M B Bush W D Gosling and P A Colinvaux Introduction

Data from palynology, taxonomy and isotopic analyses, allied to climate models, reveal the complexity of the history of Amazon ecosystems. Evidence from these records suggests that Pleistocene climatic change was neither uniform nor synchronous across the basin, but that its effects were pervasive.

A major obstacle to Amazon paleoecology is paucity of lakes containing uninterrupted sedimentary sequences spanning one or more glacial cycles. To date, the only fossil pollen records from lowland Amazon lake sediments to span the last glacial maximum (LGM) are those of Carajas (Absy et al., 1991), Maicuru (Colinvaux et al., 2001), three from the Hill of Six Lakes (Colinvaux et al., 1996; Bush et al., 2004a), together with Lakes Chaplin and Bella Vista on the southwestern forest-savanna ecotone in Bolivia (Mayle et al., 2000; Burbridge et al., 2004) (Figure 3.1). All of these are relatively small, shallow bodies of water that are vulnerable to desiccation. That these lakes retained water for most of their > 50,000-year histories is testament to the relative constancy of Amazonian precipitational regimes. However, none of these records presents an ideal archive, as they all contain sedimentary gaps or extremely slow rates of sediment accretion, are not necessarily located in optimal locations for studying past climate change, and only represent a small portion of the Quaternary.

These lake records are supplemented by the pollen history of the Amazon lowlands from sediments of the Amazon fan (Haberle, 1997; Haberle and Maslin, 1999) and records near the upper limits of wet Amazon forest on the flanks of the Andes. These lower montane records include Lake Consuelo at 1,360-m elevation in Peru (Bush et al., 2004b), and Mera (1,100m) and San Juan Bosco (970m) in Ecuador (Liu and Colinvaux, 1985; Bush et al., 1990) (Figure 3.1).

A combination of new paleoecological records, molecular phylogenies and climatic understanding has contributed to a revised view of the evolutionary origins of Amazonian biodiversity. In the past decade the suggestion that arid glacials led to

Figure 3.1. The location of paleoecological sites mentioned in the text in relation to topography. Citations are given for locations not specifically identified in text: (1) Lake Gatun (Bartlett and Barghoorn, 1973); (2) El Valle; (3) Loma Lindo (Behling and Hooghiemstra, 1999); (4) Hill of Six Lakes (Pata, Verde, and Dragao; (5) Maicuru; (6) Geral, Santa Maria, and Saracuri (De Toledo, 2004); (7) Amazon fan; (8) Tapajos; (9) Carajas; (10) sites in northeastern Brazil (Stute et al., 1995); (11) Lapa dos Brejoes and Toca da Barriguda caves (Wang et al., 2004); (12) Botuvera cave; (13) Acre transect (Pessenda et al., 1998); (14) Lakes Chaplin and Bella Vista (Mayle et al., 2000); (15) Salar de Uyuni; (16) Titicaca (Paduano et al., 2003); (17) Lake Consuelo; (18) Gentry, Werth, Parker, Vargas (Bush et al., in press); (19) Junin; (20) Mera and San Juan Bosco; (21) High Plain of Bogota.

Figure 3.1. The location of paleoecological sites mentioned in the text in relation to topography. Citations are given for locations not specifically identified in text: (1) Lake Gatun (Bartlett and Barghoorn, 1973); (2) El Valle; (3) Loma Lindo (Behling and Hooghiemstra, 1999); (4) Hill of Six Lakes (Pata, Verde, and Dragao; (5) Maicuru; (6) Geral, Santa Maria, and Saracuri (De Toledo, 2004); (7) Amazon fan; (8) Tapajos; (9) Carajas; (10) sites in northeastern Brazil (Stute et al., 1995); (11) Lapa dos Brejoes and Toca da Barriguda caves (Wang et al., 2004); (12) Botuvera cave; (13) Acre transect (Pessenda et al., 1998); (14) Lakes Chaplin and Bella Vista (Mayle et al., 2000); (15) Salar de Uyuni; (16) Titicaca (Paduano et al., 2003); (17) Lake Consuelo; (18) Gentry, Werth, Parker, Vargas (Bush et al., in press); (19) Junin; (20) Mera and San Juan Bosco; (21) High Plain of Bogota.

the contraction of rainforests into relatively small and isolated areas prompting waves of allopatric vicariance—the refugial hypothesis (Haffer, 1969)—has lost traction. Testing the veracity of this hypothesis was the focus of Amazon paleoecological research for three decades and has been exhaustively covered in recent reviews (see Colinvaux et al., 2001; see Haffer and Prance, 2001). In its most recent manifestation (Haffer and Prance, 2001), the refugial hypothesis no longer describes forests replaced by savanna, but by "intermediate forest". These ill-defined vegetation types could include any combination of elements from woody savanna to mesic forest. Haffer and

Prance (2001) also suggest that mesic forest may have persisted in the landscape along riparian corridors. According to this hypothesis not all species would have found this new landscape a barrier to dispersal, whereas others would. Furthermore, the timeframe for these changes has shifted from the last glacial period to some time in the Tertiary. The clearly defined set of predictions that provided the appeal of the original refugial hypothesis has been replaced by a nebulous wraith that defies testing. Despite these flaws, some researchers are not yet prepared to discard the refugial hypothesis. Rather than re-hash that argument in detail, we will make the following observations:

1. Molecular and genetic data for plants, birds and mammals, suggest that specia-tion was not concentrated within the ice ages, but has probably been a continuous process with most modern species appearing since the beginning of the Miocene (e.g., Zink and Slowinski, 1995; Moritz et al., 2000; Pennington et al., 2004).

2. None of the late Quaternary paleoecological records from the center of the Amazon basin provide evidence of widespread, long-term aridity sufficient to cause the fragmentation of rainforests, although there is evidence of expansion of forests in the last few thousand years at the margins (Bush, 2005).

3. Simple contrasts of wet versus dry, cold versus warm, cannot capture the variability of the glacial-interglacial cycles and the communities without modern analogs that were generated (Bush and Silman, 2004).

4. Dry forest species in modern "refugia" have not speciated and so presumably 19,000 years since deglaciation is not long enough for speciation to occur (Pennington et al., 2004).

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