Mass extinctions do not explain skew in interspecific body size distributions

In several higher animal taxa, such as mammals and birds, the distribution of species body sizes is heavily skewed towards small size. Previous studies have suggested that small‐bodied organisms are less prone to extinction than large‐bodied species. If small body size is favourable during mass extinction events, a post mass extinction excess of small‐bodied species may proliferate and maintain skewed body size distributions sometime after. Here, we modelled mass extinctions and found that even unrealistically strong body mass selection has little effect on the skew of interspecific body size distributions. Moreover, selection against large body size may, counter intuitively, skew size distributions towards large body size. In any case, subsequent evolutionary diversification rapidly erases these rather small effects mass extinctions may have on size distributions. Next, we used body masses of extant species and phylogenetic methods to investigate possible changes in body size distributions across the Cretaceous–Paleogene (K‐Pg) mass extinction. Body size distributions of extant clades that originated during the Cretaceous are on average more skewed than their subclades that originated during the Paleogene, but the difference is only minor in mammals, and in birds, it can be explained by a positive relationship between species richness and skewness that is also present in clades that originated after the transition. Hence, we cannot infer from extant species whether the K‐Pg mass extinctions were size‐selective, but they are not the reason why most extant bird and mammal species are small‐bodied.     

The end and the beginning: recoveries from mass extinctions

The evolutionary consequences of mass extinctions depend as much on the processes of survival and recovery following these biotic crises as on the patterns of extinction themselves. Paleontologists are currently documenting biotic recoveries from six major mass extinctions and several smaller biotic crises. Although the immediate responses are remarkably similar after each event, with low-diversity assemblages dominated by widespread, eurytopic species, the recovery response in the long-term is more varied. Lineages that survive the extinction can lack the resilience for recovery, whereas others vanish from the fossil record seemingly to return from the dead after several million years.     

Predicting loss of evolutionary history: Where are we?

The Earth's evolutionary history is threatened by species loss in the current sixth mass extinction event in Earth's history. Such extinction events not only eliminate species but also their unique evolutionary histories. Here we review the expected loss of Earth's evolutionary history quantified by phylogenetic diversity (PD) and evolutionary distinctiveness (ED) at risk. Due to the general paucity of data, global evolutionary history losses have been predicted for only a few groups, such as mammals, birds, amphibians, plants, corals and fishes. Among these groups, there is now empirical support that extinction threats are clustered on the phylogeny; however this is not always a sufficient condition to cause higher loss of phylogenetic diversity in comparison to a scenario of random extinctions. Extinctions of the most evolutionarily distinct species and the shape of phylogenetic trees are additional factors that can elevate losses of evolutionary history. Consequently, impacts of species extinctions differ among groups and regions, and even if global losses are low within large groups, losses can be high among subgroups or within some regions. Further, we show that PD and ED are poorly protected by current conservation practices. While evolutionary history can be indirectly protected by current conservation schemes, optimizing its preservation requires integrating phylogenetic indices with those that capture rarity and extinction risk. Measures based on PD and ED could bring solutions to conservation issues, however they are still rarely used in practice, probably because the reasons to protect evolutionary history are not clear for practitioners or due to a lack of data. However, important advances have been made in the availability of phylogenetic trees and methods for their construction, as well as assessments of extinction risk. Some challenges remain, and looking forward, research should prioritize the assessment of expected PD and ED loss for more taxonomic groups and test the assumption that preserving ED and PD also protects rare species and ecosystem services. Such research will be useful to inform and guide the conservation of Earth's biodiversity and the services it provides.     

