Vegetation changes and megafaunal extinction in South America: comments on de Vivo and Carmignotto (2004)

Abstract

The paper by de Vivo and Carmignotto (Journal of Biogeography, 31, 943–957) about the large mammal extinctions in South America is an interesting and timely contribution to the topic, but in our view suffers from some major misunderstandings that need to be addressed. One of the main assumptions of the paper is, in our view, very problematic: the authors assume that ‘under wetter and dryer climates, all regions would receive more or less rain by approximately the same amount, thus affecting vegetation physiognomy in one direction’ (p. 949). This assumption comprises the base argument for the palaeoenvironmental reconstructions envisioned by the authors expressed in their Figure 3 (p. 951). In this figure current boundaries between major plant formations are considered to be exactly the same for both the middle Holocene and the Last Glacial Maximum (LGM), the only change being an increase in ‘openess’ during the LGM, and denser vegetation in the Holocene Climatic Optimum (HCO). However, palaeoenvironmental data suggests that the spatial distribution of major plant formations was probably very different in each of these three time periods. For example, changes in the positioning of the Inter-Tropical Convergence Zone (ITCZ), in the strength of polar outbreaks, and in the presence of El Niño-Southern Oscillation (ENSO) are some of the factors that probably contributed to change significantly the boundaries between different biomes (e.g. Absy et al., 1991; Ledru et al., 1996, 2002; Baker et al., 2001; Grimm et al., 2001; Grosjean et al., 2001; Melo et al., 2001; Sifeddine et al., 2003). A second major problem is related to the supposed existence of some lineages of large mammals living ‘well into the middle Holocene’. The authors emphasize that both Cartelle (1999) and Ochsenius (1985), whose models sustain that all lineages of large mammals went extinct at the Pleistocene/Holocene boundary, ignored dates that place these animals living up to the middle Holocene. This supposition, however, based on two papers (Faure et al., 1999; Baffa et al., 2000), is far from conclusive, and in our view should not be taken into consideration for two reasons: the paper by Faure et al. (1999) gives a minimum age of c. 8490 yr bp for the megafauna, as the dates were obtained in a blackish silt organic layer above the red clayey layer that contained the large mammal bones. Moreover, between the dated organic layer and the bones there was a stalagmitic floor covering an erosional surface (Faure et al., 1999, p. 444), showing an obvious discontinuity between the bone layer and the organic layer. Thus, Faure et al.'s paper says that there were large mammals living earlier than c. 8490 yrbp, nothing else. So, we are left with the paper by Baffa et al. (2000), based on ESR dating of tooth enamel and dentine, a technique that still inspires caution (e.g. Tiemei et al., 2001), with large associated errors (for example, the same tooth provided dates of 6700 ± 1300 yr bp for dentine and 5000 ± 1600 yr bp for the enamel). We do not deny that some lineages could eventually survive into the middle Holocene, but the evidence is very weak to support that most of these mammals went extinct in the middle Holocene. On the contrary, recent datings of extinct mammals from Central Brazil tend to point to a late Pleistocene/early Holocene limit (Neves & Piló, 2003). From 47 bone samples of extinct mammals sent by us to be dated by AMS, only eight had enough collagen, and the two most recent are placed in the early Holocene (Catonyx cuvieri = 9990 ± 40 yr bp; Smilodon populator = 9260 ± 150 yr bp). A third problem is related to the concept of ‘climatic optimum’ itself. Although widely recognized in Europe (e.g. García et al., 2002; Mastronuzzi & Sansó, 2002; Kalis et al., 2003), the increase in moisture and temperature observed during the period called ‘Holocene Climatic Optimum’ is not a world-wide phenomenon. The authors acknowledge the fact that ‘for South America, the HCO is less consensual’ than in Africa (p. 950). Indeed, palaeoenvironmental studies in Brazil tend to show two opposite patterns, one of increase and another of decrease in moisture during the middle Holocene. In our view, these patterns are not necessarily mutually exclusive; they show differences in moisture that are probably associated to the already mentioned shifts in the ITCZ, to the strength of Antarctic polar masses, and even to orographic factors, such as the presence of mountains, plateaus and depressions. Perhaps the only major global trend in the middle Holocene is an increase in temperature. That is why the term ‘altithermal’ should be used instead of ‘climatic optimum’, because an increase in temperature is not necessarily associated with an increase in precipitation, as emphasized long ago by Ab'Sáber (1980). In central Brazil, for instance, the climate during the middle Holocene was probably dryer than during the early Holocene (see Araujo et al., 2003 for a review). In south-east Brazil, on the contrary, conditions were probably milder during the whole Holocene, allowing for the formation of forests. In the southern states, on the contrary, grasslands dominated the landscape during almost all the Holocene, being replaced by Araucaria forests only very recently, about 3000 yr bp. We should also not forget that Europe comprises a very small area in comparison with South America; the area of influence of a climatic trend occurring in Europe would correspond to a trend observable in a restricted portion of South America. Supported by FAPESP Grant 99/00670-7. Astolfo G. M. Araujo is a geologist with a PhD in archaeology, and a researcher at the Laboratory for Human Evolutionary Studies, Universidade de São Paulo. His interests are archaeology, Quaternary studies, and the relationship between humans and paleoenvironments. Walter A. Neves is a bioanthropologist, Head of the Laboratory for Human Evolutionary Studies, Instituto de Biocincias, Universidade de São Paulo. His main research interests are the morphometry and history of the First Americans. Luís B. Piló is a geomorphologist, and a researcher at the Laboratory for Human Evolutionary Studies, Instituto de Biocincias, Universidade de São Paulo. His main research interests are karstic environments and paleoenvironmental studies.