Reorganization of surviving mammal communities after the end-Pleistocene megafaunal extinction

Pre-Younger Dryas megafaunal extirpation at Rancho La Brea linked to fire-driven state shift

The cause, or causes, of the Pleistocene megafaunal extinctions have been difficult to establish, in part because poor spatiotemporal resolution in the fossil record hinders alignment of species disappearances with archeological and environmental data. We obtained 172 new radiocarbon dates on megafauna from Rancho La Brea in California spanning 15.6 to 10.0 thousand calendar years before present (ka). Seven species of extinct megafauna disappeared by 12.9 ka, before the onset of the Younger Dryas. Comparison with high-resolution regional datasets revealed that these disappearances coincided with an ecological state shift that followed aridification and vegetation changes during the Bølling-Allerød (14.69 to 12.89 ka). Time-series modeling implicates large-scale fires as the primary cause of the extirpations, and the catalyst of this state shift may have been mounting human impacts in a drying, warming, and increasingly fire-prone ecosystem.

Conceptual and empirical bridges between micro- and macroevolution

Explaining broad molecular, phenotypic and species biodiversity patterns necessitates a unifying framework spanning multiple evolutionary scales. Here we argue that although substantial effort has been made to reconcile microevolution and macroevolution, much work remains to identify the links between biological processes at play. We highlight four major questions of evolutionary biology whose solutions require conceptual bridges between micro and macroevolution. We review potential avenues for future research to establish how mechanisms at one scale (drift, mutation, migration, selection) translate to processes at the other scale (speciation, extinction, biogeographic dispersal) and vice versa. We propose ways in which current comparative methods to infer molecular evolution, phenotypic evolution and species diversification could be improved to specifically address these questions. We conclude that researchers are in a better position than ever before to build a synthesis to understand how microevolutionary dynamics unfold over millions of years.

Late Pleistocene megafauna extinction leads to missing pieces of ecological space in a North American mammal community

The conservation status of large-bodied mammals is dire. Their decline has serious consequences because they have unique ecological roles not replicated by smaller-bodied animals. Here, we use the fossil record of the megafauna extinction at the terminal Pleistocene to explore the consequences of past biodiversity loss. We characterize the isotopic and body-size niche of a mammal community in Texas before and after the event to assess the influence on the ecology and ecological interactions of surviving species (>1 kg). Preextinction, a variety of C₄ grazers, C₃ browsers, and mixed feeders existed, similar to modern African savannas, with likely specialization among the two sabertooth species for juvenile grazers. Postextinction, body size and isotopic niche space were lost, and the δ¹³C and δ¹⁵N values of some survivors shifted. We see mesocarnivore release within the Felidae: the jaguar, now an apex carnivore, moved into the specialized isotopic niche previously occupied by extinct cats. Puma, previously absent, became common and lynx shifted toward consuming more C₄-based resources. Lagomorphs were the only herbivores to shift toward C₄ resources. Body size changes from the Pleistocene to Holocene were species-specific, with some animals (deer, hare) becoming significantly larger and others smaller (bison, rabbits) or exhibiting no change to climate shifts or biodiversity loss. Overall, the Holocene body-size-isotopic niche was drastically reduced and considerable ecological complexity lost. We conclude biodiversity loss led to reorganization of survivors and many "missing pieces" within our community; without intervention, the loss of Earth's remaining ecosystems that support megafauna will likely suffer the same fate.

Late quaternary biotic homogenization of North American mammalian faunas

Biotic homogenization—increasing similarity of species composition among ecological communities—has been linked to anthropogenic processes operating over the last century. Fossil evidence, however, suggests that humans have had impacts on ecosystems for millennia. We quantify biotic homogenization of North American mammalian assemblages during the late Pleistocene through Holocene (~30,000 ybp to recent), a timespan encompassing increased evidence of humans on the landscape (~20,000–14,000 ybp). From ~10,000 ybp to recent, assemblages became significantly more homogenous (>100% increase in Jaccard similarity), a pattern that cannot be explained by changes in fossil record sampling. Homogenization was most pronounced among mammals larger than 1 kg and occurred in two phases. The first followed the megafaunal extinction at ~10,000 ybp. The second, more rapid phase began during human population growth and early agricultural intensification (~2,000–1,000 ybp). We show that North American ecosystems were homogenizing for millennia, extending human impacts back ~10,000 years.

