Scenario For Rainforest Evolution And Diversification

The evolution and diversification of megathermal rainforests has been dependent on, and proceeded parallel with, a succession of geological and climatic and dispersal events, controlled largely by plate-tectonic and astronomical processes, in parallel with evolutionary pressures for plants to reproduce and colonize all available land space. These events have occurred in a unique time sequence. The result is that today each geographically separated rainforest area contains its own association of species, largely descended from ancestors that were established perhaps over 70 Myr ago, and subsequently became modified, and diversified, so as to occupy the available niches within each region.

From the perspective of Quaternary studies in relation to the explanation of rainforest diversity, processes of evolution of rainforest taxa have mostly focused on the "refuge hypothesis", which maintains that the successive isolation of populations in relation to the expansion and contraction of forested areas following Quaternary Milankovich cycle driven climate changes acted as a "species pump", triggering speciation. Such an approach has paid little attention to the antiquity of rainforest species as suggested from the Tertiary fossil pollen record, and now being substantiated by molecular studies, or to the high Tertiary floristic diversities suggested by pollen diversity data. For instance, a molecular analysis of one of the fastest-evolving rainforest taxa, the species-rich Neotropical genus Inga (Fabaceae), shows that its radiation is thought to have been promoted by the later phases of Andean orogeny and the bridging of the Panama Isthmus, perhaps coupled with climatic fluctuations, (Richardson et al., 2001), but as noted by Bermingham and Dick (2001) provides little support for the idea that Pleistocene ice ages played a grandiose part in generating tropical species diversity.

This chapter attempts to show that—to understand the diversity of tropical rainforests—their development must be viewed on a much longer timescale. The climate changes which characterize the Quaternary were also taking place over much of the Tertiary period, the only difference between Quaternary and later Tertiary climate changes being one of degree of change, since "glacial" climates from the equatorial zone were clearly cooler from 2.8 Myr onward, and the vertical vegetational migration on tropical mountains over this period was likely to be more pronounced from this time onward.

From the Early Miocene to the mid-Pliocene, rainforest diversity in Southeast Asia (based on pollen-type richness) has gradually increased. A slight reduction in pollen-type richness after 2.8 Myr is not reliable in reflecting a diversity reduction in rainforest flora. Data from the Niger Delta regarding pollen-type richness suggests that West African flora has reduced significantly in diversity over the same period, with taxon losses throughout the Miocene, and with significant reductions at 11.7 and 7.0 Myr, and a particularly sharp reduction at about 2.8 Myr in the mid-Pliocene.

Palynological data from the Southeast Asian Neogene also demonstrates that the diversity of Southeast Asian flora may well have become accentuated as a result of the successive reformation of lowland vegetation on the continental shelves following periods of sea level fall over a period of at least 20 Myr (Morley, 2000a). Over this period major areas of the region have experienced everwet climates during both high and low sea level periods, with wet/dry oscillations being restricted to the Oligocene, and to some degree the Late Miocene.

In West Africa the pattern was of the alternating expansion and contraction of rainforests in relation to more open vegetation with grasslands over a period of some 30 Myr, with dry episodes—which included burning of savanna—becoming more pronounced, particularly after 7 Myr, and then again in the Late Pliocene. The depauperate nature of African rainforest flora compared with other areas is thought to be due to the decimating effect of these dry climate events, not on a Quaternary timescale, but over some 20 Myr, as emphasized by the higher number of extinctions seen in the West African Miocene pollen record than in other areas.

The scene for evolution of rainforest species is thus of gradual differentiation over a long time period with different forcing mechanisms inhibiting dispersal and isolating populations. The high diversities seen in rainforest refugia—or hot spots—are likely to relate to areas of long-term continuity of moist climates within those areas rather than to allopatric speciation driven by habitat fragmentation. The highest diversities, however, are seen where climatic stability coincides with areas which have experienced phases of orogeny and especially of plate collision, as seen from pollen data for Java following the Middle/Early Eocene collision of the Indian and Asian Plates, and for the Middle Miocene of the Sunda region following the collision of the Australian and Asian Plates. From the neotropics, molecular and biogeographical data suggest that high diversities may relate to the uplift of the Andes in the Miocene and the formation of the Panamanian Isthmus in the Pliocene.

Low equatorial floristic diversities may follow periods of cool, and particularly dry climates, as was the case following the end Eocene cooling event, when in Southeast Asia cool climate oscillations brought freezing temperatures to tropical mountains with corresponding seasonally dry lowland climates. Similarly, for equatorial Africa increased seasonality of climate from the Early Miocene onward accounts for the current depauperate nature of African rainforest flora.

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