Episodes Of Dispersal Research Articles

GENETIC STRUCTURE OF GIANT CLAM (TRIDACNA MAXIMA) POPULATIONS IN THE WEST PACIFIC IS NOT CONSISTENT WITH DISPERSAL BY PRESENT-DAY OCEAN CURRENTS.

The Pacific marine biota, particularly species with long planktonic larval stages, are thought to disperse widely throughout the Pacific via ocean currents. The little genetic data available to date has supported this view in that little or no significant regional differentiation of populations has been found over large geographical distances. However, recent data from giant clams has demonstrated not only significant regional differentiation of populations, but routes of gene flow that run perpendicular to the main present-day ocean currents. Extensive surveys of genetic variation at eight polymorphic loci in 19 populations of the giant clam Tridacna maxima, sampled throughout the West and Central Pacific, confirmed that the patterns of variation seen so far in T. gigas were not unique to that species, and may reflect a fundamental genetic structuring of shallow-water marine taxa. Populations of T. maxima within highly connected reef systems like the Great Barrier Reef were panmictic (average FST < 0.003), but highly significant genetic differences between reef groups on different archipelagos (average FST = 0.084) and between West and Central Pacific regions (average FST = 0.156) were found. Inferred gene flow was high (Ne m usually > 5) between the Philippines and the Great Barrier Reef, between the Philippines and Melanesia (the Solomon Islands and Fiji), and between the Philippines and the Central Pacific island groups (Marshall Islands, Kiribati, Tuvalu and Cook Islands). Gene flow was low between these three sets of island chains (Ne m < 2). These routes of gene flow are perpendicular to present-day ocean currents. It is suggested that the spatial patterns of gene frequencies reflect past episodes of dispersal at times of lower sea levels which have not been erased by subsequent dispersal by present-day circulation. The patterns are consistent with extensive dispersal of marine species in the Pacific, and with traditional views of dispersal from the Indo-Malay region. However, they demonstrate that dispersal along present-day ocean surface currents cannot be assumed, that other mechanisms may operate today or that major dispersal events are intermittent (perhaps separated by several thousands of years), and that the nature and timing of dispersal of Pacific marine species is more complex than has been thought.

Phylloceratina ammonoids in the Iberian Basin during the Middle Jurassic: a model of biogeographical and taphonomic dispersal related to relative sea-level changes

Phylloceratina and Lytoceratina ammonoids in the Middle Jurassic of the Iberian Chain (E. Spain) represent less than 1% of the whole of Ammonoidea. There are two intervals, however, in which their occurrence is remarkably constant: within the Upper Bajocian and at the Lower/Middle Callovian boundary. These two dispersal episodes of Phylloceratina and Lytoceratina into the Iberian Basin are regarded to reflect changes in their palaeoecological and taphonomical behaviour, as a consequence of regional sea-level changes during the Middle Jurassic. A relative rise during the Late Bajocian favoured the immigration of juvenile phyllocerataceans. Phylloceras and Adabofoloceras immigrations gave rise to monospecific assemblages, where they soon died. They did not breed or ontogenically develop in this basin. In contrast, phyllocerataceans recorded at the Lower/Middle Callovian boundary constitute polyspecific assemblages, dominated by adult individuals. These Callovian assemblages were formed by nekroplanktonic drift, related to a relative sea-level fall and general homogenization of the shelf bottom, hence favouring the concentration of remains of organisms from more open marine and oceanic areas.

