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Climatic changes during the last glacial maximum in northern South America and the Caribbean: A review
... A pesar de esto, existen pocas investigaciones relacionadas con los cambios men-0378-1844/13/10/696-09 $ 3.00/0 cionados y con los cambios paleoclimáticos y paleoambientales ocurridos durante el Cuaternario Tardío en la cuenca. Según Schubert (1988), las principales evidencias de un clima diferente durante el UMG en la región del Caribe, Suramérica y América Central se relacionan con registros sedimentológicos, geomorfológicos, pedológicos y paleontológicos. En Venezuela, los mejores registros relacionados con estos cambios climáticos se asocian con la presencia de morfología glacial en los Andes centrales, variación en las condiciones hidrológicas en las tierras bajas tropicales de los ríos y la-gos, terrazas aluviales, estudios de núcleos de sedimentos lacustrinos, cronología absoluta por 14 C, luminiscencia ópticamente estimulada y nuclidios cosmogénicos terrestres usando 10 Be, entre otros (Schubert, 1988;Stansell et al., 2005;Wesnousky, 2012). ...
... Según Schubert (1988), las principales evidencias de un clima diferente durante el UMG en la región del Caribe, Suramérica y América Central se relacionan con registros sedimentológicos, geomorfológicos, pedológicos y paleontológicos. En Venezuela, los mejores registros relacionados con estos cambios climáticos se asocian con la presencia de morfología glacial en los Andes centrales, variación en las condiciones hidrológicas en las tierras bajas tropicales de los ríos y la-gos, terrazas aluviales, estudios de núcleos de sedimentos lacustrinos, cronología absoluta por 14 C, luminiscencia ópticamente estimulada y nuclidios cosmogénicos terrestres usando 10 Be, entre otros (Schubert, 1988;Stansell et al., 2005;Wesnousky, 2012). ...
During late Pleistocene -Holocene climatic changes, the Portuguesa river watershed, Central West Plains, Venezuela, experienced environmental and hydrologic changes. The present study research is based in a morpho-sedimentary analysis and is sustained by 21 optically stimulated luminescence (OSL) datings of fluvial and eolic deposits. Recent sediments are dominated by medium and coarse sands, while the oldest deposits are dominated by medium to coarse silt. The dominant mineralogy for both deposits is quartz and kaolinite. The studied lithofacies reveal channel deposits or meander bars (AF1) and sand bars (AF2) associated with small deltas. The proximal morphological deposits indicate the development of mega-fans and fluvial terraces and the distal sediments developed channel and inundation plain deposits. The ages of the fluvial deposits are between 19000 ±1570 and 15200 ±2300ka, suggesting that the construction of the megafans started at least during the last glacial maximum (LGM), followed by cutting, paleomeander and terrace formation during the early and late Holocene (10990 ±2850 to 3850 ±840ka). The ages of the eolic deposits (14720 ±1800 y 4510 ±860ka) indicate that climate conditions during the Late Glacial and the Middle Holocene still were semiarid to sub humid , or close to present climatic conditions.
... A pesar de esto, existen pocas investigaciones relacionadas con los cambios men-0378-1844/13/10/696-09 $ 3.00/0 cionados y con los cambios paleoclimáticos y paleoambientales ocurridos durante el Cuaternario Tardío en la cuenca. Según Schubert (1988), las principales evidencias de un clima diferente durante el UMG en la región del Caribe, Suramérica y América Central se relacionan con registros sedimentológicos, geomorfológicos, pedológicos y paleontológicos. En Venezuela, los mejores registros relacionados con estos cambios climáticos se asocian con la presencia de morfología glacial en los Andes centrales, variación en las condiciones hidrológicas en las tierras bajas tropicales de los ríos y la-gos, terrazas aluviales, estudios de núcleos de sedimentos lacustrinos, cronología absoluta por 14 C, luminiscencia ópticamente estimulada y nuclidios cosmogénicos terrestres usando 10 Be, entre otros (Schubert, 1988;Stansell et al., 2005;Wesnousky, 2012). ...
... Según Schubert (1988), las principales evidencias de un clima diferente durante el UMG en la región del Caribe, Suramérica y América Central se relacionan con registros sedimentológicos, geomorfológicos, pedológicos y paleontológicos. En Venezuela, los mejores registros relacionados con estos cambios climáticos se asocian con la presencia de morfología glacial en los Andes centrales, variación en las condiciones hidrológicas en las tierras bajas tropicales de los ríos y la-gos, terrazas aluviales, estudios de núcleos de sedimentos lacustrinos, cronología absoluta por 14 C, luminiscencia ópticamente estimulada y nuclidios cosmogénicos terrestres usando 10 Be, entre otros (Schubert, 1988;Stansell et al., 2005;Wesnousky, 2012). ...
Pelas mudanças climátericas do Pleistoceno Tardio - Holoceno, o rio Portuguesa, planícies centro-occidentais, Venezuela, experimentou alterações ambientais e hidrológicas. O presente estudo foi baseado na análise morfo- sedimentar e foi sustentado por 21 datações através da luminescência opticamente estimulada (LOE) dos depósitos fluviais e eólicos da bacia. Os sedimentos recentes são dominados por areias grossas e médias; os depósitos antigos por limos grossos a meios, ambos constituidos essencialmente de quartzo e caulinite. A análise de litofácie do lote estudado permite definir depósitos de canal ou barras de meandros (AF1) e barras de areia (AF2) associados com pequenos deltas. A morfologia dos depósitos proximais indica o desenvolvimento de mega- leques e terraços fluviais, e os sedimentos distais com o desenvolvimento de depósitos de canal e da planície de inundação. A cronologia dos depósitos fluviais 19000 ±1570 e 15200 ±2320ka, sugere que a construção de mega- leques começou pelo menos durante o último máximo glacial (LGM); entre 10990 ±2850 e 3850 ±840ka, há incisão e formação de paleomeandros e terraços fluviais , durante Holoceno Meio ao Tardio. A cronologia dos depósitos eólicos (14720 ±1.800 e 4510 ±860ka) indicam que as condições climáticas ou paleoambientais durante o Glacial Tardio e o Holoceno, elas ainda eram semi- árido a sub-húmidas, ou semelhantes as do presente.
... There is substantial evidence that climate change occurring since the Last Glacial Maximum (LGM; 21 ka) altered the type and range of habitats in the Caribbean (Schubert, 1988;Orvis et al., 1997;Renken et al., 2002) and led to species extinctions or range shifts (Pregill & Olson, 1981). Within islands of the Greater Antilles, however, large genetic distances have been found between populations in adjacent habitats (e.g. ...
... A high-resolution pollen record during the Middle Pleniglacial (28-68 ka) from north-eastern Venezuela suggests that, while enhanced orographic precipitation during Greenland Stadials promoted the expansion of tropical montane forests, dry forests and savanna again predominated in the lowlands (González et al., 2008;Hessler et al., 2010). Data from fossils, pollen, eolianite deposits and geomorphic features indicate that temperatures in the Caribbean during the LGM were about 4.5°C lower and climate was substantially more arid than in the present day (Schubert, 1988;Orvis et al., 1997;Guilderson et al., 2001), there was greater highland cooling than at sea level (Roy & Lachniet, 2010), and lowland habitats in Puerto Rico were dominated by dry forest that may have extended further upslope (Renken et al., 2002). This climatic evidence, in combination with coalescent simulation results of both mtDNA and nDNA, strongly suggests that the Caguas Basin during the LGM was unsuitable for E. portoricensis population persistence. ...
