Large Land Mass Research Articles

Strontium isotope evolution of Late Permian and Triassic seawater

The 87Sr/ 86Sr values based on brachiopods and conodonts define a nearly continuous record for the Late Permian and Triassic intervals. Minor gaps in measurements exist only for the uppermost Brahmanian, lower part of the Upper Olenekian, and Middle Norian, and only sparse data are available for the Late Permian. These 219 measurements include 67 brachiopods and 114 conodont samples from the Tethyan realm as well as 37 brachiopods and one conodont sample from the mid-European Middle Triassic Muschelkalk Sea. The Late Permian/Lower Triassic interval is characterized by a steep 1.3 × 10 −3 rise, from 0.7070 at the base of the Dzhulfian to 0.7082 in the late Olenekian, a rate of change comparable to that in the Cenozoic. In the mid-Triassic (Anisian and Ladinian), the isotope values fall to 0.7075, followed again by a rise to 0.7081 in the Middle/Late Norian. The 87Sr/ 86Sr values decline again in the Late Norian (Sevatian) and Rhaetian to 0.7076. The sharp rise in the 87Sr/ 86Sr values during the Late Permian/Early Triassic was coincident with widespread clastic sedimentation. Because of the paucity of tectonic uplifts, the enhanced erosion may have been due to intermittent humid phases, during mainly an arid interval, coupled with the absence of a dense protective land plant cover following the mass extinction during the latest Permian. The apex of the 87Sr/ 86Sr curve at the Olenekian/Anisian boundary coincides with cessation of the large-scale clastic sedimentation and also marks the final recovery of land vegetation, as indicated by the renewed onset of coal formation in the Middle Triassic. The rising 87Sr/ 86Sr values from the Middle Carnian to the Late Norian coincide with the uplift and erosion of the Cimmeride-Indosinian orogens marking the closure of the Palaeotethys. The subsequent Rhaetian decline that continues into Jurassic (Pliensbachian/Toarcian boundary), on the other hand, coincides with the opening of the Vardar Ocean and its eastern continuation in the Izmir-Ankara Ophiolitic Belt. Samples from the Upper Muschelkalk are more radiogenic than the global trend. This may reflect separation of the basin from the open ocean. Due to strong meteoric influx from a large land mass in the north, the Germanic Basin became increasing brackish up section in the north and east, but because of the high evaporation rates, the salt content was not much reduced in the southern and central basin where a rich, but increasingly endemic, marine fauna survived.

Insights into orogenesis: getting to the root of a continent–ocean–continent collision, Southern Urals, Russia

The Ural mountains preserve a late Palaeozoic collision that forms a 2500 km suture in the world's largest landmass, Eurasia. Several features of the mountain belt, in particular a well-preserved crustal root, are uncharacteristic of other Palaeozoic orogens such as the Appalachians and Caledonides. Previous interpretations of the Southern Uralian root suggested that it is composed of East European Craton crust derived from the west. A new potential field data model, considered in conjunction with published seismic, heat-flow and geological data, indicates that the root is composed mainly of mafic granulite, which we interpret as oceanic arc crust originally accreted from the east, subducted eastward, and metamorphosed. A load caused by crustal lateral density variations, combined with topography, isostatically compensates root buoyancy and is thus the main cause of its preservation.