The two Early Toarcian (Early Jurassic) extinction events in ammonoids

The Early Toarcian (Early Jurassic) biological crisis was one of the ‘minor’ mass extinctions. It is linked with an oceanic anoxic event. Fossil data from sections located in northwestern European (epicontinental platforms and basins) and Tethyan (distal, epioceanic) areas indicate that Late Pliensbachian–Early Toarcian ammonoids experienced two extinction events during the Early Toarcian. The older one is linked with disruption of the Tethyan–Boreal provinciality, whereas the younger event correlates with the onset of anoxia and corresponds with the Early Toarcian mass‐extinction event. These two extinctions cannot be interpreted as episodes of a single, stepwise, event. Values of the net diversification, more than the number of extinctions, allow the two extinction events to be clearly recognized and distinguished. Values of regional net diversification for northwestern European and Tethyan faunas point to greater evolutionary dynamics in the epioceanic areas. The inclusion of Mediterranean faunas in the database proves that the ammonite turnover at the Early Toarcian mass‐extinction event was more important than previously thought. Progenitor (evolute Neolioceratoides), survivor (Dactylioceras, Polyplectus pluricostatus) and Lazarus (Procliviceras) taxa have been recognized. Different selectivity patterns are shown for the two events. The first one, linked to the disruption of the Tethyan–Boreal provinciality, has mainly affected ammonites adapted to epicontinental platforms. In the mass‐extinction event, no selectivity is recognized, because also Phylloceratina and Lytoceratina were deeply affected at species level, although their wide biogeographical distribution at clade level was a significant buffer against extinction. In contrast to Palaeozoic mass extinctions, ammonoid survivors and Lazarus taxa are characterized by complex sutures: Phylloceratina (long‐ranging ammonoids) and Polyplectus (relatively long‐ranging compared to other Ammonitina).     

Times of crisis in the evolution of the cephalopoda

The evolutionary history of the ectocochlian cephalopods is punctuated by a number of severe crises during each of which this class came very close to extinction. The crisis events follow each other at intervals of from seven to almost 300 million years and, with one exception, were not synchronous for Nautiloidea and Ammonoidea. Only at the end of the Triassic period did both groups simultaneously face the danger of extinction. Generally, the survivors of crisis situations have simple shell forms and are strikingly similar to each other. To trace the details of cephalopod evolution, the family on the taxonomic level and the stratigraphic stage on the chronological level do not provide scales fine enough to reconstruct the true course of this process. The causes of crises and “mass extinctions” are not yet understood. Most authors have approached this problem in a simplistic manner, searching for a single cause for any, or all, events of this kind. It seems that we do not even have begun to understand what the problems are.     

The complex effects of mass extinctions on morphological disparity

Studies of biodiversity through deep time have been a staple for biologists and paleontologists for over 60 years. Investigations of species richness (diversity) revealed that at least five mass extinctions punctuated the last half billion years, each seeing the rapid demise of a large proportion of contemporary taxa. In contrast to diversity, the response of morphological diversity (disparity) to mass extinctions is unclear. Generally, diversity and disparity are decoupled, such that diversity may decline as morphological disparity increases, and vice versa. Here, we develop simulations to model disparity changes across mass extinctions using continuous traits and birth-death trees. We find no simple null for disparity change following a mass extinction but do observe general patterns. The range of trait values decreases following either random or trait-selective mass extinctions, whereas variance and the density of morphospace occupation only decline following trait-selective events. General trends may differentiate random and trait-selective mass extinctions, but methods struggle to identify trait selectivity. Long-term effects of mass extinction trait selectivity change support for phylogenetic comparative methods away from the simulated Brownian motion toward Ornstein-Uhlenbeck and Early Burst models. We find that morphological change over mass extinction is best studied by quantifying multiple aspects of morphospace occupation.     

Re-assessing current extinction rates

There is a widespread belief that we are experiencing a mass extinction event similar in severity to previous mass extinction events in the last 600 million years where up to 95% of species disappeared. This paper reviews evidence for current extinctions and different methods of assessing extinction rates including species–area relationships and loss of tropical forests, changing threat status of species, co-extinction rates and modelling the impact of climate change. For 30 years some have suggested that extinctions through tropical forest loss are occurring at a rate of up to 100 species a day and yet less than 1,200 extinctions have been recorded in the last 400 years. Reasons for low number of identified global extinctions are suggested here and include success in protecting many endangered species, poor monitoring of most of the rest of species and their level of threat, extinction debt where forests have been lost but species still survive, that regrowth forests may be important in retaining ‘old growth’ species, fewer co-extinctions of species than expected, and large differences in the vulnerability of different taxa to extinction threats. More recently, others have suggested similar rates of extinction to earlier estimates but with the key cause of extinction being climate change, and in particular rising temperatures, rather than deforestation alone. Here I suggest that climate change, rather than deforestation is likely to bring about such high levels of extinction since the impacts of climate change are local to global and that climate change is acting synergistically with a range of other threats to biodiversity including deforestation.     

