Research

I study long-term diversity dynamics in order to understand the range of “normal” ecological fluctuation. Detailed historic and paleoecological records can be used to characterize these past long-term ecological dynamics, and to predict how species and ecosystems will respond to future change.

Biodiversity Change and Geologic Signals of the Anthropocene

Anthropogenic activities changed our planet over the course of the Holocene, but the scale of human impacts increased dramatically around the mid-20th century, representing the start of the Anthropocene. Global signals of these impacts and the resulting biotic and abiotic changes are captured in the geologic record. These pervasive anthropogenic impacts, including pollution, road-building, the rise of plastics, etc., are comparable in magnitude, uniqueness, and geologic preservational longevity to global changes that mark previous major geologic time intervals. To identify global and local geological signals that can be used to characterize the Anthropocene, we are examining sediment cores from Searsville Reservoir, a 126-year-old reservoir located at Jasper Ridge in the eastern foothills of the San Francisco Peninsula.

  • Learn more about our project at the Jasper Ridge Biological Preserve website and blog
  • Articles in Nature News about the Anthropocene Working Group, including our work on Searsville Reservoir
    • “Humans versus Earth: the quest to define the Anthropocene” link
    • “Anthropocene now: influential panel votes to recognize Earth’s new epoch” link

  • Article on the Anthropocene Working Group in Suddeutsche Zeitung, “‘We need new systems of knowledge production’: In Berlin, scientists met to prove that the era of the ‘Anthropocene’ has begun.” link – pdf

Abrupt Change in Ecological Systems:

How, when, and why do abrupt changes and ecological regime shifts occur? Using the extensive North American pollen record, I am applying and developing techniques to identify abrupt change in vegetation. Visit the ACES project website to learn more. Specific projects include:

  • Framework for diagnosing abrupt changes: Ratajczak et al. (2018) Abrupt changes in ecological systems: Inference and diagnosis. Trends in Ecology and Evolution. DOI: 10.1016/j.tree.2018.04.013 pdf – link
  • Intrinsic versus extrinsic drivers of abrupt Holocene Hemlock decline: Ramiadantsoa et al. (2019) The potential role of intrinsic processes in generating abrupt and synchronous tree declines during the Holocene. Ecology 100(2):e02579. DOI: 1002/ecy.2579 pdf – link – source code
  • Early warning signals of critical transitions in paleoecological time series: Stegner et al. (2019) Inferring critical transitions in paleoecological time series with irregular sampling and variable time-averaging. Quaternary Science Reviews 207: 49-63 DOI: 10.1016/j.quascirev.2019.01.009 pdf – link – source code
  • Do fires trigger abrupt changes in Northern Rockies forests during periods of climate change?: Stegner et al. (2019) Post-fire vegetation and climate dynamics in low-elevation forests over the last three millennia in Yellowstone National Park. Ecography 42:1226-1236 DOI: 10.1111/ecog.04445 link
  • Holocene resilience at the Midwest prairie-forest ecotone
  • Abrupt, community-level change in the Quaternary pollen record across North America

Conservation in a rapidly changing world:

We know from the fossil record that ecosystems are constantly in flux. However, rates of global change today are unprecedented and, among many negative outcomes, we face the likely possibility of major extinction. Paleontology can help to provide context and insight for today’s conservation challenges.

  • A new conservation paradigm: Generalizable conservation insights from the fossil record; Read the Conservation Paleobiology working group publication in Science here.

Paleo biogeography in western North America:

On-going paleoecological research on the Colorado Plateau is designed to 1) expand our knowledge of the Quaternary small vertebrates, a group under-studied in this region; 2) uncover paleo macroecological patterns and relate those to modern systems; 3) develop new techniques for identifying Quaternary vertebrate fossils; and 4) reveal how small mammals have responded to previous episodes of environmental change.

Colorado Plateau modern & historical biogeography:

I use a combination of mark-recapture surveys and data mining to understand modern mammal diversity patterns on local and regional scales, and to address questions like: How are species distributed across the landscape? Does abundance relate to environmental factors, intrinsic factors, or both? How do local diversity and abundance relate to regional patterns? And how does the modern compare to the fossil?

  • Modern vs. historical biogeography of Colorado Plateau National Parks (Stegner et al. 2017)

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