PHYSICAL AND HISTORICAL CONTEXT: WESTERN GHATS OF INDIA
The escarpment of the WG represents the elevated rim of the Deccan plateau running along India’s south-western coast (8°-20°N) at elevations c.1200m (highest peak 2,695m). The chain of mountains mediates the rainfall regime of peninsular India by intercepting the heavy south-western monsoon rainfall favourable to the establishment of evergreen forests. The humid forests of the WG are isolated from other evergreen forests of north-east India and Indochina peninsula. This is because, apart from a narrow belt in the south, the rapid decline in rainfall on eastern leeward slopes makes them unable to support wet evergreen forests (Figure 1), and to the north, deciduous formations replace the evergreen with increasing dry season lengths.
Figure 1 The annual rainfall gradient in the Western Ghats (left) with rainfall transects for different latitudes (right). Evergreen forest limits are shown in green correspond to the zones with highest rainfall (dotted line).
The heterogeneity associated with the deeply dissected topography has engendered the creation of multiple environmental conditions with drastic changes in bioclimatic conditions over space and time. The WG comprise a variety of vegetation types, from scrub forests in low-laying rain shadow areas of the plains, to deciduous and tropical wet forests up to about 1500m, and a unique mosaic of montane forests and grasslands above that.
The interaction of the summer monsoon winds with the relief results in a strong west to east rainfall gradient with high precipitation on windward slopes (2000 to >5000mm) that diminishes towards the interior (2000 to 900mm). A marked difference in precipitation seasonality correlates with the sharp decrease in rainfall beyond the crest of the WG, increasing from two dry months on the windward slopes to up to 6 months on leeward slopes in the south. Since the monsoon arrives from the south and retreats in the opposite direction, this creates another strong gradient in dry season length from two dry months in the south to up to eight in the north. An additional effect of topography is the fall in temperature with altitude: at elevations > 800m between the coast and the crest of the WG, mean temperature of the coldest month temperature is about 23°C, whereas it varies between 16-23°C at medium elevations (800-1400m), and 13.5-16°C at the highest elevations (Ramesh et al., 2010a). These basic gradients in rainfall, temperature and seasonality are major drivers of species turnover (Pascal, 1988; Bonnefille et al., 1999; Barboni et al., 2003; Ramesh et al., 2010a). A critical threshold in terms of vegetation differentiation is the transition from 4 to 5 dry months that separates the humid regions of evergreen forest from those that include deciduous elements (Gunnell, 1997). Previous work in the WG also underlined major ecological differentiation of forest types related to drought stress along the latitudinal gradient, with a transition zone between 13-14°N. Modern pollen analyses points to the dominance of Dipterocarps characterising the evergreen belt up to 14°N after which it is replaced by species of Diospyros in the semi-evergreen forests (Pascal, 1986; Barboni et al., 2003). From these bioclimatic characteristics, Pascal (1988) defined four main domains of potential forest vegetation in the WG region: wet evergreen dry evergreen, moist deciduous and dry deciduous, within which 19 floristic types are delineated (Figure 2 see details in Table 2).
DYNAMIC BIOGEOGRAPHICAL HISTORY
The vegetation of India is unique in that it was shaped by changing conditions as it crossed various paleoclimatic belts between the time of its breakup from Gondwana in the far southern hemisphere in the early Cretaceous (~130Ma), and subsequent collision with Eurasia (~50Ma) (Briggs, 2003). The separation, latitudinal displacement and collision of the Indian plate led to large-scale continental plant re-organisation, palaeoceanographic and paleoclimatic changes, and the accompanying evolution and dispersal of flora (Bajpai et al., 2012). Being embedded in the peninsular, the geological and climatic history of the WG is closely tied to that of India. Two points can be highlighted in relation to India’s complex biogeographical history: 1) rather than being isolated during its long northward drift, current studies indicate that the Indian plate would have had occasional biological contacts with other continents, resulting in the extant flora being a mix of Laurasian and Gondwanan as well as Indo-Malaysian affinity, and 2) the present-day climate is a product of geomorphological evolution.