Trilobite biogeography in the Middle Devonian: geological processes and analytical methods

Phylogenetic patterns of trilobite clades were used to deduce biogeographic patterns during the Middle Devonian, a time of active plate collision between North America (Laurentia) and other plates, coincident with several major episodes of sea-level rise and fall. The mapping of biogeographic states onto phylogenies for asteropyginid and proetid trilobites indicated that during their history these trilobite clades often shifted the areas they occupied, and also underwent vicariant differentiation, followed by range expansion, followed by subsequent vicariance. Biogeographic patterns in these individual phylogenies were evaluated and synthesized using a modified version of Brooks Parsimony Analysis, which is discussed. This method makes it possible using cladistic methods to distinguish between episodes of vicariance and episodes of dispersal. Two types of dispersal are recognized herein: (1) the individualistic responses of certain taxa in a single clade that cannot be generalized, i.e., traditional ad hoc dispersal, and (2) those patterns of congruent range expansion that are replicated across several clades. The latter are not treated as true dispersal, expansion of a taxon's range over a barrier accompanied by diversification, but rather as a result of the temporary removal of barriers to marine taxa, due either to relative sea-level rise or to the collision of formerly disjunct plates. These are interpreted as changes in the structure of areas, and this type of dispersal is referred to as geo-dispersal. Geo-dispersal was found to have occurred in the Middle Devonian trilobite fauna of Eastern North America.Biogeographic analysis indicated that Eastern North America is a strongly supported area, with the Appalachian and Michigan Basins as sister areas. Armorica and the Canadian Arctic are also sister areas. Congruence was found between area cladograms produced by vicariance and dispersal analyses for Middle Devonian trilobites, suggesting that in some cases the geological processes governing vicariance, such as sea-level changes, were the same as those that caused dispersal.

Host―parasitoid metapopulations : the consequences of parasitoid aggregation on spatial dynamics and searching efficiency

Recent studies of host-parasitoid metapopulations have shown how uniform dispersal, from a patch to its neighbouring patches, can result in the persistence of an otherwise non-persistent interaction. Here these models are extended to include density-dependent parasitoid aggregation. The fraction of dispersing hosts colonize the neighbouring patches equally, whereas parasitoid dispersal is related to the relative density of healthy hosts in each surrounding patch. Interestingly, this shows that the degree of parasitoid aggregation is associated with the self-organization of spatial structures and, in particular, spiral waves. Regions where spirals are most pronounced correspond to minimal variance in population densities. It is demonstrated, however, that parasitoid searching efficiency is highest (and mean host densities lowest) for levels of aggregation where the spatial dynamics consist of an even mixture of spirals and disordered patterns. Using an algorithmic complexity measure, it is verified that spatial transitions are also reflected in the temporal dynamics. Finally, it is demonstrated that multiple episodes of parasitoid dispersal, within a host generation, can inhibit persistence, particularly for large parasitoid movement rates.

The first Asian plesiadapoids (Mammalia: Primatomorpha)

Two new genera and species of carpolestid plesiadapoids are described from early Eocene coal beds of the Wutu Formation, Wutu Basin, Shandong Province, People's Republic China. Chronolestes simul, n. gen. and sp., possesses a reduced P-3 and hypertrophied P-4 as do all carpolestids, but contrasts with other members of this family in lacking a bladelike, polycuspidate P-4. P3-4 of Chronolestes are structurally simpler, and almost certainly more primitive, than those of any other plesiadapoid. Chronolestes is interpreted as the sister group of all other carpolestids and forms the basis for the new, monotypic subfamily Chronolestinae. Carpocristes oriens, n. gen. and sp., is a dentally derived carpolestine characterized by triple median crests on P3-4 and an S-shaped blade on P-4. The closely related North American species ''Carpodaptes'' hobackensis and ''Carpodaptes'' cygneus are transferred here to the new genus Carpocristes. Relationships among species of Carpocristes are reconstructed as (C. cygneus (C. hobackensis, C. oriens)). Potential dental synapomorphies for Plesiadapoidea, including Plesiadapidae, Saxonellidae, and Carpolestidae, are identified for the first time. Within Plesiadapoidea, available evidence weakly supports the monophyly of a Plesiadapidae + Saxonellidae clade. Altanius orlovi from the early Eocene of Mongolia is unlikely to be either a plesiadapoid or a basal carpolestid. The fossil record of North American and Asian Carpolestidae suggests at least two episodes of trans-Beringian dispersal for members of this clade during the Paleocene. The timing of these dispersal episodes is weakly constrained, but middle Torrejonian (Chronolestes dade) and latest Tiffanian or earliest Clarkforkian (Carpocristes clade) dispersal events conform with inferences regarding trans-Beringian dispersal of other groups of Paleocene mammals. The recent hypothesis that competitive exclusion by rodents may have been a causal factor underlying carpolestid extinction in North America is called into question by the co-occurrence of two carpolestids and ctenodactyloid, alagomyid, and paramyid rodents at Wutu, together with the high probability that both carpolestids and rodents coexisted in Asia through much of the Paleocene.

