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1. Floodplain environments are increasingly subject to enhancement and restoration, with the purpose of increasing their biodiversity and returning them to a more 'natural' state. Defining such a state based solely upon neoecological data is problematic and has led several authors to suggest the use of a palaeoecological approach. 2. Fossil Coleopteran assemblages recovered from multiple palaeochannel fills in south-west England were used to investigate past floodplain and channel characteristics during the mid- to late-Holocene. Ordination of coleopteran data was performed using Detrended Correspondence Analysis (DCA) and produced clear and discrete clustering. This clustering pattern is related to the nature of the environment in which assemblages were deposited and hence channel configuration and dynamics. 3. The DCA clustering pattern is strongly related to measures of ecological evenness, and a strong relationship between these indices and the composition of the water beetle assemblage within samples was revealed. Repeating the ordination with presence-absence data results in a similar pattern of clustering, implying that assemblage composition is crucial in determining cluster placement. 4. As assemblage composition is primarily a function of floodplain topography and hence disturbance regime, we attempt to relate these data to the Intermediate Disturbance Hypothesis (IDH). A significant positive correlation was found between ecological diversity (Shannon's H') and Axis 1 of all ordinations in predominantly aquatic assemblages.  相似文献   

3.
Eutrophication is a major problem for shallow lakes in the U.K. lowlands. Over the last few decades issues relating to the management and restoration of these lakes have come to the fore, including the need to develop methods for establishing realistic restoration targets. One group of lakes for which restoration is a key concern are the Attenborough Ponds in the English Midlands, an interconnected series of shallow, nutrient-enriched gravel-pit lakes. In November 1972, the highly polluted River Erewash was diverted into the Attenborough Ponds. To determine the ecological effects of this event and to establish restoration goals for this lake system, two complimentary studies were undertaken here; (i) monitoring of the ecology and sedimentary representation of diatom communities in a disturbed lake connected to the R. Erewash (Tween Pond: high nutrient concentrations, no submerged macrophytes) and a relatively undisturbed lake isolated from the R. Erewash (Clifton Pond: lower nutrient concentrations, abundant submerged macrophytes), and; (ii) examination of the sedimentary diatom record in a short sediment core collected from the chosen disturbed lake (Tween Pond). The species composition, seasonality, relative productivity and sedimentary representation of planktonic and periphytic diatom communities were very different in Tween and Clifton Ponds. In Tween Pond two major phases of planktonic diatom production/sedimentation were observed during March–May and July–September. By contrast, planktonic diatoms were restricted to a single spring peak in Clifton Pond and after May there was a switch to the dominance of periphytic diatoms associated with the development of submerged macrophytes. The 1972 diversion of the River Erewash was clearly reflected in the sedimentary diatom record from Tween Pond, by; (i) the abrupt shift to planktonic diatom dominance, and; (ii) increasing percentages of late summer–autumn associated planktonic diatom species. These changes suggest both significant nutrient enrichment and the switch from submerged macrophyte to phytoplankton dominance. Numerical matching of pre-1972 diatom assemblages with surface sediment assemblages in the gravel-pits using Principal Components Analysis (PCA) and a squared chord distance measure revealed no close analogues. Nevertheless, similarities between; (i) percentages of different diatom habitat and seasonality groups, and; (ii) pre-1972 and contemporary macrophyte survey data suggested that Clifton Pond is probably a good pre-diversion analogue for habitat structure (e.g. macrophyte biomass, composition and architecture) and phenology (e.g. diatom and macrophyte seasonality) in the Ponds. The practical value of combining space-for-time substitution and palaeoecological approaches in restoration ecology studies of shallow lakes is discussed.  相似文献   

