Taxonomic Diversity Gradients Research Articles

Global dynamics of dispersal and diversification among passerine birds

Explaining global variation in geographic and taxonomic diversity gradients represents a central focus of macroecology and macroevolution. Ultimately, these diversity gradients have been generated over deep timescales as a consequence of historical variation in rates of dispersal, diversification, and the capacity of different geographic areas to preserve taxa through time. Here, I assess the relationships among these processes, to elucidate the causes of geographic and taxonomic variation in species richness among the most speciose order of birds: the Passeriformes (c. 6,500 species). To achieve this, I comparatively analyze phylogenetic, distributional and eco-morphological trait data collated at broad taxonomic and spatial scales. The results of my analyses strongly support the prevalence of historical dispersal events across large geographic scales, in addition to spatiotemporal variation in diversification rates. These findings reflect that some lineages and some geographic regions (notably tropical mountain regions and island archipelagoes) generate and maintain a much larger amount of species diversity than others. In addition, the evolution of particular life-history and eco-morphological traits, specifically pair breeding systems, and higher wing aspect-ratios, increased rates of range expansion and diversification. In summary, geographic variation in historical diversification combined with differences in the capacity of lineages to colonize new areas, determines spatial and taxonomic differences in passerine species richness.

Triassic marine biogeography constrains the palaeogeographic reconstruction of Tibet and adjacent areas

Palaeobiogeographic studies can provide valuable constraints on palaeogeographic reconstructions. The Triassic marine bivalves from Tibet and adjacent areas, including the tectonic units of Qamdu, Qiangtang, Lhasa, Himalaya, North China, Songpan Ganzi, South China, Simao, and Indochina, and from three relevant and representative Triassic biogeographic regions, i.e. the western Tethys, Japan, and New Zealand, are rigorously analysed at the generic level. Analyses of diversity changes of the six clades and of the total bivalve fauna against latitude in the studied areas reveal that the diversity is highest along the northern margin, and gradually declines southward to the southern margin of the Tethyan Ocean during the Triassic. A dynamic Triassic palaeobiogeographic pattern is presented applying cluster analysis, and analysis of taxonomic diversity gradients and endemism to four time intervals, i.e. the Early Triassic Epoch, the Anisian, Carnian, and the Norian–Rhaetian. Three provinces, i.e. the North-Eastern Tethyan (NE), South-Eastern Tethyan (SE), and the Himalayan (HI) province, are recognised from the Early Triassic to the Carnian. The HI joined the SE and a new province, the Arctic-Pacific (AP) formed in the Norian–Rhaetian in the study area. The dynamic palaeobiogeographic pattern is used in the palaeogeographic reconstruction of the study area, such as position and timing of collision processes of the Palaeo-Tethys and Meso-Tethys during the Triassic.

Evolution of taxonomic diversity gradients in the marine realm: a comparison of Late Jurassic and Recent bivalve faunas

We still have much to learn about the evolution of taxonomic diversity gradients through geologic time. For example, have latitudinal gradients always been as steep as they are now, or is this a phenomenon linked to some form of Cenozoic global climatic differentiation? The fossil record offers potential to address this sort of problem, and this study reconstructs latitudinal diversity gradients for the last (Tithonian) stage of the Jurassic period using marine bivalves. At this time of relative global warmth, bivalves were cosmopolitan in their distribution and the commonest element within macrobenthic assemblages.Analysis of 31 regional bivalve faunas demonstrates that Tithonian latitudinal gradients were present in both hemispheres, though on a much smaller magnitude than today. The record of the Northern Hemisphere gradient is more complete and shows a steep fall in values at the tropical/temperate boundary; the Southern Hemisphere gradient exhibits a more regular decline in diversity with increasing latitude.Tithonian latitudinal gradients were underpinned by a tropical bivalve fauna that comprises almost equal numbers of epifaunal and infaunal taxa. The epifaunal component was dominated by three pteriomorph families, the Pectinidae, Limidae and Ostreidae, that may be regarded as a long-term component of tropical bivalve diversity. Of the mixture of older and newer “heteroconch” families that formed the bulk of the infaunal component, the latter radiated spectacularly through the Late Cretaceous and Cenozoic to dominate tropical bivalve faunas at the present day. This pulse of heteroconch diversification, which was a major cause of the steepening of the bivalve latitudinal gradient, provides important evidence that rates of speciation may be negatively correlated with latitude.Nevertheless, we cannot exclude the possibility that elevated extinction rates in the highest latitudes also contributed to the marked steepening of bivalve latitudinal gradients over the last 150 Myr. Rates of extinction within epifaunal bivalve taxa appear to have been higher in these regions through the Cretaceous period, but this was largely before any significant global climatic deterioration. Infaunal bivalve clades have had differential success over this time period in the polar regions. Whereas, in comparison with the Tropics, heteroconchs are very much reduced in numbers today, the anomalodesmatans are much better represented, and the protobranchs have positively thrived. We are beginning to appreciate that low temperature per se may not be a primary cause of elevated rates of extinction. Food supply may be an equally important control on both rates of speciation and extinction; those bivalves that have been able to adapt to the extreme seasonality of food supply have flourished in the polar regions.