Aim Hypotheses proposed for lineage diversification of tropical montane species have rarely been tested within oceanic islands. Our goal was to understand how basin barriers and Pleistocene climatic fluctuations shaped the distribution of diversity in Eleutherodactylus portoricensis (Eleutherodactylidae), a frog endemic to the montane rain forests of Puerto Rico.Location The north-eastern (Luquillo) and south-eastern (Cayey) mountains of Puerto Rico.Methods We generated mitochondrial DNA (mtDNA) control region sequences (c. 565 bp) from 144 individuals of E. portoricensis representing 16 localities, and sequenced 646 bp of cytochrome b and 596 bp of nuclear DNA (nDNA) rhodopsin exon and intron 1 from a subset of individuals. We conducted a phylogenetic analysis on the mtDNA sequence data and explored population substructure with maximum parsimony networks, a spatial analysis of molecular variance, and pairwise FST analysis. Coalescent simulations were performed to test alternative models of population divergence in response to late Pleistocene interglacial periods. Historical demography was assessed through coalescent analyses and Bayesian skyline plots.Results We found: (1) two highly divergent groups associated with the disjunct Luquillo and Cayey Mountains, respectively; (2) a shallow mtDNA genetic discontinuity across the La Plata Basin within the Cayey Mountains; (3) phylogeographic congruence between nDNA and mtDNA markers; (4) divergence dates for both mtDNA and nDNA pre-dating the Holocene interglacial (c. 10 ka), and nDNA suggesting divergence in the penultimate interglacial (c. 245 ka); and (5) historical demographic stability in both lineages.Main conclusions The low-elevation Caguas Basin is a long-term barrier to gene flow between the two montane frog populations. Measures of genetic diversity for mtDNA were similar in both lineages, but lower nDNA diversity in the Luquillo Mountains lineage suggests infrequent dispersal between the two mountain ranges and colonization by a low-diversity founder population. Population divergence began prior to the Holocene interglacial. Stable population sizes over time indicate a lack of demonstrable demographic response to climatic changes during the last glacial period. This study highlights the importance of topographic complexity in promoting within-island vicariant speciation in the Greater Antilles, and indicates long-term persistence and lineage diversification despite late Pleistocene climatic oscillations.
... Paleoclimatic data from the last glacial maximum (LGM) of ca. 18,000 years ago show that arid conditions prevailed in some parts of the Arnazon basin, but that the currently arid lowlands of N Colombia and N Venezuela had a more mesic climate (Schubert 1988, Kronberg et al. 1991). The cooler climates of the LGM lowered vegetation zones by 1200-1500 m and temperatures by 6-7 °C (van der Hammen 1974). ...
... Perhaps the most likely candidate for inclusion in the Variable Seedeater (s.l.) clade is the similarly heavy-billed White-collared Seedeater, s. torqueola. Given that the currently arid Caribbeari littoral of Colombia and Venezuela apparently hada more mesic climate during at least the last glacial maximum (Schubert 1988), it is possible that americana once occurred more widely here, and might even have reached Central America prior to the arrival of the "auri. ta" group. ...
The degree of relationship between the Variable (Sporophila aurita) and Wing-barred (S. americana) Seedeaters has been debated for over 80 years. The discovery of a zone of hybridization between the former and the Gray Seedeater (S. intermedia), not previously considered to be closely related because of its gray rather than black-and-white adult 0" plumage, prompted me to reexamine this question. M y working hypothesis of a close relationship between all of these forms was supported by their very similar morphology and almost perfectly complementar y distributions, the recognition of another zone of limited overlap and hybridization, and by heretofore unappreciated variation within S. intermedia itself. I conclude that S. intermedia is in effect a member of an enlarged Variable Seedeater complex, a monophyletic unit best recognized at the superspecies level. Patterns of morphological differentiation and distribution lead me to propose the recognition of four allospecies: S. corvina ( = "aurita"), Variable Seedeater; S. intermedia, Gray Seedeater; S. murallae, Caquetá Seedeater; and S. americana, Wing-barred Seedeater. Quantitative analysis of the variation within S. i. intermedia indicates that the subspecies agustini is not recognizable; likewise, synonynúzation of S. c. chocoana, suggested in a previous study, is supported. A tentative hypothesis for the historical zoogeography of the group is proposed. Accepted 5 June 1996.
... The massive amount of dry plant material from these destroyed phytocommunities are now part of the accumulated peat (Hapsari et al. 2017). • Humid climate like the one now experienced (Schubert 1988;Villagrán 1993;Latrubesse 2003). The bulk of the vegetation that formed the Orinoco peat is not very different from what we see today Montoya et al. 2009;Leal andBilbao 2011 andBallesteros et al. 2014) Lower Early Holocene 11,700 • Worldwide, most of today's tropical continental peatlands began forming, relatively close to the coastal ranges (around 10,000 years ago) (UO 2020). ...
Reconnaissance on edaphological and biological aspects was carried out in a peatland forest near the town of Piacoa in Delta Amacuro State, southeastern Venezuela. Thirteen peat samples were taken in a stratigraphic transect, using Ramsar peat extraction and analysis protocol to study tropical peatlands. Physicochemical parameters were characterized with basic laboratory analysis: color according to the Munsell scale, relative humidity, solubility in water, specific density, pH, conductivity, percentage of organic carbon and percentage of organic matter. Rapid ecological evaluation of vegetation and ichthyofauna associated with aquatic bodies was carried out. Part of a large extension of Orinoco peatlands in delta region, the study area is a marshy wetland which, according to the Ramsar classification, qualifies as forested peatlands (Xp category). Paleoclimatic, palynological and edaphological information in the literature indicates that the peatlands were formed during Holocene processes of marine transgression that occupied settling basins and lowlands where the vegetation was not very different from the current one. Samples collected at different depths (0 to 220cm) clearly show horizons with two color tones: one dark or black (associated with a good surface aeration), and a yellowish brown or brown, associated with an absence or little amount of oxygen as it goes deeper. The pH values decrease with depth (4.25 at the top and 3.9 or 3.5 at depth). The peat has a high water retention that ranges between 60% and 83%. Vegetation in the wetland is diverse: 31 species of plants within 14 families that include tree, shrub and hydrophilic grassland vegetation were identified. In associated water bodies, 41 species of fish grouped within 27 families were found
... The massive amount of dry plant material from these destroyed phytocommunities are now part of the accumulated peat (Hapsari et al. 2017). • Humid climate like the one now experienced (Schubert 1988;Villagrán 1993;Latrubesse 2003). The bulk of the vegetation that formed the Orinoco peat is not very different from what we see today Montoya et al. 2009;Leal andBilbao 2011 andBallesteros et al. 2014) Lower Early Holocene 11,700 • Worldwide, most of today's tropical continental peatlands began forming, relatively close to the coastal ranges (around 10,000 years ago) (UO 2020). ...
ABSTRACT Reconnaissance on edaphological and biological aspects was carried out in a peat near the town of Piacoa in Delta Amacuro state, southeastern Venezuela. 13 peat samples were taken, in a stratigraphic transept using Ramsar peat extraction and analysis protocol to study tropical peat lands. Physicochemical parameters were characterized with basic laboratory analysis: color according to the Munsell scale, relative humidity, solubility in water, specific density, pH, conductivity, percentage of organic carbon and percentage of organic matter. Rapid ecological evaluation of vegetation and ichthyofauna associated with aquatic bodies was carried out. Peat where we work is part of a large extension of Orinoco peat lands in delta region. The place is a marshy wetland which, according to the Ramsar classification, qualifies within the Xp category (wooded peat). Paleoclimatic, palynological and edaphological information in the literature indicates that they were formed during Holocene processes of marine transgression that occupied settling basins and lowlands where there was vegetation not very different from the current one. In samples collected at different depths (0 to 220cm), can be seen clearly horizons with two color tones: one dark or black (associated with a good surface aeration), and a yellowish brown or brown, associated with an absence or little amount of oxygen as it goes deeper. pH values decrease with depth (4.25 at the top and 3.9 or 3.5 at depth). The peat has a high water retention that ranges between 60% and 83%. Vegetation in the wetland is diverse, having identified 31 species of plants within 14 families that include tree, shrub and hydrophilic grassland vegetation. In associated water bodies, 41 species of fish grouped within 27 families were identified.