Determinants of plant species richness on small Bahamian islands

AimI investigated the determinants of plant species richness in two archipelagos, comparing the predictive power of different explanatory variables. I evaluated both conventional variables and alternative variables not commonly used in such analyses. I also investigated the effect of island location in relation to neighbouring landmasses on plant species richness and the predictive ability of regression models.LocationArchipelagos of small islands in the central Exumas and near the north‐east coast of Andros, Bahamas.MethodsI surveyed plant species richness and obtained measures of seven predictor variables: total island area, the ratio of perimeter to total area, vegetated area, the ratio of vegetated area to total area, distance to the nearest large island, elevation and protection from surrounding islands. All seven predictor variables were evaluated as determinants of plant species richness in simple and stepwise multiple linear regressions. Analyses were conducted for each archipelago overall, and then separately for three categories of islands in the Exumas. Total area, elevation, and distance were evaluated as predictors of vegetation incidence in simple and stepwise multiple logistic regressions for both archipelagos.ResultsSome expression of insular area was always the best single predictor of plant species richness in the linear regressions. Total area was a relatively poor predictor compared with other expressions of insular area. Distance, elevation, and protection explained relatively little of the overall variation in plant species number, although all variables were selected as significant in some models. A greater amount of variation in plant species richness was explained by the linear regression models in the Exumas (69.0%) compared with Andros (60.9%). Different variables were entered into the models for the three categories of islands in the Exumas, and adjusted coefficients of multiple determination ranged from 68.9% to 85.7%. In the logistic regressions, the model including total area and distance yielded almost 90% correct classification of vegetation incidence in the Exumas; no significant variables were selected for Andros. A group of exposed, outer islands supported many fewer species than more sheltered islands, on the basis of total island area or elevation.Main conclusionsThe three variables commonly used in studies of determinants of insular species richness – total island area, distance, and elevation – were relatively poor predictors in most analyses. Alternative expressions of insular area – indicative of disturbance or shape in combination with area – were usually better predictors than total area and may more realistically reflect habitable area. Alternative predictors explained similar amounts of variation in plant species richness compared with commonly used predictors, and combinations of all variables into a single stepwise model resulted in increased predictive power. The predictive power of the models tended to be higher for groups of islands that were more sheltered by neighbouring islands. Exposed islands, although separated by relatively small distances from nearby protected islands, may be impacted by storms much more severely and possess many fewer species. The location of small islands relative to large landmasses, as well as their geological histories, should be taken into account in such analyses.

Национальные и вненациональные измерения экономического развития России, XIX–XX вв.

SUMMARY: This paper has focused on a large land mass, home to a multiplicity of ethnic groups, whose members have commonly been living in a supranational polity. Here the nation-state, in the sense that Friedrich List would have understood it, has been the exception rather than the rule. Yet nationality mattered. It impinged on those tsarist officials and merchants who espoused a vision of a Russified national economy and who advanced the cause of Russification. It mattered, differently, to Soviet leaders who denounced Russian chauvinism and tolerated the creation of an ethno-territorial state within which “national” loyalties contended with Soviet supremacy. When the legitimacy of the supra-national state was questioned (in 1905, 1917 and after 1985), political opposition was expressed at least in part in national terms. But military catastrophe and socio-economic crisis, not “nationalism” per se , were the decisive motors of change. In response to Gellner, the authors’ argument is that economic changes formed part of a broader process of transformation, which allowed national claims to be asserted alongside other claims. Under tsarism, economic change contributed to the creation of national sentiment and allegiance. In the Soviet case, the state sought to mobilize the population towards the goal of socialism, but ultimately many citizens became convinced that nationalism, rather than the pursuit of Soviet-style socialism, offered them better prospects of economic improvement. After 1985, many citizens appropriated the rhetoric of nationalism as a powerful device with which to undermine the old regime. The Soviet leadership contributed to its own downfall by promoting national consolidation by territorial-administrative means. When the cupboard was shown to be bare, nationalists promised an escape into sovereignty and freedom from want. But the precise mechanisms for improved growth and welfare, as well as their ideological foundations, remain unresolved. What is taking place at the end of the twentieth century is the reiteration of national conflict and of the conflicts between different minorities in the borderlands of empire. As in 1917, these are peripheral manifestations of broader calamities, which include a strong element of socio-economic collapse. The demise of the Soviet Union has encouraged ethnic minorities – and ethnic majorities – to rethink their history. In this sense, economic history provides vital clues to the past, present and future of these troubled lands.