Biotic homogenization, which is increased similarity in the composition of species among communities, is rising due to human activities. Using North American mammal fossil records from the past 30,000 years, this study shows that this phenomenon is ancient, beginning between 12,000 and 10,000 years ago with the extinction of the mammal megafauna.

Mammal species occupy different climates following the expansion of human impacts

Cities and agricultural fields encroach on the most fertile, habitable terrestrial landscapes, fundamentally altering global ecosystems. Today, 75% of terrestrial ecosystems are considerably altered by human activities, and landscape transformation continues to accelerate. Human impacts are one of the major drivers of the current biodiversity crisis, and they have had unprecedented consequences on ecosystem function and rates of species extinctions for thousands of years. Here we use the fossil record to investigate whether changes in geographic range that could result from human impacts have altered the climatic niches of 46 species covering six mammal orders within the contiguous United States. Sixty-seven percent of the studied mammals have significantly different climatic niches today than they did before the onset of the Industrial Revolution. Niches changed the most in the portions of the range that overlap with human-impacted landscapes. Whether by forcible elimination/introduction or more indirect means, large-bodied dietary specialists have been extirpated from climatic envelopes that characterize human-impacted areas, whereas smaller, generalist mammals have been facilitated, colonizing these same areas of the climatic space. Importantly, the climates where we find mammals today do not necessarily represent their past habitats. Without mitigation, as we move further into the Anthropocene, we can anticipate a low standing biodiversity dominated by small, generalist mammals.

Dietary plasticity of North American herbivores: a synthesis of stable isotope data over the past 7 million years

Palaeoecological interpretations are based on our understanding of dietary and habitat preferences of fossil taxa. While morphology provides approximations of diets, stable isotope proxies provide insights into the realized diets of animals. We present a synthesis of the isotopic ecologies (δ¹³C from tooth enamel) of North American mammalian herbivores since approximately 7 Ma. We ask: (i) do morphological interpretations of dietary behaviour agree with stable isotope proxy data? (ii) are grazing taxa specialists, or is grazing a means to broaden the dietary niche? and (iii) how is dietary niche breadth attained in taxa at the local level? We demonstrate that while brachydont taxa are specialized as browsers, hypsodont taxa often have broader diets that included more browse consumption than previously anticipated. It has long been accepted that morphology imposes limits on the diet; this synthesis supports prior work that herbivores with ‘grazing’ adaptions, such as hypsodont teeth, have the ability to consume grass but are also able to eat other foods. Notably, localized dietary breadth of even generalist taxa can be narrow (approx. 30 to 60% of a taxon's overall breadth). This synthesis demonstrates that ‘grazing-adapted’ taxa are varied in their diets across space and time, and this flexibility may reduce competition among ancient herbivores.

The preservation potential of terrestrial biogeographic patterns

Extinction events in the geological past are similar to the present-day biodiversity crisis in that they have a pronounced biogeography, producing dramatic changes in the spatial distributions of species. Reconstructing palaeobiogeographic patterns from fossils therefore allows us to examine the long-term processes governing the formation of regional biotas, and potentially helps build spatially explicit models for future biodiversity loss. However, the extent to which biogeographic patterns can be preserved in the fossil record is not well understood. Here, we perform a suite of simulations based on the present-day distribution of North American mammals, aimed at quantifying the preservation potential of beta diversity and spatial richness patterns over extinction events of varying intensities, and after applying a stepped series of taphonomic filters. We show that taphonomic biases related to body size are the biggest barrier to reconstructing biogeographic patterns over extinction events, but that these may be compensated for by both the small mammal record preserved in bird castings, as well as range expansion in surviving species. Overall, our results suggest that the preservation potential of biogeographic patterns is surprisingly high, and thus that the fossil record represents an invaluable dataset recording the changing spatial distribution of biota over key intervals in Earth History.

Changes in the large carnivore community structure of the Judean Desert in connection to Holocene human settlement dynamics

Investigating historical anthropogenic impacts on faunal communities is key to understanding present patterns of biodiversity and holds important implications for conservation biology. While several studies have demonstrated the human role in the extinction of large herbivores, effective methods to study human interference on large carnivores in the past are limited by the small number of carnivoran remains in the paleozoological record. Here, we integrate a systematic paleozoological survey of biogenic cave assemblages with the archaeological and paleoenvironmental records of the Judean Desert, to reveal historical changes in the large carnivore community. Our results show a late Holocene (~ 3400 years ago) faunal reassembly characterized by the diminishment of the dominant large carnivoran, the Arabian leopard (Panthera pardus sbsp. nimr), and the spread of the Syrian striped hyena (Hyaena hyaena sbsp. syriaca). We suggest that increased hunting pressure in combination with regional aridification were responsible for the decrease in the number of leopards, while the introduction of domestic animals and settlement refuse brought new scavenging opportunities for hyenas. The recent extirpation of leopards from the region has been a final note to the Holocene human impact on the ecosystem.

