Effects of extinction on food web structures on an evolutionary time scale

Extinction affected food web structure in paleoecosystems. Recent theoretical studies that examined the effects of extinction intensity on food web structure on ecological time scales have considered extinction to involve episodic events, with pre-extinction food webs becoming established without dynamics. However, in terms of the paleontological time scale, food web structures are generated from feedback with repeated extinctions, because extinction frequency is affected by food web structure, and food web structure itself is a product of previous extinctions. We constructed a simulation model of changes in tri-trophic-level food webs to examine how continual extinction events affect food webs on an evolutionary time scale. We showed that under high extinction intensity (1) species diversity, especially that of consumer species, decreased; (2) the total population density at each trophic level decreased, while the densities of individual species increased; and (3) the trophic link density of the food web increased. In contrast to previous models, our results were based on an assumption of long-term food web development and are able to explain overall trends posited by empirical investigations based on fossil records.     

Extinction of the Archaeocyatha

Archaeocyaths are a group of calcified sponges almost limited to the Early Cambrian. They have undergone a rapid radiation followed by an abrupt decline even more rapid and a total extinction. Rate of this extinction is documented at the generic level and a pattern of extinction is proposed. Changes in palaeotectonics, with the consequent modifications in the environment, and a probable cooling of the climate, are the possible causes of their demise. This extinction involves the disappearance of the first reefs in which Metazoans were, at least partly, implicated.     

The youngest South American rhynchocephalian,a survivor of the K/Pg extinction

Rhynchocephalian lepidosaurs, though once widespread worldwide, are represented today only by the tuatara (Sphenodon) of New Zealand. After their apparent early Cretaceous extinction in Laurasia, they survived in southern continents. In South America, they are represented by different lineages of Late Cretaceous eupropalinal forms until their disappearance by the Cretaceous/Palaeogene (K/Pg) boundary. We describe here the only unambiguous Palaeogene rhynchocephalian from South America; this new taxon is a younger species of the otherwise Late Cretaceous genus Kawasphenodon. Phylogenetic analysis confirms the allocation of the genus to the clade Opisthodontia. The new form from the Palaeogene of Central Patagonia is much smaller than Kawasphenodon expectatus from the Late Cretaceous of Northern Patagonia. The new species shows that at least one group of rhynchocephalians not related to the extant Sphenodon survived in South America beyond the K/Pg extinction event. Furthermore, it adds to other trans-K/Pg ectotherm tetrapod taxa, suggesting that the end-Cretaceous extinction affected Patagonia more benignly than the Laurasian landmasses.     

Bumblebee community homogenization after uphill shifts in montane areas of northern Spain

Widespread alterations in species distribution and abundance as a result of global environmental change include upwards and polewards shifts driven by local extinctions in the south or at lower elevations and colonizations of newly available habitat elements in the north or at higher elevations. Although cumulative changes on patterns of community composition are also expected, studies following a community-level approach are still scarce. Here, we estimate changes in abundance and distribution of bumblebee (Bombus spp.) species over two decades along an elevational gradient to test whether these changes entailed concomitant alterations on patterns of community composition. Bumblebee species showed an overall trend to shift uphill their upper- or lower-elevational boundaries, resulting in narrower elevational ranges from one period to another, coincident with a regional warming of ca. 0.9 °C. Changes in elevational ranges were, however, mainly related to retractions of the lower limit of species distribution, rather than to variations in their upper elevational limit. Species turnover was associated with colonization and extinction events and also with variability in the relative abundance of short-, medium- and long-tongued species along the elevational gradient. Extinctions were especially relevant at medium elevations, while only communities at higher elevations had a positive net outcome between colonization and extinction events. The combination of these effects resulted in the homogenization of bumblebee assemblages, especially between medium and upper elevations. The changes reported in our study strongly match with predictions of global change driving elevational shifts in species distribution and provide the first evidence of elevational changes in bumblebees at both species and community level.     

