers of Diversification and Speciation

While it appears less likely that mutations amongst mangroves taxa have led to genetic differentiation in a process of sympatric speciation (Maynard-Smith 1966), there are other features of these taxa indicative of other recognizable processes of speciation. One is the number of closely related, sibling taxa. While it is notable that some sibling taxa have slightly different ecological preferences, the ranges of these entities often overlap. This implies that the reason for their isolation is no longer applicable, with once separate populations being reunited.

Considering all relevant evidence, it seems the diversification of mangrove plants has largely been driven by three key processes of speciation:

• (a) Allopatric speciation (Mayr 1970): incidents of isolation by land, sea, or temperature barriers blocking gene flow

• (b) Peripatric speciation (Provine 2004): founder populations (with their smaller subset of genes) multiplying rapidly into more or less vacant habitat

• (c) Parapatric speciation (Doebeli and Dieckmann 2003): the reunification of extreme ends of wide, linearly distributed species (as with the broad global shoreline surrounding the Tethys Sea) having notable diversification and genetic drift along its length

The number and diversity of families and genera with representatives specialized for mangrove habitat clearly show that the evolution and speciation of mangrove plants has taken place on multiple occasions, and independently within various families and genera. As the drivers of diversification and speciation appear strongly influenced by major geological and climatic events, it seems reasonable to conclude that respective phylogenetic pathways of each genetic entity might follow a common structure. The premise is that all taxa have been mostly subjected to the same guiding dominant influences with the same notable events of speciation.

In Fig. 2.6, a common, encompassing phylogenetic tree structure is proposed for the derivation of every mangrove plant taxa. The pathway describes how a small number of progenitors of key plant genera might have diversified from their respective origins during the last 60–90 my. During this time, mangrove taxa appear to have first dispersed east and west, then along northern and southern shores of a diminishing Tethys Sea. The dominant genera had reportedly distributed globally long before closure of the ancient Tethys Sea during the mid-Eocene period, around 45 mya (Plaziat et al. 2001). This is consistent with the presence of dominant mangrove genera in both AEP and IWP regions today. It is notable that similar patterns occur in seagrass and corals (Hoeksema 2007).

Dominant and widespread mangrove genera include Nypa, Rhizophora, Avicennia. While Nypa has no reported diversification more than its apparently sole species entity, the other two have clear diversification with speciation for common specialized ecological niches, as well as comparable distinct regional forms. This implies these taxa were influenced by a common driver, where the physical environment might have strongly guided and helped form the necessary mangrove traits and features that individually characterize each taxon. While the precise origin of these primary mangrove taxa remains unknown, the separation of South America from Africa would have provided the necessary physical circumstances consistent with the creation of mangrove traits. These circumstances include a slow and progressive increase in salinities as two massive, complex estuarine margins spread apart before becoming the southern Atlantic Ocean. And, this all would have taken place in the formative early years of flowering plants, around 90–110 mya.

It is also curious why so many groups of plants of such antiquity have so few species today, and why these plants have mostly remained unchanged throughout their long fossil record, up to 55 my. A key deduction is that events causing speciation have been rare. And, this concept is consistent with the current proposition of a common phylogenetic structure for the evolution of extant mangrove plants. This is especially the case since each instance of speciation must have been marked by notable and very unusual circumstances. While these events appear to have determined the common phylogenetic tree structure, this does in no way imply that all mangroves originated from the same place, or at the same time. But, it does however provide a means to narrow down the options for determining the individual features of each species and genotype.

For limited range species, like Mora oleifera (Duke 2013, 2014a; see Appendix 2), there are a number of important knowledge gaps. This species, restricted to the Pacific coast of Central America, appears to have originated quite recently as there are few if any fossil records. Of course, this observation would be amended if there were fossil evidence, and/or, if this species were found to have unusual dispersal limitations. While there may be other questions about its nearest relatives (nearby, there are 7–9 upland species of Mora), it is of some interest that this species has exceptionally large propagules—the largest embryo of any plant. But, it is not really known how buoyant they are, or whether they might be predated upon by small crabs, like those consuming Pelliciera propagules (Dangremond 2015). Its limited distributional range however is consistent with either poor dispersal, or that it is newly evolved, or both.

Such questions concern most, if not all, mangrove taxa. So, in the absence of further evidence, it may be concluded that species with limited distributions, might also have recent origins. For example, it was deduced for Mora oleifera therefore, that this species was likely to have arisen after closure of the Central American Isthmus, less than 3–4 mya.

Another taxon, the genus Camptostemon, appears to have been around only after 10–25 mya, and diversified as two species around 5–10 mya.

Taxa like the genus Lumnitzera, are more difficult to place. While the genus is restricted to the IWP, this does imply its post-Tethyian origins, since 40 mya. However, its subsequent diversification and its occurrence in East Africa, does suggest its presence along the Southern Tethys Eastern pathway rather than in the north. There does not appear to be any relevant major dispersal events from north to south. As such, the two species appear to have diverged with the passage of India north, leaving the other entity to possibly migrate eastward unchanged to Australia.

