Noel A. Heim

Global warming is often predicted to drive universal declines in animal body size, but empirical evidence remains equivocal. This is particularly true in marine systems, where body size distributions are influenced not only by temperature but also by oxygen and productivity, all of which are expected to be greatly altered in future oceans. Differences in regional climate trajectories and the physiology of marine taxa also suggest that body size responses will not be spatially or ecologically uniform. We address these complexities by projecting future changes in mean body size across species within ocean basins using a database of 23,329 marine mollusc species. We quantify how body size distributions change across energetic gradients and forecast assemblage level shifts of the five major classes of molluscs across all 10 oceanic basins. Under high greenhouse gas emission scenarios, we project a significant decline in body size in 68% of class–basin combinations (34 of 50), with some clades expected to shrink by ~16% in mean length by 2100. However, these responses are not universal but are governed by clade-level differences in dominant ecological and physiological strategies and the geography of climate change. Because biomass scales allometrically with length, these trends can correspond to substantial changes in the mass of the average species, from a 46% decline to a 15% increase depending on clade and region. Such widespread functional disruptions may threaten critical ecosystem services (carbon sequestration, nutrient cycling, and human food provisioning), with implications for global marine ecosystem functioning and services.

Many marine fish species are experiencing population declines, but their extinction risk profiles are largely understudied in comparison to their terrestrial vertebrate counterparts. Selective extinction of marine fish species may result in rapid alteration of the structure and function of ocean ecosystems. In this study, we compiled an ecological trait dataset for 8,185 species of marine ray-finned fishes (class Actinopterygii) from FishBase and used phylogenetic generalized linear models to examine which ecological traits are associated with increased extinction risk, based on the International Union for the Conservation of Nature Red List. We also assessed which threat types may be driving these species toward greater extinction risk and whether threatened species face a greater average number of threat types than non-threatened species. We found that larger body size and/or fishes with life histories involving movement between marine, brackish, and freshwater environments are associated with elevated extinction risk. Commercial harvesting threatens the greatest number of species, followed by pollution, development, and then climate change. We also found that threatened species, on average, face a significantly greater number of threat types than non-threatened species. These results can be used by resource managers to help address the heightened extinction risk patterns we found.

Two of the traits most often observed to correlate with extinction risk in marine animals are geographical range and body size. However, the relative effects of these two traits on extinction risk have not been investigated systematically for either background times or during mass extinctions. To close this knowledge gap, we measure and compare extinction selectivity of geographical range and body size of genera within five classes of benthic marine animals across the Phanerozoic using capture–mark–recapture models. During background intervals, narrow geographical range is strongly associated with greater extinction probability, whereas smaller body size is more weakly associated with greater extinction probability. During mass extinctions, the association between geographical range and extinction probability is reduced in every class and fully eliminated in some, whereas the association between body size and extinction probability varies in strength and direction across classes. While geographical range is universally the stronger predictor of survival during background intervals, variation among classes during mass extinction suggests a fundamental shift in extinction processes during these global catastrophes.

Ancient hyperthermal events in Earth’s history are ideal to isolate the evolutionary consequences of climate change and other environmental factors from other anthropogenic influences. A key biological trait hypothesized to be sensitive to climate change and straightforward to quantify in fossil data is body size. Most ectotherms modulate their physiological response to temperature and oxygen change in part through their body size. As hyperthermal events include both temperature increases and ocean deoxygenation, these events can have a deleterious effect on ectotherms on the ends of the body size spectrum. Large bodied ectotherms are particularly at risk, as oxygen demand increases with both body size and temperature. Previous work has assessed extinction selectivity of body size across hyperthermal events, but origination selectivity has not been assessed, which may be as important as extinction selectivity. Here, we measure extinction and origination selectivity with respect to body size for genera in six Linnean classes with robust fossil records (Rhynchonellata, Cephalopoda, Echinoidea, “bony fish” [an informal class consisting of Osteichthyes, Actinopteri, and Actinopterygii], Bivalvia, and Gastropoda). We compare selectivity during background intervals with those during hyperthermal events and their associated recovery intervals spanning the Middle Triassic to the Recent. We use capture-mark-recapture statistical models to measure extinction and origination selectivity while addressing the effects of variable sampling completeness with respect to time and body size. We find that genera with smaller body size are preferentially lost to extinction during background intervals, whereas body size is not associated with extinction probability during hyperthermal events. Conversely, originating genera are larger than average during background intervals across all size classes, but vary among classes in their body size pattern immediately following hyperthermal events. Rhynchonellate brachiopods, cephalopods, and echinoids exhibit originators that are smaller, on average, than the survivors after hyperthermal events, whereas originating bivalves and gastropods tend to be larger than the survivors. Bony fish do not exhibit size bias in origination after hyperthermal events. Overall, these results show that hyperthermal events affect both extinction and origination dynamics but have a greater effect on body size origination selectivity than extinction. The exact cause(s) for the greater effect of hyperthermal events on body size in origination versus extinction is not certain, but these results are consistent with previous findings spanning the Phanerozoic that compared body size selectivity patterns during background intervals to those associated with the Big Five mass extinction events. These results show that climate-associated perturbations change extinction and origination dynamics relative to background intervals, suggesting that climate-associated extinction and origination in the modern and future ocean may differ from geological background.