RESUMEN Se realizó un reconocimiento sobre aspectos edafológicos y biológicos en una turbera o turbal cercano a la población de Piacoa en el estado Delta Amacuro, sureste de Venezuela. Se tomaron 13 muestras de turba, en transeptos estratigráficos utilizando el protocolo de análisis y extracción de turba de Ramsar para estudiar turberas tropicales. Fueron caracterizados parámetros fisicoquímicos con análisis básicos de laboratorio: color según la escala de Munsell, humedad relativa, solubilidad en agua, densidad específica, pH, conductividad, porcentaje de carbono orgánico y porcentaje de materia orgánica. Se realizó una evaluación ecológica rápida de la vegetación y la ictiofauna asociada a los cuerpos acuáticos. La turbera donde se trabajó forma parte de una gran extensión de turberas del Orinoco en la región del delta. El lugar es un humedal pantanoso que, según la clasificación Ramsar, califica dentro de la categoría Xp (turba arbolada). La información paleoclimática, palinológica y edafológica en la literatura indica que éstas se formaron durante procesos de transgresión marina del Holoceno antiguo que ocuparon cubetas y tierras bajas donde existía una vegetación no muy diferente a la actual. En muestras recolectadas a diferentes profundidades, se pueden ver claramente horizontes con dos tonos de color: uno oscuro o negro (asociado a una buena aireación superficial), y otro marrón, pardo o amarillento, asociado a ausencia o poca cantidad de oxígeno a medida que se profundiza. Los valores de pH disminuyen con la profundidad (4,25 en la parte superior y 3,9 o 3,5 en profundidad). La turba tiene una alta retención de agua que oscila entre el 60% y el 83%. La vegetación en el humedal es diversa, habiéndose identificado 31 especies de plantas dentro de 14 familias que incluyen vegetación de árboles, arbustos y pastizales hidrofílicos. En cuerpos de agua asociados se identificaron 41 especies de peces agrupadas en 27 familias
... The Guayana Shield is one of the oldest land surfaces on earth. According to radiometric measurements, the age of its bedrock lies between 0,9 and 3,5 billion years (Schubert 1988). On top of the igneous-metamorphic basement (granites and granitoid gneisses) of the Guayana Shield, several sedimentary covers (quartzites and sandstones) were deposited, which reach a thickness of more than 2000 m. ...
... The Guayana Shield is one of the oldest land surfaces on earth. According to radiometric measurements, the age of its bedrock lies between 0,9 and 3,5 billion years (Schubert 1988). On top of the igneous-metamorphic basement (granites and granitoid gneisses) of the Guayana Shield, several sedimentary covers (quartzites and sandstones) were deposited, which reach a thickness of more than 2000 m. ...
Otto Huber destaca por sus numerosas publicaciones de estudios sobre flora, vegetación, fitogeografía, ecología y conservación realizados principalmente en Venezuela. Es probable que él sea uno de los científicos naturales más célebres del escudo de Guayana y el que ha generado el mayor número de publicaciones científicas sobre la Guayana venezolana. Este libro es el resultado de la colaboración de un grupo de amigos y colegas, los cuales deseamos celebrar no solo sus contribuciones científicas sino también su gran calidad humana, dedicación y, en muchos casos, su incondicional amistad.
... Present-day distributions of plants and animals are influenced by both contemporary and historical ecological conditions. Tropical South American ecosystems were greatly affected by global temperature fluctuations during the late Tertiary and Quaternary periods, as revealed by analyses of fossil marine microfauna, pollen from lake sediments, glacial geomorphology, sedimentology and other evidence (Ab'Sáber, 1982;Van der Hammen, 1985;Bigarella & Ferreira, 1985;Schubert, 1988;Bush et al., 1990;Absy et al., 1991;Clapperton, 1993b (Clapperton, 1993a). ...
South American climbing mice and rats, Rhipidomys, occur in forests, plantations and rural dwellings throughout tropical South America. The genus belongs to the thomasomyine group, an informal assemblage of plesiomorphous Sigmodontinae. Over 1700 museum specimens were examined, with the aim of providing a coherent taxonomic framework for future work. A shortage of discrete and consistent characters prevented the use of strict cladistic methodology; instead, morphological assessments were supported by multivariate (especially principal components) analyses. The morphometric data were first assessed for measurement error, ontogenetic variation and sexual dimorphism; measurements with most variation from these sources were excluded from subsequent analyses. The genus is characterized by a combination of reddish-brown colour, long tufted tail, broad feet with long toes, long vibrissae and large eyes; the skull has a small zygomatic notch, squared or ridged supraorbital edges, large oval braincase and short palate. Three main divisions of the genus are recognized. The R. fulviventer section contains four species - fulviventer, wetzeli, caucensis and ochrogaster - inhabiting montane forests in the northern Andes and Guiana Highlands of Colombia and Venezuela; they share grey-based ventral pelage, dark extremities, a rounded interorbital region, broad braincase, and (except for caucensis) a primitive carotid circulation pattern; R. fulviventer consists of a chain of isolated subspecies, including venustus. The monospecific R. macconnelli section is restricted to the Guiana Highlands of Venezuela; it differs in certain external characters - darker colour (especially ventrally), longer metatarsals, bicoloured tail - but is similar cranially. The R. leucodactylus section is usually paler with broad feet, stronger cranial ridges and a derived carotid circulation pattern, and occurs mainly at lower levels. It contains 13 species: couesi, leucodactylus, modicus, austrinus, latimanus (including venezuelae), nitela, emiliae, macrurus, mastacalis and four as yet unnamed. Three species occur in NE Brazil; in SE Brazil the material currently available does not permit a clear-cut arrangement.
... Parte de este complejo correspondía a tierras bajas durante el Oligoceno Tardío (29-27 Ma.), cuando aún la Península de Paraguaná, la Península de Araya y la isla de Margarita estaban bajo aguas marinas (Iturralde-Vinent, 2006 paleogeográfica no contribuye a explicar la menor riqueza de especies de hormigas en las localidades semiáridas examinadas. Por su parte, el paleoclima seco del norte de Venezuela ha estado asociado a expansiones y contracciones cíclicas de las condiciones áridas (Raven y Axelrod, 1975;Gentry, 1982;Ochsenius, 1983;Schubert, 1988;Rull 1996) y durante estos períodos de contracción, las porciones áridas de la región costera tuvieron que haber perdido comparativamente más área que la región correspondiente al Complejo Falcón-Lara. El último de estos eventos de expansión alcanzó su máximo entre 18,000 y 13,000 años antes del presente. ...
Ants are considered one of the most important zoological groups in arid and semi-arid zones of the world, where they play a key role in the characterization of the plant communities represented there; however, little is known about the abundance and diversity of the myrmecofauna associated with arid and semiarid environments of Venezuela. Despite representing a small extent of the Venezuelan territory (<5%), this type of environment can house a considerable amount of ants, with species potentially unique to the region. In this study, which examines three continental localities of the Falcón-Lara semiarid complex, in northwestern Venezuela, we used pitfall traps to sample the ant fauna in two contrasting seasons: rainy and dry. The results obtained include five subfamilies, 18 genera and 33 species, which represent the first records for the region, specifically, for localities Aregue (Lara State), Parque Nacional Cerro Saroche (Lara State) and Agua Larga (Falcón State). Taken together, the results observed for the Venezuelan localities account for a yrmecofauna with relatively high species richness compared to temperate and sub-temperate arid localities in
the American continent, but low species richness compared to semiarid localities of northern South America.
... There is tentative evidence, from the age dates at the Acre River site, that the desiccation of lake waters occurred over several mi Ilenia. Here the age (49,110 ± 900 a BP) of one of the aragonite concretions found "4 m below the uppermost clay sediments could represent the time at which the lake waters concentrated Table3(b) Minor Element Concentrations (/tgg" 1 ) and CIA Values (Acre River Sites) evidence summarized by Schubert (1988) for drier, cooler LGC climates in northern South America and the Caribbean. ...
Geochemical and geochronological analyses of samples of surficial Acre Basin sediments and fossils indicate an extensive fluvial-lacustrine system, occupying this region, desiccated slowly during the last glacial cycle (LGC). This research documents direct evidence for aridity in western Amazonia during the LGC and is important in establishing boundary conditions for LGC climate models as well as in correlating marine and continental (LGC) climate conditions.
... Una primera aproximación a la cuestión del origen de la Gran Sabana desde la perspectiva paleoecológica puede leerse en Schubert y Huber (1989). Schubert que había trabajado primeramente estudiando las glaciaciones pleistocenas en los Andes de Venezuela estuvo profundamente convencido de que los cambios climáticos cuaternarios habían tenido efectos dramáticos sobre el paisaje tropical (Schubert, 1988). La presencia de deposiciones aluviales de conglomerados de diferentes tamaños y rocas al pie de los tepuyes y en los ríos de la Gran Sabana (Schubert, 1986), fue tomado por este autor como evidencias de un clima árido en el pasado, donde lluvias torrenciales de alta energía pudieron haber originado estos patrones de transporte bajo una cobertura vegetal más rala que la actual Briceño y Schubert, 1990). ...