Extinction

A significant proportion of conservationists' work is directed towards efforts to save disappearing species. This relies upon the belief that species extinction is undesirable. When justifications are offered for this belief, they very often rest upon the assumption that extinction brought about by humans is different in kind from other forms of extinction. This paper examines this assumption and reveals that there is indeed good reason to suppose current anthropogenic extinctions to be different in kind from extinctions brought about at other times or by other factors. Having considered – and rejected – quantity and rate of extinction as useful distinguishing factors, four alternative arguments are offered, each identifying a way in which anthropogenic extinction is significantly different from other forms of extinction, even mass extinction: (1) Humans are a different kind of natural cause from other causes of extinction; (2) Extinctions brought about by humans are uniquely persistent; (3) Anthropogenic extinctions are effectively random whereas past mass extinctions are rule-bound; (4) The impact of the current anthropogenic extinction event differs from the impact of other extinction events of the past, such that future recovery may not follow past patterns. Together, these four arguments suggest that the present-day extinction event brought about by humans may be unprecedented and that we cannot clearly extrapolate from past to present recovery from extinctions. Although insufficient as justification for the claim that present-day extinctions are undesirable, the arguments provide some ammunition for conservationists' conviction that species extinction – in which humans play an accelerating role – ought to be prevented.     

The double edged sword: The demographic consequences of the evolution of self‐fertilization

Phylogenies indicate that the transition from outcrossing to selfing is frequent, with selfing populations being more prone to extinction. The rates of transition to selfing and extinction, acting on different timescales, could explain the observed distributions of extant selfing species among taxa. However, phylogenetic and theoretical studies consider these mechanisms independently, that is transitions do not cause extinction. Here, we theoretically explore the demographic consequences of the evolution of self‐fertilization. Deleterious mutations and mutations modifying the selfing rate are recurrently introduced and the number of offspring depends on individual fitness, allowing for a demographic feedback. We show that mutational meltdowns can be triggered in populations evolving near strict selfing. Populations having survived a demographic crash are more stable than ancestral outcrossing populations once deleterious mutations are purged. The relatively rapid time‐scales at which extinctions occur indicate that during evolutionary transitions the accumulation of deleterious mutations may not be the cause of extinctions observed on longer time scales, but could lead to the underestimation of transition rates from outcrossing to selfing.     

Cyclism revisited: extinction and ‘Achilles’ Heels' keep diversification in check on macroevolutionary time scales

Mass extinctions of varying magnitude prune the continuous diversification predicted by Darwinian evolutionary processes. They are caused by events that are too rare to become adaptatively accommodated. Their effects depend not only on the nature and magnitude of the triggering event but also on the state of the biosphere at the particular time. This is most clearly shown by the existence of Golden Ages preceding all Phanerozoic mass extinctions. These coincide with greenhouse periods, in which doomed clades gave rise to heteromorphs, deviating in strange ways from established bauplans. When critically examined, the seemingly ‘decadent’ morphologies of Schindewolf's ‘typolytic stages’ turn out to have been highly functional. The paradoxical link between adaptive peaks and evolutionary failure can now be explained. Specialisation tends to increase vulnerability (1) by narrowing niches and (2) by the retention of clade-specific conservative features that happen to become fatal Achilles’ Heels for entire clades in the face of a particular perturbation. Following extinctions, the availability of open niches favoured relatively rapid diversification of more innovative clades and their rise to ecological dominance (Schindewolf's ‘typogenetic stage’). Although the long-term changes can be observed only in the fossil record, Golden Biotopes in the present biosphere show that the Darwinian process may also be promoted by ecological isolation. As a result, clade histories do resemble individual biographies, but for ecological rather than orthogenetic reasons. This insight may help us to deal with the present mass extinction caused by our own species.     

Understanding extinction debts: spatio–temporal scales,mechanisms and a roadmap for future research