Investigating Biotic Interactions in Deep Time

Recent renewed interest in using fossil data to understand how biotic interactions have shaped the evolution of life is challenging the widely held assumption that long-term climate changes are the primary drivers of biodiversity change. New approaches go beyond traditional richness and co-occurrence studies to explicitly model biotic interactions using data on fossil and modern biodiversity. Important developments in three primary areas of research include analysis of (i) macroevolutionary rates, (ii) the impacts of and recovery from extinction events, and (iii) how humans (Homo sapiens) affected interactions among non-human species. We present multiple lines of evidence for an important and measurable role of biotic interactions in shaping the evolution of communities and lineages on long timescales.

Mammal Community Structure through the Paleocene-Eocene Thermal Maximum

AbstractHuman-mediated species invasion and climate change are leading to global extinctions and are predicted to result in the loss of important axes of phylogenetic and functional diversity. However, the long-term robustness of modern communities to invasion is unknown, given the limited timescales over which they can be studied. Using the fossil record of the Paleocene-Eocene thermal maximum (PETM; ∼56 Ma) in North America, we evaluate mammalian community-level response to a rapid global warming event (5°-8°C) and invasion by three Eurasian mammalian orders and by species undergoing northward range shifts. We assembled a database of 144 species body sizes and created a time-scaled composite phylogeny. We calculated the phylogenetic and functional diversity of all communities before, during, and after the PETM. Despite increases in the phylogenetic diversity of the regional species pool, phylogenetic diversity of mammalian communities remained relatively unchanged, a pattern that is invariant to the tree dating method, uncertainty in tree topology, and resolution. Similarly, body size dispersion and the degree of spatial taxonomic turnover of communities remained similar across the PETM. We suggest that invasion by new taxa had little impact on Paleocene-Eocene mammal communities because niches were not saturated. Our findings are consistent with the numerous studies of modern communities that record little change in community-scale richness despite turnover in taxonomic composition during invasion. What remains unknown is whether long-term robustness to biotic and abiotic perturbation are retained by modern communities given global anthropogenic landscape modification.

The megabiota are disproportionately important for biosphere functioning

A prominent signal of the Anthropocene is the extinction and population reduction of the megabiota—the largest animals and plants on the planet. However, we lack a predictive framework for the sensitivity of megabiota during times of rapid global change and how they impact the functioning of ecosystems and the biosphere. Here, we extend metabolic scaling theory and use global simulation models to demonstrate that (i) megabiota are more prone to extinction due to human land use, hunting, and climate change; (ii) loss of megabiota has a negative impact on ecosystem metabolism and functioning; and (iii) their reduction has and will continue to significantly decrease biosphere functioning. Global simulations show that continued loss of large animals alone could lead to a 44%, 18% and 92% reduction in terrestrial heterotrophic biomass, metabolism, and fertility respectively. Our findings suggest that policies that emphasize the promotion of large trees and animals will have disproportionate impact on biodiversity, ecosystem processes, and climate mitigation.

Human-driven losses of megafauna and megaflora may have disproportionate ecological consequences. Here, the authors combine metabolic scaling theory and global simulation models to show that past and continued reduction of megabiota have and will continue to decrease ecosystem and biosphere functioning.

Towards a Probabilistic Understanding About the Context-Dependency of Species Interactions

Observational and experimental studies have shown that an interaction class between two species (be it mutualistic, competitive, antagonistic, or neutral) may switch to a different class, depending on the biotic and abiotic factors within which species are observed. This complexity arising from the evidence of context-dependencies has underscored a difficulty in establishing a systematic analysis about the extent to which species interactions are expected to switch in nature and experiments. Here, we propose an overarching theoretical framework, by integrating probabilistic and structural approaches, to establish null expectations about switches of interaction classes across environmental contexts. This integration provides a systematic platform upon which it is possible to establish new hypotheses, clear predictions, and quantifiable expectations about the context-dependency of species interactions.