Taxonomic diversity gradients through geological time

Abstract. There is evidence from the fossil record to suggest that latitudinal gradients in taxonomic diversity may be time‐invariant features, although almost certainly not on the same scale as that seen at the present day. It is now apparent that both latitudinal and longitudinal gradients increased dramatically in strength through the Cenozoic era (i.e. the last 65 my) to become more pronounced today than at any time in the geological past. Present‐day taxonomic diversity gradients, in both the marine and terrestrial realms, are underpinned by the tropical radiations of a comparatively small number of species‐rich clades. Quite why these particular taxa proliferated through the Cenozoic is uncertain, but it could be that at least part of the explanation involves the phenomenon of evolutionary escalation. This is, in essence, a theory of biological diversification through evolutionary feedback mechanisms between predators and prey; first one develops an adaptive advantage, and then the other. However, there may also have been some form of extrinsic control on the process of tropical diversification, and this was most likely centred on the phenomenon of global climate change. This is especially so over the last 15 my Various Late Cenozoic (Neogene) vicariant events effectively partitioned the tropics into a series of high diversity centres, or foci. It has been suggested that, in the largest of these in the marine realm (the Indo‐West Pacific or IWP centre), a critical patterns of islands acted as a template for rapid speciation during glacioeustatic sea level cycles. The same process occurred in the Atlantic, Caribbean and East Pacific (ACEP) centre, though on a lesser scale. Tropical terrestrial diversity may also have been promoted by rapid range expansions and contractions in concert with glacial cycles (a modified refugium hypothesis). We are beginning to appreciate that an integrated sequence of Neogene tectonic and climatic events greatly influenced the formation of contemporary taxonomic diversity patterns.

Taxonomic diversity gradients through geological time

Taxonomic diversity gradients through geological time

The nature and origin of taxonomic diversity gradients in marine bivalves

AbstractBivalves have been fundamental in developing the understanding of large-scale biodiversity patterns in the marine realm. A new study based on 29 regional bivalve faunas indicates that both latitudinal and longitudinal gradients are not so regular in form as was once imagined. The northern hemisphere latitudinal gradient has a marked step, or inflection, at 20–30°N, and in the southern hemisphere Australia forms a distinct diversity hotspot. A longitudinal gradient runs between a large tropical high diversity focus in the southern China-Indonesia-Australia region and a somewhat smaller one in the Panamic-Caribbean region.It is unlikely that the marked asymmetry in both latitudinal and longitudinal gradients can be explained entirely by either the geometric patterns of species ranges in relation to geographical boundaries (the mid-domain effect) or the operation of contemporary, or equilibrium, factors. The role of history must be considered too, and in some cases this could involve a timescale of at least 60 Ma. For example, Australia has moved progressively northwards throughout the Cenozoic era and this seems to have led to the development of a unique assemblage of both tropical and temperate taxa around the continent. When Australia eventually collided with southeast Asia, some 15 Ma ago, three previously distinct tropical marine faunas were brought into close juxtaposition.However, something else seems to have been involved in order to generate a marked pantropical Neogene diversification event in bivalves and other marine invertebrate taxa. It is striking how this event coincided with a prolonged interval of global cooling and the suggestion has been made that glacioeustatic cycles may have promoted the formation of a tropical species diversity pump. In concert with regular and profound sea-level changes, new taxa were created in marginal tropical regions and then concentrated in certain core regions. These same glacioeustatic cycles may also have served to accentuate the latitudinal limits of many tropical taxa.As the knowledge of large-scale taxonomic diversity gradients in the marine realm has increased, so too has the realization that they are at least the partial products of major historical events.