Extinction debt refers to delayed species extinctions expected as a consequence of ecosystem perturbation. Quantifying such extinctions and investigating long‐term consequences of perturbations has proven challenging, because perturbations are not isolated and occur across various spatial and temporal scales, from local habitat losses to global warming. Additionally, the relative importance of eco‐evolutionary processes varies across scales, because levels of ecological organization, i.e. individuals, (meta)populations and (meta)communities, respond hierarchically to perturbations. To summarize our current knowledge of the scales and mechanisms influencing extinction debts, we reviewed recent empirical, theoretical and methodological studies addressing either the spatio–temporal scales of extinction debts or the eco‐evolutionary mechanisms delaying extinctions. Extinction debts were detected across a range of ecosystems and taxonomic groups, with estimates ranging from 9 to 90% of current species richness. The duration over which debts have been sustained varies from 5 to 570 yr, and projections of the total period required to settle a debt can extend to 1000 yr. Reported causes of delayed extinctions are 1) life‐history traits that prolong individual survival, and 2) population and metapopulation dynamics that maintain populations under deteriorated conditions. Other potential factors that may extend survival time such as microevolutionary dynamics, or delayed extinctions of interaction partners, have rarely been analyzed. Therefore, we propose a roadmap for future research with three key avenues: 1) the microevolutionary dynamics of extinction processes, 2) the disjunctive loss of interacting species and 3) the impact of multiple regimes of perturbation on the payment of debts. For their ability to integrate processes occurring at different levels of ecological organization, we highlight mechanistic simulation models as tools to address these knowledge gaps and to deepen our understanding of extinction dynamics.     

Mass extinction: a commentary

Four neocatastrophist claims about mass extinction are currently being debated; they are that: 1, the late Cretaceous mass extinction was caused by large body impact; 2, as many as five other major extinctions were caused by impact; 3, the timing of extinction events since the Permian is uniformly periodic; and 4, the ages of impact craters on Earth are also periodic and in phase with the extinctions. Although strongly interconnected the four claims are independent in the sense that none depends on the others. Evidence for a link between impact and extinction is strong but still needs more confirmation through bed-by-bed and laboratory studies. An important area for future research is the question of whether extinction is a continuous process, with the rate increasing at times of mass extinctions, or whether it is episodic at all scales. If the latter is shown to be generally true, then species are at risk of extinction only rarely during their existence and catastrophism, in the sense of isolated events of extreme stress, is indicated. This is line of reasoning can only be considered an hypothesis for testing. In a larger context, paleontologists may benefit from a research strategy that looks to known Solar System and Galactic phenomena for predictions about environmental effects on earth. The recent success in the recognition of Milankovitch Cycles in the late Pleistocene record is an example of the potential of this research area.     

STRESS, EXTINCTIONS AND EVOLUTIONARY CHANGE: FROM LIVING ORGANISMS TO FOSSILS

1. Natural populations are exposed to environmental stress of varying intensities. This provides a reference point for extrapolations from the living biota to fossils and vice versa. 2. Evolutionary change is likely when there are resources in excess of maintenance and survival needs. It is largely precluded at species borders by the metabolic costs of stress; from this follows climatic tracking by species. 3. A relatively small increase in abiotic stress could underlie extinctions of stress-sensitive endemic species and the spread of stress-resistant generalist and widespread species. Widespread fossil species appear resistant to extinction under the stress level of normal background extinctions. 4. Synergistic interactions among generalized stresses should increase the likelihood of extinctions, especially for stresses with energetic consequences. 5. Some marine organisms survived the K-T mass extinction event because of stress-evasion mechanisms such as stress-resistant life-cycle stages with low metabolic rates. 6. In moderately stressed and narrowly fluctuating environments, sufficient genetic variability and metabolic energy should be available to permit adaptation. In these environments phyletic gradualism is expected. 7. In highly stressed and widely fluctuating environments, a punctuated evolutionary pattern is expected whereby stasis occurs most of the time. 8. Evolutionary patterns therefore can vary depending on the details of the interaction between stress, environmental fluctuations, energy availability and genetic variability. 9. Little evolutionary change is expected when the availability of energy is severely restricted. Examples include cave animals in stable but stressed environments and ‘living fossils’ in widely fluctuating but stressed environments. 10. Since the primary effect of abiotic stress may be at the level of energy carriers, a reductionist approach permits generalisations in considering extinctions and conditions under which diversification is likely.     

The loss of indirect interactions leads to cascading extinctions of carnivores

Species extinctions are biased towards higher trophic levels, and primary extinctions are often followed by unexpected secondary extinctions. Currently, predictions on the vulnerability of ecological communities to extinction cascades are based on models that focus on bottom‐up effects, which cannot capture the effects of extinctions at higher trophic levels. We show, in experimental insect communities, that harvesting of single carnivorous parasitoid species led to a significant increase in extinction rate of other parasitoid species, separated by four trophic links. Harvesting resulted in the release of prey from top‐down control, leading to increased interspecific competition at the herbivore trophic level. This resulted in increased extinction rates of non‐harvested parasitoid species when their host had become rare relative to other herbivores. The results demonstrate a mechanism for horizontal extinction cascades, and illustrate that altering the relationship between a predator and its prey can cause wide‐ranging ripple effects through ecosystems, including unexpected extinctions.     

