The stability and collapse of marine ecosystems during the Permian-Triassic mass extinction.

The history of Earth's biodiversity is punctuated episodically by mass extinctions. These are characterized by major declines of taxon richness, but the accompanying ecological collapse has rarely been evaluated quantitatively. The Permian-Triassic mass extinction (PTME; ∼252 mya), as the greatest known extinction, permanently altered marine ecosystems and paved the way for the transition from Paleozoic to Mesozoic evolutionary faunas. Thus, the PTME offers a window into the relationship between taxon richness and ecological dynamics of ecosystems during a severe extinction. However, the accompanying ecological collapse through the PTME has not been evaluated in detail. Here, using food-web models and a marine paleocommunity dataset spanning the PTME, we show that after the first extinction phase, community stability decreased only slightly despite the loss of more than half of taxonomic diversity, while community stability significantly decreased in the second phase. Thus, taxonomic and ecological changes were unequivocally decoupled, with species richness declining severely ∼61 ka earlier than the collapse of marine ecosystem stability, implying that in major catastrophes, a biodiversity crash may be the harbinger of a more devastating ecosystem collapse.

Metaplasia of respiratory and digestive tissues in the eastern oyster Crassostrea virginica associated with the Deepwater Horizon oil spill.

Metaplasia is a well documented and deleterious effect of crude oil components on oysters. This reversible transformation of one cell type to another is a common response to petroleum-product exposure in molluscs. It has been shown experimentally in previous work that eastern oysters (Crassostrea virginica) exposed to petroleum products will exhibit metaplasia of digestive tissues. Here we document for the first time that wild adult oysters inhabiting coastal waters in the northern Gulf of Mexico during and in the aftermath of the Deepwater Horizon oil spill (2010) exhibited metaplasia in both ctenidial (respiratory and suspension feeding) and digestive tract tissues at significantly higher frequencies than geographic controls of C. virginica from Chesapeake Bay. Metaplasia included the loss of epithelial cilia, transformations of columnar epithelia, hyperplasia and reduction of ctenidial branches, and vacuolization of digestive tissues. Evidence for a reduction of metaplasia following the oil spill (2010-2013) is suggestive but equivocal.

Ecological dynamics of terrestrial and freshwater ecosystems across three mid-Phanerozoic mass extinctions from northwest China.

The Earth has been beset by many crises during its history, and yet comparing the ecological impacts of these mass extinctions has been difficult. Key questions concern the kinds of species that go extinct and survive, how communities rebuild in the post-extinction recovery phase, and especially how the scaling of events affects these processes. Here, we explore ecological impacts of terrestrial and freshwater ecosystems in three mass extinctions through the mid-Phanerozoic, a span of 121 million years (295-174 Ma). This critical duration encompasses the largest mass extinction of all time, the Permian-Triassic (P-Tr) and is flanked by two smaller crises, the Guadalupian-Lopingian (G-L) and Triassic-Jurassic (T-J) mass extinctions. Palaeocommunity dynamics modelling of 14 terrestrial and freshwater communities through a long sedimentary succession from the lower Permian to the lower Jurassic in northern Xinjiang, northwest China, shows that the P-Tr mass extinction differed from the other two in two ways: (i) ecological recovery from this extinction was prolonged and the three post-extinction communities in the Early Triassic showed low stability and highly variable and unpredictable responses to perturbation primarily following the huge losses of species, guilds and trophic space; and (ii) the G-L and T-J extinctions were each preceded by low-stability communities, but post-extinction recovery was rapid. Our results confirm the uniqueness of the P-Tr mass extinction and shed light on the trophic structure and ecological dynamics of terrestrial and freshwater ecosystems across the three mid-Phanerozoic extinctions, and how complex communities respond to environmental stress and how communities recovered after the crisis. Comparisons with the coeval communities from the Karoo Basin, South Africa show that geographically and compositionally different communities of terrestrial ecosystems were affected in much the same way by the P-Tr extinction.

Ecological continuity and transformation after the Permo-Triassic mass extinction in northeastern Panthalassa.

The Permo-Triassic mass extinction (PTME) is often implicated in the transition from the Paleozoic evolutionary fauna (PEF) to the modern evolutionary fauna (MEF). However, the exact timing and details of this progression are unknown, especially regarding the vacating and filling of functional ecological space after the PTME. Here, we quantify the functional diversity of middle Permian and Early Triassic marine paleocommunities in the western US to determine functional re-organization in the aftermath of the PTME. Results indicate that while the PTME was selective in nature, many new Triassic taxa either re-filled functional roles of extinct Permian taxa or performed the same functional roles as Permian survivors. Despite this functional overlap, Permian survivors and new Triassic taxa differed significantly in their relative abundances within those overlapping functions. This shift in numerical emphasis, driven by an increase in abundance towards more MEF-style traits, may represent a first step in the transition between the PEF and MEF. We therefore suggest that the extreme impact of the PTME had significant and permanent re-organizational effects on the intrinsic structure of marine ecosystems. Early Triassic ecosystems likely bridged the gap between the Paleozoic and modern evolutionary faunas, as newly originated Triassic taxa shared ecospace with Permian survivors, but shifted functional emphasis.

