My research focuses on assessing the biological, environmental, and anthropogenic drivers that form communities of species, that drive them apart, and how those rules change through time. I am also interested in species’ landscape use, the environmental and biological drivers of long distance migration, and the impacts of modern climate regimes and anthropogenically induced habitat fragmentation on migration strategies. While I also work in tropical and temperate systems, I am particularly drawn to how these dynamics are structured in arctic settings, where current rates of environmental changes are particularly high and available data on ecosystems are generally limited.

An over-arching research interest of mine is quantifying how well modern bone accumulations record ecological data from their source communities. By more fully understanding the quality and variety of biological data recorded in these underutilized sources of data, we can uncover critical reference points for assessing (i) recent ecological change in modern communities (the new field of “Conservation Paleobiology”), and (ii) the quality of ecological records available from fossil assemblages. Thus, I work at the nexus of biology, paleontology, geology, geography, and climatology to better understand life across the history of our planet, as well as today.

Reconstructing historical baselines from bones on the landscape

The biological data available from modern and fossil bone accumulations offer critical foundations for understanding mammal ecology, establishing baselines for evaluating modern wildlife populations, and aiding conservation and management planning.

My research in Yellowstone National Park established that temperate large mammal (ungulate) bone accumulations on landscape surfaces can faithfully record their source community’s richness and proportional abundance structure. Paired with recent surveys of living populations, bone accumulations also correctly identify species that have significantly changed in abundance over the last ten to eighty years, and the directions of those shifts (including local invasions and extirpations; Miller 2011, Behrensmeyer and Miller 2012). Because the season of death/input for some skeletal elements are identifiable (shed elk antlers are dropped in late-winter; bones of newborns are introduced in the spring), bone accumulations can be used to demarcate the spatial distribution of critical habitats (wintering and calving grounds) across a population’s geographic range (Miller 2012).

This finding was mirrored in the Arctic National Wildlife Refuge (AK), where high concentrations of shed female caribou antlers (which are dropped by females within days of giving birth) faithfully record caribou calving activity (Miller et al. 2013). Furthermore, bones decompose along stereotypical stages, allowing cohorts of input to be identified within a bone accumulation; these cohorts also track changes in species richness, relative abundances, and landscape use across decades-to-centuries (Miller 2011, 2012, Miller et al. 2013, 2014). Using ⁸⁷Sr/⁸⁶Sr geochemistry of shed antlers, I also discovered significant changes in landscape use and summering grounds for the Central Arctic Caribou Herd. This shift occurred prior to the initiation of biomonitoring surveys and following a series of ecological perturbations that included the construction of major infrastructure to support petroleum development (e.g., the Trans-Alaska Pipeline) within their summer and calving grounds (Miller et al. 2021). Slow bone weathering rates in arctic latitudes extend this insight into millennial timescales (Miller and Simpson, in press, Nature).

Pleistocene extinction dynamics and our modern biodiversity crisis

Determining what drives changes in species diversity is a fundamental goal of paleobiology and biology, and is important for contextualizing recent changes to plant and animal populations around the world. For large mammals, some of best fossil resources for studying population and community ecology through time come from late Pleistocene (“Ice Age”) permafrost deposits in Alaska and Yukon, Canada. With students and colleagues, I am using radiocarbon dated specimens paired with isotope geochemistry and microwear to explore fundamental questions related to differential species survivorship and ecology during periods of climatic and ecological change (Kelly et al. 2021). My work in these deposits is ongoing, including an undergraduate capstone project that used carnivore tooth marks on the bones of prey species (horse, bison) to test how predator pressure changed in response to large swings in prey population sizes across 40,000 years.

Sr isotopes and species mobility

To study the mobility of extinct and extant species, an increasingly popular method is to evaluate bones and teeth for ⁸⁷Sr/⁸⁶Sr. ⁸⁷Sr/⁸⁶Sr varies across space largely due to changes in surface geology and is faithfully recorded by growing biological tissues. With colleagues, I showed that models estimating how ⁸⁷Sr/⁸⁶Sr changes across space can accurately predict ⁸⁷Sr/⁸⁶Sr of biological tissues, particularly for large-bodied mammals (Crowley, Miller, and Bataille 2017). Following these positive results, I focused on evaluating changes in mobility and landscape use for caribou (Miller et al. 2021), extinct Floridian horses (Wallace et al. 2019), and mastodons (Miller et al 2022; PNAS). My mastodon study tracked patterns of landscape use for a male mastodon and was the first to document seasonal specificity of mating areas for any extinct mammal. Building on this work I am now focusing on a variety of Pleistocene and modern megafauna and (1) testing for changes in mobility of megafauna across the Pleistocene, (2) quantifying climatic drivers of mobility, and (3) evaluating how migratory patterns and individual mobility have shifted due to anthropogenic stressors and climate change.

Community assembly through time

Determining the composition, structure, and temporal stability of organismal communities are fundamental goals of ecology. While both ecologist and paleoecologists study these aspects of biology in parallel, there has been little effort to meld the two complementary disciplines to achieve a synthetic view. In collaboration with the Evolution of Terrestrial Ecosystems working group (Smithsonian Institution) I am working to aggregate datasets from plant, vertebrate, and invertebrate communities ranging in age from modern to those that thrived 100’s of millions of years ago. We are testing fundamental aspects of community ecology theory, quantifying recent changes in community assembly patterns, extinction risk (Lyons et al. 2016, Biology Letters), and β-diversity, and evaluating how current biological states relate (or are unique) to the rest of life’s recorded history. We have found that some aspects of species co-occurrence patterns (for both plants and animals) remained largely stable across 300 million years, and then changed precipitously during the Holocene; likely in response to human perturbations (Lyons et al. 2016, Nature). We also found profound shifts in the drivers of mammal communities (species co-occurrences switching from being biologically mediated to climatically mediated; Toth et al 2021, Science) and that increasing homogeneity of species across landscapes (likely driven by human activities) has been occurring for millennial longer than previously known (Fraser et al. 2022, Nature Communications).