植物群落和物种多样性动态过程及物种丧失机制放牧试验

在长期严格的沙地草场放牧试验的基础上,通过调查及多种计算和分析,揭示了在放牧条件下植物群落和物种多样性的动态演变规律及物种丧失的机制,主要结论是:(1)在不同稳定的牧压下群落外貌特征及组分发生明显的分异。(2)持续强度放牧使物种消失,α多样性下降,为理解现代物种消亡和灭绝机制提供了部分依据,现代物种的消亡和灭绝正是由于人们对于生境的破坏引起的,生境破坏引起局部地区物种的消失,这种局部消失的不断累加就可能引起物种的消亡和绝灭。(3)放牧试验对物种组成和β多样性的影响表现在时空两个方面。     

Seasonal changes in phospholipid class and class-specific fatty acid composition associated with the onset of freeze tolerance in third-instar larvae of Eurosta solidaginis

Abstract Third-instar larvae of the goldenrod gall fly Eurosta solidaginis (Diptera: Tephritidae) are freeze tolerant in winter. During freezing, cell membranes must compensate for both low temperature and partial dehydration. Documented adaptations to low temperature include increased fatty acid unsaturation and enrichment of cone-shaped phosphatides, both of which inhibit formation of gel phase lipid domains. These changes appear inconsistent with adaptations known to prevent formation of the hexagonal II phospholipid phase at low water activities, namely, increased fatty acid saturation and increased proportions of cylindrical phosphatides. To address these inconsistencies, changes in E. solidaginis phospholipid composition and class-specific fatty acid composition were studied from August to November 2002. Cylindrical phosphatides, mostly phosphatidylcholine (PC), increased transiently and significantly, from 35% of the total to nearly 50%, during the transition from freeze susceptible to freeze tolerant. Monoenes in both PC and phosphatidylethanolamine (PE) represented 35% of total fatty acids in freeze-susceptible larvae but accumulated in PC to 48% and in PE to 42% in freeze-tolerant larvae. Moreover, PC accumulated the most unsaturated acid in this species, 18:3(n-3), to a significantly greater degree than PE. This combination of changes may represent a finely tailored response to both low temperatures and freeze-induced dehydration.     

Control mechanisms of Carboniferous brachiopod zones in eastern Australia

Analysis of the Carboniferous brachiopod zones of eastern Australia shows that they were affected by two major controlling factors - eustatic changes of sea level, and a deterioration (cooling) of climate. Eustatic lowering of sea level caused the removal of the sea from narrow shelf areas and a loss of habitat, and was responsible for the first (late early to middle Visean) of two major episodes of faunal extinction. Subsequent transgression restored a modified warm-water cosmopolitan fauna to the shelf regions. Smaller changes in sea levels were probably responsible for the abrupt disappearance of many species or genera at zonal boundaries and their replacement by a new set of species and genera in succeeding zones. The second major episode of extinction was caused by the lowering of temperature in the latest Visean to early Namurian because of the rapid southerly movement of Australia. The warm water cosmopolitan fauna was eliminated and replaced by the low-diversity Gondwana fauna. Both mechanisms produced particular faunal signatures. The diversity of faunas on either side of the hiatus produced by eustatic lowering of sea level is constant in areas with uniform climatic conditions, and in warm to temperate regions there are low levels of endemism. Faunas associated with a sudden lowering of temperature suffer a significant drop in diversity but are characterized by a high level of endemism.     

Geographic signatures in species turnover: decoupling colonization and extinction across a latitudinal gradient

High latitude communities have low species richness and are rapidly warming with climate change. Thus, temporal changes in community composition are expected to be greatest at high latitudes. However, at the same time traits such as body size can also change with latitude, potentially offsetting or increasing changes to community composition over time. We tested how zooplankton communities (copepods and cladocerans) have changed over a 25–75 year time span by assessing colonization and extinction rates from lakes across an 1800 km latitudinal gradient, and further tested whether species traits predict rates of community change over time. Lake‐level dissimilarity, measured with Sorenson distance, decreased at higher latitudes. This decrease was due to higher colonization rates of cladocerans in lower latitude lakes and consistent extinction rates across the latitudinal gradient. At the species level, colonization increased with regional occupancy, and tended to be higher for smaller bodied, locally abundant, species. Local extinction rates were negatively correlated with local abundance and regional occupancy, but were not influenced by body size. None of these species‐specific characteristics changed predictably with latitude. Contrary to our expectations, low‐latitude zooplankton communities changed more rapidly than high‐latitude communities by becoming more species rich, not by losing species that were historically present. Moreover, colonization and extinction trends suggest that lakes have become increasingly dominated by species with smaller body sizes and that are already common locally and regionally. Together, these findings indicate that rates of species turnover in freshwater lakes across a latitudinal gradient are not predicted by rates of temperature change, but that turnover is nonetheless resulting in trait‐shifts that favour small, generalist species.     