4.
Sheila Hicks 《Aerobiologia》1992,8(2):220-230
Summary Aerobiology involves the study of particles present in the air. Of these, palaeoecologists are most interested in pollen and spores, specially those which come to rest on the surface of the ground. Pollen sampling in palaeoecological studies, therefore, uses a pollen trap which is placed at ground level and the sampling period is a whole season or year.Using examples from northern Finland, for each vegetation type sampled, a pollen analogue is obtained which comprises percentage presence and deposition values cm–2 year–1 for the pollen types recorded. This analogue is used to interpret pollen assemblages from the past. In general, the tree pollen provides information of the regional situation and the herbaceous pollen of specific local conditions. Both natural and man-made environments are considered. The absolute deposition values are often crucial in distinguishing between two plant communities with otherwise similar percentage pollen values. They can also be used to follow the location of the boundaries between different vegetation types through time. Seasonal records of pollen deposition are sometimes useful in distinguishing the different source of the pollen reaching a specific site. In certain instances, annual pollen deposition values can be employed to estimate the rate of accumulation of a peat deposit. In addition, long series of annual pollen deposition values can be compared with meteorological data for the same period to test for trends and correlations. This, potentially, allows both a more detailed interpretation of past climatic changes and provides a basis for models predicting futures ones.  相似文献   

5.
The North Sea is seriously threatened by a variety of pollution sources. Terrestrially derived effluents are causing extensive environmental damage and changes to ecosystems of both the offshore and coastal waters. Coastal and estuarine communities are being lost to reclamation projects, and there is the future threat of rising sea level associaed with global warming. The spatial and temporal extent of recent anthropogenic changes are largely unknown due to the paucity of background information. The possible role of palaeoecological methodology in providing ‘reference levels’ against which current status can be compared, and their importance for restoration and policy decisions, are presented. The usefulness of diatoms as environmental indicators is illustrated. The extent of natural and anthropogenic changes on coastal habitats are demonstrated by reference to the Holocene evolution of the coastline of The Netherlands. Possible profitable areas for further research are outlined,e.g. a diatom nutrient calibration data set for shallow marine embayments.  相似文献   

6.
The holdfast (attachment structure) is the most understudied aspect of the palaeoecology of the endoskeleton of fossil crinoids. A new collection of well-preserved holdfasts from a recently reopened quarry at Hunninge, Gotland, in Homerian (upper Wenlock) strata includes several morphologies. The most common are terminal dendritic radicular holdfasts (TDRHs) that may be derived from the cladid Ennallocrinus d'Orbigny. These have a consistent morphology of five, equally spaced, long radices that spread across the sea floor. These crinoids were gregarious, and TDRHs in a group commonly show the same radice orientations. The radices have a large axial canal compared with those of modern crinoids; each included, at least, nervous tissues. Taken together, these features suggest that, apart from attachment, these distinctive TDRHs may have served a sensory function. Other holdfasts in this assemblage also show monospecific aggregations, perhaps suggesting biochemical attraction such as that shown by certain other sessile invertebrates such as barnacles.  相似文献   

7.
A sample of marine invertebrates from the Late Triassic Cassian Formation (north Italy) yielded one of the most diverse Early Mesozoic fossil assemblages ever reported (c. 170 species). The assemblage was found in basin clays, but was transported from nearby carbonate platforms as indicated by fragmentation, microbial encrustation and the presence of coated grains and ooids. Most of the specimens are small (< 1 cm) reflecting both, small adult sizes and size sorting during transport. Rarefaction analysis suggests that diversity of surface collection and bulk sampling is the same. However, rank abundance, species richness and taxonomic composition differ strongly according to sampling method. Low‐grade lithification of the sediments is the main reason that high diversity can be recognized, because it facilitates disaggregation and finding of small molluscs. Sample standardization shows that the studied assemblage is much more diverse than known Early Triassic assemblages. However, its diversity is similar to that of Anisian assemblages. This suggests that recovery from the end‐Permian mass‐extinction was quite advanced in the Middle Triassic and alpha‐diversity remained high until the Late Triassic. According to current models, Early Triassic and Anisian faunas match the niche overlap phase of recovery during which diversity is built up by increasing alpha‐diversity, whereas beta‐diversity rises slowly. Subsequently, habitat width of species contracts because of increasing competition, making beta‐diversity the principal drive of overall diversity increase. The diversity pattern of various Late Triassic Cassian associations meets the predictions for the transition from the niche overlap to the habitat contraction phase.: Triassic, Cassian Formation, palaeoecology, diversity, mollusc dominance.  相似文献   