Extinction within the Paleozoic era has been studied in the past, but there still lacks a comprehensive understanding of how extinction risk changed throughout it. Our research project aims to bridge this gap by exploring extinction risk in relation to major Paleozoic phyla and ecological characteristics. Using R, we analyzed the Stanford Earth Body Size dataset, which includes extensive data (n=8816) on Paleozoic marine animals. In Step 1, regression coefficients were formed, indicating whether being in one of the 6 phyla in each period of the Paleozoic era conferred greater or less extinction risk. In Step 2, the examined ecological characteristics included ocean acidification resilience, feeding patterns, body volume, length, surface area, motility, tiering, circulatory systems, and respiratory organ type. In Step 3, 6 binomial machine learning models were created using the traits from Step 2 to determine whether an individual genus went extinct in a particular period. Our Step 1 results confirm that within these timeframes, while certain phyla have greater extinction risk, extinction risk was not uniform across these groups. Our Step 2 results show certain traits provided advantages and disadvantages for an organism’s extinction risk. One interesting pattern was that the only consistently non-significant traits were body length, area, and volume. Likewise with Step 1, extinction risk for each ecological characteristic varied across the Paleozoic. Finally, in Step 3, the results were largely successful. Most of the six models had an accuracy above 80% with the highest being 92% in the Cambrian. The areas under the Precision-Recall and the Receiver Operating Characteristic Curves were all in the acceptable (<0.6) range, demonstrating that the model has low false positive/ negative rates and is able to distinguish between what trait indicates extinction or survival for each period. Our research project identified phyla at risk of extinction in each period of the Paleozoic, determined which natural traits incited greater extinction risk, and demonstrated machine learning models trained on fossil descriptors can predict when an individual genus became extinct. Our results confirmed that extinction risk is not consistently dependent on a singular factor nor is it constant across every period of the Paleozoic era.

To determine the distribution and causes of extinction threat across functional groups of terrestrial vertebrates, we assembled an ecological trait data set for 18,016 species of terrestrial vertebrates and utilized phylogenetic comparative methods to test which categories of habitat association, mode of locomotion, and feeding mode best predicted extinction risk. We also examined the individual categories of the International Union for Conservation of Nature Red List extinction drivers (e.g., agriculture and logging) threatening each species and determined the greatest threats for each of the four terrestrial vertebrate groups. We then quantified the sum of extinction drivers threatening each species to provide a multistressor perspective on threat. Cave dwelling amphibians ( p < 0.01), arboreal quadrupedal mammals (all of which are primates) ( p < 0.01), aerial and scavenging birds ( p < 0.01), and pedal (i.e., walking) squamates ( p < 0.01) were all disproportionately threatened with extinction in comparison with the other assessed ecological traits. Across all threatened vertebrate species in the study, the most common risk factors were agriculture, threatening 4491 species, followed by logging, threatening 3187 species, and then invasive species and disease, threatening 2053 species. Species at higher risk of extinction were simultaneously at risk from a greater number of threat types. If left unabated, the disproportionate loss of species with certain functional traits and increasing anthropogenic pressures are likely to disrupt ecosystem functions globally. A shift in focus from species‐ to trait‐centric conservation practices will allow for protection of at‐risk functional diversity from regional to global scales.

Whether mass extinctions and their associated recoveries represent an intensification of background extinction and origination dynamics versus a separate macroevolutionary regime remains a central debate in evolutionary biology. The previous focus has been on extinction, but origination dynamics may be equally or more important for long-term evolutionary outcomes. The evolution of animal body size is an ideal process to test for differences in macroevolutionary regimes, as body size is easily determined, comparable across distantly related taxa and scales with organismal traits. Here, we test for shifts in selectivity between background intervals and the ‘Big Five’ mass extinction events using capture–mark–recapture models. Our body-size data cover 10 203 fossil marine animal genera spanning 10 Linnaean classes with occurrences ranging from Early Ordovician to Late Pleistocene (485–1 Ma). Most classes exhibit differences in both origination and extinction selectivity between background intervals and mass extinctions, with the direction of selectivity varying among classes and overall exhibiting stronger selectivity during origination after mass extinction than extinction during the mass extinction. Thus, not only do mass extinction events shift the marine biosphere into a new macroevolutionary regime, the dynamics of recovery from mass extinction also appear to play an underappreciated role in shaping the biosphere in their aftermath.

The half-billion-year history of animal evolution is characterized by decreasing rates of background extinction. Earth's increasing habitability for animals could result from several processes: (i) a decrease in the intensity of interactions among species that lead to extinctions; (ii) a decrease in the prevalence or intensity of geological triggers such as flood basalt eruptions and bolide impacts; (iii) a decrease in the sensitivity of animals to environmental disturbance; or (iv) an increase in the strength of stabilizing feedbacks within the climate system and biogeochemical cycles. There is no evidence that the prevalence or intensity of interactions among species or geological extinction triggers have decreased over time. There is, however, evidence from palaeontology, geochemistry and comparative physiology that animals have become more resilient to an environmental change and that the evolution of complex life has, on the whole, strengthened stabilizing feedbacks in the climate system. The differential success of certain phyla and classes appears to result, at least in part, from the anatomical solutions to the evolution of macroscopic size that were arrived at largely during Ediacaran and Cambrian time. Larger-bodied animals, enabled by increased anatomical complexity, were increasingly able to mix the marine sediment and water columns, thus promoting stability in biogeochemical cycles. In addition, body plans that also facilitated ecological differentiation have tended to be associated with lower rates of extinction. In this sense, Cambrian solutions to Cambrian problems have had a lasting impact on the trajectory of complex life and, in turn, fundamental properties of the Earth system.