Technology transfer in the space sector: an international perspective.

The following articles represent extremely interesting examples of how four different nations1 have attempted to augment the returns on their investments in space missions through government programs to encourage commercial uses of advanced technologies. Each entity’s approach to the problem recognizes the vast potential of space technology and each approach clearly reflects the way the space sector developed in that nation and the unique cultural, economic, and social situation within. The article on the European space sector is the most comprehensive, tracking the results of studies on technology transfer since the late 1970’s and summarizing more recent trends as the European space programs have evolved from infant industries into fully mature and capable space enterprises. The Canadian article emphasizes the perspective of a relatively small nation (in terms of gross domestic product) with a space program fine-tuned to a very selective set of technologies, industries, and capabilities. These reflect both the industrial expertise of Canada as well as the practical needs of a nation with a sparsely populated large land mass. The Russian article emphasizes the dramatic changes resulting from the collapse of the enormously well funded and successful Soviet space program. Russia today, with a vast portfolio of human and physical space capital, is trying to develop a technology transfer program under conditions of market insecurity, insufficient funding, and the lack of a welldeveloped legal and market-oriented economic

Mid-Proterozoic Plume-Related Thermal Event in Eastern Indian Craton: Evidence from Trace Elements, REE Geochemistry and Sr - Nd Isotope Systematics of Basic-Ultrabasic Intrusives from Dalma Volcanic Belt

Trace, REE, Sr and Nd isotopic studies have been carried out on gabbro-pyroxenite intrusives (Rb-Sr isochron age ∼ 1619±38 Ma; Sr i ∼ 0.70552±0.00002) of the Dalma volcanic belt from eastern Indian craton. Primitive mantle-normalised trace element patterns show a general depletion of high field strength elements and LREE but more or less flat pattern in most compatible elements. Chondrite-normalised REE plots show depleted LREE-flat HREE patterns [(SLREE/SHREE) N < 1, (Ce/Yb) N < 1] strikingly similar to the komatiitic and tholeiitic lavas from this belt. Nd isotopic data with mean f Sm/Nd ∼ +0.2704 and high e Nd (mean +7.8) values indicate that the source of these rocks was depleted in LREE for considerably long time. When plotted on the global e Nd evolution path for the upper mantle the Dalma intrusives fall exactly around the depleted MORB-type mantle at 1.6 Ga. Enrichment in some LILE like Rb, Ba, Th is found both in the tholeiitic lavas and the residues indicating them to be source characteristics. Positive DNb values of most of the mafic-ultramafic units (including komatiitic lavas) of this belt indicate that they originated from a mantle plume with thick envelope of hot upper mantle producing MORB-like depleted komatiites, tholeiites and intrusives. The mid-Proterozoic plume eventually rifted the continent above, forming a rapidly subsiding basin which was subsequently collapsed and compressed. The plume also caused widespread thermal events recorded in charnockitisation, migmatisation and granitisation around 1.6 Ga. This was possibly part of a global ∼1.6 Ga thermal anomaly which affected the pre-existing large landmass comprising atleast Antarctica, Australia and India (Mawson continent?).

Surplus killing by introduced predators in Australia—evidence for ineffective anti-predator adaptations in native prey species?