8.
Modern hermit crabs form associations with many organisms which encrust, bore into, or cohabit the living chambers of gastropod shells occupied by the crabs. Among these hermit crab symbionts are bryozoan species which develop massive, commonly multilayered, colonies encrusting hermit crab shells. These colonies extend the living chamber of the crab through a characteristic process of helicospiral tubular growth originating from the shell aperture. The scant information available on the ecology of Recent bryozoan‐hermit crab symbioses is reviewed. Symbioses have been recorded from intertidal to upper slope environments, and from tropical to cold temperate zones. None of the hermit crab species are obligatory symbionts of bryozoans, and the majority of the modern bryozoan species involved are also not obligatory symbionts. Fossil examples always lack the hermit crabs, which have a poor fossilization potential; however, the distinctive tubular growth pattern and other features of the bryozoans enable recognition of ancient examples of the symbiosis. The earliest inferred associations between bryozoans and hermit crabs date from the Mid Jurassic, but associations remained uncommon until the Neogene. A remarkably wide taxonomic diversity of Recent and fossil bryozoans are known or inferred symbionts of hermit crabs. The broad evolutionary pattern of the association demonstrates multiple originations of the symbiosis by bryozoans belonging to at least 5 cyclostome and 12 cheilostome families. Only the Miocene‐Recent cheilostome family Hippoporidridae has an evolutionary history closely tied to symbiosis with hermit crabs. There is no evidence for coevolution.  相似文献   

9.
Many techniques have been developed to estimate species richness and beta diversity. Those techniques, dependent on sampling, require abundance or presence/absence data. Palaeontological data is by nature incomplete, and presence/absence data is often the only type of data that can be used to provide an estimate of ancient biodiversity. We used a simulation approach to investigate the behaviour of commonly used similarity indices, and the reliability of classifications derived from these indices, when working with incomplete data. We drew samples, of varying number and richness, from artificial species lists, which represented original life assemblages, and calculated error rates for classifications of the parent lists and samples. Using these results, we estimated the Minimum Sample Richness (MSR) needed to achieve 95% classification accuracy. Results were compared for classifications derived from several commonly used similarity indexes (Dice, Jaccard, Simpson and Raup–Crick). MSR was similar for the Dice, Jaccard and Simpson indices. MSR for the Raup–Crick index was often much lower, suggesting that it is preferable for classifying patchy data, however the performance of this index was less stable than the other three in the simulations, which required an even lower MSR. MSR can be found for all presence/absence data from the contour graphs and equations as long as the absolute species richness and the beta diversity can be estimated.  相似文献   

10.
The functional morphology of Levitusia , based principally upon the type-species L. humerosa (J. Sowerby), is described and compared with living brachiopods. It is concluded that Levitusia species lived in tropical Dinantian seas with agitated waters and at shallow depths, perhaps no more than 50 m. The lithologies in which specimens are found support this view and indicate the need for some revision in our interpretations of some Dinantian sedimentary environments. Further evidence for the synonymy of L. christiani (de Koninck) in L. humerosa is presented.  相似文献   

11.
There is growing interest in the integration of macroecology and palaeoecology towards a better understanding of past, present, and anticipated future biodiversity dynamics. However, the empirical basis for this integration has thus far been limited. Here we review prospects for a macroecology–palaeoecology integration in biodiversity analyses with a focus on marine microfossils [i.e. small (or small parts of) organisms with high fossilization potential, such as foraminifera, ostracodes, diatoms, radiolaria, coccolithophores, dinoflagellates, and ichthyoliths]. Marine microfossils represent a useful model system for such integrative research because of their high abundance, large spatiotemporal coverage, and good taxonomic and temporal resolution. The microfossil record allows for quantitative cross‐scale research designs, which help in answering fundamental questions about marine biodiversity, including the causes behind similarities in patterns of latitudinal and longitudinal variation across taxa, the degree of constancy of observed gradients over time, and the relative importance of hypothesized drivers that may explain past or present biodiversity patterns. The inclusion of a deep‐time perspective based on high‐resolution microfossil records may be an important step for the further maturation of macroecology. An improved integration of macroecology and palaeoecology would aid in our understanding of the balance of ecological and evolutionary mechanisms that have shaped the biosphere we inhabit today and affect how it may change in the future.  相似文献   