Community stability and selective extinction during the Permian-Triassic mass extinction.

The fossil record contains exemplars of extreme biodiversity crises. Here, we examined the stability of terrestrial paleocommunities from South Africa during Earth's most severe mass extinction, the Permian-Triassic. We show that stability depended critically on functional diversity and patterns of guild interaction, regardless of species richness. Paleocommunities exhibited less transient instability­relative to model communities with alternative community organization­and significantly greater probabilities of being locally stable during the mass extinction. Functional patterns that have evolved during an ecosystem's history support significantly more stable communities than hypothetical alternatives.

Response of seafloor ecosystems to abrupt global climate change.

Anthropogenic climate change is predicted to decrease oceanic oxygen (O2) concentrations, with potentially significant effects on marine ecosystems. Geologically recent episodes of abrupt climatic warming provide opportunities to assess the effects of changing oxygenation on marine communities. Thus far, this knowledge has been largely restricted to investigations using Foraminifera, with little being known about ecosystem-scale responses to abrupt, climate-forced deoxygenation. We here present high-resolution records based on the first comprehensive quantitative analysis, to our knowledge, of changes in marine metazoans (Mollusca, Echinodermata, Arthropoda, and Annelida; >5,400 fossils and trace fossils) in response to the global warming associated with the last glacial to interglacial episode. The molluscan archive is dominated by extremophile taxa, including those containing endosymbiotic sulfur-oxidizing bacteria (Lucinoma aequizonatum) and those that graze on filamentous sulfur-oxidizing benthic bacterial mats (Alia permodesta). This record, from 16,100 to 3,400 y ago, demonstrates that seafloor invertebrate communities are subject to major turnover in response to relatively minor inferred changes in oxygenation (>1.5 to <0.5 mL⋅L(-1) [O2]) associated with abrupt (<100 y) warming of the eastern Pacific. The biotic turnover and recovery events within the record expand known rates of marine biological recovery by an order of magnitude, from <100 to >1,000 y, and illustrate the crucial role of climate and oceanographic change in driving long-term successional changes in ocean ecosystems.

Late Cretaceous restructuring of terrestrial communities facilitated the end-Cretaceous mass extinction in North America.

The sudden environmental catastrophe in the wake of the end-Cretaceous asteroid impact had drastic effects that rippled through animal communities. To explore how these effects may have been exacerbated by prior ecological changes, we used a food-web model to simulate the effects of primary productivity disruptions, such as those predicted to result from an asteroid impact, on ten Campanian and seven Maastrichtian terrestrial localities in North America. Our analysis documents that a shift in trophic structure between Campanian and Maastrichtian communities in North America led Maastrichtian communities to experience more secondary extinction at lower levels of primary production shutdown and possess a lower collapse threshold than Campanian communities. Of particular note is the fact that changes in dinosaur richness had a negative impact on the robustness of Maastrichtian ecosystems against environmental perturbations. Therefore, earlier ecological restructuring may have exacerbated the impact and severity of the end-Cretaceous extinction, at least in North America.

The evolutionary palaeoecology of species and the tragedy of the commons.

The fossil record presents palaeoecological patterns of rise and fall on multiple scales of time and biological organization. Here, we argue that the rise and fall of species can result from a tragedy of the commons, wherein the pursuit of self-interests by individual agents in a larger interactive system is detrimental to the overall performance or condition of the system. Species evolving within particular communities may conform to this situation, affecting the ecological robustness of their communities. Results from a trophic network model of Permian-Triassic terrestrial communities suggest that community performance on geological timescales may in turn constrain the evolutionary opportunities and histories of the species within them.

A re-consideration of the taxonomic status of Nebria lacustris Casey (Coleoptera, Carabidae, Nebriini) based on multiple datasets - a single species or a species complex?