Australian examples of surplus killing by mammalian predators were collated. These included surplus killing of native mammals and birds by foxes ( Vulpes vulpes) and stock, native mammals and native birds by dingoes ( Canis lupus dingo). We found no examples of surplus killing by feral cats ( Felis catus). Incidents collated include historical anecdotes of surplus killing by foxes as they colonised the Australian mainland, recent examples where foxes killed threatened native species at sites despite intensive management to exclude foxes, and recent examples of the killing of native species on formerly fox-free islands to which foxes gained entry. Episodes of surplus killings by foxes, other than predation on captive or closely confined animals, appeared different in kind and frequency to those documented for co-evolved predator–prey systems on the large continental landmasses. They did not appear to be uncommon events associated with synchronised births of prey species, unusual or extreme weather that disadvantaged prey species, or seasonal food caching by a predator. Rather, surplus killing events appeared to reflect ineffective anti-predator defences by prey species when encountering a novel and efficient predator to which they have had no evolutionary exposure. We suggest that surplus killing by foxes may have been a feature of, and major contributor to, the rapid mainland extinction or contraction in range of many native species in Australia. In contrast to foxes, examples of surplus killing by dingoes relate mostly to domestic stock (calves and sheep). The arrival of dingoes to the Australian continent preceded that of foxes by 3500–4000 years, but they appear not to have had the dire impact on native mammals that we attribute to foxes. This may be due to fundamental differences in hunting styles and prey size and to their sparse populations in pre-European Australia. Active persecution of non-commensal dingoes by Aborigines, the lack of free-water, and the absence of European rabbits ( Oryctolagus cuniculus) as an alternative food supply would have limited their numbers and their impact on native mammals.

Annual monsoon rains recorded by Jurassic dunes.

Pangaea, the largest landmass in the Earth's history, was nearly bisected by the Equator during the late Palaeozoic and early Mesozoic eras. Modelling experiments and stratigraphic studies have suggested that the supercontinent generated a monsoonal atmospheric circulation that led to extreme seasonality, but direct evidence for annual rainfall periodicity has been lacking. In the Mesozoic era, about 190 million years ago, thick deposits of wind-blown sand accumulated in dunes of a vast, low-latitude desert at Pangaea's western margin. These deposits are now situated in the southwestern USA. Here we analyse slump masses in the annual depositional cycles within these deposits, which have been described for some outcrops of the Navajo Sandstone. Twenty-four slumps, which were generated by heavy rainfall, appear within one interval representing 36 years of dune migration. We interpret the positions of 20 of these masses to indicate slumping during summer monsoon rains, with the other four having been the result of winter storms. The slumped lee faces of these Jurassic dunes therefore represent a prehistoric record of yearly rain events.

Palaeowaters in the aquifers of the coastal regions of southern and eastern England

Abstract The marked climatic changes that occurred during the Late Quaternary and Holocene have had a significant impact on the evolution of the groundwater systems at and near the English coastline. Lowered sea levels and the emergence of a much larger landmass over most of the past 100 ka have also ensured deeper groundwater circulation in the vicinity of the modern coastline. The impacts on the Chalk and Lower Greensand (Albian) aquifers along the English Channel and North Sea coasts are examined, using mainly geochemical and isotopic evidence, especially from borehole depth profiles and interstitial waters. Along the south coast, fresh groundwaters are found to depths of 250–300 m below OD (ordnance datum) in the Brighton-Worthing area, as well as beneath Poole Harbour, which are related to deeper circulation during lowered sea levels, controlled by the central palaeovalley of the English Channel. In contrast, pockets of saline groundwater are found, protected in east-west structures, which are considered to be little-modified Chalk formation waters. In the Albian sands, near Worthing, freshwaters dating to 7 ka are found at a depth of −450 m OD, suggesting that movement of groundwater towards the shoreline and possibly beyond is still occurring. The east-west structures also influence groundwater migration in north Kent, where fresh palaeowaters can be identified beneath saline water which invaded during the Holocene sea-level rise. In the East Midlands Sherwood Sandstone aquifer, freshwater is found to a depth of −500 m OD, showing continuous geochemical evolution probably over a period of 100 ka, although an ‘age gap’ of between c. 20 and 10 ka c. corresponds to permafrost cover. These palaeowaters in coastal and near-coastal areas remain effectively isolated from the active present-day meteoric flow system but represent high-value resources that may, in some cases, extend offshore.