HISTORICAL BIOGEOGRAPHY AND CONTEMPORARY PATTERNS OF MITOCHONDRIAL DNA VARIATION IN WHITE-TAILED DEER FROM THE SOUTHEASTERN UNITED STATES.

Mitochondrial DNA (mtDNA) was used to characterize patterns of geographic variation among white-tailed deer (Odocoileus virginianus) populations in the southeastern United States. Fifteen restriction enzymes were employed to survey and map 99 restriction sites in 142 deer from 18 localities in five southeastern states. Phylogenetic analysis revealed three primary groups of haplotypes: (1) southern Florida and the Florida Keys, (2) the remainder of peninsular Florida northward to South Carolina, and (3) the Florida panhandle westward to Mississippi. Geographical heterogeneity in haplotype frequencies suggests that stochastic lineage sorting or isolation by distance are not important determinates of mtDNA differentiation among deer populations. The pattern of mtDNA variation in white-tailed deer is concordant spatially with those observed in unrelated taxa suggesting the common influence of historical biogeographic events. The data (1) support previous hypotheses that relate contemporary patterns of intraspecific phylogeography in northern Florida to the physiogeographic history of the region; and (2) suggest that genetic differentiation in southern Florida may be attributable to episodes of Pleistocene dispersal. Despite potentially high vagility and human intervention, ecological and demographic characteristics of deer have effectively preserved the historical pattern of intraspecific mtDNA differentiation.

Temporal patterns among Phanerozoic oolitic ironstones and oceanic anoxia

Summary The stratigraphic distribution of Phanerozoic oolitic ironstones corresponds to temporal patterns recorded in common detrital deposits. These patterns, in which ironstones share significant associations with black shales, comprise successively smaller-scale sequences, each defined mainly by major transgressions and regressions, changing rates of erosion and sedimentation, and inter-regional unconformities. Two 300 Ma episodes of continental dispersal and Pangaeic assemblage of cratons encompassed the maximum rise and fall of sea level. Deposition of ironstones and black shales was essentially limited to 150–170 Ma phases (Ordovician-Devonian; Jurassic-Palaeogene) marked by extensive epicontinental seas and ineffective oxygenation of deep-water masses. Smaller-scale tectonically distinct phases of cratonic submergence, several to many tens of Ma long, were separated by briefer erosional intervals, or by longer episodes of oscillating conditions during which several interregional unconformities developed. Widespread submergence generally favoured major production of ironstones and black shales. Within this framework both were especially common during the Ordovician and Jurassic periods. A hypothetical quasiperiodic (c. 32 Ma) pattern of global climate change was accompanied by recurring relatively high sea level and expansion of oxygen-depleted water masses. Many of these phases led to widespread development of both ironstones and black shales. Episodes of fluctuating coastal onlap one to several Ma long reflect variations in sea level, sediment supply, and/or subsidence. Major successions of ironstones repeated on a similar time scale were limited to ten tectonic provinces comprising foreland basins, cratonic margins and unstable cratons, and intracratonic basins. During the favourable phases black shales and ironstones were deposited alternately in some successions. In many of the major successions an ironstone developed between each of several repeated small-scale sequences in a cadence of several hundred thousand years. Ironstones in these asymmetric units record a long lapse in normal sedimentation during the initial stage of renewed transgressions that commonly spread organic-rich mud across the shelf. The repeated association of ironstones and black shales with sequences of common sedimentary rocks helps constrain speculation about conditions controlling their origin, and suggests that ironstones reflect the more local development of productive conditions.