Competition in slow motion: the unusual case of benthic marine communities in the wake of the end‐Permian mass extinction

Changes of community structure in response to competition usually take place on timescales that are much too short to be visible in the geological record. Here we report the notable exception of a benthic marine community in the wake of the end‐Permian mass extinction, which is associated with the microbial limestone facies of the earliest Triassic of South China. The newly reported fauna is well preserved and extraordinarily rich (30 benthic macroinvertebrate species, including the new species Astartella? stefaniae (Bivalvia) and Eucochlis obliquecostata (Gastropoda)) and stems from an environmentally stable setting providing favourable conditions for benthic organisms. Whereas changes in the taxonomic composition are negligible over the observed time interval of 10–100 ka, three ecological stages are identified, in which relative abundances of initially rare species continuously increased at the cost of previously dominant species. Concomitant with the changes of dominant species is an increase in faunal evenness and heterogeneity. In the absence of both environmental and taxonomic changes, we attribute this pattern to the long‐term effects of interspecific competition, which acted at an unusually slow pace because the number of competing species and potential immigrants was dramatically reduced by the end‐Permian mass extinction. We suggest that these non‐actualistic conditions led to decreased rates of niche differentiation and hence to the delayed rediversification of benthos that characterizes the aftermath of the greatest Phanerozoic mass extinction event. A hyperbolic diversification model is proposed, which accounts for the positive relationship between the intensity of interspecific competition and the rate of niche differentiation and resolves the conundrum of delayed rediversification at a time when niche space was largely vacated.     

Alzheimer's disease: its origin at the membrane, evidence and questions

Numerous results on membrane lipid composition from different regions of autopsied Alzheimer's disease brains in comparison with corresponding fractions isolated from control brains revealed significant differences in serine- and ethanolamine-containing glycerophospholipid as well as in glycosphingolipid content. Changes in membrane lipid composition are frequently accompanied by alterations in membrane fluidity, hydrophobic mismatch, lipid signaling pathways, transient formation and disappearance of lipid microdomains, changes in membrane permeability to cations and variations of other membrane properties. In this review we focus on possible implications of altered membrane composition on beta-amyloid precursor protein (APP) and on proteolysis of APP leading eventually to the formation of neurotoxic beta-amyloid (A beta) peptides, the major proteinaceous component of extracellular senile plaques, directly involved in Alzheimer's disease pathogenesis.     

Focal topographic changes in inflammatory microcirculation associated with lymphocyte slowing and transmigration

Microcirculation is the primary mechanism for delivering lymphocytes to inflammatory tissues. Blood flow within microvessels ensures a supply of lymphocytes at the blood-endothelial interface. Whether the structure of the inflammatory microcirculation facilitates lymphocyte transmigration is less clear. To illuminate the microcirculatory changes associated with lymphocyte transmigration, we used intravital videomicroscopy to examine the dermal microcirculation after application of the epicutaneous antigen oxazolone. Intravascular injection of fluorescein-labeled dextran demonstrated focal topographic changes in the microcirculation. These focal changes had the appearance of loops or hairpin turns in the oxazolone-stimulated skin. Changes were maximal at 96 h and coincided with peak lymphocyte recruitment. To determine whether these changes were associated with lymphocyte transmigration, lymphocytes obtained from efferent lymph of draining lymph nodes at 96 h were fluorescently labeled and reinjected into inflammatory microcirculation. Epifuorescence intravital video microscopy demonstrated focal areas were associated with lymphocyte slowing and occasional transmigration. In contrast, focal loops and lymphocyte slowing were rarely observed in the contralateral control microcirculation. Results suggest that structural adaptations in inflammatory microcirculation represented by focal topographic changes may contribute to regulation of tissue entry by recirculating lymphocytes.     

The skull of Microtia, an extinct burrowing murine rodent of the late Neogene Gargano palaeoisland

The Neogene Gargano paleoisland (southern Italy) has yielded numerous fossil vertebrates, some of them showing extraordinary morphological peculiarities. Among these endemic species, the rodent genus Microtia Freudenthal, 1976, is represented by three main lineages that evolve toward gigantism. This genus is the most significant and abundant rodent represented in the Gargano palaeofaunas. Its evolutionary trends reveal an increase of size, accompanied by a complication of molar structure. Before carrying out a study on the Gargano rodent community, its structure and its evolution, it was necessary to characterize Microtia's ecological adaptations. Microtia was adapted to burrowing life, which appears to be a unique adaptation for a murine rodent, since European burrowing Plio-Pleistocene and extant rodents are only represented by arvicolids. Therefore, the extinction of Microtia is likely to be due to competition with arvicolids after the breakdown of isolation, since they occupied very similar ecological niches.     