12.
Latest Jurassic–earliest Cretaceous hydrocarbon seeps from Spitsbergen, Svalbard, are known to contain unusual fauna, lacking most of the species characteristic for roughly coeval seep deposits. This study summarizes and analyses the fauna from 16 seep carbonate bodies from Spitsbergen to explain its composition. The seeps formed in a shallow epicontinental sea with widespread deposition of fine‐grained, organic‐rich sediments. They are spread over a relatively large area and are positioned roughly in the same interval, indicating seepage over extensive areas of the palaeo‐Barents Sea. The seep fauna is very species rich and with low dominance, comprising 54 species, with a composition similar to that of Jurassic–Cretaceous normal‐marine environments of other Boreal seas. Seep‐restricted fauna is not abundant and is represented by four species only. Hokkaidoconchids and possible siboglinid worm tubes characteristic for high sulphide fluxes are rare. Apart from seep‐restricted sulphide‐mining lucinid and thyasirid bivalves, chemosymbiosis was also a source of nourishment for background solemyid and nucinellid bivalves, all of which take sulphide from infaunal sources. This all suggests a relatively weak sulphide flux. The high diversity and low dominance of the fauna and significant richness and abundance of background species is typical for shallow water seeps.  相似文献   

13.
Knowledge of the morphology, taxonomy, physiology, ecology and palaeoecology of the filamentous freshwater diatom Aulacoseira subarctica (O. Müller) Haworth is reviewed from the literature. The species is widely distributed across Northern Europe, Scandinavia and North America, Japan, China, Australia and New Zealand, in the last usually as fo. subborealis. The species is rare in the Tropics and positive identifications are lacking for Africa. The importance of correct identification is stressed because literature reports of its occurrence often need revision. A comparison is made between two sites with detailed records of its seasonal cycle. Blelham Tarn, England, is a clear-water seasonally stratified lake; Lough Neagh, Northern Ireland, is a very large turbid and unstratified lake. In spite of their contrasting characters, Aulacoseira subarctica succeeds in both lakes. In Blelham Tarn, there is a strong seasonal cycle of growth, dormancy on the sediment and further growth on re-suspension. In Lough Neagh, there is a large deposition of cells at the end of the spring bloom but few of these cells survive grazing by benthic animals. Fossil occurrences are summarized from the species’ initial appearance in the Pliocene and its later occurrence during the last inter-glacial (Eemian) deposits in NW Europe, where it is an important ecological indicator. Palaeolimnological studies are reviewed to describe the contemporary distribution and ecological preference of this widespread diatom. Aulacoseira subarctica usually appears in response to moderate increases in nutrients but is disadvantaged by further enrichment. In Blelham Tarn the species only became abundant during the twentieth century, reaching a peak between 1930 and 1960. With nutrient enrichment, abundance has declined. In Lough Neagh, it has been present for much of the post-glacial period and still thrives today in spite of the highly enriched state of the lake. The species has disappeared from many eutrophic lakes and its sustained existence in Lough Neagh is anomalous. Lough Neagh may continue to offer a favourable habitat in spite of the poor light climate because the water depth, lack of stratification and long water retention time reduce losses to a sustainable level.  相似文献   

14.
Lophialetidae is an extinct group of endemic Asiatic tapiroids that are widely distributed in the Eocene sediments of Asia. Schlosseria magister and Lophialetes expeditus are the most abundant species in this family. However, their dietary and ecological characteristics are largely unknown. For the first time, we reconstruct the palaeodiet and habitat of these two lophialetids using a combination of mesowear and stable carbon isotope analysis of fossil teeth excavated from the Erlian Basin, China. Mesowear analysis (n = 141) suggests that the dietary structure of S. magister and L. expeditus shifted from less to more abrasive diets from ~52 to ~42 Ma. Stable carbon isotope analysis (n = 137) suggests that the habitats of S. magister and L. expeditus became drier and/or more open through time. The dietary shifts of the two lophialetids are consistent with evident changes in habitat. The changes in the diet and habitat were probably related to global climate change during that time period. The gradual drop in global temperatures during the early–middle Eocene led to a drier and more open terrestrial ecosystem in the Erlian Basin, probably resulting in changes in floral composition of the environment inhabited by S. magister and L. expeditus. Hence, herbivores highly susceptible to vegetation modification had to develop new resource exploitation strategies to adapt to these changes. Schlosseria magister, considered to be the sister-group of L. expeditus and with a low level of ecological flexibility, was unable to adapt to the habitat changes finally becoming extinct at ~45 Ma.  相似文献   