... Moving away from the equator or toward dryer climates (such as the Altiplano of southern Bolivia at latitude 17-18 S), the frost and forest limits shift to lower elevations although the limit of permanent snow may be as high as 6000 m asl (Troll, 1968). It should be noted that during the last Ice Age, some 20 000 years ago, air temperatures in high tropical mountains were about 6 C lower than today, with permanent snow cover probably extending down to about 3500 m asl (Colinvaux, 1987;Schubert, 1988). Consequently, tropical (as well as temperate) high-altitude environments are of more recent origin than the surrounding lowlands. ...
This chapter describes the features of tropical high. -altitude streams, including physico. -chemical conditions, primary producers and macrophytes, fishes, and macro. -invertebrates. A particular aim was to describe similarities and differences in the environment, community composition, and functioning of these streams compared to streams in tropical lowlands as well as those at higher latitudes. One prominent difference between the conditions in high. -altitude or alpine streams at different latitudes is the less pronounced seasonality in the tropics. Tropical streams originating at high altitudes often have better water quality than streams further down. Precipitation at very high altitudes is less than that at lower elevations. Low water temperature is the most characteristic feature of high. -altitude streams. Oxygen concentration is almost constant with increasing altitude. Most alpine streams at temperate latitudes are relatively nutrient poor compared to streams at lower altitudes. The fish fauna of tropical high. -altitude streams is very poor in species. The macroinvertebrates fauna in tropical high. -altitude streams is dominated by insects. High. -altitude streams in the tropics are affected by a number of human activities, which include contamination by pesticides, sediments from fields, livestock grazing, wastewaters from human settlements and cities, acidification and contamination with heavy metals from mining, sedimentation arising from erosion of gravel pits and construction works, reductions in stream discharge due to water abstraction, and fragmentation of longitudinal connectivity caused by dams.
... There is some evidence that the Caribbean is currently warmer and wetter than it was during the Pleistocene. In the Pleistocene, a permanent snowline might have existed between 2,300 and 2,600 meters above sea level (masl) in Hispaniola, and parts of the Caribbean were more arid than they are today (Schubert and Medina 1982 ;Schubert 1988 ). Data from corals in Barbados indicate that temperatures in shallow Caribbean waters were 4°C to 6°C lower during Pleistocene glacial advances to the north (Guilderson et al. 1994 ). ...
Global change threatens ecosystems worldwide, and tropical systems with their high diversity and rapid development are of special concern. We can mitigate the impacts of change if we understand how tropical ecosystems respond to disturbance. For tropical forests and streams in Puerto Rico this book describes the impacts of, and recovery from, hurricanes, landslides, floods, droughts, and human disturbances in the Luquillo Mountains of Puerto Rico. These ecosystems recover quickly after natural disturbances, having been shaped over thousands of years by such events. Human disturbance, however, has longer-lasting impacts. Chapters reflect many years of experience in Puerto Rico and other tropical areas and cover the history of research in these mountains, a framework for understanding disturbance and response, the environmental setting, the disturbance regime, response to disturbance, biotic mechanisms of response, management implications, and future directions. The text provides a strong perspective on tropical ecosystem dynamics over multiple scales of time and space.
... Moving away from the equator or toward dryer climates (such as the Altiplano of southern Bolivia at latitude 17-18 S), the frost and forest limits shift to lower elevations although the limit of permanent snow may be as high as 6000 m asl (Troll, 1968). It should be noted that during the last Ice Age, some 20 000 years ago, air temperatures in high tropical mountains were about 6 C lower than today, with permanent snow cover probably extending down to about 3500 m asl (Colinvaux, 1987;Schubert, 1988). Consequently, tropical (as well as temperate) high-altitude environments are of more recent origin than the surrounding lowlands. ...
... However, the fact that the upwelling season is also the regional dry season suggests that changes in surface salinity need to be considered as well. Numerous studies have suggested that a more arid climate prevailed over northern South America [e.g., van der Hammen, 1974;Schubert, 1988;Markgraf, 1989;Clapperton, 1993] and over tropical Africa [Street and Grove, 1979] during the last glacial. An overall increase in net evaporation (plus lowered sea level) has been linked to significantly higher glacial salinities in marginal seas like the Mediterranean and Red Seas [Thunell et al., 1987Thunell and Williams, 1989]. ...
Seasonal trade wind-induced upwelling along the southern margin of the
Caribbean Sea occurs in response to the annual migration of the
Intertropical Convergence Zone. Laminated, high deposition rate
sediments of the Cariaco Basin, a small anoxic basin on the Venezuelan
continental shelf, clearly record large changes in the past intensity of
this upwelling. Because sediments of the Cariaco Basin are largely
unbioturbated, they offer a natural opportunity to study the stable
isotopic records of multiple planktonic foraminiferal taxa and to
evaluate their sensitivity to both the modern hydrography and temporal
changes in upwelling intensity and climate. Oxygen isotope data
(δ18O) from four dominant foraminiferal taxa are
presented for the time period covering the last 28 kyr. The
δ18O data from Globigerina bulloides, after correction
for nonequilibrium precipitation, are used as a monitor of sea surface
conditions during the winter-spring upwelling season. The
δ18O data from white Globigerinoides ruber are used as
a measure of annual-average conditions in the near surface, while pink
G. ruber data are consistent with use as an index of endmember
conditions during the summer-fall nonupwelling season. Data from the
deeper dwelling Neogloboquadrina dutertrei yield information on
conditions near the base of the local thermocline. During the last
glacial, δ18O data from G. ruber and generally reduced
interspecific differences indicate cooling of surface waters over the
Cariaco Basin by up to 4°C. This longer-term cooling does not appear
to be related to changes in upwelling intensity along the coast but may
instead reflect more regional cooling of the larger Caribbean.
Superimposed on this pattern, between 12.6 and ˜10 ka, is a
convergence of δ18O data between G. bulloides and N.
dutertrei, implying much stronger upwelling during the last
deglaciation. This scenario is consistent with other evidence for high
productivity at this time. At ˜14 ka, a sharp
δ18O depletion event observed in all taxa seems to have
been produced by increased freshwater discharge to the southern
Caribbean, suggesting either higher regional rainfall or the influence
of glacial melting in the Andes. Minimum δ18O values of
pink G. ruber around 6-7 ka record warmer summer sea surface
temperatures and/or decreased salinity in the mid-Holocene.
... La mayor parte de Paraguaná y de la franja costera ubicada al sur del istmo de Médanos están actualmente cubiertos por vegetación árida y semiárida (Sarmiento 1976). Durante los periodos glaciares, debido al clima seco que los caracteriza, este tipo de vegetación aumentó en extensión (Schubert 1988;Hillesheim 2005). Ello debe haber dificultado, pese a la amplia conexión territorial de Paraguaná con el territorio vecino, la comunicación de los reductos boscosos que había en la entonces futura península con bosques ubicados más al sur. ...
We synthesize the biological information available on the six species of cave-dwelling bats of Paraguaná Peninsula, adding new data on morphological differentiation, feeding habits, socio-economic importance, conservation, and on the characteristics of the caves in which they roost. Of these species, four are important insect pest controllers, and two are important pollen and seed vectors of native plants. Five species are strictly cavernicolous, for which reason they share two natural risk factors. First, because they roost in only three caves, their populations are extremely vulnerable to a catastrophic event in any of them. Second, because their females produce a single pup per year, their populations have a low capacity to recover, and a high probability of extinction, in the event of a major loss. The most direct threat to these bats is vandalism in caves, remediable through:1) legislation, such as the recent decree creating the “Santuario de Fauna Silvestre Cuevas de Paraguaná”; 2) gating to restrict access to the caves, a measure with numerous contraindications; 3) official wardens, so far non-existent; 4) environmental education, so far insufficient. Deforestation and indiscriminate use of pesticides are growing threats, remediable through: 5) expansion of protected areas, at present covering only 1.4% of the peninsula; 6) biological pest control; 7) crops that do not require pesticides. Wind turbines and natural collapse of caves are potential threats, remediable through: 8) mitigation of the effects of already installed wind turbines, and careful selection of sites for new ones; 9) reinforcement and repair of natural caves; 10) construction of artificial caves. For the latter initiative, we propose a design that we define as semi-spheroid with compartments. Artificial caves would accomplish five functions. First, reduce the extinction risk of bats by distributing their populations in a larger number of roosts. Second, increase the populations of insectivorous bats to boost their effect as natural pest controllers. Third, use these caves for the production of easily harvested guano to be used as a fertilizer. Fourth, serve as a tool for the biological study of cavernicolous bats and of the ecosystem services provided by them. Fifth, utilize these caves for ecotourism in benefit of local communities.
