Anthony D. Barnosky

Has the Earth’s sixth mass extinction already arrived?

Palaeontologists characterize mass extinctions as times when the Earth loses more than three-quarters of its species in a geologically short interval, as has happened only five times in the past 540 million years or so. Biologists now suggest that a sixth mass extinction may be under way, given the known species losses over the past few centuries and millennia. Here we review how differences between fossil and modern data and the addition of recently available palaeontological information influence our understanding of the current extinction crisis. Our results confirm that current extinction rates are higher than would be expected from the fossil record, highlighting the need for effective conservation measures.

Approaching a state shift in Earth’s biosphere

Localized ecological systems are known to shift abruptly and irreversibly from one state to another when they are forced across critical thresholds. Here we review evidence that the global ecosystem as a whole can react in the same way and is approaching a planetary-scale critical transition as a result of human influence. The plausibility of a planetary-scale ‘tipping point’ highlights the need to improve biological forecasting by detecting early warning signs of critical transitions on global as well as local scales, and by detecting feedbacks that promote such transitions. It is also necessary to address root causes of how humans are forcing biological changes.

Accelerated modern human–induced species losses: Entering the sixth mass extinction

Humans are causing a massive animal extinction without precedent in 65 million years. The oft-repeated claim that Earth’s biota is entering a sixth “mass extinction” depends on clearly demonstrating that current extinction rates are far above the “background” rates prevailing between the five previous mass extinctions. Earlier estimates of extinction rates have been criticized for using assumptions that might overestimate the severity of the extinction crisis. We assess, using extremely conservative assumptions, whether human activities are causing a mass extinction. First, we use a recent estimate of a background rate of 2 mammal extinctions per 10,000 species per 100 years (that is, 2 E/MSY), which is twice as high as widely used previous estimates. We then compare this rate with the current rate of mammal and vertebrate extinctions. The latter is conservatively low because listing a species as extinct requires meeting stringent criteria. Even under our assumptions, which would tend to minimize evidence of an incipient mass extinction, the average rate of vertebrate species loss over the last century is up to 100 times higher than the background rate. Under the 2 E/MSY background rate, the number of species that have gone extinct in the last century would have taken, depending on the vertebrate taxon, between 800 and 10,000 years to disappear. These estimates reveal an exceptionally rapid loss of biodiversity over the last few centuries, indicating that a sixth mass extinction is already under way. Averting a dramatic decay of biodiversity and the subsequent loss of ecosystem services is still possible through intensified conservation efforts, but that window of opportunity is rapidly closing.

Spatial Response of Mammals to Late Quaternary Environmental Fluctuations

Analyses of fossil mammal faunas from 2945 localities in the United States demonstrate that the geographic ranges of individual species shifted at different times, in different directions, and at different rates in response to late Quaternary environmental fluctuations. The geographic pattern of faunal provinces was similar for the late Pleistocene and late Holocene, but differing environmental gradients resulted in dissimilar species composition for these biogeographic regions. Modern community patterns emerged only in the last few thousand years, and many late Pleistocene communities do not have modern analogs. Faunal heterogeneity was greater in the late Pleistocene.

Distinguishing the effects of the Red queen and Court Jester on Miocene mammal evolution in the northern Rocky Mountains

Abstract Red Queen hypotheses maintain that biotic interactions are the most important drivers of evolutionary change, whereas Court Jester hypotheses regard physical-environmental perturbations, such as climate change, as most important. Tests for the biotic effects of climate change that are conducted on too large a geographic scale can falsely reject the Court Jester because climate is so complex its manifestation is in opposite directions in different geographic areas. Consequently, faunal responses vary from place to place, and lumping of data from different climate zones averages out any local faunal responses. Likewise, tests that are conducted at inappropriate temporal scales will not be effective at distinguishing between the Red Queen and Court Jester. A test at a temporal and geographic scale that takes the above considerations into account suggests a biotic response of mammals to a climatic warming event in the northern Rocky Mountains 18.5–14.0 Ma (the late-Early Miocene climatic optimum). During the environmental perturbation, mammalian species richness possibly increased, faunal turnover was pronounced, and taxa adapted to warm, arid environments became more abundant in numbers of species and density of individuals. The data are consistent with environmental change—the Court Jester—driving evolutionary change at sub-continental spatial scales and temporal scales that exceed typical Milankovitch oscillations. The Red Queen may be active at smaller temporal and geographic scales.

MAMMALIAN RESPONSE TO GLOBAL WARMING ON VARIED TEMPORAL SCALES

Abstract Paleontological information was used to evaluate and compare how Rocky Mountain mammalian communities changed during past global warming events characterized by different durations (350, ∼10,000–20,000, and 4 million years) and different per–100-year warming rates (1.0°C, 0.1°C, 0.06–0.08°C, 0.0002–0.0003°C per 100 years). Our goals were to determine whether biotic changes observed today are characteristic of or accelerated relative to what took place during past global warming events and to clarify the possible trajectory of mammalian faunal change that climate change may initiate. This determination is complicated because actual warming rates scale inversely with the time during which temperature is measured, and species with different life-history strategies respond (or do not) in different ways. Nevertheless, examination of past global warming episodes suggested that approximately concurrent with warming, a predictable sequence of biotic events occurs at the regional scale of the central and northern United States Rocky Mountains. First, phenotypic and density changes in populations are detectable within 100 years. Extinction of some species, noticeable changes in taxonomic composition of communities, and possibly reduction in species richness follow as warming extends to a few thousand years. Faunal turnover nears 100% and species diversity may increase when warm temperatures last hundreds of thousands to millions of years, because speciation takes place and faunal changes initiated by a variety of shorter-term processes accumulate. Climate-induced faunal changes reported for the current global warming episode probably do not yet exceed the normal background rate, but continued warming during the next few decades, especially combined with the many other pressures of humans on natural ecosystems, has a high probability of producing effects that have not been experienced often, if ever, in mammalian history.