This study gathered evidence from principal component analysis (PCA) of morphometric data and molecular analyses of nucleotide sequence data for four nuclear genes (28S, TpI, CAD1, and Wg) and two mitochondrial genes (COI and 16S), using parsimony, maximum likelihood, and Bayesian methods. This evidence was combined with morphological and chorological data to re-evaluate the taxonomic status of Nebria lacustris Casey sensu lato. PCA demonstrated that both body size and one conspicuous aspect of pronotal shape vary simultaneously with elevation, latitude, and longitude and served to distinguish populations from the southern Appalachian highlands, south of the French Broad, from all other populations. Molecular analyses revealed surprisingly low overall genetic diversity within Nebria lacustris sensu lato, with only 0.39% of 4605 bp varied in the concatenated dataset. Evaluation of patterns observed in morphological and genetic variation and distribution led to the following taxonomic conclusions: (1) Nebria lacustris Casey and Nebria bellorum Kavanaugh should be considered distinct species, which is a NEW STATUS for Nebria bellorum. (2) No other distinct taxonomic subunits could be distinguished with the evidence at hand, but samples from northeastern Iowa, in part of the region known as the "Driftless Zone", have unique genetic markers for two genes that hint at descent from a local population surviving at least the last glacial advance. (3) No morphometric or molecular evidence supports taxonomic distinction between lowland populations on the shores of Lake Champlain and upland populations in the adjacent Green Mountains of Vermont, despite evident size and pronotal shape differences between many of their members.

Evolutionary origins of novel conchologic growth patterns in tropical American corbulid bivalves.

We conducted a combined sclerochronologic and phylogenetic analysis to document patterns and rates of shell accretion in several subclades of related corbulids, and to explore the evolutionary origins of novel conchologic developmental patterns. We found three disparate patterns of valve development in Neogene tropical American corbulid genera. These patterns include growth through primarily radial accretion along the sagittal plane, and two derivative patterns: one characterized by initial deposition of a thin shell followed by valve thickening with little increase in valve height, and another producing a well-defined nepioconch through a marked change in the primary growth direction. We conducted a species-level phylogenetic analysis of the taxa surveyed for growth patterns, focusing on the ([Bothrocorbula+Hexacorbula]+Caryocorbula) clade. The phylogenetic distribution of shell growth patterns suggests that this clade is characterized by derivative patterns of growth. Oxygen-isotope calibrated ontogenetic age estimates of species in the derived Bothrocorbula subclade further suggest that transitions from the ancestral radial (sagittal) growth pattern to a derived pattern of growth are a function of heterochrony (peramorphosis by acceleration). These findings are significant because they link previously observed patterns of morphological constraint with a specific evolutionary process, demonstrate how morphologic constraint and innovation can be interrelated, and serve as a model for understanding the evolution of morphologic diversity in the clade as a whole. Furthermore, this study highlights the utility of sclerochronologic records as an important component of evolutionary developmental research on organisms with accretionary skeletal growth.

Trophic network models explain instability of Early Triassic terrestrial communities.

Studies of the end-Permian mass extinction have emphasized potential abiotic causes and their direct biotic effects. Less attention has been devoted to secondary extinctions resulting from ecological crises and the effect of community structure on such extinctions. Here we use a trophic network model that combines topological and dynamic approaches to simulate disruptions of primary productivity in palaeocommunities. We apply the model to Permian and Triassic communities of the Karoo Basin, South Africa, and show that while Permian communities bear no evidence of being especially susceptible to extinction, Early Triassic communities appear to have been inherently less stable. Much of the instability results from the faster post-extinction diversification of amphibian guilds relative to amniotes. The resulting communities differed fundamentally in structure from their Permian predecessors. Additionally, our results imply that changing community structures over time may explain long-term trends like declining rates of Phanerozoic background extinction.

Comment on "Statistical independence of escalatory ecological trends in Phanerozoic marine invertebrates".

The analysis of Madin et al. (Reports, 12 May 2006, p. 897) of Phanerozoic diversity failed to support expected correlations between carnivores and noncarnivores, leading the authors to reject escalation as an important macroevolutionary process. The test, however, is based on a flawed model of causality, and the ecological groups are improperly delineated with regard to the hypothesis.

Complex morphological variability in complex evaporitic systems: thermal spring snails from the Chihuahuan Desert, Mexico.

Thermal springs in evaporitic environments provide a unique biological laboratory in which to study natural selection and evolutionary diversification. These isolated systems may be an analogue for conditions in early Earth or Mars history. One modern example of such a system can be found in the Chihuahuan Desert of north-central Mexico. The Cuatro Cienegas basin hosts a series of thermal springs that form a complex of aquatic ecosystems under a range of environmental conditions. Using landmark-based morphometric techniques, we have quantified an unusually high level of morphological variability in the endemic gastropod Mexipyrgus from Cuatro Cienegas. The differentiation is seen both within and between hydrological systems. Our results suggest that this type of environmental system is capable of producing and maintaining a high level of morphological diversity on small spatial scales, and thus should be a target for future astrobiological research.