Variation in leaf-cutter ant (Atta sp.) densities in forest isolates: the potential role of predation

The potential role of predation as a factor underlying variation in leaf-cutter ant (Atta sp.) densities was investigated on recently isolated land-bridge islands in Lago Guri, Venezuela. A survey of mature Atta colonies revealed a sharp increase in densities on small and medium islands (5.6 and 2.3 colonies ha−1 respectively) compared to large islands (0.72 colonies ha−1) and the mainland (0 nests in 5.5 ha). Incipient colonies showed the reverse trend with higher densities on larger landmasses. Cage experiments were conducted to test the impacts of predation on the survival of incipient and young Atta colonies. Protection by wire-mesh cages increased the survival of both incipient and young colonies at all sites. The survival of unprotected incipient and young colonies was significantly reduced on larger landmasses relative to small islands. These results are consistent with burrow survey data that provided indirect evidence of higher armadillo densities on larger landmasses. Findings suggest that reduced predation is a potentially important factor underlying observed variation in Atta density patterns in the Guri islands.

Epidemic Development and Virulence in 1995–1998 of Puccinia coronata, a Potential Biocontrol Agent of Wild Oats on San Clemente Island

Abstract The biology of islands differs from that of large land masses in having less complex ecosystems. Introduced exotic weeds are often a major threat to fragile island ecosystems because of their expansion into habitats previously occupied by endemic species. San Clemente Island, 120 km off the California coastline, is an example of this process in which numerous exotic weed species have been introduced and some are endangering the native flora. Crown rust of oats caused by Puccinia coronata f.sp. avenae was investigated as a potential biocontrol agent against two wild oat species, Avena barbata and Avena fatua, introduced on San Clemente Island. Epidemiology and virulence of this rust were studied. The island was surveyed from 1995 to 1998 for occurrence of P. coronata on wild oats. Wild oats were found sprouting in the northern part of the island shortly after autumn rainfall and subsequently covered the main grasslands of the island. The rust also appeared first on the northern part of the island and progressively spread south. However, disease severities in the south were considerably lower than those in the north. Diverse virulence types, although related to Californian and Mexican forms, were detected among the isolates. The potential use of P. coronata as an augmentative biocontrol agent for wild oat species on San Clemente Island is discussed.

Wegener and his Theory of Continental Drift

Alfred Wegener, an astronomer by training and meteorologist by profession, visualised that there was once a single very large landmass on the surface of the earth. Gradually it broke up into smaller fragments which drifted away to form the present continents. In a few cases, the drifting fragments subsequently collided, coalesced and formed larger landmasses again. Wegener could thus explain in a simple way (i) similarities in shapes of continental coast lines separated by wide oceans, (ii) similarities in the rocks and fossils occurring on such coasts and in the fauna and flora inhabiting them, (iii) the formation of several major mountain ranges of the world, and (iv) the evidence in rocks on past climates of the earth.

Inequality and the Directionality of History.

I argue that history is genealogy and succession in an economic context of interactions among participants. Inequality is a ubiquitous property of economic interaction among responsive, metabolizing entities. The prevailing party tends to be larger, to have a higher productivity or metabolism, to engage in a larger number of interactions, and to perform more functions at a higher level. The crucial economic consequence of inequality is that prevailing entities and the conditions they create disproportionately influence the economy as a whole. This leads to a pervasive increase in energy flux, both globally and per capita, among economic dominants, observable in the history of life as well as in the development of the human economy. Unpredictable disturbances such as mass extinctions temporarily reverse this trend. Comparative studies of the biotas of islands and of large land masses point to limitations on the evolution of energy-intensive dominants. Because entities with a large economic domain typically restrict rather than eliminate subordinates, the trend toward increasing energy flux will rarely be expressed within individual clades. History's directionality is imposed by effects of energy-intensive dominants on the distribution and characteristics of all other participants in the economies they help to fashion.