Diverse biogeographical origins
It is now believed that, rather than the « biotic ferry » or Noah’s ark model (Hedges, 2003), the Indian plate was less isolated from neighbouring landmasses during its long northward drift than previously thought (Briggs, 2003; Ali & Aitchison, 2008). While there is agreement that there were prolonged periods of isolation to which we can attribute unique assemblages of ancient plant and animal taxa (Biju & Bossuyt, 2003; Roelants et al., 2004; Van Bocxlaer et al., 2012), and phylogenetic relationships show that a number of taxa in India are basal to south-east Asian lineages in support of their Gondwanan origin and the « out of India hypothesis » (Conti et al., 2002; Karanth, 2006), mixing did take place with tropical flora and fauna of neighbouring landmasses (Rust et al., 2010).
In terms of Gondwanan ancestry, the Gondwanan vicariance hypothesis postulates that a subset of such lineages was already present on the Indian plate since mid to late Cretaceous. Competing hypotheses propose their arrival through alternative routes during the Tertiary: migration from Africa to Asia facilitated by a northern mid-latitude corridor of frost-free climates during the Eocene (~40 Ma, boreotropical migration hypothesis); overland dispersal across Arabia at the Miocene Climatic Optimum (18-16 Mya, Miocene geodispersal hypothesis); and transoceanic dispersal aided by the ‘stepping stone’ effect of islands in the Indian Ocean (Thomas et al., 2015). The biota is thought to have been assembled through multiple colonisation events of lineages of Gondwanan and other ancestry from Africa, Eurasia and south-east Asia, especially as new areas would have opened up with the extinction of many existing Gondwanan elements triggered by late Cretaceous (~65Ma) flood basalt volcanism after the Indian plate crossed the Reunion mantle plume (Samant & Mohabey, 2009).
Knowledge of the origin and epoch of diversification of lineages is important because species are best adapted to abiotic conditions under which their lineages originated (Ricklefs, 2006; Bartish et al., 2016). In the WG, monospecific families with Laurasian affinities (e.g Magnoliaceae, Ericaceae, Myrsinaceae) are mostly found at medium to higher elevations whereas monospecific families of Gondwanan affinity are characteristic of low to medium elevations subject to short dry seasons (Gimaret-Carpentier et al., 2003). Carlucci et al.,(2016) found that Magnoliids, which generally show conserved preferences for Gondwana-like montane or moist and shady habitats, tracked these conditions in the Andean forests. Along the same vein, species of different lineages might vary in their responses to climate-induced habitat change, depending on their evolutionary heritage. For e.g. lineages that diversified in a warmer epoch might still carry adaptations to such a warmer climates while others might be more sensitive to climate change as suggested by Bartish et al., (2016).
Lastly, mounting molecular evidence that ancient Gondwanan lineages from the Cretaceous survived in WG despite dramatic climatic changes and volcanism, points to the existence of refugial conditions here. Joshi and Karanth (2013) show evidence of relict humid forest-dwelling taxa in southern WG refugia having diversified and dispersed into other parts of the WG under favourable conditions commencing in the Palaeocene. Indeed, recent pollen analyses from western and northern India indicate the re-establishment of wet evergreen forests in the late Palaeocene after the extensive volcanic activity (Prasad et al., 2009). This study revealed the striking similarity of extant pollen flora of WG endemic plants with 28 most common fossil pollen taxa of the early Palaeogene (~50-55 Mya). The southern WG is thus thought to have served as refugia for wet evergreen species that were widely distributed prior to the volcanic period as well as a centre of origin for others. The high diversity is a legacy of the globally warm interval of this period, when the Indian plate experienced long periods of high precipitation and low seasonality in the low latitude equatorial region. These conditions were key factors in the widespread distribution of the tropical rain forest community in the Indian subcontinent (Bajpai et al., 2012).
