A species is considered to be "rare" if it exhibits any one of the following attributes: (1) naturally occurs in a narrow geographical area, (2) occupies only one or a few specialised habitats, (3) forms only small population(s) in its range. An "endemic" species, however, grows naturally in a single geographical area, the size of which could be either narrow or relatively large. Not all endemic species are rare, just as not all rare species must necessarily be endemic. Many rare and/or endemic species exhibit one or more of the following attributes which make them especially prone to extinction: (1) narrow (and single) geographical range, (2) only one or a few populations, (3) small population size and little genetic variability, (4) over-exploitation by people, (5) declining population sizes, (6) low reproductive potential, (7) the need for specialised ecological niches, (8) growth that requires stable and nearly constant environments. When habitats of a rare and/or endemic species are damaged and/or fragmented by various human activities, the distribution ranges and population sizes of the species will be reduced, leaving them vulnerable to extinction at a much higher rate than other comparable species. Species that experience any of the above attributes must be given priority and monitored and managed carefully in an effort to promote genetic conservation.

Origination and extinction of stromatoporoid genera show that the rate of origination exceeded that of extinction from the early Early Devonian (Lochkovian) through the early Middle Devonian (Eifelian). For the remainder of the Devonian (Givetian-Famennian), extinction outpaced origination. During the Early and Middle Devonian, tropical to subtropical marine habitats were divided into the Eastern Americas Realm and the Old World Realm—in North America these were separated by the Transcontinental Arch and the Canadian Shield. Eustatic sea level, which had been rising since the Emsian, reached a point late in the Givetian (Taghanic Onlap) where the land barrier was breached, and Old World taxa invaded the Eastern Americas. Sea level continued to rise episodically through most of the Frasnian, followed by an abrupt fall in conjunction with the Frasnian-Famennian extinction. It is concluded that the decrease in stromatoporoid genus diversity, which occurred as Old World and Eastern Americas faunas came into competition, continued with further sea-level rise during the Frasnian. This lowered diversity put the stromatoporoids in a vulnerable position for extinction when climatic disturbances occurred at the end of the Frasnian.

We look at classifying extinction risks in three different ways, which affect how we can intervene to reduce risk. First, how does it start causing damage? Second, how does it reach the scale of a global catastrophe? Third, how does it reach everyone? In all of these three phases there is a defence layer that blocks most risks: First, we can prevent catastrophes from occurring. Second, we can respond to catastrophes before they reach a global scale. Third, humanity is resilient against extinction even in the face of global catastrophes. The largest probability of extinction is posed when all of these defences are weak, that is, by risks we are unlikely to prevent, unlikely to successfully respond to, and unlikely to be resilient against. We find that it’s usually best to invest significantly into strengthening all three defence layers. We also suggest ways to do so tailored to the classes of risk we identify. Lastly, we discuss the importance of underlying risk factors – events or structural conditions that may weaken the defence layers even without posing a risk of immediate extinction themselves.

Invasive predators can impose strong selection pressure on species that evolved in their absence and drive species to extinction. Interactions between coexisting predators may be particularly strong, as larger predators frequently kill smaller predators and suppress their abundances. Until 3500 years ago the marsupial thylacine was Australia's largest predator. It became extinct from the mainland soon after the arrival of a morphologically convergent placental predator, the dingo, but persisted in the absence of dingoes on the island of Tasmania until the 20th century. As Tasmanian thylacines were larger than dingoes, it has been argued that dingoes were unlikely to have caused the extinction of mainland thylacines because larger predators are rarely killed by smaller predators. By comparing Holocene specimens from the same regions of mainland Australia, we show that dingoes were similarly sized to male thylacines but considerably larger than female thylacines. Female thylacines would have been vulnerable to killing by dingoes. Such killing could have depressed the reproductive output of thylacine populations. Our results support the hypothesis that direct killing by larger dingoes drove thylacines to extinction on mainland Australia. However, attributing the extinction of the thylacine to just one cause is problematic because the arrival of dingoes coincided with another the potential extinction driver, the intensification of the human economy.

Loss of species will directly change the structure and potentially the dynamics of ecological communities, which in turn may lead to additional species loss (secondary extinctions) due to direct and/or indirect effects (e.g. loss of resources or altered population dynamics). Furthermore, the vulnerability of food webs to repeated species loss is expected to be affected by food web topology, species interactions, as well as the order in which species go extinct. Species traits such as body size, abundance and connectivity might determine a species’ vulnerability to extinction and, thus, the order in which species go primarily extinct. Yet, the sequence of primary extinctions, and their effects on the vulnerability of food webs to secondary extinctions, when species abundances are allowed to respond dynamically, has only recently become the focus of attention. Here, we analyse and compare topological and dynamical robustness to secondary extinctions of model food webs, in the face of 34 extinction sequences based on species traits. Although secondary extinctions are frequent in the dynamical approach and rare in the topological approach, topological and dynamical robustness tends to be correlated for many bottom–up directed, but not for top–down directed deletion sequences. Furthermore, removing species based on traits that are strongly positively correlated to the trophic position of species (such as large body size, low abundance, high net effect) is, under the dynamical approach, found to be as destructive as removing primary producers. Such top–down oriented removal of species are often considered to correspond to realistic extinction scenarios, but earlier studies, based on topological approaches, have found such extinction sequences to have only moderate effects on the remaining community. Thus, our result suggests that the structure of ecological communities, and therefore the integrity of important ecosystem processes could be more vulnerable to realistic extinction sequences than previously believed.