History of the Celebes Sea Basin based on its stratigraphic and sedimentological record

The stratigraphic and sedimentary history of the Celebes Sea Basin provides the basis for a tectonic model for its evolution which is consistent with new regional plate reconstructions. Cores drilled on Leg 124 of the Ocean Drilling Program (sites 767 and 770) form a stratigraphic record from the basement to the sea floor in just under 800 metres of sedimentary succession. The basement itself is basalt with a strong MORB affinity. Radiolaria in pelagic mudstones at the base of the succession indicate a late middle Eocene age. A very condensed pelagic sequence (90 m in 20 Ma) suggests a setting which was distal from sources of terrigenous clastics and volcaniclastics and below the CCD. These characteristics are similar to the West Philippine Sea Basin (drilled on DSDP Legs 31 and 59), which also has oceanic basement, dated at 44–42 Ma, and Paleogene pelagic strata. These data are therefore consistent with any plate reconstruction model which has these two basins forming as part of the same ocean basin. During the Miocene quartz-rich sandy turbidites were deposited in the Celebes Sea Basin. This influx of terrigenous clastics occurred for a brief period in middle to late Miocene times (c. 10 Ma). The nearest large continental landmass which could have acted as the source of this mature siliciclastic detritus was the island of Borneo, and the onset of this clastic supply may be related to uplift caused by collision events on the island. Cessation of terrigenous clastic supply to the basin centre occurred as southward subduction of the underlying oceanic crust beneath the northern arm of Sulawesi began. It is speculated that the development of a trench along the southern margin of the basin acted as a trap for detritus coming from Borneo, diverting material from the basin centre.

Initiation of ultrahigh‐pressure metamorphism and its significance on the Proterozoic–Phanerozoic boundary

Ultrahigh‐pressure (UHP) metamorphism began at 540–530 Ma near the Precambrian–Cambrian boundary, and repeated more than 10 times through the Phanerozoic, indicating one of the most prominent tectonic aspects of the Phanerozoic Earth. The preceding subduction zone metamorphism back to the Archean had been of the intermediate‐ to low‐P type. An apparent change in the P/T conditions of subduction zone metamorphism occurred at the Precambrian–Cambrian boundary at ca 544 Ma. At ∼ 200 million years earlier than the initiation of UHP metamorphism, blueschist facies metamorphism occurred along the northern margin of the Tarim craton. A transitional period between the Proterozoic and Phanerozoic at 750–540 Ma corresponds to the initiation of blueschist facies metamorphism at the subduction zone; the cooling Earth dropped its geothermal gradient which intersected the blueschist facies P–T field. An abrupt change in the geothermal gradient from 22–15 °C/km (Archean–Proterozoic) to 6–7 °C/km (Phanerozoic) to initiate UHP metamorphism may correspond to a rather abrupt increase of slab‐pull driving force at the beginning of the Phanerozoic. Large amounts of heat loss from the Earth's interior may have occurred just before 540 Ma; presumably it may correspond to the duration from the breakup of supercontinent Rodinia at 750 Ma, to the formation of the next supercontinent Gondwana at 540 Ma.Through the transient period 750–540 Ma the most extensive glaciation in Earth's history ended at ca 600 Ma, when the world's largest‐scale unconformity covered nearly all the continents on the Earth. Huge landmasses from ∼ 5 to 30% of the Earth's surface first raised above sea‐level during 600–540 Ma; such change may have initiated since ca 750 Ma, and repeated four times until 540 Ma. Emergence of large continental masses above sea‐level resulted in extensive erosion and diversity of surficial environment, and significant modification in the composition of seawater and world climates. Coincidentally or accidentally with the breakup of the latest Proterozoic supercontinent, the Cambrian life explosion began after the transient period in which the Ediacara fauna evolved along rifted seaways with hot springs in a partly rifted supercontinent.The complex scenario of these phenomena may be triggered and linked by a sudden increase in the plate‐driving force of slab‐pull during 750–540 Ma, which allowed buoyant continental crust to be subducted to a 150 km depth and subjected to UHP metamorphism. The increased slab‐pull force reflects an increase in plate thickness, and is the surface manifestation of the loss of a large amount of internal heat from the Earth's interior before the initiation of UHP metamorphism. It occurred in a short time span (750–540 Ma) from rifting of Rodinia to amalgamation of the next supercontinent Gondwana. Since then, old slabs with deep oceans survived, large landmasses raised above sea‐level, the surface environment diversified, and the explosion of life accelerated before and after the mass extinction that occurred at 550–540 Ma.