... Palynologlcal evidences for a drier climate during the last full glacial have been documented in Ghana (Maley, 1987) and Congo (Preuss, 1990), but a continuous record of vegetation evolution is lacking for the last glacial period in South America lowlands (Markgraf, 1989). However, for the 0921-8181/93/$06 00 © 1993 -Elsevier Soence Publishers B V All rights reserved last 12,000 years quite a lot of results are available: the ram forest was largely degraded before 10,000 yr B P. in some parts of Amazoma (Absy and Van der Hammen, 1976; Van der Hammen, 1991) and Central America (Schubert, 1988, Markgraf, 1989), short-term modifications of the floristic composition (Absy, 1975(Absy, , 1979Liu and Cohnvaux, 1988) and regional regression of the forest (Servant et ai., 1981(Servant et ai., , 1989) occurred during the Holocene in some areas of the present humid tropical zone of South America. ...
... It is now known that the lowland landscapes and vegetation cover of northern South America were strongly affected by climate change. Palynological and geomorphological evidence from the Gran Sabana, the Llanos of the Orinoco Basin, and Lake Valencia, all in Venezuela, have shown the last glacial Ž maximum climate to have been much drier than today Schubert, 1988;Schubert and . Salgado Labouriau, 1987 . ...
New evidence is provided for the aeolian origin of the upper unit of the Boa Vista Formation, found in the eastern part of the Brazilian state of Roraima and in neighboring Guyana. This unit has two geomorphologically/sedimentologically distinct domains: a sandy and a silty realm. Located to the east and southeast (upwind) of the silty realm, the sandy realm includes very highly eroded, inactive parabolic and linear dunes and sand sheets. An extensive field of parabolic dune remnants was detected in an orbital radar image and in low-altitude air photos of the interfluve of the Cauamé and Uraricoeira rivers, near the town of Boa Vista. Along the downwind periphery of the sandy realm is a thin mantle of silty sediments forming a flat plain and penetrating into intermontane valleys. The eroded fossil dunes, oriented 50–60°E, and the downwind grain size selection suggest a late Pleistocene dry period with trade wind direction the same as that of the modern dry season. A more recent dry period, probably Holocene, is suggested by the presence of many well preserved circular depressions in the upper unit of the Boa Vista formation, interpreted as circular pans or blowout hollows.
... Evidencias obtidas de muitas regiões da América tropical indicam que alterações climático-vegetacionais extensivas ocorreram durante os últimos milhões de anos (revisões recentes incluem as de Ab 'Saber 1982, Tricart 1985, Bigarella & Ferreira 1985, Haffer 1987a, Schubert 1988, Hoppe & Schobbinghaus 1991. Dentre os dados examinados nessas revisões há os seguintes: os sedimentos no Lago Valência, no norte da Venezuela, registram pelo menos quatro ciclos de enchimento e dessecação do lago; campos de dunas nos lhanos do leste da Colômbia e sudoeste da Venezuela, ao norte da Amazônia, foram ativos durante o fim do Pleistoceno; flutuações vegetacionais nessas regiões têm sido documentadas através de estudos de paleopolén. ...
... Van der 0031-0182/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0031-0182(94) E0175-S Hammen, 1974E0175-S Hammen, , 1981E0175-S Hammen, , 1982E0175-S Hammen, , 1986aE0175-S Hammen, ,b, 1988E0175-S Hammen, , 1991Schubert, 1988;Markgraf, 1989), lake levels are generally high during the middle Pleniglacial interval (ca. 60,000 to ca. 26,000 yr B.P.), while glacier extension was maximal. ...
New data on the vegetational history and Quaternary geology of Amazonia permit an improved reconstruction of past environments in Amazonia during the last glacial period. Although limited, data from Rondonia, Carajas and Guyana show that, in certain areas, savanna-type vegetation and savanna forest had replaced the rain forest during the late Pleniglacial (ca. 22,000–13,000 yr B.P.). The Amazonian forest may have been split up into one major west Amazonian and several other medium-size forest areas. This suggests a decline in rainfall of 500 to 1000 mm (a reduction of 25 to 40%). Temperatures may also have been 2° to 6°C (4±2°C) lower than today, possibly substancially influencing Amazonian vegetation. During the humid middle Pleniglacial (55,000–26,000 yr B.P.), rivers carried a lot of water and sediment, resulting in the deposition of lower terrace sediments with one dry interruption around 40,000 yr B.P. (Carajas and Katira). In Carajas, there is evidence of dry periods ocurring at about 40,000 yr B.P., and during the early Pleniglacial (ca. 60,000 yr B.P.). Rivers carried little water and incised into the low terrace sediments during the dry late Pleniglacial. Water levels rose during the late glacial (13,000–10,000 yr B.P.) or at the beginning of the Holocene (10,000 yr B.P.). Sedimentation in (and of) the present inundation valleys commenced after that.
... The Pentomacrus-group is widespread in Cuba and Jamaica, today not longer in Hispaniola. Its presence in Hispaniola during the Lower Miocene confirms the observations given by SCHUBERT (1988) andPENNEY (1999;2000), according to which the Antilles were not drastically affected by the Pleistocene glaciations. Fig. 2. Plectromerus tertiarius nov. ...
A new fossil species, Plectromerus tertiarius, included in amber of Dominican
Republic (Lower Miocene) is described. The belonging to the genus Plectromerus Haldeman, 1847, today overall widespread in the West Antilles, and its relationships with current species are discussed.
Further observations and a key about this genus are added.
... It is not my aim here to provide ad hoc explanations to the observed pattern. However, I suspect that important contributing factors could be 1) area, i.e. the decreasing regional extension of higher altitudinal zones (Rahbek 1997, Sanders 2002, 2) age of habitats (Rohde 1999, Townsend et al. 2000 because the Ecuadorian Andes mountains are geologically relatively young (about 20 million years) and were recently (during the Pleistocene) covered by ice caps down to an altitude of about 3500 m (Colinvaux 1987, Schubert 1988 and 3) harsh environmental conditions, here defined as the low temperatures in the mountains (Rohde 1992(Rohde , 1999. ...
Altitudinal patterns in diversity of macroinvertebrate families at different spatial scales (stone, stream and altitude) were studied by collecting stone samples from six streams at each of the three altitudes: lowlands (400 m), midlands (2000 m) and highlands (3800m), in the equatorial Andes of Ecuador. Stream sites were characterised by a number of physico-chemical parameters and the fauna by several indices of richness, diversity and evenness. A MDS ordination on the composition of the fauna clearly separated the streams in three groups according to altitude. The invertebrate fauna was dominated by insects, mainly Ephemeroptera, Trichoptera and Diptera at all three altitudes. Odonata and Hemiptera were relatively rich in lowland streams, scarcely present in the midland streams and absent in the highland streams (as was the case for the less diverse orders Lepidoptera and Megaloptera). Four of the five measures of stream diversity correlated significantly with altitude and temperature. In addition, seven environmental parameters were correlated with one or more of the diversity measures. Of these parameters, stream width, riparian vegetation cover and coarse detritus cover were inter-correlated with altitude and temperature. With the effect of altitude removed, the number of families, the Jackknife richness estimate and the evenness were positively correlated with an index of physical stream stability. The effect of region (altitude) explained more of total variability in family richness than that among streams within regions. The mean number of families was 8.2, 4.9 and 4.1 per stone, 26.5, 19 and 13.3 per locality (stream), and 44, 37 and 27 per region at the three altitudes, respectively. Thus, both local and regional richness decreased approximately linearly with increasing altitude. In contrast, beta diversity (taxon turnover among streams) increased with altitude. The higher richness in the lowland streams appeared at the smallest spatial scale (stone), and was therefore not due to higher beta diversity among stones within streams. Local richness was nearly linearly related to regional richness, indicating non-saturated local communities, even in lowland streams.