The Aztec fish fauna (Devonian) of Southern Victoria Land: Evolutionary and biogeographic significance

Abstract The Aztec fauna is one of the most diverse fossil vertebrate faunas from Antarctica, with over 30 taxa belonging to the four major groups of jawed fishes (acanthodians, placoderms, osteichthyans, chondrichthyans), and one group of agnathans (thelodontids). Biogeographically the strongest affinity is with eastern Australia, as part of the East Gondwana Province. Biostratigraphically the Aztec succession includes probably the youngest known non-marine thelodont agnathans at the base, and the oldest known phyllolepids at the top. This transition, and an association of species of Turinia and Bothriolepis , is not seen in any other sequence. Most or all of the Aztec sequence is regarded as Givetian (late Middle Devonian) in age. The Aztec fauna contains older and/or more primitive representatives of several groups typical of younger (late Palaeozoic) strata in Euramerica, suggesting a biotic dispersal episode between Gondwana and Euramerica in the Late Devonian. Centres of origin and dispersal hypotheses in Palaeozoic biogeography can be related to disappearance of faunal barriers as manifested in different stratigraphic ranges in different areas. The change for Devonian vertebrates from the endemic patterns seen in the Early Devonian to the close similarity of Famennian faunas between East Gondwana and Euramerica suggests disappearance of a major faunal barrier in the Late Devonian, presumably the ocean between Gondwana and Euramerica.

Is female-biased sex determination in lemmings caused by staying together for warmth?

Two kinds of lemming have evolved genetic modifications of sex determination that result in surpluses of daughters. Female-biased sex ratios can evolve when mating occurs between neighbouring individuals who are more related than if mating occurred randomly. Two proposed sources of such ‘viscous’ gene flow in lemmings arc cyclical changes in population density and mosaic habitat. Alternatively, perhaps cold climate favours winter aggregation and inhibits the dispersal of winter-born offspring, which would Nature and mate with close relatives; dispersal and outbreeding would occur during the warm months. Thus the episodes of dispersal and inbreeding would be seasonal rather than density-dependent and the supposition of discontinuous habitat is obviated.

Metallogeny and tectonic development of the Tasman Fold Belt System in New South Wales