Shallow-Water Marine Gammaridean Amphipods of Mauritius Island

Over 34000 specimens of marine gammaridean amphipods were collected from 34 sites of varying wave exposure in the intertidal/shallow subtidal zones of Mauritius (19°59'-20°32'S 57°18'-57°47'E, Indian Ocean) in April to July, 1995. Substrates included algae, seagrass, coral, sand, and sponges. Sixty-nine species are reported, 41 of which are new records for the island; 19 species are undescribed. One hundred-and-thirty species are now known from Mauritius and of these 32% appear to be endemic. Many Mauritian amphipods are widely distributed in the Indo-Pacific. Fifty-seven percent of the Mauritian amphipods are shared with Madagascar (the closest large land mass). The numbers of species in amphipod families in Mauritius are comparable with those of Hawaii and Fiji.

BIRD COMMUNITIES IN TRANSITION: THE LAGO GURI ISLANDS

We report on the bird communities of a set of 12 7-yr-old forested land-bridge islands in Lago Guri, a 4300 km2 hydroelectric impoundment in the State of Bolivar, Venezuela. Birds were censused on all islands and at mainland control sites by spot mapping in 1993, and via point counts in 1995. Instead of orderly “nested sets” of species on landmasses of graded size, the species composition of small (≈1 ha) and medium (11–12 ha) islands was highly variable. Spot mapping substantiated the occurrence of 58 species of forest-nesting birds, collectively, on the 11 small and medium islands, which supported means of only 9 and 12 resident species, respectively. No species was found on all islands, and only five species were found on as many as 7 of the 11 small and medium islands. The mean number of islands per resident species was 3.0 for the 11 islands. Transients of many species were detected on all islands, but were more frequent on near (≤0.5 km from a larger landmass) than on far islands (≥1.0 km from a larger landmass). A large majority of the populations on the 1-ha islands consisted of a single pair, implying that few such populations could have survived since isolation without new colonizations to replenish individuals lost to dispersal and mortality. Notwithstanding low species numbers, both census methods indicated that avian densities were approximately twice as high on the 1-ha islands as on the mainland. In contrast, two of three medium-sized islands supported anomalously low densities of approximately one-fifth the mainland level. Both islands retain relict populations of capuchin monkeys (Cebus olivaceus). One hundred percent of artificial nests set out on one of these islands were raided, whereas no more than 30% of nests were lost on any of the other islands or on the mainland. For small and medium islands, we conclude that the founding communities present when the waters of Lago Guri reached their final level have already collapsed and been reconstituted. Species loss on some islands may have been accelerated by relict populations of predators, such as capuchins. Changes in composition thus appear to have been driven by a combination of biological (nest predation) and stochastic processes (high turnover).