Due to the cyclic regeneration of forest vegetation (Aubreville 1938 cited in Richards 1952; Oldeman 1989) shifting small-scale mosaics of different habitat patches exist in tropical lowland forests, e.g., tree-fall gaps, overgrown forest edges, dense pioneer vegetation, and increasingly mature forest vegetation closing the gaps. Successional vegetation zones along shifting river courses and patches of different vegetation types on extensive floodplains provide habitat mosaics at a somewhat larger scale. These habitat mosaics, together with the structurally and floristically complex tropical forest vegetation, maintain the ecological heterogeneity necessary for the coexistence of the numerous species of neotropical forest birds. Climatic and paleoclimatic changes caused cyclic disturbance processes at a regional scale. Paleogeographic changes in the distribution of land areas and oceans (or inland seas) altered many distribution patterns on a continental scale.
The concept of landscape sensitivity was introduced into thinking about geomorphological systems by Brunsden and Thornes (1979) and has been developed in the context of environmental change (Thomas and Allison 1993). When resistance to change is exceeded by the magnitude of the disturbing forces the environmental system will adjust to create a new equilibrium. However, that equilibrium will not be attained if the frequency of high magnitude events, sufficient to cause further disturbance, is greater than the ‘relaxation time’ of the system. Some attributes of environmental systems, and equally, some areas of natural landscapes, are more sensitive to change than others. Both the materials (rocks, saprolites, sediments, soils) of the landscape and its morphological components and their arrangement (aspects of slope and landscape pattern) contribute to its sensitivity and when these, mainly landform, characteristics prove stable those parts of the landscape can be said to have a high ‘factor of safety’, a concept borrowed from engineering (Brunsden 1980, 1993; Brunsden and Thornes 1979). Where the landscape has very low resistance to change (high sensitivity) it may said to be ‘fragile’.
Conflicting opinions regarding the relative importance of dispersal versus vicariance in understanding Caribbean biogeography continue to stimulate lively debate, despite recent advances in regional tectonics, geology, palaeogeography and palaeontology. The Greater Antilles are young geographical features, probably mid-Miocene and Hispaniola has a rich terrestrial invertebrate fossil record in the form of Dominican Republic amber inclusions. Spiders are common in Dominican amber and all fossil species belong in extant families. The island is unique in terms of its known spider fauna, in that a similar number of families have described species recorded on Hispaniola from both fossils in amber and from the extant fauna. It is also the region of the world where the amber fauna is most similar to the Recent fauna, and Miocene Hispaniolan spider biodiversity was presumably similar to that at present. Fossils form empirical data with both phylogenetic and temporal implications and are of paramount importance for understanding historical biogeography. They can play a decisive role in falsifying hypotheses proposed solely on the distributions of extant taxa. It is also possible to generate hypotheses for both fossil and extant faunas that are ultimately falsibiable through the discovery of new Hispaniolan taxa. The amber spider fauna has clear affiliations to the present South American fauna, so how did they colonize the island from the mainland: dispersal or vicariance? The high Miocene biodiversity (including taxa with poor dispersal capabilities) on a geologically young island would seem to add more weight in favour of the vicariance model for explaining on-island spider lineages, although dispersal cannot be excluded for some of the highly dispersive taxa. However, a case-by-case approach is required in order to determine the relative contributions of the different models. The future research direction for understanding the biogeographical patterns will lie in the application of cladistic and phylogenetic biogeographical approaches. It is interesting to note that preliminary obersvations based on pedipalp morphology suggest some fossil taxa are more derived than extant forms. The contribution that the Dominican (and Mexican) amber fossil fauna can make to our understanding of the origins of Hispaniolan biodiversity, and Caribbean biogeography in general, should not be underestimated.
The modern Orinoco Delta is the latest of a series of stacked deltas that have infilled the Eastern Venezuelan Basin (EVB) since the Oligocene. During the late Pleistocene sea-level lowstand (20,000 to 16,000 yrs BP), bedrock control points at the position of the present delta apex prevented the river channel from incising as deeply as many other major river systems. Shallow seismic data indicate that the late Pleistocene Orinoco incised into the present continental shelf, where it formed a braided-river complex that transported sediment to a series of shelf-edge deltas. As sea level rose from 16,000 to 9,500 yrs BP, the Orinoco shoreline shifted rapidly landward, causing shallow-marine waves and currents to form a widespread transgressive sand unit. Decelerating sea-level rise and a warmer, wetter climate during the early Holocene (9,500 to 6,000 yrs BP) induced delta development within the relatively quiet-water environment of the EVB embayment. Sea level approached its present stand in the middle Holocene (6,000 to 3,000 yrs BP), and the Orinoco coast prograded, broadening the delta plain and infilling the EVB embayment. Significant quantities of Amazon sediment began to be transported to the Orinoco coast by littoral currents. Continued progradation in the late Holocene caused the constriction at Boca de Serpientes to alter nearshore and shelf hydrodynamics and subdivide the submarine delta into two distinct areas: the Atlantic shelf and the Gulf of Paria. The increased influence of littoral currents along the coast promoted mudcape development. Because most of the water and sediment were transported across the delta plain through the Rio Grande distributary in the southern delta, much of the central and northwestern delta plain became sediment starved, promoting widespread accumulation of peat deposits. Human impacts on the delta are mostly associated with the Volcán Dam on Caño Manamo. However, human activities have had relatively little effect on the delta processes and environments.
The soils associated with most tropical forests are exposed to significant leaching. This leaching has weathered the soils’ mineral suite and reduced their capacity to bind nutrients, exacerbating the potential for nutrient loss from the forest or any derived ecosystem such as slash-burn agriculture. Phosphorus is often the most limiting nutrient in ecosystems on weathered tropical soils. Examples are given for P budgets of South American forests and for an estimate of P loss from a highly dystrophic rain forest. Despite a biological turnover of 6% of the forests P every year, only about 0.001% of the total P stocks are lost annually, indicating that biogeochemical P recycling is highly efficient. The mechanisms for such efficient cycling are internal retranslocation of P, and a litter and root mat with abundant mycotrophism which result in an effective separation of the biological P cycle from a potentially P-fixing soil. In the examples of less dystrophic forests from NE Brazil, P retention in the litter mat is less efficient and P uptake from the soil is more important. Therefore, the potential for non-forest land use is greater in NE Brazil, although P tranformations to less available forms limit the length of cultivation cycles. Few data exist on alternative land uses, such as tree crops or agroforestry, and the complex methodology of studying P has limited the number of experiments aiming at an understanding of the complete set of P transformations which determine an ecosystem’s resilience to disturbance and limit net P losses.
The fish faunas of continental South and Central America constitute one of the greatest concentrations of aquatic diversity on Earth, consisting of about 10 percent of all living vertebrate species. Historical Biogeography of Neotropical Freshwater Fishes explores the evolutionary origins of this unique ecosystem. The chapters address central themes in the study of tropical biodiversity: why is the Amazon basin home to so many distinct evolutionary lineages? What roles do ecological specialization, speciation, and extinction play in the formation of regional assemblages? How do dispersal barriers contribute to isolation and diversification? Focusing on whole faunas rather than individual taxonomic groups, this volume shows that the area's high regional diversity is not the result of recent diversification in lowland tropical rainforests. Rather, it is the product of species accumulating over tens of millions of years and across a continental arena.
The Guayana shield is one of the oldest land surfaces of the world, consisting of Precambrian rocks between 0.9 and 3.5 billion years old (Schubert et al. 1986). The shield is stratified with a granitic base and a huge overlaying cover of sandstone. The latter suffered intensive erosion, producing the masses of white quartzitic sand which are distributed all over the shield and its adjacent areas. Remnants of the sandstone layer are huge table mountains, the tepuis.
A synthesis of the two contrasting climates of the upper Quaternary in the Southern Hemisphere, based on continental geological indicators, suggests that the Antarctic Anticyclone is the most important climatic system in the hemisphere. According to glaciological and oceanographic studies, the Antarctic Anticyclone covered an area in the LGM that was double of that during the Hypsithermal. This phenomenon produced a consequent shifting of the climatic belts by approximately 10° of latitude to the north in the LGM and a migration to the south during the Hypsithermal. Oceanic and continental anticyclones were active systems in shaping climates in South America, Australasia, and Southern Africa. These structures were enhanced throughout the LGM (provoking generalized droughts), and weakened at the Hypsithermal (permitting more rains in tropical latitudes). The results presented here were obtained by integrating our own results with regional and continental syntheses of other authors. The paleoclimatic proposals are amply validated by more than 1500 dates in the four continents. Geological and geomorphological features, such as dunefields, paleosols, fluvial terraces and lake levels were the main tools employed in the work.