Evolution of taxonomic diversity gradients in the marine realm: evidence from the composition of Recent bivalve faunas

A major new inventory of living marine Bivalvia (Mollusca) is based on 29 regional faunas. These again pick out strong latitudinal and longitudinal gradients in taxonomic diversity, but there are indications that the patterns are not so regular as previously thought. There are signs of asymmetry between the Northern and Southern Hemisphere latitudinal gradients, with the former tending to be more regular than the latter. Northern gradients are also characterized by a marked inflection at approximately 30°N, and the three Australian provinces seem to form a distinct “hot-spot” in the Southern Hemisphere. The larger of the two tropical high-diversity foci (the southern China-Indonesia-NE Australia one) appears to be much more nearly arcuate in plan view than oval and is closely associated with the world's richest development of coral reefs.A taxonomic and stratigraphic analysis reveals that the steepest latitudinal gradients are associated with the youngest bivalve clades. The most striking pattern is that shown by the heteroconchs, an essentially infaunal taxon that radiated extensively throughout the Cenozoic era. Steep gradients are also characteristic of the relatively young anomalodesmatan and arcoid clades and, somewhat surprisingly, the predominantly epifaunal pteriomorphs. Although the latter taxon falls within an older (i.e., “late Paleozoic-Jurassic”) group of clades, it is apparent that certain elements within it (and in particular the Pectinidae) radiated extensively in the latest Mesozoic-Cenozoic. A small but significant component of the later stages of the adaptive radiation of the Bivalvia comprised epifaunal taxa.The presence of the steepest latitudinal gradients in the youngest clades provides further evidence that the Tropics have served as a major center of evolutionary innovation. Even though some sort of retraction mechanism cannot be completely ruled out, these gradients are most likely the product of primary radiations. Clade history can be an important determinant of contemporary large-scale biodiversity patterns. The markedly lower diversity of some bivalve clades, such as the heteroconchs, in the high-latitude and polar regions may simply reflect the fact that they are not yet fully established there. In a way that is reminiscent of the onshore-offshore radiation of certain benthic marine invertebrate taxa, it may take periods of tens or even hundreds of millions of years for bivalve clades to disseminate fully across the earth's surface.The persistent spread of taxa from low- to high-latitude regions should perhaps come as no great surprise, as the tropical ocean is very much older than either of the polar ones. The late Cretaceous-Cenozoic evolutionary radiation of the Bivalvia was accompanied by a marked deterioration in global climates, and many new groups have yet to be fully assimilated into cool- and cold-water benthic ecosystems.

Marine biodiversity: patterns and processes

Foreword: the value of diversity Crispin Tickell 1. Marine biodiversity in its global context M. H. Williamson 2. Gradients in marine biodiversity J. S. Gray 3. Pelagic biodiversity M. V. Angel 4. Biological diversity in oceanic macrozooplankton: more than counting species A. C. Pierrot-Bults 5. Large scale patterns of species diversity in the deep sea benthos M. A. Rex, R. J. Etter and C. T. Stuart 6. Diversity, latitude and time: patterns in the shallow sea A. Clarke and J. A. Crame 7. High benthic species diversity in deep-sea sediments: the importance of hydrodynamics J. D. Gage 8. Diversity and structure of tropical Indo-Pacific benthic communities: relation to regimes of nutrient input J. D. Taylor 9. Why are coral reef communities so diverse? A. J. Kohn 10. The biodiversity of coral reef fishes R. F. G. Ormond and C. M. Roberts 11. The historical component of marine taxonomic diversity gradients J. A. Crame and A. Clarke 12. Population genetics and demography of marine species J. E. Neigel 13. Discovering unrecognised diversity among marine molluscs J. Grahame, S. L. Hull, P. J. Mill and R. Hemingway 14. Ecosystem function at low biodiversity - the Baltic example R. Elmgren and C. Hill 15. Land-seascape diversity of the US east coast coastal zone with particular reference to estuaries G. C. Ray, B. P. Hayden, M. G. McCormick-Ray and T. M. Smith 16. The development of mariculture and its implications for biodiversity M. C. M. Beveridge, L. G. Ross and J. A. Stewart 17. Protecting marine biodiversity and integrated coastal zone management J. S. H. Pullen 18. Conserving biodiversity in north-east Atlantic marine ecosystems K. Hiscock Index.