Rainforest dynamics after the Miocene
The present-day climate in the WG is a product of geomorphological evolution. In the Eocene (45 Mya) tropical forests covered most of the Deccan plate (Meher-Homji, 1989) before the creation of the WG escarpment and the subsequent collision with Laurasia. The upliftment of the Himalayas and subsequent establishment of the monsoon regime brought about a major shift in climate, with the onset of aridification in large parts of the Indian peninsula after the mid-Miocene (Morley, 2000; Guo et al., 2008; Patnaik et al., 2012). As the arid zone spread southwards, previously widespread wet evergreen forests were largely replaced by deciduous forests to the north and east of the WG (Meher-Homji, 1983; Patnaik & Prasad, 2016). Orographic rainfall provided the necessary humid environment and habitat conditions to ensure the persistence of wet evergreen forests on the western slopes of the WG (e.g. Roelants et al., 2004). The WG was thus isolated from other persistent wet zones and evergreen forests in North-east India and Indo-China at this point. Within the WG itself, the seasonal reversing monsoon winds introduced a south to north gradient of seasonality during this period (Gunnell, 1997; Patnaik et al., 2012; Patnaik & Prasad, 2016).
ENDEMISM AND DIOECY IN THE WESTERN GHATS
The distribution and high rates of endemism (over 56% for evergreen species, Pascal et al., 2004) in the WG raise questions regarding the past history and origin of the flora and their possible evolution from the time that the WG was isolated from other evergreen formations in India. The high rates of endemism are comparable to that of oceanic islands, where endemism may be due to the fact that, fortified by its insularity the flora underwent an evolution giving rise to endemic species (cradle); or, protected by a (sea) barrier, might have escaped the onslaught of unfavourable climatic fluctuations/ biological instabilities which affect larger landmasses, thus serving as a refugia for relict flora (museum).
Two points are of particular interest: Firstly, species richness and endemism are not uniformly distributed along the WG. The southernmost regions that have the most favourable conditions with high (but not excessive) annual rainfall and low seasonality are the richest in species (Ramesh & Pascal, 1997; Barboni et al., 2003; Davidar et al., 2005). The rate of endemism among evergreen trees decreases from ~70% in the southernmost WG to ~60-65% up to about 12°N, followed by a drastic decline from 38% to 25% up to about 14-15°N. In the extreme northern WG it is less than 10% (Ramesh, 2001). Additionally, birds, amphibians, and fishes also exhibit similar patterns of decreasing diversity and endemism towards the north (Dahanukar et al. 2004; Aravind and Gururaja, 2010 in Joshi and Karanth, 2013). Most of these studies have identified contemporary ecological factors like seasonality, productivity, climate, and low seasonality to explain these patterns. However, speciation and biogeographic processes are poorly understood for the WG biota (Joshi & Karanth, 2013).
To summarise what the literature in the previous section points to: within the WG, the southern forests have been least affected by past climatic changes and studies show that they have served as recurrent refugia since the early Eocene. This stretch of forests (8°-13°N) is biogeographically delineated from forests further north that experienced lower historical climate and habitat stability, having experienced multiple alternations in climate and alterations in habitat – with accompanying cycles of contraction and colonisation of species distributions – dating from the early Palaeocene flood basalt volcanism, to the Miocene aridity, and the more recent Quaternary climate fluctuations that the southern forests were spared from. The northern forests are both at the limit of the Deccan traps (lava deposits from Cretaceous volcanism) and have been subject to greater seasonality since the mid-Miocene. The effect of more recent climate fluctuations, especially reduced rainfall in Quaternary, was also stronger in northern WG according to global circulation models (GCM). Secondly, the presence of co-generic wet evergreen species belonging to species-rich genera in habitats that are environmentally distinct has led previous work to allude to ‘ecological vicariance’ along a south-north gradient and between western windward and eastern leeward slopes, based on a capacity to tolerate lengthening dry seasons (Pascal, 1988; Gimaret-Carpentier et al., 2003). Many of these genera comprise predominantly dioecious species. The high proportion of dioecious species overall in the WG (20% of tree species, Krishnan & Ramesh, 2005), which increases slightly for endemic species, thus also raises questions regarding whether there were ecological or evolutionary constraints on the assemblage and current distribution of species that may be linked to their reproductive systems.