The northwestern corner of New South Wales consists of the paratectonic Late Proterozoic to Early Cambrian Adelaide Fold Belt and older rocks, which represent basement inliers in this fold belt. The rest of the state is built by the composite Late Proterozoic to Triassic Tasman Fold Belt System or Tasmanides. In New South Wales the Tasman Fold Belt System includes three fold belts: (1) the Late Proterozoic to Early Palaeozoic Kanmantoo Fold Belt; (2) the Early to Middle Palaeozoic Lachlan Fold Belt; and (3) the Early Palaeozoic to Triassic New England Fold Belt. The Late Palaeozoic to Triassic Sydney—Bowen Basin represents the foredeep of the New England Fold Belt. The Tasmanides developed in an active plate margin setting through the interaction of East Gondwanaland with the Ur-(Precambrian) and Palaeo-Pacific plates. The Tasmanides are characterized by a polyphase terrane accretion history: during the Late Proterozoic to Triassic the Tasmanides experienced three major episodes of terrane dispersal (Late Proterozoic—Cambrian, Silurian—Devonian, and Late Carboniferous—Permian) and six terrane accretionary events (Cambrian—Ordovician, Late Ordovician—Early Silurian, Middle Devonian, Carboniferous, Middle-Late Permian, and Triassic). The individual fold belts resulted from one or more accretionary events. The Kanmantoo Fold Belt has a very restricted range of mineralization and is characterized by stratabound copper deposits, whereas the Lachlan and New England Fold Belts have a great variety of metallogenic environments associated with both accretionary and dispersive tectonic episodes. The earliest deposits in the Lachlan Fold Belt are stratabound Cu and Mn deposits of Cambro-Ordovician age. In the Ordovician Cu deposits were formed in a volcanic are. In the Silurian porphyry CuAu deposits were formed during the late stages of development of the same volcanic are. Post-accretionary porphyry CuAu deposits were emplaced in the Early Devonian on the sites of the accreted volcanic arc. In the Middle to Late Silurian and Early Devonian a large number of base metal deposits originated as a result of rifting and felsic volcanism. In the Silurian and Early Devonian numerous SnW, Mo and base metal—Au granitoid related deposits were formed. A younger group of MoW and Sn deposits resulted from Early—Middle Carboniferous granitic plutonism in the eastern part of the Lachlan Fold Belt. In the Middle Devonian epithermal Au was associated with rifting and bimodal volcanism in the extreme eastern part of the Lachlan Fold Belt. In the New England Fold Belt pre-accretionary deposits comprise stratabound Cu and Mn deposits (pre-Early Devonian): stratabound Cu and Mn and ?exhalite Au deposits (Late Devonian to Early Carboniferous); and stratabound Cu, exhalite Au, and quartz—magnetite (?Late Carboniferous). S-type magmatism in the Late Carboniferous—Early Permian was responsible for vein Sn and possibly AuAsAgSb deposits. Volcanogenic base metals, when compared with the Lachlan Fold Belt, are only poorly represented, and were formed in the Early Permian. The metallogenesis of the New England Fold Belt is dominated by granitoid-related mineralization of Middle Permian to Triassic age, including SnW, MoW, and AuAgAs Sb deposits. Also in the Middle Permian epithermal AuAg mineralization was developed. During the above period of post-orogenic magmatism sizeable metahydrothermal SbAu(W) and Au deposits were emplaced in major fracture and shear zones in central and eastern New England. The occurrence of antimony provides an additional distinguishing factor between the New England and Lachlan Fold Belts. In the New England Fold Belt antimony deposits are abundant whereas they are rare in the Lachlan Fold Belt. This may suggest fundamental crustal differences.

Density/Height Profile and Long-range Dispersal of First-instar Gypsy Moth (Lepidoptera: Lymantriidae)

The density/height profile of first-instar gypsy moth, Lymantria dispar (L.), was determined by sampling with airplanes up to 800 m above a moderately dense population of the moth in forests around Driftwood, Pa. A model is proposed suggesting that these flightless larvae were lifted there by atmospheric turbulence and that horizontal movement of convective cells due to wind could transport the larvae up to 19 km in a single dispersal episode. Rapid expansion of the gypsy moth population range along the Appalachian Mountains can be explained by atmospheric transport of a small proportion of the larval population.

On the moulding of population viscosity by natural selection

In this paper, we explore the conjoint evolution of dispersal and social behaviour. The model investigated is of a population distributed over a number of sites each with a carrying capacity of two adults and an episode of dispersal in the juvenile stage. The fertilities are governed by whether an individual and its neighbour are selfish or co-operative. It is shown that the best dispersal strategy for the co-operative genotype always involves lower levels of dispersal; and further that ecological conditions favouring low levels of dispersal increase the selective advantage of a co-operative genotype. Given this positive feedback, we suggest that in any taxon viscosity and co-operativity will tend to be correlated and bimodally distributed. Hence we predict the existence of two kinds of animal societies; viscous and co-operative (e.g. quasi-social wasps such as Mischocyttarus), and non-viscous and selfish (e.g. communal sphecid wasps such as Cerceris), and relatively few social groups with intermediate levels of co-operativity and viscosity. We also suggest that when one of the two sexes disperses, it will be the sex with lower potential for co-operative behaviour.

