Explanations for latitudinal diversity gradients must invoke rate variation

The latitudinal diversity gradient (LDG) describes the pattern of increasing numbers of species from the poles to the equator. Although recognized for over 200 years, the mechanisms responsible for the largest-scale and longest-known pattern in macroecology are still actively debated. I argue here that any explanation for the LDG must invoke differential rates of speciation, extinction, extirpation, or dispersal. These processes themselves may be governed by numerous abiotic or biotic factors. Hypotheses that claim not to invoke differential rates, such as ‘age and area’ or ‘time for diversification’, eschew focus from rate variation that is assumed by these explanations. There is still significant uncertainty in how rates of speciation, extinction, extirpation, and dispersal have varied regionally over Earth history. However, to better understand the development of LDGs, we need to better constrain this variation. Only then will the drivers of such rate variation – be they abiotic or biotic in nature – become clearer

Integrating ecology and evolution in deep time: using Ecological Niche Modeling to study species' evolutionary responses to climate change from the Pliocene to the present-day biodiversity crisis

The aim of my dissertation was to elucidate how environmental changes have influenced evolutionary and distributional patterns in the near-shore molluscan fauna of the Atlantic Coastal Plain (southeastern U.S.) over the past three million years. Disentangling the long-term evolutionary responses of species to environmental change is important for understanding the mechanisms controlling evolutionary processes and for assessing how current and future climate changes will impact Earth's biodiversity. My dissertation was comprised of three chapters that integrated both paleontological and neontological data to study the molluscan record of the Atlantic Coastal Plain. The first study in my dissertation focused on 14 extant marine mollusk species and their potential responses to future climate changes over the next ~100 years. Two hypotheses were tested: that suitable areas will shift northwards for these species, and that they will show varied responses to future climate change based on species-specific niche attributes. I found that species were not predicted to shift pole-ward, but rather showed varied responses to future warming. Many of the studied species will be hard hit by future climate changes, such that over 20% of their suitable area will disappear by the end of this century. The second study statistically analyzed whether the niches of mollusk species remained stable across three million years of profound environmental changes. Prior to this research, the long-term evolutionary dynamics of species' niches to differing climatic regimes remained uncertain, even though the question is vital to understanding the fate of biodiversity in a rapidly changing world. I found that species' tolerances were statistically similar from the Pliocene to the present-day, which suggest that species will respond to current and future warming by altering distributions to track suitable habitat, or, if the pace of change is too rapid, by going extinct. The last study tested whether niche breadth and/or geographic range size was a better predictor of extinction selectivity for mollusk species from the Pliocene. I hypothesized that species that went extinct post Pliocene would have smaller geographic ranges and smaller niche breadths compared with those species that are still extant. I found that only realized niche breadth (i.e., the breadth of the environment actually occupied by a species) and geographic range size, rather than fundamental niche breadth, are inversely related to extinction probability. This finding has implications for assessing which species are more at risk as a consequence of current and future climate changes, and helps to sharpen our understanding of which macroevolutionary processes shape patterns of diversity over evolutionary time scales. Together, these studies indicated that abiotic, environmental factors play a fundamental role in governing species' distributions in deep time. More specifically, species did not seem to rapidly evolve in response to new environmental conditions, but tracked preferred habitat or faced extirpation if conditions exceeded their tolerance limits. These findings can be used to ensure that paleobiology does not become the biology of the future

Biogeography and Evolution of the Araneae: A Synthetic Approach

Various methods are used to study the evolution and biogeography of the Araneae through time. Two new fossil spider species were described from Miocene Dominican amber and French Cretaceous amber. A preliminary biogeographic analysis was performed on the former in order to elucidate the biogeographic origins of the genus to which it belongs. Further, ecological niche modeling, a biogeographic technique used to delineate the set of tolerances and limits in multidimensional space that define where a species is potentially able to maintain populations, was undertaken on the brown recluse (Loxosceles reclusa) spider for extant distributions and potential future distributions given climate change. A methodological analysis addressing how error within species occurrence points influences model quality within ecological niche modeling was conducted. Results indicated that studies of lower spatial resolution are valid, if enough data are utilized; this has implications for using ecological niche modeling in the fossil record

Forecasting climate-driven habitat changes for Australian freshwater fishes

Aims: Climate change is expected to have profound effects on species' distributions into the future. Freshwater fishes, an important component of freshwater ecosystems, are no exception. Here, we project shifts in suitable conditions for Australian freshwater fishes under different climate change scenarios to identify species that may experience significant declines in habitat suitability. Location: Australia. Methods: We use MAXENT bioclimatic models to estimate the effect of climate change on the suitable conditions for 154 species of Australian freshwater fishes, of which 109 are endemic and 29 are threatened with extinction. Suitable conditions for freshwater fish species are modelled using three different Earth System climate models (ESMs) under two different emission scenarios to the year 2100. For each species, we examine potential geographic shifts in the distribution of suitable conditions from the present day to 2100 and quantify how habitat suitability may change at currently occupied sites by the end of this century. Results: Broadscale poleward shifts in suitable conditions are projected for Australian freshwater fishes by an average of up to 0.38° (~180 km) across all species, depending on the emission scenario. Considerable loss of suitable conditions is forecast to occur within currently recognized distributional extents by 2100, with a mean projected loss of up to 17.5% across species. Predicted geographic range shifts and declines are larger under a high-emission scenario. Threatened species are projected to be more adversely affected than nonthreatened species. Main Conclusions: Our models identify species and geographic regions that may be vulnerable to climate change, enabling freshwater fish conservation into the future