Bird Communities in Transition: The Lago Guri Islands

We report on the bird communities of a set of 12 7-yr-old forested land-bridge islands in Lago Guri, a 4300 km2 hydroelectric impoundment in the State of Bolivar, Venezuela. Birds were censused on all islands and at mainland control sites by spot mapping in 1993, and via point counts in 1995. Instead of orderly “nested sets” of species on landmasses of graded size, the species composition of small (≈1 ha) and medium (11–12 ha) islands was highly variable. Spot mapping substantiated the occurrence of 58 species of forest-nesting birds, collectively, on the 11 small and medium islands, which supported means of only 9 and 12 resident species, respectively. No species was found on all islands, and only five species were found on as many as 7 of the 11 small and medium islands. The mean number of islands per resident species was 3.0 for the 11 islands. Transients of many species were detected on all islands, but were more frequent on near (≤0.5 km from a larger landmass) than on far islands (≥1.0 km from a larger landmass). A large majority of the populations on the 1-ha islands consisted of a single pair, implying that few such populations could have survived since isolation without new colonizations to replenish individuals lost to dispersal and mortality. Notwithstanding low species numbers, both census methods indicated that avian densities were approximately twice as high on the 1-ha islands as on the mainland. In contrast, two of three medium-sized islands supported anomalously low densities of approximately one-fifth the mainland level. Both islands retain relict populations of capuchin monkeys (Cebus olivaceus). One hundred percent of artificial nests set out on one of these islands were raided, whereas no more than 30% of nests were lost on any of the other islands or on the mainland. For small and medium islands, we conclude that the founding communities present when the waters of Lago Guri reached their final level have already collapsed and been reconstituted. Species loss on some islands may have been accelerated by relict populations of predators, such as capuchins. Changes in composition thus appear to have been driven by a combination of biological (nest predation) and stochastic processes (high turnover).

Mechanism and sequence of assembly and dispersal of supercontinents

The term continent implies a major land mass, with adjacent shelves and possessing one or more internal stable regions known as cratons. In geological time very large land masses had originated as supercontinents and superoceans formed outboard of these. Motions of former cratons and continents have been investigated paleomagnetically and by tectonic tracing. Thus, supercontinents were formed and fragmented in the process of continental drift and ocean spreading. The growth of supercontinents is the consequence of collision with other continents, smaller continental fragments, as well as island arcs, accretionary prisms, and a variety of oceanic plateaux. Dispersal of supercontinents occurred as fragmentation of larger continental masses into smaller blocks took place. There is some evidence that growth and dispersal of supercontinents developed periodically every few hundred million years. The best known relatively young dispersion of a supercontinent is that of Pangea. However, the amalgamation of this supercontinent was more complex and some of its aspects are disputed among those who invoke normal plate tectonic processes and those who suggest the operation of superplumes. The amalgamation of continental fragments occurred by: (i) the development of cold subsiding regions in the mantle; (ii) the adherence of island arcs: or, (iii) the movement of diverse terranes and their collision and accretion to continental masses and those to each other. Several similar scenarios have been suggested in Archean and Proterozoic times. At present Archean plate tectonics are still controversial, but abundant evidence suggests that collisional and accretionary events occurred even then. In the Lower-Middle Proterozoic times amalgamation and accretion became significant, while in the Upper Proterozoic other new cratons originated, possibly by island arc accretion. It is suggested that the break up of continental plates as a consequence of distentional events was prompted by convective changes in the mantle.

Temperature depression in the lowland tropics in glacial times

Equatorial air temperatures at low elevations in the New World tropics are shown by pollen and other data to have been significantly lowered in long intervals of the last glaciation. These new data show that long recognized evidence for cooling at high elevations in the tropics were symptomatic of general tropical cooling and that they did not require appeal to altered lapse rates or other special mechanisms to be made to conform with conclusions that equatorial sea surface temperatures (SSTs) were scarcely changed in glacial times. The new data should be read in conjunction with recent findings that Caribbean (SSTs) were lowered in the order of 5 ° C, contrary to previous interpretations. Thus these accumulating data show that low latitudes as well as high were cooled in glaciations. In part the earlier failure to find evidence of low elevation cooling in the lowland tropics resulted from the data being masked by strong signals for aridity given by old lake levels in parts of Africa and elsewhere. Global circulation models used to predict future effects of greenhouse warming must also be able to simulate the significant cooling of the large tropical land masses at glacial times with reduced greenhouse gas concentrations. Plants and animals of the Amazon forest and similar ecosystems are able to survive in wide ranges of temperatures, CO2 concentrations, and disturbance, though associations change constantly.