Granitic inselbergs represent xeric islands with a patchy distribution world-wide. Comparative studies on the floristic inventory of these ecosystems give an illuminating glimpse into mechanisms of diversity The Venezuelan study area comprises the northwestern margin of the Guayana Shield Here the granitic outcrops, locally known as `lajas', are grouped along a macro-climatic gradient In total, 614 species of vascular plants are recorded Characteristic families and endemic elements are assessed, with the result that eighty-six species are obviously restricted in their occurrence to laja sites of the Guayana region Evaluation of the distributional ranges of these species reveals that a northern and a southern phytogeographical subunit of the inselberg flora can be distinguished. In the area where the two subunits overlap, a centre of endemism (Atures centre of endemism) can be located. The effects of climate and isolation on floristic diversity are discussed, giving emphasis to the importance of intermediate isolation for florisitic diversification
The Chimborazo (6,310 m)–Carihuairazo (5,102 m) massif is one of the largest ice-capped central volcanic complexes in the northern Andes. Combined evidence from volcanic and glacial landforms and sediments suggests cyclical evolution during the Pleistocene. Effusive eruptions of mixed high-silica andesite (SiO 2 %wt c. 60) predominated and built the bulk of the edifice. Explosive activity developed as the parental magma evolved to dacite-rhyolite (SiO 2 %wt 64–74), culminating with cone collapse and large-scale debris avalanching. Post-collapse activity evolved from the production of high-silica andesite to terminate with monogenetic eruptions of basic andesite (SiO 2 %wt 54–56) from flank fissures. The last eruption occurred before 11,000 yBP.
The interstratification of volcanic and glacial deposits shows that glaciers expanded and contracted several times during the later Pleistocene, while the volcanic edifices were evolving. Glaciers expanded to altitudinal limits of 3,400–3,600 m during the early last glaciation and reached similar limits sometime after 33,000 yBP; an intervening interstadial interval lasted for 10,000 y. By 20,000–18,000 yBP, glaciers receded slightly because of decreased precipitation, but later readvances culiminated at 12,000–10,000 yBP and during the last 5,000 y. Glacier reconstruction and estimation of former equilibrium line altitudes suggest that the mean annual temperatures during the full glacial, late-glacial and Neoglaciation intervals were lower than now by c. 5–6°C, 2–3°C and 1°C, respectively, but these may be underestimates because of the assumption that precipitation was constant.
Ice core and ocean-drilling records now confirm the evidence from lakes, rivers and mires, concerning temperature depression in the humid tropics during the LGM by at least 5°C on land and this accords with the recent CCM1 model output. There is less agreement about fluctuation in precipitation amount, intensity and seasonality during the late Quaternary. Pollen records remain indeterminate because of the absence of sites in lowland rainforest locations and a widespread hiatus in sedimentation spanning the LGM. Alluvial sediments have often been ignored by specialists from other disciplines, but they also provide a valuable source of palaeoenvironmental information that reflects catchment-wide conditions. Current evidence shows that prolonged sedimentation took place in Stage 3 at sites in S America and SE Asia, and this was followed by incision. For up to 9 ka, spanning the LGM streams either produced little sediment or were characterised by braided channels and sometimes fan deposits, until the transition towards an interglacial mode took place after 12,700 BP (). In many places, this transition was marked by large floods, incision and deposition of coarse sediment. But as the Holocene climates stabilised and fluctuations of runoff reduced in amplitude, so meandering rivers developed floodplains with thick overbank silts and clays. These changes have regional and intercontinental patterns, not everywhere identical in bracketing dates, but demonstrating major changes to the fluvial systems as a result of profound changes to the hydrology of forested tropics during the last 60 ka.
AS FLUTUAÇÕES climático-vegetacionais causadas pelos ciclos astronômicos de Milan-kovitch provocaram mudanças globais na distribuição de florestas tropicais e demais vegetações não-florestais antes e durante o Cenozóico (Terciário-Quaternário). Os biomas continentais de florestas e vegetações não-florestais mudaram continuamente sua distribuição durante o seu passado geológico, fragmentando-se em blocos isolados, expandindo-se e juntando-se novamente sob condições climáticas alternadas entre secas e úmidas. Entretanto, durante as diversas fases climáticas, comunidades de plantas e animais fragmentaram-se e as espécies mudaram suas distribuições de maneira individual. Existem, para o Quaternário, dados de campo indicando mudanças na vegetação da Amazônia. A teoria dos Refúgios postula a persistência de grandes manchas de florestas tropicais úmidas durante os períodos secos do Terciário e do Quaternário, especialmente aquelas localizadas próximo de superfícies rebaixadas, sobretudo nas porções periféricas da Amazônia. Essas áreas são, provavelmente, a origem de muitas espécies e subespécies de plantas e animais existentes hoje em dia. Os "refúgios" úmidos podem ter sido separados por vários tipos de savana e florestas secas, como também por outros tipos de vegetação intermediária de climas sazonalmente secos. A quantidade e o tamanho dos refúgios durante os diferentes períodos de seca continuam desconhecidos. Indícios biogeográficos da existência de refúgios florestais anteriores incluem áreas de endemismo e zonas de contato entre espécies e subespécies de pássaros e outros animais da floresta amazônica nitidamente definidos. Essas áreas representam zonas de distinta descontinuidade biogeográfica num ambiente florestal contínuo. Modelos alternativos para a formação de barreiras na Amazônia que conduzem à especiação alopátrica incluem as seguintes hipóteses: do Rio, dos Refúgios do Rio, da Densidade do Dossel, da Perturbação da Vicariânia, do Museu e várias hipóteses paleogeográficas, das quais alguns aspectos poderiam ser aplicáveis a certos períodos na evolução da biota.
Some phytogeographical relationships of the Colombian Guayana are commented. The analysis considered four groups of flowering plants: Axonopus (Poaceae: Panicoideae: Paniceae), Raddiella (Poaceae: Bambusoideae: Olyrodae), Rapateaceae and Malpighiaceae. The areas considered were the Sierras Chiribiquete, La Lindosa, La Macarena, and the rocky sandstone outcrops of El Tuparro, Araracuara, and Las Lajas of Guainía. Most species (70 %) are distributed in the Guayana Region; of these, about 16 % are endemic to the Colombian Guayana; while 30 % occur in the Neotropics. Results based on a phenetic biogeographical study support the hypothesis that the Sierra de La Macarena is an area belonging to the Colombian Guayana phytogeographical complex insted of to the Andean group. I propose that the Sierra de La Macarena belongs to Araracuara District of the Western Guayana Province
Dated alluvial stratigraphies indicative of late Quaternary environmental change in the humid tropics have increased, but the database remains inadequate and the intensity and duration of wet-dry oscillations and responses of hillslopes and river systems remain poorly understood. Dry conditions at the Last Glacial Maximum were marked by semi-arid landforms with reduced stream activity. Large palaeofloods, valley-floor erosion, channel cutting and flood deposition occurred at the Pleistocene-Holocene transition after 13 1000 BP. Distinctive floodplains and stratigraphies characterized by multiple shifts from lateral to vertical accretion were built initially over a period of nearly 2ka after 9500 BP during the early Holocene pluvial in Africa, Kalimantan and Amazonia during and after re-establishment of the lowland rainforests. Several wet-dry climatic oscillations followed in the mid-Holocene period and are marked by alluvial cut and fill sequences and by slightly thinner but coarse textured floodplain overbank sediments.
This paper critically reviews reconstructions of cultural development in prehistoric Amazonia and argues for the primacy of regional and interregional exchange in generating the complex distributions of ethno-linguistic identities traced by linguists and archaeologists in the area. This approach requires an explicit abandonment of notions of migrating "peoples" in favor of modern anthropological understandings of ethnicity and ethnogenesis. Further, the paper discusses the significance of such a regional system perspective on Amazonian ethnogenesis for the ongoing debate on the extent of social stratification and agricultural intensification on the floodplains and wet savannas of lowland South America. It concludes that the emergence of Arawakan chiefdoms and ethnic identities in such environments after the first millennium BC signifies the occupation of a niche defined in terms of both ecology and regional exchange but also that it transformed both these kinds of conditions. In these processes, ethnicity, social stratification, economy, and ecology were all recursively intertwined.