Spatial standardization of taxon occurrence data—a call to action

The fossil record is spatiotemporally heterogeneous: taxon occurrence data have patchy spatial distributions, and this patchiness varies through time. Large-scale quantitative paleobiology studies that fail to account for heterogeneous sampling coverage will generate uninformative inferences at best and confidently draw wrong conclusions at worst. Explicitly spatial methods of standardization are necessary for analyses of large-scale fossil datasets, because nonspatial sample standardization, such as diversity rarefaction, is insufficient to reduce the signal of varying spatial coverage through time or between environments and clades. Spatial standardization should control both geographic area and dispersion (spread) of fossil localities. In addition to standardizing the spatial distribution of data, other factors may be standardized, including environmental heterogeneity or the number of publications or field collecting units that report taxon occurrences. Using a case study of published global Paleobiology Database occurrences, we demonstrate strong signals of sampling; without spatial standardization, these sampling signatures could be misattributed to biological processes. We discuss practical issues of implementing spatial standardization via subsampling and present the new R package divvy to improve the accessibility of spatial analysis. The software provides three spatial subsampling approaches, as well as related tools to quantify spatial coverage. After reviewing the theory, practice, and history of equalizing spatial coverage between data comparison groups, we outline priority areas to improve related data collection, analysis, and reporting practices in paleobiology

Climate change is an important predictor of extinction risk on macroevolutionary timescales

Anthropogenic climate change is increasing rapidly and already impacting biodiversity. Despite the importance for future projections, understanding of the underlying mechanisms by which climate mediates extinction remains limited. We present an integrated approach examining the role of intrinsic traits vs. extrinsic climate change in mediating extinction risk for marine invertebrates over the past 485 million years. We found that a combination of physiological traits and the magnitude of climate change are necessary to explain marine invertebrate extinction patterns. Our results suggest that taxa previously identified as extinction resistant may still succumb to extinction if the magnitude of climate change is great enough

Ecological niche conservatism spurs diversification in response to climate change

Lengthy debate has surrounded the theoretical and empirical science of whether climatic niche evolution is related to increased or decreased rates of biological diversification. Since species can persist for thousands to millions of years, these questions cross broad scales of time and space. Thus, short-term experiments may not provide comprehensive understanding of the system, leading to the emergence of contrasting opinions: niche evolution may increase diversity by allowing species to explore and colonize new geographic areas across which they could speciate – or – niche conservatism might augment biodiversity by supporting isolation of populations that may then undergo allopatric speciation. Here, we use a simulation approach to test how biological diversification responds to different rates and modes of niche evolution. We find that niche conservatism promotes biological diversification, whereas labile niches—whether adapting to conditions available or changing randomly—generally led to slower diversification rates. These novel results provide a framework for understanding how Earth–life interactions produced such a diverse biota

Climate change is an important predictor of extinction risk on macroevolutionary timescales

Anthropogenic climate change is increasing rapidly and already impacting biodiversity. Despite its importance in future projections, understanding of the underlying mechanisms by which climate mediates extinction remains limited. We present an integrated approach examining the role of intrinsic traits versus extrinsic climate change in mediating extinction risk for marine invertebrates over the past 485 million years. We found that a combination of physiological traits and the magnitude of climate change is necessary to explain marine invertebrate extinction patterns. Our results suggest that taxa previously identified as extinction resistant may still succumb to extinction if the magnitude of climate change is great enough.</p

Human brains preserve in diverse environments for at least 12 000 years

The brain is thought to be among the first human organs to decompose after death. The discovery of brains preserved in the archaeological record is therefore regarded as unusual. Although mechanisms such as dehydration, freezing, saponification, and tanning are known to allow for the preservation of the brain on short time scales in association with other soft tissues (≲4000 years), discoveries of older brains, especially in the absence of other soft tissues, are rare. Here, we collated an archive of more than 4400 human brains preserved in the archaeological record across approximately 12 000 years, more than 1300 of which constitute the only soft tissue preserved amongst otherwise skeletonized remains. We found that brains of this type persist on time scales exceeding those preserved by other means, which suggests an unknown mechanism may be responsible for preservation particular to the central nervous system. The untapped archive of preserved ancient brains represents an opportunity for bioarchaeological studies of human evolution, health and disease

Experimental evidence that clay inhibits bacterial decomposers: Implications for preservation of organic fossils

Exceptionally preserved organic fossils are commonly associated with clay-rich horizons or directly with clay minerals. It has been posited that interactions between clay minerals and organic tissues inhibit enzymatic reactions or protect carcasses in such a way that decay is impeded. However, interactions between clay minerals and the biological agents of decay, especially bacteria, may be at least as important to preservation potential. Here we show that clays of particle size <2 μmm in suspensions exceeding 10 mg/ml in concentration inhibit the growth of Pseudoalteromonas luteoviolacea, a marine heterotrophic bacterium involved in the decay of marine animals. Such clay-microbe interactions can contribute to exceptional preservation, and specific examples may play a role in shaping the distribution of Konservat- Lagerstätten through time.</p