QUANTIFYING PAST AND PRESENT ENVIRONMENTAL CONDITIONS
For a synthetic climatic characterisation, we performed a Principal Component Analysis (PCA) of the 19 bioclimatic variables as well as elevation at a resolution of 5 arc-minutes (~10 km2) obtained from WorldClim database (Hijmans et al., 2005) covering the study area. The three first axes explained 90.42% of the overall bioclimatic variability and yielded synthetic variables for subsequent analyses. Variables relating to temperature (bio 1-11) and elevation (Alt) contribute strongly to PCA Axis 1 (Figure 3); Precipitation seasonality and precipitation of the driest month/quarter (bio 15 and bio 14/17) contribute to PCA Axis 2 and lastly, other precipitation variables such as annual rainfall contribute most strongly to PCA Axis 3. See Table S1 in Annexe 1 for details on the bioclimatic variables from WorldClim.
These variables, devoid of the co-linearity effects among the initial variables, thus summarized the main environmental gradients present in the WG (Figure 4c), namely, (i) the temperature-elevation gradient (range of mean temperature of the coldest month: 3-22°C); (ii) the N-S precipitation seasonality (duration of dry season) gradient (range of months with rainfall < 100 mm: 3-8) and (iii) the rainfall gradient (annual precipitation: 484 to 6032mm). These three primary components are consistent with the three main climatic drivers of vegetation change formerly identified in the WG (Pascal, 1988; Bonnefille et al., 1999; Barboni et al., 2003). We also considered past environmental conditions of the Last Glacial Maximum (LGM; ~21 kyr BP) and the Last Inter-Glacial (LIG; ~ 120-140 kyr BP) to investigate the possible scenarios of species distribution changes. These periods represent important climatic extremes in the last 150 kyr. The same 19 bioclimatic variables, as described above, downscaled from Global Circulation Models (GCM) output, were retrieved for the LGM from the WorldClim server.
Table of contents :
PART GENERAL INTRODUCTION
A. BACKGROUND: HISTORICAL PERSPECTIVE
1. ECOLOGY AND BIOGEOGRAPHY: ‘DUEL’ VIEWS
2. ECOLOGICAL NICHE, EVOLUTIONARY DYNAMICS
B. BACKGROUND: THE REGIONAL SCALE
1. MEETING IN THE MIDDLE
2. BRIDGING THE GAP: METHODOLOGICAL APPROACHES
3. HYPOTHESES AND EXPECTATIONS
MATERIAL AND METHODS
A. PHYSICAL AND HISTORICAL CONTEXT: WESTERN GHATS OF INDIA
1. TOPOGRAPHIC AND HABITAT HETEROGENEITY
2. DYNAMIC BIOGEOGRAPHICAL HISTORY
3. ENDEMISM AND DIOECY IN THE WESTERN GHATS
B. THE DATA
1. QUANTIFYING PAST AND PRESENT ENVIRONMENTAL CONDITIONS
2. VARIATION IN FLORISTIC COMPOSITION ALONG ENVIRONMENTAL GRADIENTS
3. PHYLOGENETIC DATA
CHAPTER 1 PAST POTENTIAL HABITATS SHED LIGHT ON THE BIOGEOGRAPHY OF ENDEMIC TREE SPECIES OF THE WESTERN GHATS BIODIVERSITY HOTSPOT, SOUTH INDIA (JOURNAL OF BIOGEOGRAPHY)
CHAPTER 2 PHYLOGENETIC DIVERSITY IN THE WESTERN GHATS BIODIVERSITY HOTSPOT REFLECTS ENVIRONMENTAL FILTERING AND PAST NICHE DIVERSIFICATION OF TREES
CHAPTER 3 ROLE OF DIOECY IN THE ECOLOGICAL AND EVOLUTIONARY DYNAMICS OF TREES IN THE WESTERN GHATS: A LEGACY OF ARIDIFICATION?
GENERAL DISCUSSION AND CONCLUSION
2. THE STORY THAT CONGENERIC ENDEMICS TOLD: NICHE LABILITY
3. CONSERVATION OF ANCESTRAL PREFERENCES IN SOUTHERN REFUGIA
4. MACROEVOLUTIONARY CONTINGENCY
5. LIMITATIONS OPEN PERSPECTIVES
6. IMPLICATION FOR CONSERVATION