A study of soil chemical properties of eleven soils present in different geographycal locations where palm trees (Copernicia tectorum) grow in Venezuela gave the following relevant results: 1) ten soils have at least one layer with exchangeable Mg higher than exchangeable Ca (Mg/Ca>1.00; magnesic layer); 2) eight soils have at least one layer with more than 15% exchangeable Na (sodic layer); 3) ten soils have layers with predominance of Na+Mg in the exchangeable complex (Na + Mg >50%, sodic‐magnesic layers); 4) very acid soil pH values (3.75 to 5.00) on the soil surface layers increasing markedly (even to alkaline values) with depth; and 5) all profiles have very low available P values. The common occurrence of these rather unusual chemical properties, especially the first three ones on most of these soils were considered as evidence of their role as fundamental edaphic factors on palm tree adaptation and distribution. These results also indicate that, besides the already described soil physical (high clay content, slow permeability) and environmental conditions (seasonal flooding and drought, and yearly burning), unusual soil chemical properties seem to be also involved with the ecological conditions associated with the presence of palm trees.
The most extensive biomes of tropical South America, in particular the Amazonian Forest, are the richest in total number of bird species and number of endemic species. On the other hand, relative diversity of species and endemic species (number of species per 10,000 km of biome) is highest in areally restricted and/or isolated biomes like the Pacific Colombian‐Ecuadorian Forest, the Andean Montane Forest and the Pantepui Montane Forest. Local species richness is highest in Amazonia, especially in upper Amazonia where 400 to 430 forest bird species (over 500 species, if water‐birds and migrants are included) occur at individual localities. Very high species richness characterizes also the avifaunas of the forests which extend, from upper Amazonia, far eastward in the hilly country both to the north and to the south of the central Amazon Valley. By contrast, local species diversity is considerably reduced in central Amazonia. The avifaunas at individual Amazonian localities are five to six times richer in species than north Temperate Zone avifaunas. “Excess” tropical species richness is explained by several factors, viz. tighter species packing, additional tropical guilds and increased size ranges of tropical guilds. Interspecific competition has a prominent role in determining the range limits of numerous parapatric bird species in the lowlands as well as along the slopes of the Andes. The total number of forest bird species at individual sites decreases with elevation in the Andes closely paralleling a similar trend of decreasing forest stature. During Cenozoic time (Tertiary‐Quaternary), tropical and high latitude avifaunas have been affected by similar vicariance processes (which probably caused extensive faunal diversification), viz. paleogeographic changes in the distribution of land and flooded areas, separation by broad rivers, and climatic‐vegetational fluctuations (leading to the isolation of restricted populations in temporal ecological refugia).
Pedological analyses and thermoluminescence dating were carried out on two soil profiles in the alluvial plain of the Orituco River (856′N, 67°25′W), Venezuela, to determine if the chemical properties of the soil subhorizons are related to the paleoclimatology of the region. The results of the chemical analyses show that a natric horizon is present in both sampling sites (river levee and flood plain) at about 50 cm depth, and that CaCO3 is present only in the lower subhorizons. They also show that pH and exchangeable Na and Mg increase with depth, and that there is a predominance of Na plus Mg over Ca throughout the soil column. On the other hand, the TL dating results show that the age of the sediment increases with depth in both sites, that the age of the natric horizon is 4.3–8.7 ka, and that the rate of sedimentation is greater at the river levee than at the flood plain. Since natric horizons are found only in soils developed in arid climates, and could not have formed under the present savanna climate of the region, the results suggest that the natric horizon found in the alluvial plain corresponds to the period of aridity that was prevalent at low elevations in the neotropics during the Last Glacial Maximum, and that the non natric upper subhorizons, where the acid cation Al is predominant and CaCO3 is absent, were developed during the humid period that followed aridity in the Early Holocene.
Landsat Thematic Mapper images covering the entire 3.9 million km of forested Brazilian Amazon reveal natural change and disturbance occurring on a scale of decades to centuries. These include: 92,000 km of bamboo forests undergoing synchronous mortality and regrowth; 1500 km of recently active dune fields; 900 km of recent downburst blowdowns; > 500 km of recent forest fire scars; and an unknown area of forest mortality from flooding of high ground and alluvial forests. Fire subclimax fern savannas created by Yanoama Indians cover an additional 600 km. Natural and indigenous disturbances and synchronized phenologies are therefore responsible for a dynamic spectral bahavior in large portions of the Amazon Basin. Disturbances and disturbance indicators not easily detected include blowdown sites > 30 yrs old, blowdowns < 30 hectares in size, liana forests and babassu palm forests.
Glacial lake sediments and glacial geomorphology in Valle de las Morrenas, a glacial trough on the north face of Cerro Chirripó, Costa Rica, provide evidence on high-altitude Pleistocene conditions in Central America. The most recent glacier in the valley (Chirripó stage I) receded very rapidly near the end of the Younger Dryas chronozone. Radiocarbon dates on basal organic sediments from lakes beneath upper, middle, and lower limits of that glacier fall close together, and two-sigma calibrated ages overlap for the period 9700–9600 cal yr B.P. Earliest datable transition sediments from the central lake date to 12,360–11,230 cal yr B.P. Larger, older moraines, and associated trimlines, allowed reconstruction of three paleoglaciers (Chirripó stages II, III, and IV). Computer analysis of hypsometry using published tropical-glacier vertical mass balance profiles yields ELAs of 3506–3523, 3515–3537, and 3418–3509 m, respectively; Chirripó II ELA-estimate positions applied to Chirripó I yield an ELA of 3538–3546 m. We infer minimal temperature depressions of 7.4–8.0°C for the Chirripó I–IV stages. Modeling the behavior of modern tropical glaciers yields basinwide net accumulation estimates of 440–620, 550–830, and 960–1760 mm yr−1 for the Chirripó II, III, and IV stages.
Climatic-vegetational fluctuations due to astronomical Milankovitch cycles caused global changes in the distribution of tropical forests and nonforest vegetation during the Cenozoic (Tertiary-Quaternary) and before. Forest and nonforest biomes on the continents changed continuously in distribution during the geological past, breaking up into isolated blocks and again expanding and coalescing under the varying dry to humid climatic conditions. However, plant and animal communities disrupted and species changed their distributions individualistically during the various climatic phases. Field data indicating vegetational changes in Amazonia exist for the Quaternary. The refuge theory postulates that extensive patches of humid rainforests persisted during dry periods of the Tertiary and Quaternary, especially near areas of surface relief in peripheral portions of Amazonia, where many extant species and subspecies of plants and animals probably originated. The humid 'refugia' may have been separated by various types of savanna and dry forests as well as other intermediate vegetation types of seasonally dry climates. The number and size of refugia during different dry periods remain unknown. Biogeographic evidence for the former existence of forest refugia include areas of endemism and sharply defined contact zones between species and subspecies of Amazonian forest birds and other animals which represent zones of conspicuous biogeographic discontinuity in a continuous forest environment. Alternative models of barrier formation in Amazonia leading to allopatric speciation include the river hypothesis, river-refuge hypothesis, canopy-density hypothesis, disturbance-vicariance hypothesis, museum hypothesis and various paleogeography hypotheses, some aspects of which may be applicable to certain periods in the evolution of the biota.
This chapter describes the geologic, geographic, and ecological context of the location of Luquillo Mountains, particularly the factors affecting the response mechanisms of terrestrial and aquatic ecosystems to disturbance. It describes the existing conditions of the physical environment, chemical environment, and the biota of the Luquillo Mountains as they respond to disturbances. It then merges the decade-long research about the Mountains with the other tropical ecosystems around the globe.
Palaeodunes were examined on the eastern margin of the Rio Branco–Rupununi savanna, northeast Amazonia. Optical dating suggests that the onset of aeolian activity was between 17 000 and 15 000 yr ago, just after the Last Glacial Maximum. Both the palaeodune axes and modern dominant wind directions have northeast to east-northeast directions, implying no significant shift in atmospheric circulation patterns over northeast Amazonia during the Late Pleistocene to Holocene. Major regional climate change events, such as the Younger Dryas, do not appear to have had any effect on the rates of aeolian deposition at the study site. Aeolian activity appears to have continued to the present day, showing a remarkably constant deposition rate of around 0.13 m kyr−1 initially, increasing smoothly to the present. Until more palaeodunes in northern Amazonia are dated, it is impossible to determine if this record of gradual aeolian deposition is a reliable regional palaeoclimate indicator, rather than being the result of local bioclimatic and geomorphological effects. Copyright © 2004 John Wiley & Sons, Ltd.
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