A summary of estimated ages for mangrove-related representatives from all 32 genera is shown in Table 2.3. While the deductions may be altered and amended in the future as new information comes to hand, the proposed common phylogenetic tree structure does offer a practical working understanding of how and where each mangrove species evolved and dispersed to locations where they exist today.

2.11 Summary and Conclusions

In summary, there are six key deductions and findings raised in this treatment:

• 1. The mangrove niche is inhabited by around 80 tree and shrub species (including 11 hybrid intermediates) of 32 genera from 17 families worldwide. These mangrove species are widely distributed, but constrained by natural barriers of temperature, water distance, and land masses. For the most part, the effectiveness of each barrier is self-evident, marked by genetic differences or endpoints, as genetic discontinuities.

• 2. The natural distribution of individual species and genotypes are mostly distinct with few occupying the same range. Taxa are individually constrained by their respective tolerances for key functional factors like temperature, precipitation and evaporation, salinity, soil type, inundation frequency and tidal regimes, as well as individual adaptations in anatomy, physiology, phenology, propagule buoyancy and dispersal longevity, and establishment ability.

• 3. There are significant instances where extant dispersal barriers have been fully effective in recent times, leaving suitable habitat niches vacant. Because mangrove plants all have buoyant, water-dispersed propagules, the effectiveness of land mass barriers are self-evident. This is complicated for barriers of climate (temperature, rainfall) and water distance because these are dependent on individual capabilities and tolerances of each plant type. But, in all cases, when people intervene by introducing a plant type to habitat previously unoccupied by that genotype, this proves two things: a) that suitable habitat was available; and b) that the introduced entity was unable to disperse and arrive there naturally.

• 4. There are a number of genetic discontinuities that lack obvious current-day barriers. These can only be explained by past geological circumstances where previous barriers are no longer effective, like Wallace’s’ line in the IWP. The presence of these occurrences implies there can be appreciable lags in genetic mixing and natural dispersal amongst previously established populations. The relevance and role of each discontinuity is determined by its location; its geological age; and, the level of taxonomic differences in responsible biota.

• 5. Species distributions worldwide are divided naturally into two global regions of the Indo West Pacific (IWP) and Atlantic East Pacific (AEP), distinguished by relatively high level taxonomic differences in families and genera. Genetic hot spots are present in each region, but that in the IWP is well recognized for its concurrent and comparable concentrations of mangroves, corals, sea grasses, reef fishes; collectively and individually known for its globally high diversity as the hot spot triangle. These common diversity patterns across different plant and animal taxa demonstrate the overwhelming dominance of geo-physical circumstances over individual functional differences in biota. In this way, plant and animal life appears more to have been shaped and driven by their individual responses to the changing geophysical environment, rather than the other way around.

• 6. In conclusion, this treatment offers the novel hypothesis that all mangrove species have dispersed, diversified, and evolved following more or less, common, overall phylogenetic pathways formed by the overwhelming geophysical occurrences over the last 100 my. And, it is within these constraints that each taxon displays its individual differences depending on their own circumstances in genesis location and date, as well as their biological capabilities and tolerances in growth and mutation, including their notable attributes of propagule buoyancy, longevity, and establishment success.

Appendices

Appendix 1

Distribution Maps . Listed are 24 distribution maps (A–X) showing 32 genera with 70 species of mangrove plants. Hybrid species are not included because their ranges for the most part match the overlapping distributions of parental taxa. For more references, specific descriptions, images, and distributional maps of each species, refer to the World Mangrove e-book app (Duke 2013, 2014a).

Fig. 2.7

(A) Distribution of mangrove species of the genus Nypa, showing extant, introduced, and fossil occurrences. Species: N. fruticans van Wurmb 1781. (B) Distribution of mangrove species of the genus Acrostichum, showing extant, introduced, and fossil occurrences. Species: A. aureum L. 1753, A. danaeifolium Langsdorff and Fischer 1810, A. speciosum Willd. 1810. (C) Distribution of mangrove species of the genus Rhizophora, showing extant, introduced, and fossil occurrences. Species: R. mucronata Lam. 1804, R. stylosa Griff. 1854, R. apiculata Blume 1827, R. mangle L. 1753, R. samoensis (Hochr.) Salvoza 1936, R. racemosa Meyer 1818. (D) Distribution of mangrove species of the genus Avicennia, showing extant, introduced, and macro fossil occurrences. Species: A. marina (Forssk.) Vierh. 1907, A. alba Blume 1826, A. rumphiana Hallier f. 1918, A. officinalis L. 1753, A. integra N.C. Duke 1988, A. germinans (L.) Stearn 1958, A. bicolor Standley 1923, A. schaueriana Stapf and Leechman ex Moldenke 1939. (E) Distribution of mangrove species of the genus Pelliciera, showing extant and fossil occurrences. Species: P. rhizophorae Planchon & Triana 1862. (F) Distribution of mangrove species of the genus Sonneratia, showing extant, introduced, and fossil occurrences. Species: S. alba J.Sm. in A.Rees 1816, S. apetala Buch.-Ham. 1800, S. griffithii Kurz 1871, S. ovata Backer 1929, S. caseolaris (L.) Engl. 1897, S. lanceolata Blume 1851