Did incumbency play a role in maintaining boundaries between Late Ordovician brachiopod realms?

Studies of ecosystem level changes in the geological record have found that the major extinction events eliminated many incumbent clades that had been ecologically dominant for long intervals. Surviving clades that had not been able to compete with the extinct incumbents were then able to evolve adaptations that allowed them to move into the niches vacated by the incumbents. Underlying this pattern is the inability of clades that do not occupy a particular niche to evolve adaptations that would permit them to compete with incumbent clades that are already successfully occupying that niche. The zoogeographic distributions of brachiopods in the Late Ordovician of Laurentia may also have been maintained by incumbency, which was disrupted by the end-Ordovician extinction event. Following the extinction event, an Early Silurian zoogeographic reorganization occurred, during which surviving clades evolved into the vacated epeiric sea niches in the Early Silurian. Just as incumbency plays a role in long-term evolutionary patterns, zoogeographic realms and provinces are also partially maintained by incumbency.     

Extinction and replacement among predatory mammals in the North American late Eocene and Oligocene: Tracking a paleoguild over twelve million years

Changes in taxonomic and morphologic diversity within the paleoguild of predatory mammals were explored within a mammalian chronofauna spanning a twelve million year interval, from the latest Eocene to the end of the Oligocene of North America (36–24 ma). The timespan encompassed a modest extinction event among terrestrial mammals (circa 34 ma) followed by a period of relative stability. Morphological diversity was assessed with estimates of body mass, relative tooth size, and tooth shape. Principal component, nearest‐neighbor, and minimum‐spanning‐tree analyses were used to compare morphological diversity and species packing within predator paleoguilds in the mid‐Chadronian (37–34 ma), Orellan (34–32), Whitneyan (32–29.5), and early Arikareean (29.5–24) land mammal ages. Species richness of predators throughout the interval was relatively constant, fluctuating between 15 and 18 total taxa. Moreover, despite significant differences in taxonomic composition and a modest extinction event among terrestrial mammals, morphological diversity within the paleoguild was very similar in the Chadronian and Orellan. In the Whitneyan and especially the early Arikareean, the diversity of feeding adaptations among species declined slightly, largely due to the loss of several highly specialized meat‐eaters (creodonts, nimravids) and the addition of small omnivores (canids).     

On density and extinction in continuous population models

Survival analyses, investigations of extinction and persistence, are executed for populations represented by a nonautonomous differential equation model. The population is assumed governed by density dependent and time varying density independent demographic parameters. While traditional approaches to extinction postulate extinction on an infinite time horizon and at zero abundance level, survival analysis is developed not only for this traditional setting but also on a finite time horizon and at a nonzero threshold level. A main conclusion is that extinction of a temporally stressed population is determined by a totality of density independent and density dependent factors.     

Classification and evolution of metazoan traces at a topological level

Topological fidelity of metazoan traces formed by metazoan behaviour is little influenced by compaction, diagenesis, continuous soft-sediment deformation and biostratinomy, substrate consistency, etc., whereas all of these can greatly alter the Euclidean geometric attributes of metazoan traces. Morphological characteristics of trace fossils can be distinguished and described objectively by both topological and Euclidean geometric parameters. The former constitute the basis of ichnoorder and ichnofamily. On the basis of topological criteria, metazoan traces can be classified as 4 ichnoorders and 22 ichnofamilies, consisting of 9 basic and 3 combined topological configurations. At a topological level, the behavioural diversity and complexity indicated by metazoan topoichnotaxa remain fairly stable in the Phanerozoic. All ichnoorders, 75% of ichnofamilies and all 9 basic topological configurations of metazoan traces are formed in the late Neoproterozoic, and all ichnofamilies, a combination of topological configurations and the most complex and highest level of topological configurations of metazoan traces, occurred in the early Cambrian. The evolution of metazoan traces can be expressed in three ranks. Changes at ichnoorder level constitute the first evolution, which is associated with the advent of kingdoms (animalia and plantae, etc.) and phyla (Ediacara and bilaterian, etc.), and the first level of palaeoecological and palaeoenvironmental changes, such as the appearance or disappearance of an ecosystem (Precambrian biomat). The first evolution terminated in the late Neoproterozoic. Changes at the ichnofamily level constitute the second evolution, which is associated with the advent of important phyla such as coelomate animal explosion and the second level of palaeoecological and palaeoenvironmental changes, such as structural changes within an ecosystem. The second evolution terminated in the early Cambrian. Changes at the ichnogeneric and ichnospecific levels constitute the third evolution, which is associated with the appearance or disappearance of the organic taxa lower than phylum, such as dinosaurs and birds, etc., and the third and fourth levels of palaeoecological and palaeoenvironmental changes, such as community-type level changes, within an established ecological structure and community level. The third evolution has been taking place since the Proterozoic.     