Bipolar Molluscs and Their Evolutionary Implications

The phenomenon of bipolarity, one of the major disjunct distribution patterns on the face of the earth, has been investigated repeatedly since the mid-nineteenth century. Running through the many hypotheses that have been put forward to account for its occurrence, it is possible to detect two persistent themes: it is usually interpreted within a dispersal framework, and it is generally believed to be of comparatively recent origin. To many authors, the phenomenon is intimately linked to the Plio-Pleistocene glaciations. Recent palaentological investigations have extablished that bipolarity can now be traced back to at least the Early Jurassic period (i.e. 200 m.y.a.). Here it is well marked in the Pliensbachian stage by a variety of pectinacean bivalve taxa. Further bivalves indicate probable Middle Jurassic examples, but the phenomenon is more clearly seen in the Late Jurassic, especially in the Tithonian stage. At this time, in-oceramid, buchiid and oxytomid bivalve occurrences at northern hemisphere localities such as arctic Canada, N.W. Europe, Siberia, N.E. USSR and Japan can be matched with those in southern South America, Antarctica and Australasia. A striking Early Cretaceous (Aptian-Albian) bipolar pattern for the oxytomid Aucellina may be complemented by several infaunal bivalves, brachiopods and at least one gastropod. There is strong circumstantial evidence to suggest that bipolar molluscs continued to develop through the Cenozoic era. Such is the level of generic and subfamilial differentiation within certain living forms as to suggest that they must be the product of a considerable evolutionary history. It is likely that present-day distribution patterns of prosobranch gastropod groups such as th whelks (Buccinidae), together with certain fissurellids, littorinids, naticids and turrids, can be related to a late Paleogene-early Neogene phase of bipolarity. Many amphitropical taxa, in both the marine and terrestrial realms, have probable late Neogene-Pleistocene origins. It is possible to set the Jurassic and Cretaceous examples of bipolarity within a largely vicariant framework based upon the disintegration of the Pangean supercontinent. In this way the widespread ranges of putative Triassic ancestors were disrupted by tectonic processes in low latitude regions, although it should be emphasized that major climatic and oceanographic changes were almost certainly involved too. Similarly, it is possible to view late Paleogene earl Neogene bipolarity as a vicariant event, but this time with climatic change identified as the single most important agent. Widespread or cosmopolitan distributions are held to have formed during global cool phases (such as the late Eocene-early Miocene) only to be disrupted by global warming (such as in the late early Miocene). It is even possible to view Plio-Pleistocene patterns as, at leat in part, the products of vicariant events caused by rapid temperature and sea level shifts. Clearly, there is an urgent need here for more critical taxonomic data to test these various hypotheses. Phylogenetic studies of groups such as the Mesozoic bivalve superfamily Monotoidea and the Cenozoic Buccinidae, in particular, should constitute future rigorous tests. In so doing, they should also provide much useful information on the relative roles of dispersal and vicariance in promoting global disjunction in marine faunas. Repeated formation of bipolar patterns through geological time may have had important implications for modes of speciation and phenomena such as the origin of taxonomic diversity gradients.

Taxonomic Diversity Gradients of Ectoprocts and Bivalves and Their Geologic Implications

Important attributes of some taxonomic diversity gradients of marine organisms include an “ocean effect” and a “margin effect.” As examples of the former, both ectoprocts and bivalves are twice as diverse in the Pacific Ocean as in the Atlantic Ocean. As examples of the latter, bivalves and, less clearly, ectoprocts have their greatest within-ocean diversity along the western margin of the Pacific and the eastern margin of the Atlantic Ocean. Correct determination of the paleo-equator may be severely hampered unless the data include samples from the Pacific Basin. Although ectoprocts and bivalves are equally diverse at low latitudes, ectoprocts have markedly more species per geographic area at high latitudes.