15.
Abstract: Nautiloids of the superfamily Rutoceratoidea from the late Emsian (late Early Devonian) of the Prague Basin (Czech Republic) are commented upon. Species recognized include the hercoceratids Hercoceras mirum, H.? transiens, Ptenoceras proximum, P. nudum, P. minusculum and Anomaloceras anomalum, as well as the rutoceratids Adelphoceras bohemicum, Homoadelphoceras devonicans, Pseudorutoceras bolli and Goldringia? devonicans. In addition, four new species are described: Parauloceras regulare sp. nov., Roussanoffoceras chlupaci sp. nov., Otomaroceras sp. nov. and Goldringia sp. nov. Morphology and distribution patterns of Pragian and late Emsian rutoceratoid faunas from the Prague Basin are compared. They show that an increased diversity was accompanied by a higher level of specialization of rutoceratoids, which manifested itself in low abundance, increased facies dependence and greater variation in shell size during the Early Devonian. The evolution of sculpture and a contracted aperture, both regarded as protective adaptive features, was also examined, but no adaptive trend towards more pronounced sculpture and constriction of the aperture was found to have occurred in the Early Devonian. A more distinctive sculpture was, however, observed in shallow‐water assemblages of P. proximum in comparison with deeper‐water faunules, and two additional cephalopod species were examined in order to obtain comparative data. The presence of distinct sculpture patterns in coeval shallow‐ and deeper‐water assemblages suggests limited migration between them and consequently reflects some degree of territoriality in Devonian nautiloids. New data on early shell development in P. proximum are presented. During the Chote? Event, rutoceratoid generic diversity dropped dramatically, one family became extinct and the Early Devonian diversification of the group came to an end. The recovery of nautiloid faunas was slower than that of other cephalopods and associated, unrelated invertebrates. The absence of change in abundance patterns between Pragian and late Emsian rutoceratoid faunas, i.e. prior to and subsequent to ammonoid radiation, suggests that the appearance and radiation of the latter group in the early Emsian did not affect the structure of nautiloid assemblages, i.e. these two clades did not occupy the same niches.  相似文献   

16.
Comparisons of morphologies and modes of life of the Ordovician fossil Tetradium Dana, 1846, with chaetetid sponges, tabulate corals, and Recent rhodophytes show that several defining Tetradium characteristics are incompatible with its chaetetid and tabulate classifications. All Tetradium characteristics fit a rhodophyte identification, however, as a calcified uniaxial corticated florideophyte. It is argued from functional morphology that the fundamental subsquare cross-section of the Tetradium tube is an adaptation for close packing, which implies that the basic growth form is compact and should have developed where conditions were optimal. More open forms are derived from it and probably occupied less-favourable environments.
Palaeoecological studies from the Ottawa Valley, Canada, show that the Tetradium growth form is correlated with environmental stress and became more open as salinity increased: i.e. the compact T. fibratum form lived in normal marine conditions, radiating bundles of T. cellulosum tubes in low to middle hypersalinity and single T. syringoporoides tubes in high hypersalinity. Different Tetradium growth forms from the study area are phenotypic variants of a single species, rather than different species and genera. Therefore, classifications that divide Tetradium into different species and genera based only on growth form have no biological basis. There is little evidence of interspecific interactions with Tetradium .  相似文献   