How many flowering plant species are there? Where are they? How many are going extinct, and how fast are they doing so? Interesting in themselves, these are questions at the heart of modern conservation biology. Determining the answers will dictate where and how successfully conservation efforts will be allocated. Plants form a large taxonomic sample of biodiversity. They are important in themselves and directly determine the diversity of many other taxonomic groups. Inspired by conversations with Peter Raven, we set out to provide quantitative answers to these questions. We argue that there are 450,000 species, two thirds of which live in the tropics, a third of all species are at risk of extinction, and they are going extinct 1000 to 10,000 times the background rate. In obtaining these results, we point to the critical role of dedicated taxonomic effort and biodiversity monitoring. We will only get a good answer to the age-old question of “how many species are there?” when we understand the population biology and social behavior of taxonomists. That most missing species will be found in biodiversity hotspots reaffirms their place as the foci of extinction for decades to come. Important, but not yet addressed, are future studies of how long plant species take to become extinct in habitat fragments. These will deliver not only better estimates of extinction rates, but also the critical timeframe of how quickly one needs to act to prevent extinctions.

Many modern extinction drivers are shared with past mass extinction events, such as rapid climate warming, habitat loss, pollution and invasive species. This commonality presents a key question: can the extinction risk of species during past mass extinction events inform our predictions for a modern biodiversity crisis? To investigate if it is possible to establish which species were more likely to go extinct during mass extinctions, we applied a functional trait-based model of extinction risk using a machine learning algorithm to datasets of marine fossils for the end-Permian, end-Triassic and end-Cretaceous mass extinctions. Extinction selectivity was inferred across each individual mass extinction event, before testing whether the selectivity patterns obtained could be used to 'predict' the extinction selectivity exhibited during the other mass extinctions. Our analyses show that, despite some similarities in extinction selectivity patterns between ancient crises, the selectivity of mass extinction events is inconsistent, which leads to a poor predictive performance. This lack of predictability is attributed to evolution in marine ecosystems, particularly during the Mesozoic Marine Revolution, associated with shifts in community structure alongside coincident Earth system changes. Our results suggest that past extinctions are unlikely to be informative for predicting extinction risk during a projected mass extinction.

Understanding which species might become extinct and the consequences of such loss is critical. One consequence is a cascade of further, secondary extinctions. While a significant amount is known about the types of communities and species that suffer secondary extinctions, little is known about the consequences of secondary extinctions for biodiversity. Here we examine the effect of these secondary extinctions on trophic diversity, the range of trophic roles played by the species in a community. Our analyses of natural and model food webs show that secondary extinctions cause loss of trophic diversity greater than that expected from chance, a result that is robust to variation in food web structure, distribution of interactions strengths, functional response, and adaptive foraging. Greater than expected loss of trophic diversity occurs because more trophically unique species are more vulnerable to secondary extinction. This is not a straightforward consequence of these species having few links with others but is a complex function of how direct and indirect interactions affect species persistence. A positive correlation between a species’ extinction probability and the importance of its loss defines high‐risk species and should make their conservation a priority.

Understanding which species might become extinct and the consequences of such loss is critical. One consequence is a cascade of further, secondary extinctions. While a significant amount is known about the types of communities and species that suffer secondary extinctions, little is known about the consequences of secondary extinctions for biodiversity. Here we examine the effect of these secondary extinctions on trophic diversity, the range of trophic roles played by the species in a community. Our analyses of natural and model food webs show that secondary extinctions cause loss of trophic diversity greater than that expected from chance, a result that is robust to variation in food web structure, distribution of interactions strengths, functional response, and adaptive foraging. Greater than expected loss of trophic diversity occurs because more trophically unique species are more vulnerable to secondary extinction. This is not a straightforward consequence of these species having few links with others but is a complex function of how direct and indirect interactions affect species persistence. A positive correlation between a species' extinction probability and the importance of its loss defines high-risk species and should make their conservation a priority.

Three fear-conditioning experiments with rat subjects examined the effects of extinction in multiple contexts on a final relapse (renewal) effect that occurred when the extinguished fear cue was tested in a new context (Experiments 1 and 3) or in the context in which fear conditioning had first occurred (Experiment 2). Rats that received extinction in three contexts demonstrated more fear during extinction than rats that received the same number and temporal distribution of extinction trials in one context; extinction was partially lost with each context switch. Although extinction in multiple contexts thus had an impact on extinction behavior, it did not reduce the size of the final renewal effect. Fear during extinction was occasionally positively correlated with fear during final testing, but the two were never negatively correlated. The results suggest that extinction in multiple contexts does not necessarily weaken fear renewal, and that extinction procedures that generate high levels of responding in extinction do not necessarily make extinction learning less context-specific.