Did southern Western Ghats of peninsular India serve as refugia for its endemic biota during the Cretaceous volcanism?

Abstract

The Western Ghats (WG) of south India, a global biodiversity hotspot, has experienced complex geological history being part of Gondwana landmass and encountered extensive volcanic activity at the end of Cretaceous epoch. It also has a climatically and topographically heterogeneous landscape. Thus, the WG offer a unique setting to explore the influence of ecological and geological processes on the current diversity and distribution of its biota. To this end, three explicit biogeographical scenarios were hypothesized to evaluate the distribution and diversification of wet evergreen species of the WG – (1) southern WG was a refuge for the wet evergreen species during the Cretaceous volcanism, (2) phylogenetic breaks in the species phylogeny would correspond to geographic breaks (i.e., the Palghat gap) in the WG, and (3) species from each of the biogeographic subdivisions within the WG would form distinct clades. These hypotheses were tested on the centipede genus Digitipes from the WG which is known to be an ancient, endemic, and monophyletic group. The Digitipes molecular phylogeny was subjected to divergence date estimation using Bayesian approach, and ancestral areas were reconstructed using parsimony approach for each node in the phylogeny. Ancestral-area reconstruction suggested 13 independent dispersal events to explain the current distribution of the Digitipes species in the WG. Among these 13 dispersals, two dispersal events were at higher level in the Digitipes phylogeny and were from the southern WG to the central and northern WG independently in the Early Paleocene, after the Cretaceous Volcanism. The remaining 11 dispersal events explained the species’ range expansions of which nine dispersals were from the southern WG to other biogeographic subdivisions in the Eocene-Miocene in the post-volcanic periods where species-level diversifications occurred. Taken together, these results suggest that southern WG might have served as a refuge for Digitipes species during Cretaceous volcanism.

South-west Sri Lanka: a floristic refugium in South Asia

“…These hypothetical processes, examined using phylogenetic reconstructions combined with fossil-calibrated molecular dating techniques, help to advance our understanding of the historical assembly, by revealing alternative lineage accumulation through time. It has been reported that Southern Western Ghats have served as such a refuge during Deccan volcanism for scolopendrid centipede genus Digitipes, a taxon confined to wet evergreen forests and the taxa commenced their range expansion to the northern areas of the Western Ghats in the post-volcanic periods in the Paleocene (Joshi and Karanth, 2013).…”

“…Consequently, such refugial hotspots of biological diversity are indeed among the best laboratories for identification of and analysis for conservation through prioritized phylogeographic research (Couvreur et al, 2008, Joshi andKaranth, 2013).…”

“…Many taxa unique and endemic to the Indian Plate appear to have been eliminated by massive extinctions of the majority of India's rain forests caused by a) rapid latitudinal change in India's position during the Early Cretaceous until its collision with Eurasia some 35 Ma and b) the Neogene climate changes and associated volcanic activities (Ali & Aitchison, 2008;Morley, 2000). During the Late Miocene, shifts towards more seasonal climates led to the disappearance of rain forests across much of India, restricting them to SW regions of India and Sri Lanka (Morley, 2000, Couvreur et al, 2011Joshi and Karanth, 2013;Prasad et al, 2009). During these inter-pluvial dry periods, both South Indian and Sri Lankan rainforests must have suffered massive extinction, having retreated to refugia in the steep inner valleys of the mountains (Datta-Roy and Karanth, 2009;Prasad et al, 2009;Joshi and Karanth, 2013).…”

“…During the Late Miocene, shifts towards more seasonal climates led to the disappearance of rain forests across much of India, restricting them to SW regions of India and Sri Lanka (Morley, 2000, Couvreur et al, 2011Joshi and Karanth, 2013;Prasad et al, 2009). During these inter-pluvial dry periods, both South Indian and Sri Lankan rainforests must have suffered massive extinction, having retreated to refugia in the steep inner valleys of the mountains (Datta-Roy and Karanth, 2009;Prasad et al, 2009;Joshi and Karanth, 2013).…”

Deep and wide valleys drive nested phylogeographic patterns across a montane bird community

“…Although no systematic studies have been conducted comparing species of differing ecologies and dispersal abilities, an impact of this Gap has also been found in small, restricted species like frogs [33] and some plants [34] and even in large-bodied, wide-ranging species like elephants [35]. However, certain older taxa like centipedes [36] and some long-lived trees [37] do not appear to be affected by this Gap. Although the Shenkottah and the Palghat Gaps are geologically very old ( pre-Cambrian), the impact on recent and relatively vagile colonizers like birds, especially those with small ranges, is likely to be through more recent climatic effects [38].…”