Megafauna biomass tradeoff as a driver of Quaternary and future extinctions

Earths most recent major extinction episode, the Quaternary Megafauna Extinction, claimed two-thirds of mammal genera and one-half of species that weighed >44 kg between ≈50,000 and 3,000 years ago. Estimates of megafauna biomass (including humans as a megafauna species) for before, during, and after the extinction episode suggest that growth of human biomass largely matched the loss of non-human megafauna biomass until ≈12,000 years ago. Then, total megafauna biomass crashed, because many non-human megafauna species suddenly disappeared, whereas human biomass continued to rise. After the crash, the global ecosystem gradually recovered into a new state where megafauna biomass was concentrated around one species, humans, instead of being distributed across many species. Precrash biomass levels were finally reached just before the Industrial Revolution began, then skyrocketed above the precrash baseline as humans augmented the energy available to the global ecosystem by mining fossil fuels. Implications include (i) an increase in human biomass (with attendant hunting and other impacts) intersected with climate change to cause the Quaternary Megafauna Extinction and an ecological threshold event, after which humans became dominant in the global ecosystem; (ii) with continued growth of human biomass and todays unprecedented global warming, only extraordinary and stepped-up conservation efforts will prevent a new round of extinctions in most body-size and taxonomic spectra; and (iii) a near-future biomass crash that will unfavorably impact humans and their domesticates and other species is unavoidable unless alternative energy sources are developed to replace dwindling supplies of fossil fuels.

The Impact of the Species–Area Relationship on Estimates of Paleodiversity

Estimates of paleodiversity patterns through time have relied on datasets that lump taxonomic occurrences from geographic areas of varying size per interval of time. In essence, such estimates assume that the species–area effect, whereby more species are recorded from larger geographic areas, is negligible for fossil data. We tested this assumption by using the newly developed Miocene Mammal Mapping Project database of western North American fossil mammals and its associated analysis tools to empirically determine the geographic area that contributed to species diversity counts in successive temporal bins. The results indicate that a species–area effect markedly influences counts of fossil species, just as variable spatial sampling influences diversity counts on the modern landscape. Removing this bias suggests some traditionally recognized peaks in paleodiversity are just artifacts of the species–area effect while others stand out as meriting further attention. This discovery means that there is great potential for refining existing time-series estimates of paleodiversity, and for using species–area relationships to more reliably understand the magnitude and timing of such biotically important events as extinction, lineage diversification, and long-term trends in ecological structure.

Effects of Quaternary Climatic Change on Speciation in Mammals

An ongoing controversy in evolutionary biology is the extent to which climatic changes drive evolutionary processes. On the one hand are “Red Queen” hypotheses, which maintain that climatic change is less important than biotic interactions in causing evolutionary change. On the other hand are “Court Jester” models, which recognize climatic change as a very important stimulus to speciation. The Quaternary Period (the last 1.8 million years), characterized by multiple climatic changes in the form of glacial–interglacial transitions, offers a fertile testing ground for ascertaining whether cyclical climatic changes that operate at the 100,000-year time scale appreciably influence evolutionary patterns in mammals. Despite the increased potential for isolation of populations that should occur with multiple advances and retreats of glaciers and rearrangement of climatic zones, empirical data suggests that speciation rates were neither appreciably elevated for Quaternary mammals, nor strongly correlated with glacial–interglacial transitions. Abundant evidence attests to population-level changes within the Quaternary, but these did not usually lead to the origin of new species. This suggests that if climatic change does influence speciation rates in mammals, it does so over time scales longer than a typical glacial–interglacial cycle.

Morphological change in Quaternary mammals of North America

1. Quaternary mammals and evolutionary theory: introductory remarks and historical perspective Robert A. Martin and Anthony D. Barnosky 2. A method for recognizing morphological stasis Deborah K. Anderson 3. Mosaic evolution at the population level in Microtus pennsylvanicus Anthony D. Barnosky 4. Variogram analysis of paleontological data Andrew P. Czebieniak 5. Morphological change in quaternary mammals: a role for species interactions? Tamar Dayan, Daniel Simberloff and Eitan Tchernov 6. Rates of evolution in Plio-Pleistocene mammals: six case studies Philip D. Gingerich 7. Patterns of dental variation and evolution in prairie dogs, genus Cynomys H. Thomas Goodwin 8. Quantitative and qualitative evolution in the giant armadillo Holmesina (Edentata: Pampatheriidae) in Florida Richard C. Hulbert, Jr. and Gary S. Morgan 9. Evolution of mammoths and moose: the holarctic perspective Adrian M. Lister 10. Evolution of hypsodonty and enamel structure in Plio-Pleistocene rodents Larry D. Martin 11. Patterns of variation and speciation in Quaternary rodents Robert A. Martin 12. Decrease in the body size of white-tailed deer (Odocoileus virginianus) during the late Holocene in South Carolina and Georgia James R. Purdue and Elizabeth J. Reitz 13. Short-term fluctuations in small mammals of the late Pleistocene from Eastern Washington John M. Rensberger and Anthony D. Barnosky 14. Size change in North American Quaternary jaguars Kevin Seymour 15. Ontogenetic change of Ondatra zibethicus (Arvicolidae, Rodentia) cheek teeth analyzed by digital image processing Laurent Viriot, Jean Chaline, Andre Schaff and Eric Le Boulenge 16. Morphological change in woodrat (Rodentia: Cricetidae) molars Richard J. Zakrzewski Index.