Rapid genomic changes in interspecific and intergeneric hybrids and allopolyploids of Triticeae.

Allopolyploidy is preponderant in plants, which often leads to speciation. Some recent studies indicate that the process of wide hybridization and (or) genome doubling may induce rapid and extensive genetic and epigenetic changes in some plant species and genomic stasis in others. To further study this phenomenon, we analyzed three sets of synthetic allopolyploids in the Triticeae by restriction fragment length polymorphism (RFLP) using a set of expressed sequence tags (ESTs) and retrotransposons as probes. It was found that 40-64.7% of the ESTs detected genomic changes in the three sets of allopolyploids. Changes included disappearance of parental hybridization fragment(s), simultaneous appearance of novel fragment(s) and loss of parental fragment(s), and appearance of novel fragment(s). Some of the changes occurred as early as in the F1 hybrid, whereas others occurred only after allopolyploid formation. Probing with retrotransposons revealed numerous examples of disappearance of sequences. No gross chromosome structural changes or physical elimination of sequences were found. It is suggested that DNA methylation and localized recombination at the DNA level were probably the main causes for the genomic changes. Possible implications of the genomic changes for allopolyploid genome evolution are discussed.     

Ecological consequences of Late Quaternary extinctions of megafauna

Large herbivorous vertebrates have strong interactions with vegetation, affecting the structure, composition and dynamics of plant communities in many ways. Living large herbivores are a small remnant of the assemblages of giants that existed in most terrestrial ecosystems 50 000 years ago. The extinction of so many large herbivores may well have triggered large changes in plant communities. In several parts of the world, palaeoecological studies suggest that extinct megafauna once maintained vegetation openness, and in wooded landscapes created mosaics of different structural types of vegetation with high habitat and species diversity. Following megafaunal extinction, these habitats reverted to more dense and uniform formations. Megafaunal extinction also led to changes in fire regimes and increased fire frequency due to accumulation of uncropped plant material, but there is a great deal of variation in post-extinction changes in fire. Plant communities that once interacted with extinct large herbivores still contain many species with obsolete defences against browsing and non-functional adaptations for seed dispersal. Such plants may be in decline, and, as a result, many plant communities may be in various stages of a process of relaxation from megafauna-conditioned to megafauna-naive states. Understanding the past role of giant herbivores provides fundamental insight into the history, dynamics and conservation of contemporary plant communities.     

Biotic effects of the Chicxulub impact,K–T catastrophe and sea level change in Texas

Biotic effects of the Chicxulub impact, the K–T event and sea level change upon planktic foraminifera were evaluated in a new core and outcrops along the Brazos River, Texas, about 1000 km from the Chicxulub impact crater on Yucatan, Mexico. Sediment deposition occurred in a middle neritic environment that shallowed to inner neritic depths near the end of the Maastrichtian. The sea level fall scoured submarine channels, which were infilled by a sandstone complex with reworked Chicxulub impact spherules and clasts with spherules near the base. The original Chicxulub impact ejecta layer was discovered 45–60 cm below the sandstone complex, and predates the K–T mass extinction by about 300,000 years.Results show that the Chicxulub impact caused no species extinctions or any other significant biotic effects. The subsequent sea level fall to inner neritic depth resulted in the disappearance of all larger (> 150 μm) deeper dwelling species creating a pseudo-mass extinction and a survivor assemblage of small surface dwellers and low oxygen tolerant taxa. The K–T boundary and mass extinction was identified 40–80 cm above the sandstone complex where all but some heterohelicids, hedbergellids and the disaster opportunistic guembelitrids went extinct, coincident with the evolution of first Danian species and the global δ¹³C shift. These data reveal that sea level changes profoundly influenced marine assemblages in near shore environments, that the Chicxulub impact and K–T mass extinction are two separate and unrelated events, and that the biotic effects of this impact have been vastly overestimated.