The rise and fall of the Refugial Hypothesis of Amazonian speciation: a paleoecological perspective

Abstract

The refugial hypothesis is treated as the definitive history of Amazonian forests in many texts. Surprisingly, this important theoretical framework has not been based on paleoecological data. Consequently, a model of Amazonian aridity during the northern hemispheric glaciation has been accepted uncritically. Ironically, the Refuge Hypothesis has not been tested by paleobotanical data. We present a revision of the concept of Neotropical Pleistocene Forest Refuges and test it in the light of paleocological studies derived from pollen analysis of Amazonian lake sediments deposited during the last 20,000 years. Our analysis is based primarily on paleoenvironmental data obtained from sites in Brazil and Ecuador. These data are contrasted with those that favor the hypothesis of fragmented tropical forests in a landscape dominated mainly by tropical savannas under an arid climate. The Ecuadorian data set strongly suggests a 5 o C cooling and presence of humid forests at the foot of the Andes, during the last Ice Age. The same climatic and vegetational scenario was found in the western Brazilian Amazon. On the other hand, somewhat drier conditions were observed in the central Amazon, but the landscape remained a forested landscape during the supposedly arid phases of the Late Quaternary. Data obtained from the Amazon Fan sediments containing pollen derived from extensive sections of the Amazon Basin, were fundamental to the conclusion that the predominance of savannas in this region is not supported by botanical data. Our revision of the assumptions derived from the Refuge Hypothesis indicates that it has succumbed to the test now permitted by a larger paleocological data set, which were not available during the golden age of this paradigm, when indirect evidence was considered satisfactory to support it. Key words: Amazonia, refugia, fossil pollen, glaciations, Pleistocene, Holocene, Miocene, phylogeny, speciation diversity Resumo -Bush, M.B and Oliveira, P.E. Apogeu e declínio da Hipótese dos Refúgios para a especiação na Amazônia: uma perspectiva paleoecológica. Biota Neotrop. Jan/Abr 2006 vol. 6, no. 1, http://www.biotaneotropica.org.br/v6n1/pt/ abstract?point-of-view+bn00106012006. ISSN 1676-0611 A Hipótese dos Refúgios Florestais do Pleistoceno é aceita como a história definitiva da Amazônia por muitos autores. Surpreendentemente, este importante conceito não foi baseado em dados paleoecológicos. Como conseqüência, hoje temos um modelo teórico intimamente conectado à visão de uma Amazônia árida, durante o período de maior atividade glacial no hemisfério norte. Ironicamente, a Hipótese dos Refúgios não foi testada por dados paleobotânicos. Por isso, apresentamos uma revisão desse conceito e testamos a sua validade frente aos estudos paleoecológicos, derivados de análises palinológicas de sedimentos lacustres da Amazônia, depositados nos últimos 20.000 anos. Nossa análise baseia-se, principalmente, em dados paleoambientais obtidos em regiões da Amazônia do Brasil e do Ecuador, os quais são contrastados com informaçõ...

Phylogeny and historical biogeography of gnateaters (Passeriformes, Conopophagidae) in the South America forests

“…For many years the Pleistocene refuge hypothesis (Haffer, 1969;Vanzolini and Williams, 1970) was assumed to represent the principal mechanism to explain diversification in the Amazon basin and other lowland rainforest regions in South America (Carnaval and Moritz, 2008;Whitmore and Prance, 1987). However, not all paleoecological data supported the presence of the postulated non-forested barriers during the Pleistocene (Bush and de Oliveira, 2006;Colinvaux et al, 2000), and phylogenetic analyses indicated that most diversification events in this region predate the Pleistocene climatic oscillations (Hoorn et al, 2010).…”

Brazilian Atlantic Forest lato sensu: the most ancient Brazilian forest, and a biodiversity hotspot, is highly threatened by climate change

“…It is made up of a mixture of species that have evolved from native vegetation from the era in which South America was separating from Africa, 65 million years ago, with the original Ombrophylus dense forest cover. On the other hand, in the northeast and in the south, the Atlantic Forest expanded and retracted during the Quaternary (Brown Jr., 1987;Behling and Negrelle, 2001;Ledru et al, 2005;Bush and Oliveira, 2006). There is also abundant information about possible past connections between the Atlantic Forest and the more recent Forests of the Amazon Basin (Thomas et al, 1998;Costa, 2003) During the last 500 hundred years, the Atlantic Forest has been exploited and destroyed, being replaced first by sugarcane in the NE region (XVI century), and then later by coffee in Rio de Janeiro and São Paulo (XVIII and XIX century), by cattle ranching in São Paulo and Minas Gerais (XIX and XX century), by cocoa in Bahia (XX century), and more recently by Eucalyptus forest for cellulose and paper production.…”

Chloroplast DNA Microsatellites Reveal Contrasting Phylogeographic Structure in Mahogany (Swietenia macrophylla King, Meliaceae) from Amazonia and Central America

“…These seasonally dry forests are areas of "ecological tension" between the Amazonian and Cerrado biomes and are bounded by the evergreen rain forests to the north and by savannas to the south. Seasonally dry forests, which provide optimum habitat for mahogany, probably expanded during the cool and dry glacial intervals (Pennington et al 2000;Bush and Silman, 2004;Bush and Oliveira, 2006;Colinvaux et al 1996Colinvaux et al , 2001) and mahogany population sizes would have been correspondingly larger and possibly more continuous in some areas. During the wetter and warmer Holocene, rainforests expanded south and eastwards replacing the deciduous forests and savannas The high level of private haplotype diversity found in the Marajoara population, in contrast to other Amazonian populations, suggests additional intricacy to the pattern.…”