Fig. 2.8

(G) Distribution of mangrove species of the genus Bruguiera, showing extant, introduced, and fossil occurrences. Species: B. gymnorhiza (L.) Savigny ex Lam. & Poiret 1798, B. cylindrica (L.) Blume 1828, B. hainesii C.G. Rogers 1919, B. parviflora (Roxb.) Griff. 1836, B. sexangula (Lour.) Poir. 1816, B. exaristata Ding Hou 1956. (H) Distribution of mangrove species of the genus Ceriops, showing extant, introduced, and fossil occurrences. Species: C. tagal (Perr.) C.B. Robinson 1908, C. australis (C.T.White) Ballment, T.J. Sm. & J.A. Stoddart 1988, C. decandra (Griff.) Ding Hou 1958, C. pseudodecandra Sheue, Liu, Tsai and Yang 2010, C. zippeliana Blume 1849. (I) Distribution of mangrove species of the genus Aegiceras, showing extant and fossil occurrences. Species: A. corniculatum (L.) Blanco 1837, A. floridum Roemer & Schultes 1819. (J) Distribution of mangrove species of the genus Heritiera, showing extant and fossil occurrences. Species: H. littoralis Aiton 1789, H. fomes Buch.-Ham. 1800. (K) Distribution of mangrove species of the genus Laguncularia and Lumnitzera, showing native and introduced occurrences. Species: Laguncularia racemosa (L.) Gaertn. f. 1805; Lumnitzera racemosa Willd. 1803, Lumnitzera littorea (Jack) Voigt 1845. (L) Distribution of mangrove species of the genus Aegialitis. Species: A. annulata R.Br. 1810, A. rotundifolia Roxburgh 1824

Fig. 2.9

(M) Distribution of mangrove species of the genus Barringtonia and Pavonia. Species: B. racemosa (L.) Spreng. 1826; P. paludicola Nicolson ex Fryxell 1989, P. rhizophorae Killip. ex Kearney 1954. (N) Distribution of mangrove species of Acanthus and Muellera genera. Species: A. ebracteatus Vahl 1791, A. ilicifolius L. 1753; M. moniliformis L.f. 1781. (O) Distribution of mangrove species of the genus Camptostemon. Species: C. philippinense (Vidal) Becc. 1898, C. schultzii Mast. 1872. (P) Distribution of mangrove species of the genus Mora and Xylocarpus. Species: M. oleifera (Triana) Ducke 1925; X. granatum J.König 1784, X. moluccensis (Lam.) M. Roemer 1846. (Q) Distribution of mangrove species of the genus Kandelia. Species: K. candel (L.) Druce 1914, K. obovata C.R. Sheue, H.Y. Liu & W.H. Yong 2003. (R) Distribution of mangrove species of the genus Crenea and Scyphiphora. Species: C. patentinervis (Koehne) Standl. 1947; S. hydrophylacea C.F. Gaertn. 1791

Fig. 2.10

(S) Distribution of mangrove species of the genus Conocarpus and Pemphis, showing native and introduced occurrences. Species: C. erectus L. 1753; P. acidula J.R. Forst. & G. Forst. 1775. (T) Distribution of mangrove species of the genus Brownlowia, Diospyros, and Tabebuia. Species: B. tersa (L.) Kosterm. 1959; D. littoralis (R.Br.) Kosterm. 1977; T. palustris Hemsl. 1882. (U) Distribution of mangrove species of the genus Osbornia. Species: O. octodonta F.Muell. 1863. (V) Distribution of mangrove species of the genus Cynometra. Species: C. iripa Kostel. 1835.(W) Distribution of mangrove species of the genus Dolichandrone. Species: D. spathacea (L.f.) K. Schum. 1889. (X) Distribution of mangrove species of the genus Excoecaria.

Species: E. agallocha L. 1759

Appendix 2

Table of Species Groupings . Presented are tables showing proposed phylogenetic groupings of 32 mangrove genera (A: with 8 genera, B: with 11 genera, and C: with 13 genera) with selections of key species in each of the 15 global spatiotemporal groupings discussed in the chapter.

Table 6a Proposed phylo-genetic groupings of 8 of 32 mangrove genera and all respective species in 15 spatio-temporal groups

Table 6b Further proposed phylogenetic groupings of 11 of 32 mangrove genera and all respective species in 15 spatiotemporal groups

Table 6c Further proposed phylo-genetic groupings of 13 of 32 mangrove genera and all respective species in 15 spatio-temporal groups