From success to persistence: Identifying an evolutionary regime shift in the diverse Paleozoic aquatic arthropod group Eurypterida,driven by the Devonian biotic crisis

Mass extinctions have altered the trajectory of evolution a number of times over the Phanerozoic. During these periods of biotic upheaval a different selective regime appears to operate, although it is still unclear whether consistent survivorship rules apply across different extinction events. We compare variations in diversity and disparity across the evolutionary history of a major Paleozoic arthropod group, the Eurypterida. Using these data, we explore the group's transition from a successful, dynamic clade to a stagnant persistent lineage, pinpointing the Devonian as the period during which this evolutionary regime shift occurred. The late Devonian biotic crisis is potentially unique among the “Big Five” mass extinctions in exhibiting a drop in speciation rates rather than an increase in extinction. Our study reveals eurypterids show depressed speciation rates throughout the Devonian but no abnormal peaks in extinction. Loss of morphospace occupation is random across all Paleozoic extinction events; however, differential origination during the Devonian results in a migration and subsequent stagnation of occupied morphospace. This shift appears linked to an ecological transition from euryhaline taxa to freshwater species with low morphological diversity alongside a decrease in endemism. These results demonstrate the importance of the Devonian biotic crisis in reshaping Paleozoic ecosystems.     

Selective extinction and rapid loss of evolutionary history in the bird fauna

The extinction of species results in a permanent loss of evolutionary history. Recent theoretical studies show that this loss may be proportionally much smaller than the loss of species, but under some conditions can exceed it. Such conditions occur when the phylogenetic tree that describes the evolutionary relationships among species is highly imbalanced due to differences between lineages in past speciation and/or extinction rates. I used the taxonomy by C. G. Sibley and B. L. Monroe Jr to estimate the global loss of bird evolutionary history from historical and predicted extinctions, and to quantify the ensuing changes in balance of the bird phylogenetic tree. In the global bird fauna, evolutionary history is being lost at a high rate, similar to the rate of species extinction. The bird phylogenetic tree is highly imbalanced, and the imbalance is increased significantly by anthropogenic extinction. Historically, the elevated loss of bird evolutionary history has been fuelled mostly by phylogenetic non-randomness in the extinction of species, but the direct effect of tree imbalance is substantial and could dominate in the future.     

The present, past and future of human-caused extinctions

This paper re-evaluates whether we are really at the start of a mass extinction caused by humans. I consider the present, past and future of human-caused extinctions. As regards the present, estimates of extinction rates based on Red Data Books underestimate real values by a large factor, because the books evaluate only those species that have attracted specific attention and searches. Especially in tropical areas with few resident biologists, many poorly known species go extinct without having been the object of specific attention, and others disappear even before being described. A 'green list' of species known to be secure is needed to complement 'red books' of species known to be extinct. As regards the past, it is now clear that the first arrival of humans at any oceanic island with no previous human inhabitants has always precipitated a mass extinction in the island biota. Well-known victims include New Zealand's moas, Madagascar's giant lemurs, and scores of bird species on Hawaii and other tropical Pacific islands. Late-Pleistocene or Holocene extinctions of large mammals after the first arrival of humans in North America, South America and Australia may also have been caused by humans. Hence human-caused mass extinction is not a hypothesis for the future but an event that has been underway for thousands of years. As regards the future, consideration of the main mechanisms of human-caused extinctions (overhunting, effects of introduced species, habitat destruction, and secondary ripple effects) indicates that the rate of extinction is accelerating.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Early onset of secondary extinctions in ecological communities following the loss of top predators

The large vulnerability of top predators to human-induced disturbances on ecosystems is a matter of growing concern. Because top predators often exert strong influence on their prey populations their extinction can have far-reaching consequences for the structure and functioning of ecosystems. It has, for example, been observed that the local loss of a predator can trigger a cascade of secondary extinctions. However, the time lags involved in such secondary extinctions remain unexplored. Here we show that the loss of a top predator leads to a significantly earlier onset of secondary extinctions in model communities than does the loss of a species from other trophic levels. Moreover, in most cases time to secondary extinction increases with increasing species richness. If local secondary extinctions occur early they are less likely to be balanced by immigration of species from local communities nearby. The implications of these results for community persistence and conservation priorities are discussed.