Cotton Rat Invasion of Sand Pine Scrub Habitat

Cotton rats ( Sigmodon hispidus ) invaded sand pine scrub in central Florida in 1974. One episode of dispersal was observed in January–March and a second commenced in May and ceased in November. Invasion was not observed on three study grids during 1973, 1975, or 1976. Small numbers of immigrant cotton rats settled on the grids during 1974 and breeding took place; however, 60% of the individuals tagged and released were not recaptured. These latter animals were presumed to have continued to disperse. Supplemental food on one of the study grids did not prevent the decline of cotton rats in 1974. The sand pine scrub appeared to have acted as a dispersal sink for cotton rats which underwent dispersal from more optimal habitat in surrounding pine flatwoods. Dispersing animals were healthy, equally divided between the sexes, and included all weight classes.

Biogeography of Devonian vertebrates

Five faunal provinces based on vertebrates are proposed for Early Devonian time, as follows: a cephalaspid province (Euramerica), an amphiaspid province (Siberia), a tannuaspid province (Tuva); a galeaspid-yunnanolepid province (South China), and a wuttagoonaspid-phyllolepid province (East Gondwana). It is suggested that certain major groups of early vertebrates differentiated in isolation on these continental areas as freshwater environments were invaded, and the interrelationships of some agnathans and placoderms endemic to these areas are analyzed using the cladistic vicariance biogeographic method of Platnick & Nelson (1978). Two possible episodes of biotic dispersal are identified; between Laurentia and Baltica at the end of the Silurian, and between Gondwana and Euramerica during the Late Devonian. Amongst placoderm fishes a Gondwana origin is suggested for Phyllolepis, antiarchs indicate affinity between Gondwana and South China, and euarthrodires may have evolved in Euramerica. Phylogenetic analysis of other test groups is required to resolve conflicting evidence of area interconnections provided by agnathans and placoderms, and to establish the existence of general patterns in the Devonian distribution of endemic vertebrate taxa.

Behavioral Limitation of Round-Tailed Ground Squirrel Density

The two principal hypotheses of behavioral population regulation-territorial limitation and social stress-were tested in a 4-yr study of a high density, free-living population of round-tailed ground squirrels, Spermophilus tereticaudus, in Arizona. After copulation in early March, 22 to 3 1% of the adult squirrels, predominantly d 5, seemed to disperse. In late June, 29 to 45% of the juveniles, predominantly d d again, seemed to disperse. During the 2 yr of intensive study, these dispersal episodes were the major losses from the population, and they were correlated with the onsets of territorial behavior among adults in March and among juveniles in June. This correlation supports the hypothesis that territorial behavior limits population density. However, intensive observation and 2 field experiments involving removal and introduction of juveniles and provision of supplemental food indicate that dispersal was not statistically related to local density, food availability, or aggression. Territorial aggression did not cause emigration of adults or native-born juveniles, though it probably prevented immigration of foreign-born juveniles, during the summer months. Dispersal was not suffi- cient to limit population density increase. There appeared to be an innate sex difference in juvenile dispersal tendency, perhaps including a differential response to food availability and litter size. In 1 yr, spring density was unusually high, and many squirrels were wounded in February and March. The population subsequently showed classical symptoms of physiological stress, including reduced repro- duction and poor survival. This episode supports the hypothesis that high density leads to socially induced physiological stress, which eventually reduces density. However, the stress did not prevent the squirrel density from reaching a much higher level in the following spring. There were relatively few wounds and no signs of stress at the higher density. Physiological stress was not predictably related to denstiy and was not sufficient to limit density increase. In summary, behavioral factors were not sufficient to limit squirrel density when food was abundant and predation rate was low.

Dispersal of first-instar gypsy moth larvae in relation to population quality

Field studies of dispersal by first instar gypsy moth larvae indicate that almost all larvae undergo an initial dispersal episode. However, in laboratory studies large larvae (from large eggs) disperse more frequently than small larvae (from small eggs) in the presence of favored food. Large larvae may be better adapted for dispersal. When larvae encounter unacceptable food or are denied food, larvae disperse more frequently and dispersal by small larvae is nearly as frequent as dispersal by large larvae. Factors affecting egg size may contribute to shifts in dispersal patterns of gypsy moth larvae and distribution of populations.