17.
A humerus and a coracoid from the Early Eocene Wasatch Formation in the Washakie Basin of south‐western Wyoming are the oldest materials (by ~2 million years) of the pelecaniform Limnofregata (Aves) and represent a new large species, Limnofregata hutchisoni sp. nov. This fossil is the oldest known member of the frigatebird lineage. Other than its large size relative to Limnofregata azygosternon and Lhasegawai, the new material is very similar morphologically to other known Limnofregata specimens. The size of this new species is comparable to the largest living species (e.g. Fregata minor and Fregata magnifiscens) and much larger than the two described species of Limnofregata. This fossil indicates that the hard minimum date previously advocated for molecular calibration of the split between Fregatidae and Suloidea is an underestimate by approximately two million years. The presence of early pelecaniform bird lineages (represented by Limnofregata and Masillastega) in limnic ecosystems prior to their known occurrences in marine deposits/habitats appears to indicate that some clades of pelecaniform birds may have undergone an evolutionary transition from freshwater to marine habitats in a pattern reminiscent of what has been suggested during the evolution of pinnipeds or that their palaeoecology included broader niches ranging across a variety of aquatic habitats. That transition in habitat occupation and the origin of many of the characteristic biological aspects present in the crown frigatebird clade likely occurred during a significant temporal gap (> 45 million years) in the fossil record of the frigatebird lineage after these earliest occurrences in the Early Eocene and before the oldest records of the extant Fregata species in the Pleistocene.  相似文献   

18.
Type and referred specimens of Coniasaurus crassidens from the Lower Chalk (Upper Cretaceous; Cenomanian) of southeast England, are re-described. The type is a left maxilla associated with 14 dorsal vertebrae. The maxilla is elongate, bears a low ascending process, and has a long and posteriorly positioned external narial margin. The first maxillary tooth is pointed and bears a groove on the labial face; more posterior maxillary teeth are increasingly rounded and bulbous, and have a single groove on the labial face. Mandibles assigned to Coniasaurus cf. C. crassidens possess teeth of similar form; mandibular bones include the dentary, splenial, angular, coronoid, prearticular, and surangular. A number of features show important similarities to later mosasaurs and contemporaneous groups such as dolichosaurs. These new data provide a very different picture of coniasaurs and their mode of life in the early Upper Cretaceous. The functional morphology of coniasaur teeth is unique and shows occlusion between the lingual platforms of the upper teeth with the crowns of the lower teeth. Coniasaurs can be considered as analgous to small sauropterygians in terms of general morphology, habitats, and trophic structure. Coniasaur distributions in the Cenomanian and Turonian of Europe and North America are similar to the palaeobiogeographic patterns of other organisms living in the Tethys and SuperTethys Seaway.  相似文献   

19.
Peat layers are well represented in the Holocene coastal deposits of the southern North Sea and provide evidence as to the extent and nature of the fens and bogs that occupied the region in the mid and late Holocene. While natural processes contributed to their demise, without human interference extensive areas of peatland would remain. We review the characteristics of the vegetation of these peatlands along with the processes that influenced their development. Spatial and temporal trends are explored through the use of palaeogeographic maps from three areas: the East Anglian Fenland, the Romney Marsh area and the Netherlands. The palaeoecological evidence indicates that eutrophic vegetation promoted by rising relative sea level (RSL) dominated in the mid Holocene, with a trend towards the development of oligotrophic and ombrotrophic vegetation in the late Holocene as the rate of RSL rise declined. Nevertheless, areas of eutrophic vegetation appear capable of long-term stability with areas of fen woodland and herbaceous fen persisting at some locations for several thousand years in the mid and late Holocene. Areas of active peat growth in the region are now largely confined to small remnants within agricultural settings. To retain their characteristic biodiversity these remnants have been managed using traditional practices, although their small size and fragmented distribution limits their biodiversity value. Biodiversity concerns and the ecosystem services peatlands provide, notably carbon sequestration and flood attenuation, underlie recent restoration projects. These efforts are likely to receive additional impetus as a consequence of rising water levels, given projected rates of RSL rise. Future large-scale restoration can be informed by a greater understanding of the processes that formed and sustained coastal peatlands in the past. We identify advances in palaeoenvironmental research that could enhance restoration efforts and help maximise the ecosystem services delivered through such projects.  相似文献   

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